Materials for Tribology

MATERIALS FOR TRIBOLOGY

TRIBOLOGY SERIES Advisory Board W.J. Bartz (Germany, F.R.G.) R. Bassani (Italy) B. Briscoe (Gt. Britain) H. Czichos (Germany, F.R.G.) D. Dowson (Gt. Britain) K. Friedrich (Germany, F.R.G.) N. Gane (Australia)

Vol. 1 Vol. Vol. Vol. VOl. Vol. VOl. Vol.

2 3 4 5 6 7 8

VOl. 9 VOl. 10 Vol. 11 VOl. 12 Vol. 13 Vol. 14 Vol. 15 Vol. 16 Vol. 17 Vol. 18 VOl. 19 VOl. 20

W.A. Glaeser (U.S.A.) M. Godet (France) H.E. Hintermann (Switzerland) K.C. Ludema (U.S.A.) T. Sakurai (Japan) W.O. Winer (U.S.A.)

Tribology - A Systems Approach to the Science and Technology of Friction, Lubrication and Wear (Czichos) Impact Wear of Materials (Engel) Tribology of Natural and Artificial Joints (Dumbleton) Tribology of Thin Layers (Iliuc) Surface Effects in Adhesion, Friction, Wear, and Lubrication (Buckley) Friction and Wear of Polymers (Bartenev and Lavrentev) Microscopic Aspects of Adhesion and Lubrication (Georges, Editor) Industrial Tribology - The Practical Aspects of Friction, Lubrication and Wear (Jones and Scott, Editors) Mechanics and Chemistry in Lubrication (Dorinson and Ludema) Microstructure and Wear of Materials (Zum Gahr) Fluid Film Lubrication - Osborne Reynolds Centenary (Dowson et al., Editors) Interface Dynamics (Dowson et al., Editors) Tribology of Miniature Systems (Rymuza) Tribological Design of Machine Elements (Dowson et al., Editors) Encyclopedia of Tribology (Kajdas et al.) Tribology of Plastic Materials (Yamaguchi) Mechanics of Coatings (Dowson et al., Editors) Vehicle Tribology (Dowson et al., Editors) Rheology and Elastohydrodynamic Lubrication (Jacobson) Materials for Tribology (Glaeser)

TRIBOLOGY SERIES, 20

MATERIALS FOR TRIBOLOGY William A. Glaeser Engineering Mechanics Division, Battelle, Columbus, Ohio, USA

ELSEVIER Amsterdam London New York Tokyo

1992

ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

L i b r a r y o f Congress C a t a l o g i n g - i n - P u b l i c a t i o n Data Giaeser. W i l l i a m A. M a t e r i a l s for t r i b o l o g y / W i l l i a m A. Glaeser. p. cm. -- I T r i b o l o g y s e r l e s ; 20)

Includes b i b l i o g r a p h i c a l references and i n d e x . ISBN 0-444-88495-5 (U.S.) 1. T r l b o l o g y . I . T i t l e . 11. Series. TJ1075.G53 1992 621.8'9--dc20

9 1-4835 1

CIP ISBN 0 444 88495 5

0 1992 ELSEVIER SCIENCE PUBLISHERS B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V., Copyright & Permissions Department, P.O. Box 521, 1000 A M Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the publisher. No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Printed in The Netherlands

DEDICATION

T h i s book i s d e d i c a t e d t o my w i f e , B e t t y , who has k e p t me going

on t h i s long p r o j e c t . W.A. Glaeser

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vii

CONTENTS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

............................ INTRODUCTION ........................... METALLURGY OF STEELS ....................... C a r b o n Steels . . . . . . . . . . . . . . . . . . . . . . . . . SELECTION OF STEEIS . . . . . . . . . . . . . . . . . . . . . . . . PEARLITIC STEELS ......................... MARTENSITIC STEELS ........................

8

22

.........................

24

INTRODUCTION

.

CHAPTER 1 STEELS

STAINLESS STEELS

9

10 13

18

18

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 . . . . . . . . . . . . . . . . . . . . . . . . . . 37 BEARING STEELS . . . . . . . . . . . . . . . . . . . . . . . . . 41 MANGANESE STEELS SELECTING STEELS FOR MINING AND CONSTRUCTION INDUSTRIES . . . . . . 4 1 CHAPTER 2 . COPPER BASE BEARING MATERIALS . . . . . . . . . . . . . . . .4 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 INTRODUCTION 48 T I N BRONZES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 LEADED T I N BRONZES 51 COPPER LEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 ALUMINUM BRONZE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 MANGANESE BRONZE . . . . . . . . . . . . . . . . . . . . . . . . . 56 BERYLLIUM COPPER . . . . . . . . . . . . . . . . . . . . . . . . . . 56 POROUSBRONZES BEARING PROPERTIES OF COPPER BASE MATERIALS . . . . . . . . . . . . 5 9 TOOL STEELS

viii

. . . . . . . . . . . . . . . . . . . . . . . . . . . 63 ELECTRICAL CONTACTS . . . . . . . . . . . . . . . . . . . . . . . . 64 CHAPTER 3 . SOFT METAL BEARING MATERIALS . . . . . . . . . . . . . . . . . 69 SOFT METALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 72 BABBITTS OR WHITEMEAL . . . . . . . . . . . . . . . . . . . . . . . GEAR BRONZES

. . . . . . . . . . . . . . . . . . . . . . . . 78 Bearing Materials . . . . . . . . . . . . . . . 79

Babbitt Fatigue Aluminum Based

. . . . . . . . . . . . . . . . . .81 ALLOY SELECTION AND DESIGN . . . . . . . . . . . . . . . . . . . . 84 87 ZINC BEARING ALLOYS . . . . . . . . . . . . . . . . . . . . . . . . 88 GOLD. SILVER AND INDIUM . . . . . . . . . . . . . . . . . . . . . . B i m e t a l . T r i m e t a l M a t e r i a1 s

CHAPTER4

. CAST IRON

INTRODUCTION

.......................... ...........................

89

90

PROPERTIES OF CAST IRONS

.....................

91

METALLURGY OF CAST IRONS

.....................

94

. . . . . . . . . . . . . . . . . . 103 . SURFACE HARDENING . . . . . . . . . . . . . . . . . . . . . . . . . 106 WEAR PROPERTIES OF CAST IRONS . . . . . . . . . . . . . . . . . . 1. 0 7 112 HIGH ALLOY CAST IRONS . . . . . . . . . . . . . . . . . . . . . . . 114 CHAPTER 5 . CARBON GRAPHITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 115 USAGE I N TRIBOLOGY 119 WEAR OF CARBON GRAPHITE . . . . . . . . . . . . . . . . . . . . . . MECHANICAL SEALS . . . . . . . . . . . . . . . . . . . . . . . . . 122 124 SLEEVE BEARINGS . . . . . . . . . . . . . . . . . . . . . . . . . . 126 THRUST BEARINGS AND VANES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 CARBON BRUSHES HEAT TREATMENT OF CAST IRONS

ix

CHAPTER 6

. CERAMICS AND S P E C I A L ALLOYS

INTRODUCTION CERAMICS

. . . . . . . . . . . . . . . . 130 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

.............................

132

. . . . . . . . . . . . . . 139 . Heat T r e a t a b l e Ceramics . . . . . . . . . . . . . . . . . . . . 145 CERAMIC TOOLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 8 STRUCTURE AND PROPERTIES OF CERAMICS

. . . . . . . . . . . . . . . . 1. 5 0 151 CERMETS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 GLASSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . REFRACTORY METALS AND ALLOYS . . . . . . . . . . . . . . . . . . .1 5 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 SUPER ALLOYS CERAMIC ROLLING CONTACT BEARINGS

. . . . . . . . . . . . . . . . . 1. 6 6 H i g h T e m p e r a t u r e P r o p e r t i e s O f S u p e r A l l o y s . . . . . . . . . .1 6 8 CHAPTER 7 . POLYMERIC MATERIALS . . . . . . . . . . . . . . . . . . . . . 177 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 . . . . . . . . . . . . . . . . . . . . . . 180 PROPERTIES OF POLYMERS MATERIALS FOR NUCLEAR REACTORS

DESIGN OF PLASTIC BEARINGS

....................

182

. . . . . . . . . . . . . . . . . . . . 187 H i g h T e m p e r a t u r e P l a s t i c s . . . . . . . . . . . . . . . . . . .1 9 4 ELASTOMERS-RUBBER . . . . . . . . . . . . . . . . . . . . . . . . . 200 PLASTICS USED I N TRIBOLOGY

. . . . . . . . . . . . . . . . . . . . . 201 BEARINGS . . . . . . . . . . . . . . . . . . 2.0 5

PROPERTIES OF ELASTOMERS DESIGN OF ELASTOMER

. . . . . . . . . . . . . . . . . . . . . . . . 207 . . . . . . . . . . . . . . . . . . . . . . . . . . 209 WEAROFRUBBER REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1 1 APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 .2 1 INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 TYPES OF ELASTOMERS

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XI

LIST

OF

TABLES

OF

MATERIAL

PROPERTIES

Carbon steels Mechanical properties ...............................................

14

Physical properties .................................................

15

Stainless steels Mechanical properties ...............................................

24

Physical properties .................................................

25

Tool steels Mechanical properties ...............................................

33

Physical properties .................................................

34

Roll ing contact bearing steels Mechanical properties ...............................................

36

Physical properties .................................................

37

Relative hardnesses o f Steels. Minerals and Carbides .....................

45

Porous bearing materials PV limits .......................................

58

Wear properties o f bearing bronzes .......................................

59

Copper base materials Mechanical properties ...............................................

66

Physical properties .................................................

67

Soft metal elements mechanical & physical properties .....................

71

xii

Soft metal bearing alloys mechanical & physical properties ...............73 Mulilayer bearing material load capacities...............................

83

Cast iron mechanical and physical properties .............................

92

Cast irons abrasion resistance ..........................................

110

Carbon graphite Mechanical properties ..............................................

117

Physical properties ................................................

118

Seal wear properties ...............................................

120

PV values ..........................................................

125

Carbon electric brush properties ...................................

128

Ceramics Mechanical and physical properties ...........................

133

.

138

Thermal shock resistance...........................................

141

Critical velocities ................................................

144

Modified zirconias. mechanical & physical properties ............... 147 Cermets. mechanical and physical properties .............................

154

Glasses, mechanical and physical properties .............................

156

Wear of superalloys .....................................................

159

High temperature alloys Mechanical properties ..............................................

162

Physical properties ................................................

163

Cobalt base alloys, abrasive wear properties ............................

165

xiii

High temperature m a t e r i a l s E f f e c t o f temperature on thermal c o n d u c t i v i t y ......................

170

E f f e c t o f temperature on s h o r t t i m e t e n s i l e s t r e n g t h ............... 171 E f f e c t o f temperature on Young's Modulus ...........................

172

E f f e c t o f temperature on thermal expansion .........................

173

E f f e c t o f temperature on thermal d i f f u s i v i t y .......................

174

E f f e c t o f temperature on thermal s t r e s s r e s i s t a n c e f a c t o r .......... 175 E f f e c t o f temperature on s p e c i f i c heat c a p a c i t y ....................

176

Plastics Mechanical p r o p e r t i e s ..............................................

180

Physical p r o p e r t i e s ................................................

181

PV values ..........................................................

184

D e f l e c t i o n temperatures ............................................

198

Wear r a t e s .........................................................

199

Elastomers Hardness comparisons ...............................................

202

Mechanical p r o p e r t i e s ..............................................

203

Physical p r o p e r t i e s ................................................

204

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1

INTRODUCTION

M a t e r i a l s used i n t r i b o l o g i c a l a p p l i c a t i o n s are, o r t h e most p a r t , common m a t e r i a l s used f o r general e n g i n e e r i n g a p p l i c a t i o n s . There a r e some m a t e r i a l s designed s p e c i f i c a l l y f o r b e a r i n g s as, f o r i n s t a n c e b a b b i t t s , leaded bronze, a1 uminum-t i n , woven PTFE-g1 ass, s i n t e r e d bronze- 1ead-PTFE and T r i b a l oy. Many conventional e n g i n e e r i n g m a t e r i a l s have been adapted t o t r i b o l o g i c a l uses. For instance, A I S I 52100 s t e e l and M50 t o o l s t e e l a r e b o t h used i n r o l l i n g c o n t a c t bearings. Special h e a t t r e a t m e n t s have been developed f o r t h e s e two a l l o y s t o enhance t h e i r endurance under r o l l i n g c o n t a c t c o n d i t i o n s . There a r e many o t h e r examples o f a d a p t i o n o f m a t e r i a l s t o s p e c i f i c t r i b o l o g i c a l a p p l i c a t i o n s . These w i l l be found i n t h e ensuing c h a p t e r s . U n f o r t u n a t e l y , f o r t h e t r i b o l o g i s t , i n f o r m a t i o n on m a t e r i a l s he uses i s s c a t t e r e d t h r o u g h t h e l i t e r a t u r e , causing c o n s i d e r a b l e waste o f t i m e i n searching f o r d e s i r e d m a t e r i a l p r o p e r t i e s . Often, t h e p a r t i c u l a r m a t e r i a l o f i n t e r e s t i s found b u r i e d i n l i s t i n g s o f o t h e r m a t e r i a l s n o t used f o r b e a r i n g s , gears, brakes, o r machine t o o l s . The i n t e n t o f t h i s handbook i s t o p r o v i d e a comprehensive r e f e r e n c e f o r m a t e r i a l s used i n t r i b o l o g i c a l a p p l i c a t i o n s . The a u t h o r has used h i s l o n g experience i n s e l e c t i n g m a t e r i a l s f o r a wide v a r i e t y o f f r i c t i o n and wear a p p l i c a t i o n s t o develop a d a t a base o f m a t e r i a l s f o r t r i b o l o g y . I n a d d i t i o n i n f o r m a t i o n has been s e l e c t e d f r o m t h e l i t e r a t u r e on t h e b e h a v i o r o f these m a t e r i a l s i n bearings, seals, gears, brakes, c l u t c h e s , w i r e rope, v a l v e s cams and wear s u r f a c e s and i s i n c l u d e d i n t h e d e s c r i p t i v e t e x t . The m a t e r i a l s have been grouped i n f a m i l i e s , r e l a t i n g t o t h e i r compos t i o n . The f o l l o w i n g c l a s s i f i c a t i o n s have been s e l e c t e d :

CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER

1. 2. 3. 4. 5. 6. 7.

STEELS COPPER BASE BEARING MATERIALS SOFT METAL BEARING MATERIALS CAST IRON CARBON GRAPHITE CERAMICS AND SPECIAL ALLOYS POLYMERIC MATERIALS

2

Each c a t e g o r y i n t h e l i s t i n g on t h e p r e v i o u s page i s t h e s u b j e c t o f a c h a p t e r i n t h e handbook. A s h o r t t a b l e i s p r o v i d e d a t t h e b e g i n n i n g o f each chapter l i s t i n g t h e ranges o f s e l e c t e d p r o p e r t i e s f o r t h e m a t e r i a l s i n t h a t c h a p t e r . I n a d d i t i o n t h e r e a r e s h o r t summaries o f t h e t r i b o l o g i c a l a p p l i c a t i o n s t h i s c l a s s o f m a t e r i a l s i s used f o r . Therefore, on t h e f i r s t page o f each c h a p t e r one can f i n d a g u i d e f o r s e l e c t i o n o f m a t e r i a l s - a t l e a s t f o r t h e f i r s t c u t . Tables o f m a t e r i a l p r o p e r t i e s a r e i n c l u d e d i n t h e body o f each c h a p t e r . The p r o p e r t i e s chosen f o r a g i v e n c l a s s o f m a t e r i a l s r e p r e s e n t those p r o p e r t i e s r e l e v a n t t o t h e t r i b o l o g i c a l use o f t h e m a t e r i a l s i n t h e c h a p t e r . I n o r d e r t o keep t a b l e s i z e t o a reasonable l e v e l , several p r o p e r t i e s t a b l e s may be used i n one c h a p t e r . For instance, i n Chapter 8 ( S o f t M e t a l s ) , a separate t a b l e i s p r o v i d e d f o r pure metals such as g o l d , s i l v e r , t i n , e t c . A second t a b l e i s p r o v i d e d f o r b a b b i t t s , b o t h l e a d based and t i n based. The p r o p e r t i e s d a t a f o r t h e s e l e c t e d m a t e r i a l s have been o b t a i n e d f r o m a number o f sources. The data, t h e r e f o r e , a r e o n l y as good as t h e source from which t h e y have been taken. Some data, i n p a r t i c u l a r , b e a r i n g l o a d c a p a c i t y , wear c o e f f i c i e n t s and PV values have been developed i n t h e B a t t e l l e T r i b o l o g y Laboratory. Other sources i n c l u d e t h e B a t t e l l e Defense M a t e r i a l s I n f o r m a t i o n Center, A l l o y s D i g e s t , ASM M e t a l s Handbook, M e t a l s Reference Book, Wear, Proceedings o f ASME Wear Conferences and ASME Wear C o n t r o l Handbook. C e r t a i n m a t e r i a l s p r o p e r t i e s a r e q u i t e s p e c i f i c f o r t r i b o l o g y - as, f o r instance, PV, wear c o e f f i c i e n t , maximum b e a r i n g s t r e n g t h , hardness and c o e f f i c i e n t o f f r i c t i o n . Other p r o p e r t i e s such as t e n s i l e s t r e n g t h and s p e c i f i c h e a t a r e i n more g e n e r a l use b u t have a p p l i c a t i o n i n c e r t a i n t r i b o l o g i c a l areas. C o e f f i c i e n t o f f r i c t i o n i s v e r y s p e c i f i c b u t p r e s e n t s a v e r y d i f f i c u l t problem i n i t s meaning, accuracy and r e l e v a n c e . C o e f f i c i e n t o f f r i c t i o n i s used i n several mathematical r e l a t i o n s h i p s as f o r i n s t a n c e , t h e f o r m u l a f o r power l o s s i n a s l e e v e b e a r i n g : HP

=

7.93 f L RPM D x

[l]

Where: coefficient o f f r i c t i o n Normal l o a d on b e a r i n g , l b RPM = S h a f t speed, RPM D = S h a f t diameter, i n . f L

= =

3

If, in the above relation, the friction coefficient is measured while the bearing is in operation under the load and lubrication condition in question, the power calculation should be accurate. I f , however, the friction coefficient must be assumed, errors amounting to several orders of magnitude are possible. Unfortunately, friction coefficient is not a material property like tensile strength which can be assigned to each material. Instead, it is a proportionality constant which indicates that tangential resistive force varies 1 inearly with normal bearing load: F = f L Generally, under well controlled surface conditions, this proportionality holds. However, under very heavy loads, frictional heating or changing surface environment, the proportionality no longer applies. If one measures friction forces in a bearing, considerable fluctuation can be detected in the friction level as the bearing runs. Depending on the sensitivity and response characteristics o f the friction measuring device, friction force variations can appear very complex. This presents a problem in selection of a friction value. Often, an average value is used. However, spectrum analysis may be more appropriate - especially when friction peaks are of concern. Because of the above mentioned barriers to the definition of reliable friction values, the author of this handbook has refrained from including friction coeffient values for many of the materials. Some exceptions have been made, however, for self lubricating materials like carbon graphite and ptfe to indicate the level of friction reduction. Estimates of friction coefficient ranges for material classes have been included to show the effect of self lubricating materials on friction levels for comparison purposes. The material properties chosen for inclusion in this handbook were selected on the basis of frequency of use in tribological applications. Some properties, like fracture toughness, are appropriate to a class of materials -namely ceramics. Fracture toughness influences wear or erosion rates of brittle materials.

4

A r e l a t i o n s h i p has been developed b y Evans and M a r s h a l l [ 2 ] i n which a b r a s i v e wear r e s i s t a n c e i s r e l a t e d t o f r a c t u r e toughness and modulus:

0.5 0.625 Abrasive Wear Resistance

=

[KIc

H

0.8 ]/ (E/H)

Where: H KIc E

= = =

Hardness F r a c t u r e Toughness Young's Modulus

Thermal c o n d u c t i v i t y i s s i g n i f i c a n t f o r e s t i m a t i o n o f temperature developed b y f r i c t i o n a l h e a t i n g . Functions r e l a t i n g t o " f l a s h temperature" a r e used i n t h e p r e d i c t i o n o f l u b r i c a n t f i l m breakdown. F l a s h temperature, o r l o c a l temperature s p i k e s a t s l i d i n g c o n t a c t s generated b y f r i c t i o n a l h e a t i n g i s used i n t h e a n a l y s i s o f t h i n f i l m o r boundary l u b r i c a t e d surfaces t o estimate threshold s l i d i n g v e l o c i t i e s f o r contact f a i l u r e . Czichos [ 3 ] has summarized t h e work o f B l o k , Jaeger, Holm and Archard [ 4 , 5, 6, 71 i n t h e f o l l o w i n g r e l a t i o n s h i p :

dT = 0 . 2 5 CNL Where :

f = friction coefficient

g p

g r a v i t y constant p e n e t r a t i o n hardness = density c = s p e c i f i c heat k = thermal d i f f u s i v i t y v = velocity C = c o n s t a n t ; 1 - 0.5, depending on t h e v a l u e o f L dT = f l a s h temperature r i s e above ambient =

=

5

F l a s h temperature i s i n f l u e n c e d b y l o a d and speed, d e f o r m a t i o n mode and t h e thermal p r o p e r t i e s o f t h e c o n t a c t i n g m a t e r i a l s . Not o n l y i s t h e dT v a l u e i n f l u e n c e d , b u t t h e f o r m u l a must be a l t e r e d f o r s p e c i f i c c o n d i t i o n s . As can be seen, t h e above analyses r e q u i r e a number o f p h y s i c a l and mechanical properties f o r the materials i n s l i d i n g contact. I n d r y s l i d i n g systems where m a t e r i a l s w i t h l o w thermal c o n d u c t i v i t y a r e used such as i n h i g h speed s h a f t f a c e s e a l s and c l u t c h e s and brakes, thermal shock can be a s i g n i f i c a n t f a c t o r i n t h e wear process. I f f r i c t i o n a l l y developed s u r f a c e h o t spots develop r a p i d l y and a r e quenched, f r a c t u r e o f b r i t t l e m a t e r i a l s from t h e thermal shock can r e s u l t . Thermal shock can cause a c a t a s t r o p h i c i n c r e a s e i n wear. Thermal shock r e s i s t a n c e i s a d i f f i c u l t parameter t o d e f i n e o r measure i n a m a t e r i a l . Mehrotra [8] has developed a r a n k i n g system f o r comparing t h e r e l a t i v e thermal shock r e s i s t a n c e o f v a r i o u s ceramic m a t e r i a l s . He uses two r e l a t i o n s : KIc/Ea KIck/Ea Where: KIc E a k

= = = =

F r a c t u r e toughness E l a s t i c Modulus C o e f f i c i e n t o f l i n e a r thermal expansion Thermal c o n d u c t i v i t y

R a t i o (1) does n o t c o n t a i n thermal c o n d u c t i v i t y and can be used t o check o u t m a t e r i a l s w i t h l o w thermal expansion and E as p o t e n t i a l l y good i n thermal shock r e s i s t a n c e . S i a l o n i s one o f those m a t e r i a l s and i t performs w e l l as a h i g h speed c u t t i n g t o o l . R a t i o ( 2 ) c o n t a i n s thermal c o n d u c t i v i t y and w i l l s o r t o u t t h e e f f e c t o f thermal c o n d u c t i v i t y on thermal shock r e s i s t a n c e f o r ceramic m a t e r i a l s w i t h s i m i l a r expansion and e l a s t i c p r o p e r t i e s . S i l i c o n c a r b i d e , w i t h i t s h i g h thermal c o n d u c t i v i t y , a l s o has t h e h i g h e s t thermal shock r e s i s t a n c e among t h e ceramics.

6

Dow and Burton [9] have developed a relationship for determining velocity at which hot spots developed on a sliding surface begin move about the surface, causing an increase in the wear rate. This called thermal mechanical instability (TEM). The relationship is as Vcr =

a critical to rapidly process is follows:

16 K2 [ ( f a E )2 ~ k z ]

Where: Vcr

=

K

=

f

=

a

=

E k

= =

p

=

z

= =

c

critical sliding speed thermal conductivity coefficient of friction coefficient of linear thermal expansion Young's modulus thermal diffusivity; (Klpc) density width of slider specific heat

The thermoelastic instability criterion has been used in the estimation of excessive wear for given ceramic materials for cylinder liner and piston ring materials for advanced low heat loss diesels [ l o ] . In this work, materials like partially stabilized zirconia were predicted to produce excessive wear by thermal shock because of their low thermal diffusivity. Silicon carbide or silicon nitride, on the other hand, were predicted to not develop thermal instability under diesel operating conditions. These predictions were verified by experiment Note that the coefficient of friction is one parameter in the equation. t was derived experimentally in the above work. The preceding examples show that there are a number of material properties needed to solve equations used in tribologica applications. These properties include hardness, tensile strength, density, Young's modulus, specific heat, fracture toughness, thermal conductivity and thermal expansion coefficient. In add it ion, some tribological properties are useful for selection of materials. These include PV, maximum bearing strength, and maximum operating temperature. Other properties of use to tribologists include electrical resistivity and melting point. In the study of high strain deformation effects in the near surface region in the wear zone, stacking fault energy is of use as well as the work hardening coefficient at very high strain levels.

7

These two p r o p e r t i e s a r e n o t r e a d i l y a v a i l a b l e f o r a l l m a t e r i a l s . Some s i m p l e b i n a r y a l l o y s and pure m e t a l s have these values recorded. T h e r e f o r e a l i m i t e d number o f m a t e r i a l s w i l l have t h i s i n f o r m a t i o n p r o v i d e d i n t h i s handbook. F i n a l l y , something must be s a i d about t h e wear r e s i s t a n c e o f e n g i n e e r i n g m a t e r i a l s . There a r e a number o f equations which have been developed f r o m wear t e s t d a t a and used i n s p e c i f i c a p p l i c a t i o n s such as brakes and c l u t c h e s , f a c e seals, automobile t i r e s , m i n i n g machinery and machine t o o l s f o r e s t i m a t i n g component l i f e f o r g i v e n c o n d i t i o n s . For these examples, some f i g u r e o f m e r i t o r measure o f wear r e s i s t a n c e i s o f t e n used. S p e c i f i c wear, a b r a s i v e wear r e s i s t a n c e f a c t o r o r wear c o e f f i c i e n t a r e some examples. Wear c o e f f i c i e n t i s p r o b a b l y used more t h a n any o t h e r parameter. I t i s used c o n s i s t e n t l y i n t h e ASME Wear C o n t r o l Handbook [ll]. Wear d a t a i s b e i n g r e p o r t e d i n t h e l i t e r a t u r e u s i n g t h e wear c o e f f i c i e n t . Wear c o e f f i c i e n t s have been i n c l u d e d i n some c h a p t e r s , u s i n g s e l e c t e d d a t a o b t a i n e d i n w e l l documented and accepted t e s t s .

The ASTM has developed an a b r a s i o n t e s t f o r l o w s t r e s s a b r a s i o n c o n d i t i o n s . Low s t r e s s a b r a s i o n i s a form o f a b r a s i o n i n which a b r a s i v e p a r t i c l e s r u b over a s u r f a c e under a c o n t a c t s t r e s s which does n o t r e s u l t i n f r a c t u r e o f t h e p a r t i c l e s . I n t h e ASTM t e s t , sand passes between a r o t a t i n g r u b b e r wheel and t h e t e s t specimen. The ASTM Dry Sand/Rubber Wheel Abrasion Tests a r e s p e c i f i e d under G 65-81. The wear values f r o m t h i s t e s t a r e r e p o r t e d i n volume l o s s i n c u b i c m i l l i m e t e r s p e r ASTM procedure. The volume l o s s i s determined f o r a s p e c i f i e d number o f wheel r e v o l u t i o n s under a g i v e n wheel f o r c e . Data from these t e s t a r e q u i t e r e p r o d u c i b l e . Data a r e p r o v i d e d f o r some m a t e r i a l s i n t h i s handbook. Coatings f o r c o n t r o l o f wear a r e f i n d i n g an i n c r e a s i n g use i n machinery. The economical advantage o f u s i n g a l o w c o s t and ‘ e a s i l y formed m a t e r i a l as t h e b u l k o f a p a r t w i t h a small amount o f h i g h c o s t wear r e s i s t a n t m a t e r i a l l i k e a c a r b i d e a p p l i e d t o t h e s u r f a c e s expected t o see wear i s b e i n g c o n s i d e r e d more and more i n new designs. The importance o f t h e s e m a t e r i a l s i n t r i b o l o g y cannot be ignored. However, t h e t o p i c o f wear r e s i s t a n t c o a t i n g s i s l a r g e enough t o t a k e up an e n t i r e volume. I n a d d i t i o n , new developments i n c o a t i n g technology such as diamond and diamond-1 i k e c o a t i n g s a r e coming a l o n g r a p i d l y . Therefore, c o a t i n g s a r e n o t covered i n d e t a i l i n t h i s t e x t . Some u s e f u l t a b l e s , e x t r a c t e d from t h e l i t e r a t u r e , a r e i n c l u d e d i n t h e appendix. Readers a r e d i r e c t e d t o o t h e r t e x t s on wear r e s i s t a n t c o a t i n g s i n c l u d i n g B u d i n s k i ’ s [12].

8

CHAPTER

1 -

STEELS

MELTING POINTS 1400 - 1500'C HARDNESS

180 - 790 VICKERS

TENSILE STRENGTH 100 - 2700 MPa YOUNGS MODULUS

6.9Et4

- 2.OEt5 MPa

THERM COND 25 - 50 WATT/m k THERM EXPANS 1.1

- 1.5 Et05/'C

DENSITY 6.4 - 8 . 0 Et03 Kg/m3

TYPES OF STEEL ALLOYS USED I N TRIBOLOGY Carbon S t e e l - Forged o r hardened b y h e a t t r e a t m e n t A l l o y S t e e l - C o n t a i n i n g a l l o y i n g elements t o improve h a r d e n a b i l i t y A u s t e n i t i c S t a i n l e s s S t e e l - Not hardenable b y heat t r e a t m e n t M a r t e n s i t i c S t a i n l e s s S t e e l - Heat t r e a t a b l e t o harden Tool S t e e l - Hot hardness, r e s i s t a n c e t o a b r a s i o n APPLICATIONS FOR STEELS Shafting o r bearing journals Gears B a l l and r o l l e r b e a r i n g s Tools and d i e s Wire rope Wheels and r a i l s Fasteners Pumps and compressors Knives Thrust bearings Drill bits

9

INTRODUCTION

Steels have a wide and diversified use for Tribologica applications. Utilization, rather than specific formulation has been the rule for this class of materials. A few steels have been modified in compo ition and heat treatment methods for use in bearings. A I S I 52100 steel is used for ball and roller bearings and is subjected to special vacuum melting practices for inclusion control and is given a special heat treatment to minimize residual austenite content and to ensure dimensional stability. These practices are discussed later in the chapter. A I S I 440C stainless steel is also used for rolling contact bearings - principally for elevated temperature and some corrosive environments. This alloy has been modified to refine the carbide structure for roll ing contact applications. Some tool steels with the required grinding properties have also been utilized for elevated temperature rolling contact bearings. More recently, hardened steels made by powder metallurgy techniques have been developed to provide better control over microstructure and inclusion content to improve contact fatigue life of high speed aircraft bearings. Manganese steel or Hadfield steel is used in the mining, earth moving and railroad industries where high toughness, impact resistance and abrasive wear resistance are needed. This alloy contains 1% carbon and 11 to 14% manganese. It is an austenitic steel which transforms to martensite when abraded heavily. This results in a tough core with a hard skin that continually renews itself as it is worn off. This alloy is capable of extensive work hardening under abrasive impact conditions. Steel is used extensively in machinery for load bearing components like shafting, gearing, housings, cable, thrust surfaces, etc. For efficiency, steel surfaces are being coated with wear resistant materials. In this way, the structural strength of the steel can be used for large components while small amounts of expensive wear resistant material is applied to specific areas where it is needed. Stainless steel is used extensively in reactor components where resistance to heat and corrosion is essential. It also finds use in the petrochemical and food processing industries for much the same reasons. Stainless steel is also used in cutlery, flat ware and cooking utensils.

10

METALLURGY OF STEELS Two abundant elements, i r o n and carbon a r e t h e b a s i s f o r t h e s t e e l i n d u s t r y . Carbon i s s o l u b l e i n i r o n i n small amounts and s t e e l s cover a range o f carbon - i r o n a l l o y s f r o m 0.005 w t % carbon t o 2.00 w t % carbon. The i r o n - c a r b o n e q u i l i b r i u m diagram, w e l l e s t a b l i s h e d from y e a r s o f research, p r o v i d e s a map o f p o s s i b l e phases based on temperature and carbon c o n t e n t . An i r o n - c a r b o n e q u i l i b r i u m diagram f o r carbon w t % up t o 6.67% i s shown i n f i g u r e 1.1. The p o r t i o n o f t h e diagram f r o m 0% t o 2.0% carbon covers s t e e l s . H i g h e r carbon c o n t e n t s a r e t y p i c a l f o r c a s t i r o n s . I r o n - c a r b o n - s i l i c o n diagrams a r e used i n c a s t i r o n m e t a l l u r g y and a r e discussed i n c h a p t e r 4 ( c a s t i r o n ) . R e f e r r i n g t o f i g u r e 1.1, s e v e r a l phases o f i r o n - c a r b o n a r e shown: a u s t e n i t e , f e r r i t e , gamma i r o n and c e m e n t i t e . These a r e a l l e q u i l i b r i u m phases. Gamma i r o n i s n o t o f consequence i n s t e e l m e t a l l u r g y and w i l l n o t be discussed. A u s t e n i t e e x i s t s a t e l e v a t e d temperature (above 727'C). Carbon i s an a u s t e n i t e s t a b i l i z e r and i s more s o l u b l e i n a u s t e n i t e (2.11 wt%) t h a n i n f e r r i t e (0.022wt%). As temperature i s reduced from 1148'C, t h e s o l u b i l i t y o f carbon i n a u s t e n i t e decreases and carbon i s r e j e c t e d as i r o n c a r b i d e (Fe3C) o r cementite. Two phases, c e m e n t i t e and a u s t e n i t e e x i s t t o g e t h e r as shown i n t h e diagram. A t 727'C, a u s t e n i t e c o n v e r t s t o f e r r i t e and c o n s i d e r a b l e carbon i s r e j e c t e d from s o l u t i o n as c a r b i d e and t h e two phases, f e r r i t e and c e m e n t i t e e x i s t together. F o r t u n a t e l y f o r engineers, carbon r e j e c t i o n i s a v e r y s l u g g i s h process and w i t h r a p i d enough c o o l i n g f r o m t h e a u s t e n i t e r e g i o n , a s u p e r s a t u r a t e d s o l u t i o n o f carbon i n i r o n r e s u l t s . The l a t t i c e s t r a i n caused b y t h e excess carbon produces a phase change t o body centered t e t r a g o n a l ( a d i s t o r t e d c u b i c s t r u c t u r e ) w i t h v e r y h i g h hardness. T h i s s t r u c t u r e i s n o t shown on t h e e q u i l i b r i u m diagram and i s known as m a r t e n s i t e . M a r t e n s i t e i s d e s i r a b l e as a h i g h l y wear r e s i s t a n t form o f s t e e l . M a r t e n s i t e r e q u i r e s r a p i d c o o l i n g as achieved b y quenching. Slower c o o l i n g r a t e s w i l l produce o t h e r m i c r o s t r u c t u r e s having e q u a l l y u s e f u l e n g i n e e r i n g p r o p e r t i e s . These m i c r o s t r u c t u r e s i n c l u d e b a i n i t e ( a m a r t e n s i t e - l i k e s t r u c t u r e ) and p e a r l i t e ( a l a m e l l a r s t r u c t u r e o f f e r r i t e sandwiched between c e m e n t i t e p l a t e s ) . The hardness o f t h e s e m i c r o s t r u c t u r e s - e s p e c i a l l y m a r t e n s i t e and b a i n i t e a r e s i g n i f i c a n t l y i n f l u e n c e d b y carbon c o n t e n t . F i g 1.2 shows hardness as a f u n c t i o n o f carbon content f o r martensite. The above m e t a l l u r g i c a l f a c t o r s make f o r a wide v a r i e t y o f s t e e l s produced b y v a r y i n g amounts o f carbon and d i f f e r e n t heat t r e a t m e n t s . T h i s makes s t e e l a very v e r s a t i l e engineering material.

11

3

Figure 1.1 Iron-carbon equilibrium diagram

12

Not only are there a large variety of steels to choose from, but within those grades of steels, various heat treatments are p o s s i b l e resulting in a variety of microstructures. The microstructure of steel can have an effect on wear resistance.

1100 1000 900

800

> 700

I 6 ln 600 (I)

bainilic microslruclures

f 500

2

400

300 200 100

0 0

0.20 0.40 0.60 0.80 1.00 1.20

Carbon, %

Figure 1.2 Hardness as a function of carbon content for martensite

13

S t e e l s can e x i s t i n t h r e e b a s i c s t a t e s : a u s t e n i t i c , m a r t e n s i t i c and p e a r l i t i c . There are many v a r i a t i o n s on these s t a t e s i n c l u d i n g m i x t u r e s o f each, m a r t e n s i t i c o r a u s t e n i t i c m a t r i x c o n t a i n i n g c a r b i d e s , mixed f e r r i t e and p e a r l i t e , b a i n i t e e t c . Each one o f t h e s e s t a t e s has c h a r a c t e r i s t i c wear p r o p e r t i e s . Other p r o p e r t i e s such as f r a c t u r e toughness, c o r r o s i o n r e s i s t a n c e , c o s t , m a c h i n a b i l i t y , f a t i g u e s t r e n g t h , creep r e s i s t a n c e and t e n s i l e s t r e n g t h come i n t o c o n s i d e r a t i o n when choosing a grade o f s t e e l f o r a p a r t i c u l a r a p p l i c a t i o n . We w i l l d i s c u s s these b a s i c s t e e l m i c r o s t r u c t u r e s i n d i v i d u a l l y . Wear r e s i s t a n c e can be c o n t r o l l e d i n s t e e l s b y heat t r e a t m e n t , b y c a r b u r i z i n g o r n i t r i d i n g , b y work hardening o r b y t h e a p p l i c a t i o n o f hard c o a t i n g s . These f a c t o r s w i l l be discussed s e p a r a t e l y .

Carbon Steels Carbon s t e e l s can e x i s t i n a number o f d i f f e r e n t s t a t e s : p e a r l i t i c , b a i n i t i c , m a r t e n s i t i c and a u s t e n i t i c . Carbon s t e e l s r e v e r t t o t h e a u s t e n i t i c s t a t e when heated t o t h e temperature range 750'C t o 100O'C. A u s t e n i t e decomposes t o c a r b i d e s and f e r r i t e ( i r o n ) a t room temperature. The s t e e l a l s o changes d e n s i t y d u r i n g t h e change f r o m a u s t e n i t e t o another phase so t h a t t h e development o f m a r t e n s i t e f r o m a u s t e n i t e r e s u l t s i n an expansion o f t h e p a r t . T h i s e f f e c t can be c r u c i a l i n o p e r a t i o n o f b e a r i n g s and w i l l be discussed 1a t e r . Mechanical and p h y s i c a l p r o p e r t i e s and composition o f s e l e c t e d carbon s t e e l s a r e found i n t a b l e s 1.1 a,b,& c . P l a i n carbon s t e e l s having more t h a n 0.4'' C c o n t e n t can be hardened b y heat t r e a t m e n t . With a l l o y a d d i t i o n s o f N i o r Cr e t c t h e c r i t i c a l carbon c o n t e n t can be reduced below 0.4%. P e a r l i t e i s an e q u i l i b r i u m phase i n s t e e l c o n s i s t i n g o f f e r r i t e sandwiched between c e m e n t i t e ( i r o n c a r b i d e ) p l a t e s . I t forms d u r i n g slow c o o l i n g below t h e e u t e c t o i d temperature (around 600'C). P e a r l i t e forms from t h e decomposition o f t h e h i g h temperature a u s t e n i t e phase produced b y h e a t i n g t h e s t e e l above 160OaC and h o l d i n g l o n g enough t o d i s s o l v e a l l c a r b i d e s . P e a r l i t e e x h i b i t s s u p e r i o r wear r e s i s t a n c e t o a s - c a s t s p h e r i o d i z e d s t e e l . The p e a r l i t e phase w i l l r e s i s t deep p e n e t r a t i o n o f h a r d a b r a s i v e p a r t i c l e s and m i n i m i z e t h e depth o f plowing.

14

Table l . l a Mechanical P r o p e r t i e s of Carbon S t e e l s

MATERIAL

1040 1040 1095 1095 1118 1118 4320 4340 4340 4620 4620 4820 4820 81845 81845 8620 8620 9310 9310 Austenitic-Mn C 1080 C1080 Conversion f a c t o r s : MPa x 0.145 = k s i

FORM

CARBURIZED

CARBURIZED CARBURIZED CAST CARBURIZED CARBURIZED CAST

TEMPER HEAT TREAT Annealed HT,300F t e HT 500F t e ANNEALED HT,300F t e Annealed Annealed Annealed HT, 500F t e Annealed HT, 300F t e HT, 300F t e Annealed Annealed HT,400F t e HT,300F t e Annealed HT,3OOF t e Annealed HT,1850F t e HT,400F t e Annealed -

HARDNESS VICKERS DPH 150 540 710 200 360 165 225 260 550 230 740 690 230 195 617 789 180 694 200 200 404 224

TENSILE YOUNGS STRENGTH MODULUS MPa MPa 552 896 1480 690 7 58 448 752 979 1724 607 827 1379 7 58 64 1 2041 1296 607 1241 689 965 1303 820

2.07E+05 2.07E+05 2.07E+05 2.07E+05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.07E+05 2.07Et05 2.07E+05 2.07Et05 2.07Et05 2.07Et05 2.07Et05 2.00Et05 2.07Et05 2.07Et05

15

Tab1 e 1. l b

P h y s i c a l P r o p e r t i e s Carbon Steel s

MATERIAL

1040 1118 4320 4340 4820 81845 8620 9310 Aust.-Mn C 1080

MAX OP TEtjP C

KIc MPa m4 55.00 167.00 77.00

260 538

77.00 22.00

204

DENSITY THERM CON kg/ WATT/m CU METER K 7,75Et03 7.86Et03 7.85Et03 7.75Et03 7.86Et03 7.75Et03 7.75Et03 7.75Et03 8.03Et03 7.75Et03

THERM EXP R E S I S m/m micro

/'C

50.69 51.55 38.06 37.54 45.00

1.12E-05 1.22E-05 1.13E-05 1.46E-05 1.55E-05 1.26E-05 1.48E-05 1.46E-05 1.84E-05 1.47E-05

37.50 46.71 12.63 46.71

HEAT CAPACITY ohm-cm J/kg/'K

19.00 14.00 30.00 30.00 26.00 30.00 20.00 68.30 18.00

5.OEt2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 4.6Et2 5.OEt2 5.OEt2 4.2Et2

Conversion f a c t o r s : kg/m

3

x 3.613E-5

=

l b / c u i n : kg/m

W/m/K x 0.578 = BTU/ft/hr/'F J/kg/K x 2.388E-4 =BTU/lb/'F

3

x 0.001

=

gm/cc ( S G )

or c a l / g / C

Table l . l c Composition o f Carbon S t e e l A l l o y s MATERIAL 1040 C 1095 C 1118 C 4320 C 4340 C 4620 C 4820 C 81845 C 8620 C 9310 C Austenitic-Mn C1080

COMPOSITION .4,Mn 1.0,Mn .16,Mn .2,Mn .4,Mn .2,Mn .2,Mn .45,Mn .2,Mn .1,Mn

.7,P .04,S .05,Si .2,Fe 98.6 .4,P .04,S .05,Fe 98.6 1.4,Fe 98.44 .55,Si .25,Ni 1.7,Cr .5,Mo .25 .7,P .025,S .025,Si .3,Ni 1.7,Cr .8,Mo .25,Fe 96 .55,Si .3,Ni 1.7,Mo .25 .6,P .035,S .04,Si .3,Ni 3.5,Mo .25,Fe 95 .8,Si .3,Ni .3,Cr .4,Mo .1,B .0005,Fe 98 .8,Si .2,Cr .5,Ni .6,Mo .2,Fe 97.5 .5,Si .2,Cr 1.2,Ni 3.2,Mo 1,Fe 94.2 C 1.0,Mn 12,Si .2,Fe 87 C .8,Mn . 7 , S i .2,Fe 98

16

Rapid quenching of austenite produces martensite. This i s a single phase containing dissolved carbon with a highly stressed lattice. The result is a very hard, abrasion resistant material. Martensite is the most wear resistant phase which can be developed in plain carbon steel. However it is relatively brittle until tempered by reheating after quenching. Selection of the correct quench rate is governed by the temperature-timetransformation (TTT) characteristics of the alloy. The chart in figure 1.3 shows TTT characteristics for a plain carbon steel. The C curve at the upper temperatures reflects the rate of phase change at a given temperature. For instance, if one slowly cools from austenite to below the eutectoid temperature and holds at about 700'C until the C curve is passed through (cooling curve 1) a coarse pearlite is formed after quenching when well within the knee of the curve. Holding at a lower temperature still above the knee of the curve (cooling curve 2) produces finer pearlite. If one cools rapidly to below the knee of the C curve (say about 500'C) and holds until the transformation is complete, one forms bainite (cooling curve 3 ) . Bainite is a feathery or acicular phase somewhat more wear resistant than pearlite. On quenching rapidly below the knee of the C curve until one passes through the Ms line , martensite i s formed (cooling curve 4 ) . Note that the martensite transformation i s not completed even at temperatures close to the boiling point of water (M90 line). This means that at room temperature there can still be some metastable austenite in the martensite. The retained austenite can transform when heavily strained and cause small, but important dimensional changes in a part. Note that the complete transformation does not occur until below freezing temperature. This is why heat treatment of ball bearing steel involves subzero cooling. This will be discussed under bearing steels.

17

800

700

600

h

," 500

coarse pearl ite

I

v 0)

L

3 c,

2

400

aJ cz

fij + 300

200

100

0

lo-'

Irn

2 4 s

tf

I

!

Id

Ih I

I

I

I

I

I

I

I

I

10

10'

I 03

I 04

I

10'

Time (seconds) Figure 1.3 Temperature-Time-Transformation Diagram for Plain Carbon Steel

18

SELECTION OF STEELS

Pearlitic Steels: Carbon Steels are inexpensive and ideal for large components. Steels are probably the most used materials in conditions requiring wear resistance. A great deal of information on the wear properties and methods for selection for steels can be found in the ASM metals handbook 1131.

Pearlitic steels are hardened by heat treatment. By heating the part to the austenitizing temperature and quenching, a microstructure is developed containing lamellar pearlite composed o f plates o f iron carbides (cementite) sandwiching iron or ferrite. Free carbides may also be present. The resulting microstructure is about five times harder than the hardness of the original austenite. Wear resistance increases with increasing amount of pearlite. Increased carbon content raises the percentage of carbides and pearlite in the structure as well as the hardness and wear resistance. Increased cooling rate reduces the carbide lamellar spacing and increases the resistance to abrasion. Reducing the area o f ferrite in the structure reduces the chances of adhesion and galling. In addition, the increased hardness provides better support for oxide films which also inhibit wear. The wear process produces extreme plastic deformation in the near surface region of steel. The extent of the plastic flow will, to some extent, govern the amount of wear. The pearlitic structure will tend to minimize the depth of heavy deformation. As the structure is subjected to contact stress and shear, the average spacing in the lamellar structure is reduced by deformation processes and essentially increases the resistance to penetration. Toughness - Although wear resistance increases monotonically with hardness, there is a hardness level above which the metal toughness decreases to a point where microfracture in the surface becomes part of the wear process and wear increases. A chart showing the effect of toughness on wear resistance i s shown in figure 1.4.

19

A

Glass

0 Tool Steel 0

7-

Gray Cast Iron

Nodular Cast Iron

6-

-

0

5-

I

-

I

9

-3 4-

I

u a 8

-e

atm

c

0

ul

i

01

a

:2- I P f g I.

0

I

3-

0

a 0

I

1

F i g u r e 1.4

E f f e c t o f M a t e r i a l Toughness on Wear Resistance [14]

20

H a r d e n a b i l i t y - There i s some c o n f u s i o n between t h e terms hardness and h a r d e n a b i l i t y . Hardness i s t h e r e s i s t a n c e t o p e n e t r a t i o n o r s c r a t c h i n g on a g i v e n l o c a t i o n on a p a r t . H a r d e n a b i l i t y i s t h e d e p t h t o which a s t e e l p a r t can be hardened t o a s e l e c t e d hardness l e v e l f o r a g i v e n c o o l i n g r a t e d u r i n g h e a t t r e a t i n g . Thus, i f a one i n c h diameter s t e e l s h a f t must have a hardness o f Rc 50 h a l f way i n t o t h e c e n t e r , an a l l o y must be chosen which w i l l have t h e h a r d e n a b i l i t y r e q u i r e d t o produce t h a t hardness p a t t e r n w i t h a reasonable quench r a t e . Hardness i s c o n t r o l l e d b y quench r a t e . Quench r a t e depends on t h e r a t e o f d i f f u s i o n o f heat i n t o a b l o c k o f m e t a l . That i s , t h e h i g h e s t quench r a t e i s a t t h e s u r f a c e o f t h e p a r t . The f u r t h e r one goes i n t o t h e p a r t , t h e l o w e r t h e quench r a t e . A t some depth, one reaches a p o i n t where no hardening occurs. Thus, i n s e l e c t i n g a s t e e l and i t s heat t r e a t m e n t , one must c o n s i d e r t h e t h i c k n e s s o f t h e s e c t i o n and t h e depth t o which a c e r t a i n hardness l e v e l must be maintained. For instance, i n a s t e e l gear, a h i g h hardness i s r e q u i r e d on t h e s u r f a c e t o r e s i s t s c u f f i n g . High hardness i s a l s o r e q u i r e d below t h e s u r f a c e t o t h e d e p t h a t which t h e maximum shear s t r e s s o c c u r s t o r e s i s t contact fatigue. The t o o t h must a l s o have a g i v e n c o r e s t r e n g t h and f r a c t u r e toughness l e v e l t o p r e v e n t bending f a i l u r e . The s e l e c t i o n o f t h e p r o p e r a l l o y must n o t o n l y c o n s i d e r maximum hardness and toughness a c h i e v a b l e b u t a l s o t h e h a r d e n a b i l i t y and i t s e f f e c t on t h e hardness g r a d i e n t t h r o u g h t h e t o o t h s e c t i o n . A g r i n d i n g b a l l f o r b a l l m i l l i n g can l o s e c o n s i d e r a b l e p a r t o f i t s volume t o wear and s t i l l be a usable p a r t . Therefore, t h r o u g h hardness o r h i g h hardness t o t h e c e n t e r o f t h e b a l l i s r e q u i r e d f o r an economic r e s u l t . The h a r d e n a b i l i t y o f t h e a l l o y s e l e c t e d f o r t h e b a l l , then, w i l l be an e s s e n t i a l c o n s i d e r a t i o n and w i l l be d i f f e r e n t f r o m t h a t f o r t h e gear.

21

Required: 45 HRC at 3/4 radius for a 44 mm diameter shaft, oil quenched 120

E

100

E

L -

c1I

80

n L 0

60 4 0-

20 0 4

0

12

8

16

20

Distance from quenched end, 1/16 in. 70 I

6 5 ....................

i ...................~....................

..... .............;.....................

....................

................... j................... j ....................

...................

30 0

4

8

12

16

Results: Minimum hardenability o f 4140H produces 49 HRC

Figure 1.5 Example o f Use o f Hardenability Charts for 4140H Steel

20

22

H a r d e n a b i l i t y d a t a f o r s t e e l s can be found i n t h e f o r m o f end quench and e q u i v a l e n t c o o l i n g r a t e c h a r t s . An example i s shown i n f i g u r e 1.5 [15]. ( F o r more d e t a i l s on h a r d e n a b i l i t y and h a r d e n a b i l i t y curves see ASM M e t a l s Handbook, n i n t h e d i t i o n , 1978, pgs 471-525). I n f i g u r e 1.5, an A I S I 4140H s t e e l w i t h a carbon c o n t e n t o f 0.38% i s r e q u i r e d t o have a hardness o f a t l e a s t 48RC a t t h e t h r e e - q u a r t e r - r a d i u s o f a 1.75 i n c h b a r when quenched i n o i l . E n t e r i n g t h e e q u i v a l e n t c o o l i n g r a t e c h a r t a t 1.75 in.(44.5mm) diameter and f i n d i n g t h e e q u i v a l e n t d i s t a n c e from t h e quenched end o f a t e s t standard b a r (0.4 i n ) , one e n t e r s t h e h a r d e n a b i l i t y c h a r t a t 0.4 inches f r o m t h e quenched end and f i n d s a hardness v a l u e o f 49 Rc. Thus t h e a l l o y s e l e c t e d i s satisfactory.

Martensitic Steels: The m a r t e n s i t i c s t r u c t u r e i n s t e e l s , produced b y heat t r e a t i n g ( u s u a l l y b y a r a p i d quench) i s t h e most wear r e s i s t a n t s t r u c t u r e i n carbon s t e e l s . For t h e same hardness, p e a r l i t i c s t e e l s w i l l n o t r e s i s t a b r a s i v e wear as w e l l as m a r t e n s i t i c s t e e l s . B a i n i t e , a s t r u c t u r e s i m i l a r t o m a r t e n s i t e and achieved w i t h slower quench r a t e s has about t h e same wear r e s i s t a n c e as m a r t e n s i t e , b u t a g r e a t e r toughness. High carbon m a r t e n s i t e i s u s u a l l y tempered a t about 230 C f o r s t r e s s r e l i e f and an i n c r e a s e i n f r a c t u r e toughness and has t h e b e s t wear r e s i s t a n c e o f any o f t h e s t e e l s t r u c t u r e s . I f m a r t e n s i t i c s t e e l must be tempered below 50 HRC (as, f o r i n s t a n c e t o p r e v e n t c h i p p i n g under impact l o a d s ) , i t i s i n f e r i o r t o p e a r l i t i c s t e e l a t t h e same hardness. M a r t e n s i t e i s a metastable s u p e r s a t u r a t e d s o l u t i o n o f carbon i n i r o n . T h i s produces a v e r y f i n e homogeneous needle-1 i k e c r y s t a l s t r u c t u r e which w i l l show no p r e f e r e n t i a l wear o f a second phase. However, m a r t e n s i t e w i l l t e n d t o decompose and produce c a r b i d e s when heated. Therefore, i t i s n o t recommended f o r temperatures much above 200°C. T h i s i n c l u d e s s u r f a c e h e a t i n g f r o m f r i c t i o n . High carbon m a r t e n s i t i c s t e e l s w i l l o f t e n c o n t a i n some r e t a i n e d a u s t e n i t e f r o m t h e heat t r e a t m e n t . As l o n g as t h e percentage o f r e t a i n e d a u s t e n i t e i s low, i t can be b e n e f i c i a l i n p r o v i d i n g some toughening and under heavy a b r a s i o n w i l l t r a n s f o r m t o untempered m a r t e n s i t e and w i l l be more r e s i s t a n t t o wear. I n r o l l i n g c o n t a c t bearings, and i n c l o s e f i t t i n g p a r t s , r e t a i n e d a u s t e n i t e can be d e t r i m e n t a l because i t w i l l cause a s w e l l i n g o f t h e p a r t when i t transforms, r e s u l t i n g i n b i n d i n g o f small c l e a r a n c e b e a r i n g s .

23

Austenite and Ferrite: The addition of sufficient manganese to steels will stabilize austenite at room temperature. If the austenite is stabilized sufficiently so that it does not transform even under plastic deformation, the steel will show better wear resistance than ferritic steels with the same carbon content. Manganese i s used in tough impact resistant steels for mining and dirt moving machinery to increase abrasion resistance. Martensite is formed during impacting and will add resistance to gouging with little reduction in toughness. To summarize: the relative wear resistance of various steel microstructures, an increase in wear resistance is noted as one goes from pearlite through austenite t o martensite. This trend is shown graphically in figure 1.6 - a plot of gouging wear ratio vs carbon content. Note that the highest carbon iron-base alloys are the high chromium white irons. The chart shows little increase in gouging wear resistance for these alloys - however - they have an important role in certain abrasive wear conditions as is described in chapter 4. (Cast Irons)

0

1.o

2.0

3.0

4.0

Carbon Content, 96

Figure 1.6 Gouging Wear Ratio as a Function of Carbon Content and Microstructure [16]

24

Stainless Steels There a r e t h r e e b a s i c t y p e s o f s t a i n l e s s s t e e l used i n wear c o n t r o l : A u s t e n i t i c o r 18-8 s t a i n l e s s s t e e l Martensitic stainless steel PH s t a i n l e s s s t e e l

The mechanical and p h y s i c a l p r o p e r t i e s and chemical c o m p o s i t i o n o f a s e l e c t e d group o f s t a i n l e s s s t e e l s a r e found i n Table 1.2 a,b, & c .

Table 1.2a

Mechanical P r o p e r t i e s o f S t a i n l e s s S t e e l s

MATERIAL

SS SS SS SS SS SS SS

ss SS SS SS SS SS SS SS SS SS

15-5PH 16-25-6 17-4PH 304 304HN 316 316 347 410 410 440C 440C 440CM ALLEGHENY A-286 NITRONIC 60 UHB AEB-L UNILOY 19-9DL

Conversion f a c t o r s : MPa x 0.145 = k s i

CONDITION

HEAT TREAT

WROUGHT H900 HOT ROLLED Hardened WROUGHT H900 ANNEALED

HOT WORK

Bar

Annealed Cold R o l l ANNEALED HT, 1000 t e Annealed Annealed HT, 600F t e F u l l y aged Annealed Hot R o l l ed

HARDNESS DPH

TS MPa

1379 1103 1379

MODULUS MPa

420 320 400 150 200 150 200 150 257 135 257 650

1.96Et05 1.97E+05 1.96Et05 1.97Et05 2.00Et05 1.96Et05 1.96E+05 1.93E+05 1.96E+05 1.96Et05 2.00Et05 2.00Et05 2.00E+05

689 586 758 620 758 517 862 1379

330 200 310 217

1000 717 2.00Et05 689 2.07Et05 751

Table 1.2b P h y s i c a l P r o p e r t i e s o f S t a i n l e s s S t e e l s STAINLESS GRADE SS SS SS SS SS SS

ss

SS SS SS SS SS SS SS

MAX TEMP DENSITY OC Kg/cu METER

15-5PH 16-25-6 17-4PH 304 304HN 316 347 410 440C 440CM ALLEGHENY A-286 NITRONIC 60 UHB AEB-L UNILOY 19-9DL

500 649 500 900 700 649 815 649 316 760 760

7.75€+03 8.03€+03 7.75E+03 8.03E+03 8.03E+03 8.03E+03 7.89E+03 7.68E+03 7.68E+03 7.68E+03 7.90E+03 7.66E+03 7.75E+03 7.92E+03

CONDUCT. WATT/m/K

EXPANS. m/m/oC

16.20 17.0 24.90 29.41

1.12E-05 1.60E-05 1.12E-05 1.74E-05 1.91E-05 1.12E-05 1.87E-05 1.17E-05 1.02E-05

1427 1427 1427 1482 1538

20.0

1.78E-05

1427

28.35

1.10E-05

649

17.82 17.8 16.20

Conversion f a c t o r s : kg/m

3

W/m/K

x 3.613E-5 x 0.578

=

=

l b / c u i n : kg/m

x 0.001

BTU/sqft/ft/hr/'F

J/kg/K

x 2.388E-4 =BTU/lb/'F

m/m/"C

x 0.55

=

3

in/in/'F

o r cal/g/-C

M.P. C

=

gm/cc (SG)

RESIST. ohm-cm

HEAT CAP. J/kg/K

77.00

4.2E+02

77.00

4.2€+02

74.00 72.00 57.00 60.00

4.2E+02 4.2E+02 4.2E+02 4.2E+02 4.2E+02

98.0 98.2 58

4.2E+02 4.2E+02

26

A u s t e n i t i c s t a i n l e s s s t e e l : This class of steels is used to a great extent for

its superior corrosion resistance. Having the most effective balance of carbon, chromium and nickel for corrosion resistance, these steels resist many chemical reagents, sea water, 1 iquid metals, rusting and reactor grade 316'C (600OF) water. These are the 300 series steels ( AISI 304, 316, etc) and contain about 18% chromium, 8% nickel and 0.1 carbon. They are notorious for their tendency for adhesion and galling. Even though they quickly develop a tough passivating oxide film when exposed to air, their tendency for low work hardening rates causes easy rupture of the oxide and virgin metal contact. Austenitic stainless steels cannot be hardened by heat treatment and therefore have no easy means for improving wear resistance. Two series of austenitic stainless steels are available: 200 series and 300 series. The 200 series have a portion of the nickel replaced by manganese or nitrogen. The 200 series stainless steels respond well to work hardening - achieving maximum values in the stainless alloys. However, 200 series stainless steels are not used as much as other grades in tribological applications. Austenitic stainless steels are used for high temperature, aggressively corrosive conditions and nuclear reactor appl ications for the most part. These steels resist corrosion in 316'C reactor water and in chemical processing plants operating at temperatures up to 900 C. The chromium - nickel combination produces an oxide in oxidizing conditions that is very slow growing and tough. As long as this oxide is not disrupted, corrosion is insignificant over long periods of exposure. However, this oxide provides little protection from wear. Since the 300 series stainless steels have relatively low carbon content for optimum static corrosion resistance, they do not have the carbides present in wear resistant steels. In addition, 300 stainless steels are highly susceptible to adhesion and galling. There is little wear data in the literature for austenitic stainless steels. Most of the data is found in reactor handbooks. ASM metals handbook shows the gouging wear resistance of AISI 316 stainless and it has the lowest resistance of all other steels tested. The tendency for austenitic stainless steels to gall and seize - especially when self-mated presents a serious problem when they are needed in an aggressive environment and sl iding contact is involved. Schumacker has attributed high nickel content in 300 series stainless alloys to sensitivity to galling [17,18]. Bhansali [19] has suggested that the high nickel content of 300 series stainless steels increases the stacking fault energy of the alloy. Stacking fault energy has an influence on the deformation mode of metals

27

High s t a c k i n g f a u l t energy a l l o w s h e a v i e r d e f o r m a t i o n i n a metal t h a n does low s t a c k i n g f a u l t energy - f o r a g i v e n shear s t r e s s l e v e l . B h a n s a l i concludes t h a t s t a c k i n g f a u l t energy can i n f l u e n c e g a l l i n g o r adhesion t e n d e n c i e s b u t n o t general wear p r o p e r t i e s o f an a l l o y . T h i s i s t r u e because t h e s u r f a c e condition ( n a t i v e oxide, l u b r i c a n t , contamination) i s a strong m o d i f i e r o f t h e wear p r o p e r t i e s o f a m a t e r i a l . However, i f t h e c o n t a c t s t r e s s i s such t h a t t h e d e p t h o f p l a s t i c d e f o r m a t i o n i s l a r g e , near s u r f a c e p l a s t i c f l o w can cause f o l d i n g o f t h e s u r f a c e l a y e r s i n t o t h e subsurface r e g i o n , exposing f r e s h metal t o 1 i k e l y adhesion and g a l l i n g . However,the near s u r f a c e d e f o r m a t i o n o c c u r r i n g w i t h wear i n v o l v e s such h i g h l e v e l s o f s t r a i n , t h a t t h e d e f o r m a t i o n modes a s s o c i a t e d w i t h s t a c k i n g f a u l t energy l e v e l s a r e o f t e n l o s t i n h i g h s t r a i n modes. I t i s t r u e , however, t h a t n i c k e l c o n t e n t does seem t o i n f l u e n c e t h e tendency f o r g a l l i n g i n stainless steels. Carbon c o n t e n t and n i c k e l c o n t e n t have s i g n i f i c a n t i n f l u e n c e on t h e work hardening c o e f f i c i e n t o f s t a i n l e s s s t e e l s . F o r 18Cr-8Ni s t e e l s , carbon c o n t e n t below 0.06% r e s u l t s i n a drop i n work hardening c o e f f i c i e n t [20]. T h i s i s shown i n f i g u r e 1.7. The e f f e c t o f n i c k e l c o n t e n t on work hardening exponent i s shown i n f i g u r e 1.8. Note t h a t g o i n g from 8% t o 12% r e s u l t s i n a s i g n i f i c a n t drop i n t h e work hardening exponent. T h i s i s f u r t h e r evidence t h a t 316 s t a i n l e s s should have p o o r g a l l i n g r e s i s t a n c e .

F i g u r e 1.7 E f f e c t o f Carbon Content on Work Hardening F a c t o r i n S t a i n l e s s S t e e l [21]

28

Table 1 . 2 ~Chemical Composition o f S t a i n l e s s S t e e l s

MATERIAL

CHEMICAL

COMPOSITION

SS 15-5PH C .07,Mn 1.0,P .04,S .03,Si l , C r 14.5,Ni 4,Cu 3,Cb .3,Fe 76 SS 16-25-6 C .12,Mn 2,Si l , C r 16,Ni 25,Mo 6,Fe 49.9 SS 17-4PH C .05,Cr 16.5,Ni 4.0,Cu 4.0,Fe 75.5 SS 304 C .08,Mn 2,Si l , C r 18,Ni 8,Fe 71 SS 304HN C .08,Mn 2 , P .045,S .03,Si l , C r 18,Ni 8,N .2,Fe 70.6 SS 316 C . l , C r 18,Ni 14,Mo 3,Fe 65 ss 347 C .08,Cr 18,Ni l l , T i . 8 Fe 70 SS 410 C .15,Mn 1.0,Si .5,CR 1 2 , Fe 86.5 SS 440C C 1,Mn 1.25,Si l , P .04,S .04,Cr 18,Mo .75,Fe 78 SS 440CM C .08,Mn 1 , S i .6,Cr 15,Ni 26,Mo l , T i 2,Va .3 Fe 54 SS ALLEGHENY A-286 C l,Mn 1 , S i 1,Cr 15,Mo 4,Va .15,Fe 87.25 SS NITRONIC 60 C .1,Mn 8,Si 4,Cr 17,Ni l , N 1,Fe 63 SS UHB AEB-L C .68,Mn .6,Si .38,Cr 13.2,Fe 85.2 SS UNILOY 19-9DL C .4,Mn 1 , S i .5,Cr 19,Ni 9,Mo 1.5,W 1.5, CbtTa .4,Fe 67

I55

145

e

2

155

v


.r

c,

.

I

I

I I

I

I

5 7

W CT

Lead babbitts

Alumium-tin

A1 -Cu-Sn

Figure 3.8 Relative bearing performance for various bearing materials

86

Aluminum-lead was an inexpensive alternative but had inherent problems in manufacture. Note, from the aluminum-lead constitution diagram in figure 3.9 that lead is virtually insoluble in aluminum at room temperature. A 5% lead content can exist in solution at 815 C, allowing mixing and casting of the two constituents. The cast alloy would be a mixture of free lead globules in an aluminum matrix. This would be similar to a copper-lead bearing material. Two problems had to be overcome: (1) the lead would be susceptible to corrosion and ( 2 ) over 2% lead produces a weak bond when bonding to a steel backing. The first problem was solved by using a lead-tin babbitt with sufficient tin t o provide corrosion resistance. The second problem was solved by developing a casting method that produced a graded lead content throughout a solid section. By machining the cast plate so that one side contained minimum free lead and the other side contained 5% lead, the material could rolled t o a sheet, cut into bearing inlays and bonded to the steel backing with the low lead side.

Figure 3.9 Aluminum-lead constitution diagram

87

ZINC BEARING ALLOYS

Zinc is an inexpensive substitute for babbitt or bronze materials. A1 hough not found in general use, large bearings such as hot mill runout table bearings are an appropriate application, saving approximately 20% over bronze. Zinc alloy bear ngs were used in Germany during World War I1 because of critical copper shortages. Zinc-aluminum a loys are used for plain bearings. Two commercial alloys are listed in table 3.2. Note that the alloys contain 12 to 27% aluminum. Sometimes zinc die casting alloys are used so that a cast housing can serve as a bearing, eliminating the necessity of casting in or pressing in a bronze bearing. The zinc bearing alloys can be used in the cast condition, much the same as bronze bearings. Zinc-aluminum alloys have a number of advantages besides economy. For instance, their low elastic modulus allows zinc bearings to conform to elastic bearing loads. Zinc bearing materials can support large loads for slow moving, grease lubricated conditions. In fact, they will support loads which leaded bronze bearings are used for [61]. Zinc bearing alloys have limitations that must be considered if their use is anticipated. Thermal expansion rates for zinc alloys are higher than bronze or cast iron and therefore, journal bearings made of zinc a loy require larger clearances than used in equivalent bronze bearings. Zinc alloys also loose strength quickly when heated. These alloys are also sens tive to lubrication starvation. Loss of lubricant can result in adhesion and galling of zinc alloy bearings .

88

GOLD, SILVER AND INDIUM

The precious metals gold, silver and platinum are used as coatings on harder metals to provide both corrosion protection and solid lubrication. These metals provide soft, easily sheared coatings as long as the coating is thin enough. The metals are used in thin coatings electrodeposited or sputter coated on metal substrates. There is an optimum coating thickness for minimum friction conditions as was discussed early in the chapter. Wear will eventually remove the coating and therefore a compromise between minimum friction and maximum life may be necessary when using this approach. Gold and platinum are used in electrical contacts not only because of their desired electrical conductivity but also for their corrosion resistance especially atmospheric in industrial atmospheres. Their ability to act like solid lubricants also is desirable for moving electrical contacts. Very thin gold coatings are used on connections for removable printed circuit boards used in high quality intstruments and computers. These coatings will last for several removals and insertions before they wear off. Gold coatings have been used on precision instruments operating in vacuum or inert environments. This includes coatings on precision ball bearings. Unalloyed or low alloy gold is susceptible to adhesive type wear and resulting noisy low energy electrical contacts. In addition, precious metal electrical contacts will develop "friction polymers" during sl iding contact in atmospheres containing organics. This can provide effective lubrication - but if polymer develops in sufficient quantity, it can separate the contacting metal surfaces. For these reasons, it is often desirable to use a lubricant with a sliding electric contact. Silver has been used in heavy duty engine journal bearings for aircraft and diesels. Silver is plated over a steel backing and an overlay of lead and a flash coating of indium is used t o provide breakin and corrosion protection. Silver has also been used as an antiscuffing coating in engine cylinders.

89

CHAPTER

4 - CAST

IRON

MELTING POINTS

HARDNESS

150

-

THERM EXPANS DENSITY

'C

895 VICKERS

TENSILE STRENGTH YOUNGS MODULUS

1160 - 1450

110 - 689 MPa 1.03Et05

1.24E-05

0.27 kg/mJ

TYPES OF CAST IRONS Gray Cast I r o n White I r o n Nodular Cast I r o n Ma1 1eabl e Cast I r o n High A l l o y I r o n s

APPLICATIONS FOR CAST IRONS Automotive c r a n k s h a f t s and connecting r o d s Piston r i n g s Brakes and c l u t c h e s Gears D i e blocks Grinding b a l l s Machine ways and s l i d e s Crushers Cams and t a p p e t s Valves Pumps Wheels and r o l l e r s f o r heat t r e a t furnaces Crane wheels

-

-

2.21Et05 MPa

2.5E-06 K

90

INTRODUCTION

Cast iron is an iron-carbon-silicon alloy with carbon content between 2.5 and 4.0 percent. The alloy can also contain silicon, manganese, sulphur and phosphorous. The alloying additions are used to modify the structure and properties of cast iron. Cast iron has been used in machinery since the beginning of the industrial revolution. It is a low cost material with structural strength similar to steel and can be used for the fabrication of large structures such as machine bases, engine blocks and large brackets. It is ideal for casting because of its ease of flow when molten. Cast irons are known for their good tribological properties. They are resistant to wear in boundary lubrication conditions and can be heat treated to be resistant to aggressive abrasive conditions. Cast irons can be used at elevated temperatures (gray iron can be used at 400' C ) . There are many grades of cast irons and often the terms of reference can be confusing. Gray cast iron, the most used of the irons, is an iron-carbonsilicon alloy in which much of the carbon exists as flake graphite. The matrix in which the graphite is imbedded can be ferrite (pure iron), pearlite, bainite or martensite. The particular matrix structure can be developed by heat treatment.Gray cast iron is so named because when it is fractured, the fracture face looks gray because of the free graphite in it. White iron is cast iron in which the carbon exists as iron carbides. When white iron is fractured the fracture face looks white owing to the presence of a large fraction of carbides. White iron can be achieved by rapid quenching a normally gray iron composition. Malleable iron is an iron-carbon alloy in which the carbon exists as nodular graphite. Malleable iron is formed from white iron by heat treating. The result is an alloy with much improved fracture toughness and increased elastic modulus (compared to gray cast iron). The hardness, of course, is reduced. This material resembles steel in mechanical properties and yet can be fabricated by casting - a low cost advantage over steel. Heat treatment required to achieve the nodular structure however, is long and expensive. Nodular or Ductile or Spherulitic Graphite (SG) iron is an iron-carbon alloy in which the carbon again exists as graphite nodules or spheres. In this alloy, the nodular graphite is formed by addition of small amounts of spheroidizing agents such as magnesium or cerium during casting. Nodular iron is less expensive than malleable because it does not require extensive heat treating to achieve its nodular structure.

91

High alloy cast irons are conventional cast irons with extra amounts of chromium, silicon, molybdenum or nickel added to improve abrasion resistance, corrosion resistance or for high hot hardness requirements. These alloys generally have hard chromium carbides or have an austenitic matrix. Some of the familiar alloys in this category are the Ni-hards and the Ni-resists. High chromium white irons are found in the mining and ore processing industries. PROPERTIES OF CAST IRONS

The general characteristics of each of the classes of cast irons are summarized in table 4 . 1

Table 4 . 1 Characteristics o f the Five General Types o f Cast Iron Type of Iron

Toughness

Hardness

Elastic Modulus

Wear Resistance

White Iron

Low

High

High

High

Gray Iron

Moderate

Moderate

Low *

Moderate

Ductile Iron

High

Moderate

Higher

Moderate

Malleable Iron

High

Moderate

15% greater Moderate than ductile

High Alloy Iron

Low

High

Same as steel

Best abras erosion resistance

&

* Some gray irons have elastic moduli almost half that of steel, The mechanical and physical properties o f cast irons are summarized in table 4 . 2

92

Table 4.2 Mechanical and Physical P r o p e r t i e s o f Cast I r o n s

MATERIAL

A536 NODULAR ABEX PACE DURIRON HC-250 MEEHANITE AQ N I-HARD NI-HARD 4 NI-RESIST 1 Pearl i t i c M a l l e a b l e

HARDNESS VICKERS DPH

350 380 530 600 196 655 560 150 248

HARDNESS BRINELL BHN

TENSILE STRENGTH MPa

974 606 110 689 340 379 620 207 723

331 360

500 550 196 587 522 150 248

YOUNGS MAX OP MODULUS TEMP 'C GPa

168.90 197.17 124.09 217.16 152.00 172.35 172.35 103.41 172.35

D E N S I T Y CONDUCT Kg/ WATT/m CU METER K

.

x 3.613E-5 = l b / c u i n : kg/m

3

44.70 17.30

816 538

7.47E+03 7.20E+03

39.79

1.27E-05 1.94E-05

7,30E+03

40.00

1.00E-05

x 0.001 = gm/cc ( S G )

W/m/K x 0.578 = B T U / s q f t / f t / h r / ' F J/kg/K x 2.388E-4 = BTU/lb/'F or cal/g/.C m/m/'C

x 0.55 = i n / i n / ' F

CHEMICAL COMPOSITION

MATERIAL

A536 NODULAR ABEX PACE DURIRDN MEEHANITE AQ N 1-HARD NI-HARD 4 NI-RESIST 1 P e a r l i t i c Malleable

C C C C C C C C

J/kg/'K

816

MPa x ,145 = k s i 3

/'C

HEAT CAPACITY

8.47E-06 4.19E+02 1.24E-05 5.44E+02 8.90E-06 1.02E-05 7.44E+00 9.36E-06

Conversion f a c t o r s :

kg/m

m/m

7,64E+03 7,20E+03 7,47E+03 7.34E+03 7.60E+03

399

3.5.Si 2,Mn 1,P ,065.Mg .04,Fe 93 (approx comp. ) 1.6.Mn 1 . 5 . S i 2.0,Cr 28,Ni 2,Mo 2,Fe 63 .85,Si 14.5,Mn .5,P .07.S .08, Fe 84 3,Mn 2,Si 1.8,Cu .5,Ni 2,Mo .5,Fe 90.2 3.5,Si .5.Ni 4,Cr 2,Fe 89.5 3 . 5 . S i 1.5.Mn.5.Ni 6 , c r 8,Fe 80.5 3,Si 1.5.Ni 15,Cu 6,Cr 2,Fe 71.5 2.5.P 1 . 0 , s . 6 . S i 1 . 0 , Fe 95

14.50

EXPAN

93

Elastic modulus. The graphite structure in gray cast irons influences the modulus of elasticity. Elastic modulus drops with increasing graphite content. The effect of flake graphite is much greater than nodular graphite. Angus [ 6 2 ] reports that the ratio of (elastic modulus CI/elastic modulus steel) ranges between 0.4 and 0.7 in as-cast pearlitic gray cast iron.In most cast irons there is no proportional limit in the tensile stress-strain diagram. Elastic moduli are determined by a tangent to the stress strain curve at 0.10 strain. Poisson’s ratio. Poisson’s ratio in grey iron tends to fall as tensile stress increases owing to the opening up of graphite cavities. It generally remains constant under compressive stresses. Angus [ 6 3 ] shows a range of values from 0.25 to 0.08 depending on the load and graphite structure. Impact resistance. Gray cast iron exhibits moderate impact resistance especially in low phosphorous containing alloys. When the phosphorous content exceeds 0.7%, impact resistance drops. Nodular iron shows much higher impact resistance than gray iron. White iron and high alloy cast irons exhibit low impact resistance. Thermal conductivity. The thermal conductivity of gray cast iron increases with increasing carbon content - as long as the carbon exists as graphite. Pearlite will tend to reduce conductivity. Nodular iron exhibits a 20 to 30 percent lower thermal conductivity than gray iron. White iron and high alloy cast irons also exhibit lower thermal conductivities because of the carbide content. Gray iron is similar to steel in thermal conductivity levels. Electrical resistivity. The electrical resistivity of cast iron ranges between 25 and 80 microhms/cm. Carbon and silicon content are the most effective

moderators of resistivity. Flake graphite produces high resistivity and the finer the graphite flakes, the lower the resistivity. For instance, gray cast iron with coarse graphite flakes has a resistivity of 104 microhms/cc while fine flake iron is 77.4 microhms/cc. Ferritic nodular iron has the lowest resistivity of the cast irons (-60 microhms-cm). Magnetic properties. Although cast irons are not as good permanent magnets as magnet steels, the temperature coefficient of magnetic loss and impact loss are much lower in cast iron. Nonmagnetic cast irons are produced by adding austenite stabilizers to iron. Nickel and manganese alloys are used. Damping capacity. Damping properties of gray cast irons are an important advantage in their use in bearings. Gray cast iron has exceptionally high damping capacity. Compared with mild steel, ferritic gray iron damping capacity is almost an order of magnitude greater. Thus, when fretting, stick slip or noise is a problem, gray cast iron is a good candidate to consider.

94

Hardness. Gray cast iron is often graded according to hardness. Care must be exercized, however, that the way in which the hardness reading is taken, the rate of cooling in the area where the hardness is determined and the microstructure of the iron are considered. A casting can experience various cooling rates depending on section thicknesses and therefore have a range of hardness levels over a component, depending on where the hardness readings are taken. Gray iron, with its heterogeneous structure of soft graphite in pearl ite and some carbides represents a range of hardnesses. Hardness measurements should provide an average of this range. Thus the Brine11 hardness system is used because the indentor covers a large enough area to include all phases in the structure. The hardness values for gray irons shown in this chapter have been converted to Vickers from BHN to maintain consistency in terms for the text. Since hardness values for gray cast iron are an average of soft and hard phases, it is dangerous to compare the hardness of cast iron with the hardness of steels when trying to choose a wear resistant material. A high carbon gray iron might exist as fully hardened martensite matrix with soft graphite imbedded and have a Rockwell C hardness of 40 - 45 but could be very wear resistant and virtually unmachinable because of its hard constituents. A steel of the same hardness might not compare at all in wear resistance and machinabil ity. METALLURGY OF CAST IRONS

The iron-carbon-silicon phase diagram, shown in figure 4 . 1 , shows that for the cast irons, the possible microstructures include iron carbide, pearlite and transformed austenite. I f the cooling rate is slow, the austenite transforms to pearlite and graphite. However, if the cooling rate is sufficiently rapid, carbon forms carbides rather than graphitizing. When the iron-carbon-silicon diagram is compared with the iron-carbon diagram shown in the upper right corner, it can be seen that addition of silicon changes the eutectic compositions (lowers the carbon content for the eutectic) and complicates the diagram with ranges o f transformation rather than sharp phase change boundaries. Gray iron can be pearlitic or ferritic depending on the speed of cooling during casting. Its structure consists of flake graphite in a ferrite or pearlite matrix. The structure is shown in figure 4 . 2 .

95

Nodular iron can be formed when a nodularizing agent is added to the alloy. The structure is different from gray iron only in morphology of the graphite phase. A an example of nodular structure is shown in figure 4.3. Note the ferrite surrounding the nodules of graphite. The ferrite shown in figure 4.3 is excessive in amount. The ferrite can all but be eliminated with controlled casting processes.

,

I

Carbide

I

I

I

I

2.0

10

I

i i

1 3.0

Carbon Content-Percent

4

I

Iec

0

By Weight

a. Iron-carbon diagram 900

+

,

800 IFerrite Austenite Carbide

+

700

600

"C Carbon Content-Percent

B y Weight

Figure 4.1 Iron-carbon-silicon equilibrium diagram (2% silicon) (from Walton [63]

96

Figure 4.2 Microstructure o f Gray Iron

Figure 4 . 3 Nodular iron

2M642

Figure 4 . 4

3Mo 7 V White iron

97

As s t a t e d above, w h i t e i r o n r e s u l t s from r a p i d c o o l i n g , p r e v e n t i n g e q u i l i b r i u m s t r u c t u r e s shown i n f i g u r e 4 . 1 f r o m f o r m i n g a t room temperature. An example o f t h e m i c r o s t r u c t u r e f o r w h i t e i r o n i s shown i n f i g u r e 4.4. The room temperature composition o f w h i t e i r o n c o n s i s t s o f d e n d r i t e s o f p e a r l i t e surrounded by i r o n c a r b i d e s . I t i s c a l l e d w h i t e i r o n because when f r a c t u r e d , i t s f r a c t u r e f a c e i s s i l v e r y w h i t e . Gray c a s t i r o n f r a c t u r e f a c e s l o o k g r a y because o f t h e g r a p h i t e f l a k e s i n t h e s t r u c t u r e . The h i g h carbon c o n t e n t and s i l i c o n i n c a s t i r o n s make them e x c e l l e n t c a s t i n g a l l o y s . They a r e v e r y f l u i d i n t h e l i q u i d s t a t e and do n o t f o r m d i f f i c u l t s u r f a c e f i l m s d u r i n g c a s t i n g . The i r o n - c a r b o n diagram ( f i g 4 . l a ) shows a e u t e c t i c o r l o w m e l t i n g p o i n t a t about 4.3% carbon. A t t h a t composition, t h e i r o n m e l t s a t about 1150 C. C a s t i n g i s f a c i l i t a t e d b y t h e l o w e s t m e l t i n g temperature. (Most commercial c a s t i r o n s c o n t a i n between 2.5 and 4 p e r c e n t carbon). A d d i t i o n o f s i l i c o n o r phosphorous w i l l l o w e r t h e carbon l e v e l a t which t h e e u t e c t i c occurs. S i l i c o n a l s o a l t e r s t h e c o m p o s i t i o n o f p e a r l i t e phase and t h e s o l u b i l i t y o f carbon i n a u s t e n i t e . Note t h a t t h e a d d i t i o n o f two p e r c e n t s i l i c o n t o c a s t i r o n reduces t h e carbon l e v e l a t which t h e e u t e c t i c occurs t o about 3.6 p e r c e n t as shown i n f i g u r e 4.1. The c a s t i r o n s , t h e r e f o r e d i f f e r s i g n i f i c a n t l y f r o m s t e e l s i n t h a t t h e y c o n t a i n a p p r e c i a b l y more carbon and g e n e r a l l y a l s o c o n t a i n s i l i c o n .

The i r o n - c a r b o n - s i l i c o n composition o f c a s t i r o n s makes p o s s i b l e a v a r i e t y o f m i c r o s t r u c t u r e s depending on t h e r a t e o f c o o l i n g and t h e e u t e c t i c p o i n t . S i l i c o n extends t h e t r a n s f o r m a t i o n range as w e l l . One can produce gray, w h i t e o r n o d u l a r i r o n depending on c o m p o s i t i o n and c o o l i n g r a t e . O f t e n t h e t y p e o f i r o n can be r e l a t e d t o t h e carbon e q u i v a l e n t (CE). The f o l l o w i n g f o r m u l a i s used t o determine t h e CE: CE

=

T o t a l Carbon% t ( S i % t P%)/3

T h i s v a l u e can be used i n t h e i r o n - c a r b o n diagram i n f i g u r e 4 . l a as t h e carbon c o n t e n t . For instance, an i r o n w i t h carbon c o n t e n t o f 3.2%, s i l i c o n 2% and phosphorous 1.3% would have a CE o f 4.3 and would be a t t h e e u t e c t i c p o i n t . The diagram i n f i g u r e 4.5 ( f r o m Walton) [ 6 4 ] shows t h e e f f e c t o f carbon s i l i c o n c o n t e n t on t h e t y p e o f i r o n s which f o r m d u r i n g s o l i d i f i c a t i o n . The carbon and s i l i c o n l e v e l s a t which t h e s t r u c t u r e changes form s t e e l t o c a s t i r o n can be seen i n t h e diagram. The d o t t e d l i n e s d e f i n e t h e zone i n which most commercial c a s t i r o n s f a l l . Note t h a t w h i t e i r o n s t e n d t o appear a t t h e lower CE l e v e l s .

98

As an iron composition cools from the melt, it will reach a temperature at which austenite will begin to form. This is shown in figure 4.1 by the vertical line representing a CE of 3.5. At the eutectic temperature, the remaining melt rejects carbon in iron carbide form and a eutectic mixture o f iron carbide and austenite occurs. As the CE is increased, the amount o f eutectic mixture increases and the amount of austenite decreases. Although iron carbide forms more easily than graphite because it does not require complete separation of carbon from iron, graphite is the most prevalent constituent o f commercial cast irons. Silicon is a graphitizing agent and therefore serves to promote graphite in the final structure after solidification resulting in gray iron. In addition, slow cooling and high carbon content promote graphitization. Rapid cooling rate favors iron carbide and white iron structure.

+

%c 3%

Si =4.3

. ~ u c t i l ed n s

4.0

I

c

e

2

3.c

I

I

c

” c V

5 2.0

1.c

C

I

I

I

1.o

2.0

3.0

Silicon Content-Percent

Figure 4.5 Silicon and carbon content ranges for various cast irons

99

A s t h e c o o l i n g proceeds i n t h e diagram i n f i g u r e 4.1,

t h e s o l u b i l i t y o f carbon i n a u s t e n i t e decreases and f u r t h e r carbon i s r e j e c t e d f r o m t h e s o l u t i o n and i t d e p o s i t s on t h e s u r f a c e o f t h e e x i s t i n g g r a p h i t e f l a k e s . When t h e f i n a l phase change temperature range i s reached, t h e a u s t e n i t e c o n v e r t s t o f e r r i t e and i r o n c a r b i d e - o r p e a r l i t e . The a l l o y composition and t h e c o o l i n g r a t e through t h i s i n t e r v a l w i l l have an i m p o r t a n t e f f e c t on t h e amount o f f r e e f e r r i t e ( s o f t i r o n w i t h v e r y low carbon c o n t e n t ) i n t h e p e a r l i t e m a t r i x . I n i r o n c a s t i n g , t h e f i n e n e s s o f p e a r l i t e and i t s carbon c o n t e n t w i l l depend on t h e a n a l y s i s o f t h e a l l o y and i t s c o o l i n g r a t e b u t a p o r t i o n o f t h e m a t r i x t h a t i s p e a r l i t e i s determined b y t h e amount o f carbon t h a t remains i n s o l u t i o n . Wear r e s i s t a n c e r e q u i r e s l o w f e r r i t e c o n t e n t and f i n e p e a r l i t e s t r u c t u r e

White iron White i r o n s f o r m f r o m a l l o y s l o w i n s i l i c o n c o n t e n t , w i t h CE s u f f i c i e n t l y below t h e e u t e c t i c p o i n t o r w i t h s u f f i c i e n t r a p i d c o o l i n g r a t e . The s t r u c t u r e o f w h i t e i r o n c o n s i s t s o f d e n d r i t e s o f p e a r l i t e surrounded by i r o n c a r b i d e .

Gray iron Gray i r o n , s u i t a b l e f o r t r i b o l o g i c a l use, cons s t s o f t y p e A f l a k e g r a p h i t e imbedded i n a f i n e p e a r l i t e m a t r i x w i t h l i t t l e f r e e f e r r i t e . When s p e c i f y i n g g r a y c a s t i r o n hardness, p e a r l t e s t r u c t u r e and g r a p h i t e s t r u c t u r e should be included.

Ma1 1 eabl e and nodul ar irons I n f i g u r e 4.5, one f i n d s a zone i n t h e 1.5 s i l i c o n - 2 . 5 carbon c o o r d i n a t e s marked m a l l e a b l e i r o n . T h i s d e f i n e s t h e compositions o f w h i t e i r o n s which can be transformed t o m a l l e a b l e i r o n b y h e a t i n g i n t o t h e a u s t e n i t i c zone and h o l d i n g a t temperature f o r a l o n g p e r i o d o f t i m e . T h i s decomposes t h e metastable c a r b i d e s t o produce i r r e g u l a r aggregates o f g r a p h i t e . Nodular i r o n , on t h e o t h e r hand i s produced b y a d d i t i o n o f magnesium r e s u l t i n g i n t h e f o r m a t i o n o f s p h e r i c a l nodules o f g r a p h i t e d u r i n g c a s t i n g .

N i Resist and N i Hard irons There i s o f t e n c o n f u s i o n about t h e d i f f e r e n c e s between these two c l a s s e s o f c a s t i r o n s . N i Resist. i s an a u s t e n i t i c g r a y o r n o d u l a r i r o n developed c h i e f l y f o r c o r r o s i o n and o x i d a t i o n r e s i s t a n c e . N i Hard i s a l o w a l l o y chrome-nickel w h i t e i r o n developed f o r abrasion r e s i s t a n c e .

100

N i c k e l i n i r o n promotes g r a p h i t e f o r m a t i o n , suppresses p e a r l i t e and s t a b i l i z e s a u s t e n i t e . Up t o 4.5% n i c k e l i s used t o promote m a r t e n s i t i c s t r u c t u r e and i s t h e b a s i s f o r t h e N i Hard a l l o y s . Chromium i s added t o a c t as a c a r b i d e s t a b i l i z e r and t o o f f s e t t h e g r a p h i t i z i n g i n f l u e n c e o f n i c k e l . S i l i c o n i s k e p t t o a l o w l e v e l i n t h e m a r t e n s i t i c N i Hards. Above 6.5% n i c k e l , a u s t e n i t e i s r e t a i n e d i n t h e c a s t i r o n s t r u c t u r e . N i R e s i s t a l l o y s c o n t a i n n i c k e l , copper & manganese i n v a r i o u s combinations. These a d d i t i o n s r e s u l t i n an a u s t e n i t i c s t r u c t u r e s t a b l e down t o room temperature. T h i s i s what t h e N R e s i s t a l l o y s a r e based on. N i c k e l c o n t e n t s above 18% produce an a l l austen t i c i r o n which i s nonmagnetic.

High chromium w h i t e irons High chromium i r o n s are used i n t h e m i n i n g i n d u s t r y f o r t h e i r g r e a t a b r a s i o n r e s i s t a n c e . As has been mentioned, chromium suppresses g r a p h i t e f o r m a t i o n and s t a b i l i z e s c a r b i d e s . With h i g h carbon l e v e l s , chromium a t between 12 and 27 p e r c e n t and some molybdenum, a v e r y hard w h i t e i r o n i s produced. A u s t e n i t i c i r o n s heat t r e a t e d t o m a r t e n s i t e have much b e t t e r a b r a s i o n r e s i s t a n c e than p e a r l i t i c i r o n s . The r e l a t i v e a b r a s i o n r e s i s t a n c e s o f these h i g h a l l o y i r o n s a r e shown i n f i g u r e 4.6 ( f r o m Diesburg and B o r i k ) [ 6 5 ] . S i l i c o n i s k e p t a t a l o w l e v e l t o m i n i m i z e t h e tendency t o form p e a r l i t e . Molybdenum h e l p s t o suppress p e a r l i t e and s p h e r o i d a l c a r b i d e s a f e r r i t e matrix. Hardenability i s a l s o increased. High chromium w h i t e i r o n s can be q u i t e b r i e . However, w i t h t h e r i g h t composition and m i c r o s t r u c t u r e , abrasion r e s s t a n t h i g h chromium i r o n s w i t h good toughness can be produced.

High Silicon or Duriron irons I r o n s w i t h 14 t o 24 p e r c e n t s i l i c o n e x h i b i t h i g h c o r r o s i o n r e s i s t a n c e t o a c i d s , e s p e c i a l l y h y d r o c h l o r i c a c i d . The s i l i c o n a d d i t i o n produces a tough, c o r r o s i o n r e s i s t a n t s i l i c o n o x i d e f i l m . Carbon i s k e p t t o n o t l e s s t h a n 0.35 p e r c e n t and n o t more t h a n 1 p e r c e n t . The c o n s t i t u t i o n diagram f o r these a l l o y s i s shown i n f i g u r e 4.7. Note t h a t a t 14 p e r c e n t s i l i c o n one has a s o l i d s o l u t i o n o f s i l i c o n i n gamma i r o n . The carbon e x i s t s as g r a p h i t e . A t s i l i c o n c o n t e n t above 15.2%, a new phase, n, appears. The n phase which appears a t between 825 and 1030' C i s so s l u g g i s h i n changing phase t h a t i t w i l l e x i s t i n t h e room temperature a l l o y .

101

High s i l i c o n i r o n i s a l o w s t r e n g t h b r i t t l e m a t e r i a l . Residual t e n s i l e s t r e s s e s can be annealed o u t b y h e a t i n g i n an a n n e a l i n g furnace. High s i l i c o n i r o n has e x c e l l e n t wear r e s i s t a n c e t o wet a b r a s i v e .

High phosphorous i r o n s Phosphorous increases t h e f l u i d i t y o f i r o n and has been used f o r c a s t i n g o f t h i n s e c t i o n s where f l o w p r o p e r t i e s o f t h e m e l t a r e i m p o r t a n t . A phosphorous c o n t e n t o f 0.2% produces an i r o n phosphate o r s t e a d i t e phase o f small d i s p e r s e d phosphate p a r t i c l e s which e t c h w h i t e i n t h e m i c r o s t r u c t u r e . A phosphorous c o n t e n t o f more t h a n 0.5% w i l l r e s u l t i n a continuous s t e a d i t e phase surrounding t h e p e a r l i t e c o l o n i e s . T h i s tends t o e m b r i t t l e t h e a l l o y . S t e a d i t e i s v e r y hard and i t s presence i n i r o n improves d r y wear r e s i s t a n c e .

\ 0.10

\

\

\ \

'

Spheroidite Matrix ( O I Mo)

0.09

\

\

\

\ \

0.08

\

W

Pearlite Matrix

Y

W

k

5

\

0.07

\

0.

5

15Cr 2Mo 1Cu (3 3C) 17Cr 1 5Mo 1 c u (3 oc) 0.06 0 0 0 0 l 8 C r O t o 3Mo 1Cu (2 9C) 2 o* 18Cr 2Mo 1Cu ( 3 OC) M vv 18Cr 2Mo 1Cu (3 3C)

01 AA

Y)

3

\

'

\

\

770+\

\ \

'

\

\

\

620'\

0.05

0.04 Matrix 0.03

Matrix I

I

I

10

20

30

I

40

1

I

50

60

HRC

f i g u r e 4 . 6 Abrasion Resistance o f High Chromium I r o n s ( P i n Test)

102

SILICON,

Wt

"/o

Figure 4.7 Iron-Sil icon Equil ibrium Diagram (from Angus

[66] )

103

HEAT TREATMENT OF CAST IRONS

Heat treatment is used to alter the microstructure of the as-cast product. Structural elements which can be influenced include graphite, ferrite, cementite (iron carbide), pearl ite, austenite and martensite. Hardening and softening the alloys are reversible processes. That is, irons hardened by quenching - like steel - can be annealed and irons that have been annealed to soft ferrite can be hardened by increasing the combined carbon content from the graphite phase. Carbides can be dissociated to graphite - but the graphite cannot be altered - it is a stable phase. A substantial improvement in wear resistance can be obtained by a variety of

hardening treatments. These include quenching and tempering, martempering, austempering, induction hardening flame hardening, nitriding and carbon i trid ing .

Stress Relief... In many tribolog cal applications, the dimensional stabi i ty of the bearing or rubbing surface is important to the precision of operat on of the device. Internal casting stresses are difficult to prevent in cast iron foundry practices. The residual stresses can cause gradual distortion as parts are machined. Internal stresses can also cause creep in a part if it is subject to elevated temperatures. Some plastic yielding can occur in castings as they age at room temperature. Stress relief by heat treatment can reduce residual stresses up to 80%. Heating a part for a few hours at 600' C will suffice.

... Cast iron can be heat treated to a martensitic structure just like steel. However, the resulting structure will contain flake or nodular graphite as it occurred in the original casting. To heat treat successfully, the cast iron must start out pearlitic in structure. In this condition, a part can be machined after casting and then the appropriate heat treatment carried out followed by a finish grind. This applies to both gray iron and nodular iron.

Hardening

The time-temperature-transformation diagram is used in designing heat treatments just as it is in steel heat treatment. A typical diagram for 2% silicon iron is shown in figure 4.8.

104

BHN 1200

Pearlite 202

Fine Pearlite 285

Bainite 401

MF

I

I 1

\ I 102

Schematic transformation diagram for a low silicon gray iron illustrates isothermal transformation at three different temperatures. Martensite formstion is by interrupted quenching.

Martensite 555 I 104

I 106

Time, Seconds

Figure 4.8 Time-Temperature-Transition Diagram for Cast Iron (from Walton)

105

Heat treatment begins with austenitizing the part by heating to above the critical temperature - the temperature at which pearlite decomposes to austenite (containing carbon in solution). The part must be held at temperature long enough to complete the decomposition of pearl ite and solution of carbon in the iron. Ferritic cast iron would require a very long furnace time to produce enough dissolved carbon because the only source of carbon is from the graphite - a more stable form than iron carbide. The austenitizing temperature ranges between 850 - 880' C (1560 - 1600' F) and is influenced by the silicon content. Increasing silicon content increases the critical temperature. The best response to hardening is obtained with cast irons containing total carbon between 2.8 and 3.2 percent and silicon between 1.3 and 2.1 percent. Quenching is done in oil. Water quenching may cause cracking in some parts. The TTT diagram indicates that a full quench in oil i s rapid enough to produce a martensitic structure. The diagram in figure 4.8 shows several heat treatment methods to achieve a variety of structures. Slow cooling keeps the process above the knee of the transformation curve and produces a pearlitic structure. If the part is quenched in a constant temperature bath and held while transformation takes place, a variety of compositions are possible depending on the holding temperature - as shown in figure 4.8. Thus a fine pearlite or a bainite can be produced with the difference in hardness as shown. This process of isothermal transformation is called austempering. The process not only is useful to control ultimate microstructure, but also allows large sections to cool slowly, preventing large temperature gradients with ensuing residual stresses or cracking. The shock of rapid cooling, then,is eliminated. If the quench is interrupted at a temperature well below the knee of the TTT curve but above the martensite transition temperature, held for a time long enough for the temperature gradients to level out and then quenched to martensite, a hardened part is produced with improved mechanical properties over one that is fully quenched. The interrupted quench is known as martempering and is often recommended for wear resistant parts.

** Quenching and tempering of gray and nodular irons can reduce corrosion resistance to dilute HCL.

106

Surface Hardening. .. Pearl itic and nodular cast iron can be surface hardened to white iron hardness levels for maximum wear resistance combined with toughness of the bulk material. Flame hardening, laser hardening and induction hardening will produce about the same results as in steel. Case hardness levels as high as 600 Vickers (55Rc) can be achieved by these means. Case depths are generally about 1.8 mm ( 70 mils). Noses on cam shafts can be hardened to over 600 Vickers with chromium contents from 0.2 to 0.7 percent. Flame hardening can produce pitting in cast iron with coarse graphite structure. Close control of the flame hardening conditions is required if pitting is to be avoided in even moderately coarse grained graphite gray irons 1631. Nitriding ... Gaseous nitriding can be used as a surface hardening method in special alloy cast irons containing aluminum and chromium. This will produce a surface hardness of 900 Vickers ( 67Rc) and also provides residual compressive stresses - further inhibiting surface fatigue type wear. Gray and nodular iron can be nitrided by cyaniding or the salt bath method. The resulting surface hardness is 790 Vickers (64Rc) and the case depth is about 10 pm (0.4 mils). Cyaniding not only improves wear resistance, reduces gall ing tendencies, improves fatigue strength but a1 so improves corrosion resistance. Hardenability . . . The same principles used in determining hardenabil ity of steels discussed in chapter 1 apply to cast irons. Alloying is used to improve the hardenability of cast irons - principally Cr, Ni and Mo are the alloying elements used.

107

WEAR PROPERTIES OF CAST IRONS

Pearlitic gray and nodular cast irons are the least expensive and easiest to use of wear resistant materials. The role o f graphite in the structure is complex but does appear to provide some solid lubrication when lubricant starvation occurs or under dry sliding conditions. The graphite phase also will inhibit tendency for adhesive contacts to grow and develop galling. Graphite will hold liquid hydrocarbon lubricants and provide a surface lubricant reservoir for dry starts or temporary loss of lubricant. Graphite can be detrimental to some rubbing applications. Under high contact stress, flake graphite can contribute the breakout of metal flakes during wear. The process is illustrated in figure 4.9. The SEM micrograph in (a) shows the flaking type of wear associated with high friction and heavy contact stress that can occur in pearlitic gray cast iron. The worn surface has been sectioned in (b) to show the subsurface process leading to the accelerated wear. As surface deformation occurs under heavy contact stress and high friction,a shallow layer smears out and forces metal to move around the graphite, opening long subsurface fissures. The fissures extend to the surface producing loose laps which break off as flakes. If this process occurs, hardening by treatment is in order. Ferrite caps ... These features develop in nodular iron as a result of the graphite nodules. High contact stress and friction will cause the ferrite phase surrounding the graphite nodules to flow over the nodules. The flowed material will not adhere to the graphite and will break off as small, work hardened "caps"[67]. The process is shown in figure 4.10. Ferrite caps can be formed during the machining process and should be removed before the part is put in service. The caps shown in figure 4.10 were in an as machined crankshaft for a gas compressor. The broken out caps embedded in the babbitt bearings, eventually accumulating sufficient concentration to cause abrasion of the crankshaft. Effect of microstructure ... Much has been written on the effect of gray iron microstructure on its wear properties. Walton summarizes information up to 1970 and the references are numerous. As has been noted the graphite phase is influential in wear resistance and both the average size and distribution can be important. Graphite size must be large enough to be effective. At the same time the number of flakes must be limited and the distribution should be even. The effect of concentration of graphite flakes on wear is shown in figure 4.11 [68]. Fine pearlite and minimum amounts of ferrite are also desirable for optimizing wear resistance.

108

a. Surface wear (SEM micro)

b. Microstructure o f wear

Figure 4.9 Effect o f flake graphite on wear of gray iron under high contact stress

Figure 4.10 Ferrite caps on ground nodular iron shaft

109

5

.12

a

a

.lo

0

- .08 v)

C

22 .06 CJ .04 .02

0

Figure 4.11 Effect of graphite flake size on wear of gray iron (from Walton)

... Cavitation is deleterious to graphitic gray cast iron. Gray irons with large graphite flakes and soft matrix are particularly susceptible. Ductile irons with martensitic matrix are the most resistant to cavitation.

C a v i t a t i o n erosion

110

Running-in of Gray Cast Iron ... Engine cylinders made o f gray cast iron will wear less provided they are run-in properly. The running-in process not only smooths the surface so that hydrodynamic lubrication of piston rings becomes more likely, but also a thin layer of iron oxide and graphite has been detected [69]. Presumably, this coating inhibits asperity welding and adhesive wear. Abrasion Resistance.. . Gray cast iron in the pearl itic condition does not perform well under severe abrasion conditions. By severe conditions is meant high stress abrasion, where the contact stresses are large enough to crush the abrasive medium and the abrasive is harder than the pearlite matrix. Heat treating the cast iron to achieve a martensitic structure improves the abrasion resistance although the flake graphite phase limits the resistance to high contact stress. Heat treated nodular iron behaves better in high stress abrasion. Figure 4.12 shows the increasing difference in abrasion resistance between the two types of cast iron as the surface stress is increased [70]. The relative abrasion resistance of cast irons is summarized in table 4.3

Table 4.3 Abrasion Resistance of Cast Irons * Material

Hardness V i ckers

Abrasion Factor**

565

0.4

Martensitic AISI 4150 Steel 750

0.6

Alloyed White Cast Iron

550

0.8

Unalloyed White Cast Iron

425

1.0

Gray Cast Iron

200

1.5

Ni -Hard (martensi t ic)

* From ASM Metals Handbook, 9th Edition [71] ** Ratio of weight loss o f sample to weight loss of AISI 1020 Steel

111

25

-+Tool

Steel

P

20

4 -Gray Cast Iron

/

- * -Nodular Iron

/

/

In

z

6

15

ai

e

m

/

a

8 3 .-% VI

E n a

/

lo

P 5

0

0

1

2

3

4

Surface Pressure, MPa

Figure 4.12 Abrasion resistance o f gray and nodular irons

5

112

HIGH ALLOY CAST IRONS

Low alloy white irons have been the cheapest abrasion resistant cast irons in use for many decades. The white irons have austenitic, pearlitic or martensitic matrixes with iron carbide inclusions. The martensitic grades exhibit the best resistance to abrasion. However, in order to achieve the martensitic structure, a rapid quench i s required and this results in a very brittle product. Therefore, either selected surface zones are made white iron by casting against "chill surfaces" or the irons are alloyed with appropriate amounts of nickel and chromium to encourage martensite production with slower cool ing rates. These are the Nihards, containing 3-5% Ni, 1-2.5% Cr & 2.5-3.5% C. The need for greater resistance to abrasive wear in the mining and ore processing industries has resulted in a series of high alloy cast irons containing 6-28% Cr, 1-4% C and other alloying constituents including Mo, Ni, Cu and Va. These alloys can be heat treated to produce ferritic, austenitic or martensitic matrix containing eutectic carbides - mostly chromium carbides. These carbides are much harder than iron carbides and harder than quartz -the most prevalent hard abrasive found in mining. The martensitic and austenitic high alloy cast irons are mostly used for abrasion resistance. The ferritic grades are used for high temperature oxidat ion resistance. The relative abrasion resistance of some high alloy cast irons are summarized in figure 4.6 [72]. Note that the austenitic white irons perform slightly better than the martensitic. The work hardening properties and tendency to transform to martensite under high shear stresses may be the explanation. Of course, high carbon content austenite is known to be very wear resistant. The low carbon variety is soft and prone to galling. Brittleness of high alloy irons are of some concern - especially in applications involving impact such as ore crushers, ball mills and pneumatic tools. Fracture toughness in these alloys can be controlled by microstructure and alloy content. Fracture toughness for high chromium-molybdenum irons as a function of carbide content is shown in figure 4.13 [73]. Note that the austenitic white irons demonstrate higher fracture toughness at lower carbide content. Thus, in view of the good abrasion resistance of austenitic high alloy irons, they might be considered in applications requiring toughness as well.

113

There are a large number o f high alloy cast irons. Each one has been developed for a given abrasion and corrosion problem. Choice o f the appropriate alloy may require the aid of an alloy producer.

0

10

20

30

Carbide Volume, %

Figure 4.13 Fracture toughness o f white irons

40

50

114

CHAPTER

5 .

CARBON

GRAPHITE

MELTING POINT..

. ...4375'C

HARDNESS RANGE.. .15 - 2 8 KNOOP DENSITY RANGE ...1.7 - 1.9 g/CC ELASTIC MODULUS ...7Et3 - 1.5Et5 MPa ELEC RESISTIVITY .....1.5 - 6.2 Mill iohm-cm

GRADES OF CARBON-GRAPHITE: Resin bonded Carbon (Phenolic filled with ground flake graphite) Hard Carbon (calcined coke t powdered graphite self bonded) High Thermal Conductivity Carbon Graphite (Graphitized carbon heated to 2760,248>'C) Metal Impregnated Carbon Graphite Resin Impregnated Carbon Graphite Oxidation Inhibited High Temperature Graphite Soft Machinable Carbon Graphite APPLICATIONS FOR CARBON GRAPHITES: Water 1 ubricated bearings Seal rings Cryogenic bearings Gas Bearings Electric brushes Aircraft brakes Pump vanes

115

INTRODUCTION Carbon g r a p h i t e i s a v e r y i n e r t s o l i d s t r u c t u r a l m a t e r i a l manufactured f r o m m i x t u r e s o f carbon and ground up n a t u r a l and manufactured g r a p h i t e . The carbon can s t a r t i n t h e f o r m o f lamp b l a c k , p e t r o l e u m coke and p i t c h coke. C a l c i n e d p i t c h coke, used i n h a r d carbon g r a p h i t e grades, i s h a r d enough t o s c r a t c h most metals. Various mixes o f carbon and g r a p h i t e , o f t e n bonded w i t h p i t c h , a r e heated and c o m p l e t e l y carbonized o r heated t o a " g r a p h i t i z i n g temperature" (above 2760'C). The c a r b o n i z i n g o f t h e b i n d e r produces a s t r u c t u r a l s o l i d . The e l a s t i c modulus can be l o w o r q u i t e h i g h depending on t h e c o n s t i t u e n t s . G r a p h i t i z i n g a l t e r s t h e s t r u c t u r e o f t h e b i n d e r and carbonaceous f i l l e r s somewhat. X-ray d i f f r a c t i o n i n d i c a t e s t h a t t h e carbon-carbon bond d i s t a n c e changes. However, t h e g r a p h i t i z a t i o n t r e a t m e n t can produce 2 t o 5 t i m e s increase i n thermal and e l e c t r i c a l c o n d u c t i v i t y . P o r o s i t y can be c o n t r o l l e d so t h a t a range o f f r o m 20% t o l e s s t h a n .01% p o r o s i t y can be o b t a i n e d i n carbon g r a p h i t e s . D e t a i l s o f t h e manufacture o f c a r b o n - g r a p h i t e can be found i n two e x c e l l e n t t e x t s b y Paxton & Shobert on manufactured carbon [74,75].

USAGE IN TRIBOLOGY Carbon g r a p h i t e i s a s e l f l u b r i c a t i n g m a t e r i a l w i t h h i g h r e s i s t a n c e t o c o r r o s i o n b y most aggressive media except s t r o n g l y o x i d i z i n g a c i d s . It w i l l r e t a i n i t s s t r e n g t h a t v e r y h i g h temperatures. The chemicals t h a t a t t a c k carbon g r a p h i t e a r e l i s t e d i n Table 5.1. The temperature l i m i t a t i o n s o f carbon g r a p h i t e a r e r e l a t e d t o o x i d a t i o n . I n an o x i d i z i n g atmosphere, carbon g r a p h i t e i s s a t i s f a c t o r y up t o about 500'C. With o x i d a t i o n i n h i b i t o r a d d i t i v e s , t h e upper l i m i t can be pushed t o 650'C. A carbon e l e c t r o d e i n a carbon a r c lamp operates i n open a i r a t about 4000'K and o x i d i z e s a t a r a t e o f about 3 inches p e r hour. The bending s t r e n g t h and t e n s i l e modulus o f carbon g r a p h i t e a c t u a l l y increases somewhat w i t h i n c r e a s i n g temperature. Carbon g r a p h i t e i s a m a t e r i a l w i t h l o w thermal expansion (about 1/4 t h a t o f s t e e l ) . Thermal c o n d u c t i v i t y o f t h e h i g h e r c o n d u c t i v i t y grades i s comparable t o aluminum. The m a t e r i a l i s v e r y d i m e n s i o n a l l y s t a b l e d u r i n g thermal e x c u r s i o n s and i s n o t s u s c e p t i b l e t o thermal shock. Carbon g r a p h i t e i s a l o w modulus m a t e r i a l - about 10 t o 15 t i m e s l o w e r t h a n s t e e l . I t i s a l s o b r i t t l e and r e q u i r e s c a r e i n h a n d l i n g i n c l u d i n g m i n i m i z a t i o n o f impact l o a d s and t e n s i l e s t r e s s s t a t e s when used i n d e s i g n o f machine elements.

116

Table 5 . 1 Chemicals corrosive to carbon graphite Bromine Chlorine, hot 1 iquid Chromic acid Chrome pl at ing solution Fluorine Hydrofluoric acid Iodine Nitric acid Sulfuric acid, over 75% concentration

The lubricating ability of carbon graphites depends on adsorbed water or organic materials. When used in vacuum or very low humidity conditions, carbon graphite will not lubricate and will tend to wear rapidly, releasing a powder wear debris which is deliquescent. Specially treated carbon graphites are available for use in vapor free environments. A special high altitude brush grade was developed during World War I 1 for use in electric motors in high flying planes. The properties of some commercial carbon graphites used in tribological applications are summarized in Tables 5.2 and 5.3 The composition of the various grades in tables 5 . 2 and 5.3 are shown in Table 5.4.

117

Table 5.2 Mechanical P r o p e r t i e s o f Carbon Graphites

MATER IAL

P-29 P - 658RC 486 G- 1 PM- 103 Pyrol y t i c Graphite P-03 P-15 P-2w P-5 P - 5Ag P-9 s-95 Conversion f a c t o r s : MPa x 0.145 = k s i

FORM

HARDNESS TENSILE YOUNGS MAXIMUM VICKERS STRENGTH MODULUS TEMP DPH MPa MPa ' c

Molded Molded

540 730

Molded Molded CVD Baked Bab Imprg Baked Baked Ag Imprg Baked

180 660 600 750 320 680 820 480 650

48.32 48.32 6.89 20.68 68.90 689.40 37.92 62.05 27.58 41.36 68.94 27.58 96.52

1.80Et04 2.4Et04 2.07Et04 1.38Et05 2.76Et04 1.24Et04 2.62Et04 6.89Et03 1.38Et04 2.62Et04 1.45Et04 1.38Et04

230 260 399 204 400 538 204 288 316 260 288 371

118

Table 5.3

Physical p r o p e r t i e s o f c a r b o n - g r a p h i t e

MATER IAL

DENSITY kg/ CU METER

Graphite C r y s t a l P-29 P - 658RC 486 G- 1 PM- 103 Pyrol y t i c Graphite P-03 P-15 P-2w P-5 P - 5Ag P-9 s-95

*

2.26Et03 2.26Et03 1.80Et03 1.83Et03 1.66Et03 1.94Et03 5.90Et03 1.94Et03 1.94Et03 2.41Et03 1.66Et03 1.66Et03 2.35Et03 1.66Et03 8.30Et03

THERMAL EXPANS CONDUCTIVITY m/m Watt/m k /'C

400 6 12 8.6 1.73 43.00 69.20 10.38 17.30 8.65 25.95 12.11 138.40

0.36E - 06 3.00E-05 1.60E-06 1.50E-06 9.18E-06 8.10E-06 9.36E-06 6.66E-06 7.74E-06 4.60E-06 9.90E-06 8.91E-06 4.60E-06 3.96E-06 3.96E-06

P a r a l l e l t o basal planes

t Normal t o basal planes

Con e r s i o n f a c t o r s : 3 kg/m x 3.613E-5 = l b / s u i n : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

Y

J/kg/K x 0.238E-4 =BTU/lb/'

F

=

gm/cc (SG)

RESIT micro ohm-cm

HEAT CAPACITY J/g K

4.00 4.00 3.8

8.5* 8.5t

2.50

0.60 4.10 1.80 4.50 2.0 3.80

119

Table 5.4 Composition of Carbon-Graphite Grades MATERIAL P-29 P-658RC 486 G-1 PM-103 Pyrolytic Graphite P-03 P-15 P-2w P-5 P- 5Ag P-9 s-95 T-0054

COMPOSITION Polyester impregnated Resin impregnated Mechanical carbon Molybdenum disulf ide, metal bonded Carbon graphite C, Babbitt Carbon graphite Carbon graphite C, Silver Carbon graphite Graphite, inorganic impreg

WEAR OF CARBON GRAPHITE

Carbon graphite wears by abrasion. Usually the surface against which it slides is harder and its asperities will tend to grind material off the carbon surface. Therefore, surface roughness of the mating part in a carbon graphite sliding system is very important to wear control. When carbon graphite is lubricating effectively, a transfer film develops on the counter surface and graphite vs graphite results. Under these conditions, wear rate is minimum and transfer continues as the transfer film wears away. The wear of the transfer film is slow. As with other tribological materials, the wear rate of carbon graphite is rapid during a short "wear-in period and then levels off to a very low linear process. The wear-in period represents the initiation and development of a transfer film. Most bearing appl ications for carbon graphite involve relatively low bearing stress. Wear does not follow a linear relationship with load. After a critical load, wear rate increases rapidly. Bearing stress should be kept below 50 psi for reasonable life. 'I

120

Mating materials are an important consideration when using carbon graphite in sliding contacts. Those carbon graphites containing hard carbons, calcined petroleum, charcoal, glassy carbon require stell ite, chromium plate, ceramics (Tungsten Carbide, Alumina) as mating surfaces. Chromium plate, ceramics, hardened steel, cast iron, and carbon graphite make good mating materials for "soft" carbon graphites containing high percentages of natural graphite and soft metal or polymer impregnants. Aluminum and bronze or brass are inferior as mating materials. Massaro [76] has demonstrated the importance of selecting the correct mating materials for seal ring applications. Using the materials shown in Table 5.5, tested seals made up of various combinations. Face type seals were run for 100 hours sealing 46'C water. The shafts turned at 1750 rpm resulting in a rubbing speed of 9.1 m/sec. The results for three seal nose contact pressures are shown in Table 5.6. Note the significant difference in carbon ring wear when reaction bonded silicon carbide was used as compared with tungsten carbide.

Table 5.5 Materials used in Massaro's face seal wear tests Carbon Material Density Hardness Material Identification gm/cc Scler C* Seal nose Grade A Hard Carbon-graphite Resin impreg.

1.83

95

Grade B Medium Hard Carbon-graphite Resin impreg.

1.80

85

Grade C Medium hard Carbon-graphite Resin impreg

1.80

84

*Schlerescope C

121

Table 5.6 Wear data

-

nose materials v s ring materials sealing warm water

Ring material Fused Alumina WC Contact press. kPa 500 770

Si Graphite* 1500

Nose Grade A Frict Coeff. Nose wear** Ring wear

.08 .051

.12 .0025

.07 .09

.0002

.0003

.oooo

Nose grade B Frict coeff. Nose wear Ring wear

.10 .134

.09 .017

.0002

.oooo

.07 .162 .0002

Nose grade C Frict coeff. Nose wear Ring wear

.ll .220 .0000

*Siliconized graphite ** Wear rate, mm/100 hr

.14

.06

.009

.oooo .oooo

.oooo

122

MECHANICAL SEALS

One of the most important uses for carbon graphite is in mechanical seals. Some typical seal designs in which carbon graphite is used are shown in Fig. 5.1. Nose pieces in face seals, rings in turbine seals, vanes in positive displacement pumps and rings for dry gas piston compressors use carbon graphite as a basic material. Carbon graphite is an ideal material for these purposes because of several properties. First of all, it is self lubricating and, therefore less apt to damage the smooth, flat surfaces required for effective sealing. Secondly, it has a low elastic modulus which helps in providing conformity to a parallel surface to minimize th seal gap over the entire contact region. Thirdly, it can be manufactured to a high degree of flatness and precision. Fourthly, it has a structure that lends itself to the generation of hydrodynamic lubricating films between para lel flat surfaces. And finally, it is inert.

'sup

1. Anti-Rotation Device 2. cup 3. Extended Length 4. Primary-Seal Carrier 5. Primary-Seal Ring, Carbon 6. %a1 Bore 7. Sealing Face 8. Sealing-Face Inner Diameter 9. Sealing-Face Outer Diameter

10. SealingFace Width 11. Seal Nose 12. Seal Outer Diameter 13. Secondary Seal (ORing) 14. Secondary -Seal Land 15. Secondary-Seal-Land Diameter 16. Washer 17. Wave Spring

a. Face Seal

1. Back Ring, Carbon 2. Coil Spring 3. Cover Ring, Carbon 4. Garter Springs 5. Primary-Seal Ring, Carbon

6. Runner 7. Seal Housing 0. Shaft 9. Spring Adapter

b. Circumferential Seal

Figure 5.1 Typical seal designs using carbon seal elements

123

When sealing gases, the self lubricating property and ability to finish to a flat very smooth surface makes carbon graphite work. The sealing elements are brought together by spring pressure or a combination of spring pressure and gas pressure. Sliding contact occurs at the sealing surface unless gas film support is designed into the system. Owing to its low modulus, a seal ring, reacting to the pressure drop across it will elastically deform. The result is a lifting of the carbon graphite surface away from its contact on the high pressure side and contact on the low pressure side. Under proper conditions, the surfaces can be separated a very small amount by a process known as leakage hydrodynamics [77]. If the seal is to be exposed to changing pressure differentials, it is best to try to select a carbon with a high elastic modulus to minimize distortion. If there are hydrocarbons present in the gas being sealed, heating o f the seal interface becomes important. At high sliding speeds, the seal interface can generate sufficient heat to cause polymerization of hydrocarbons and a buildup of a soft solid on the carbon and the mating surface. This material will cause leakage. This effect can be minimized by using a grade of carbon graphite with some mild abrasive characteristics. The carbon graphite will tend to clear the hydrocarbon from the mating surface as it deposits. Carbon graphite grades with distinct hard phase in a soft matrix will tend to wear to a surface topography that encourages hydrodynamic lubrication in a liquid environment. The microstructure shown in Fig. 2.2 consists of fine crystallites of graphite distributed in an amorphous matrix. The wear rates of the two phases are different and the graphite grains tend to project from the surface after wear in. These small plateaus can act like pad bearings because of the fluid dynamics of the liquid flowing past them. In this way two parallel flat surfaces can be supported on a very thin fluid film, thus reducing wear and friction [78]. The gap developed between the sealing surfaces is small enough so that leakage of a viscous fluid is negligible. The process i s complicated by the tendency for the carbon graphite to elastically deform under the pressure gradient across the seal. Care must be taken in design of the seal components that advantage is taken of the self generating hydrodynamic film.

124

There are a number of grades of carbon graphite used in seals. Each grade has a somewhat different microstructure. Generally they contain a graphite phase , some porosity and, possibly a resin impregnant. Figure 5.2 shows the microstructure of ATJ-S. The black areas are pores - but since they are not interconnected, the permeablility is low. The white accicular phase is graphite.

Figure 5.2

Microstructure of ATJ-S carbon graphite

SLEEVE BEARINGS

Carbon graphite is best known for its use in water lubricated bearings. Grades which are not influenced by radiation flux were used as pump bearings in the first successful nuclear reactors. Carbon is virtually immune to gamma, beta and alpha radiation. There is little effect from neutron irradiation at dosages below 10 neutrons per cc. In these applications, the bearings had to perform in reactor grade water at 145'C. Pressurized water is highly corrosive to many metal bearing alloys but does not effect carbon graphite at all. In addition, the carbon graphite provided insurance for re1 iable operation of the pumps even in case of loss of fluid. The bearings were capable of operating without lubricant for a period of time. The immunity to galling or seizure of carbon graphite provided additional impetus for their use. Precipitation hardened stainless steel was used as shafting for these bearings [79].

125

Carbon graphite expansion rate is about 1/3 of that for steel and therefore, the effect on operating clearance must be considered when sizing bearings. For water lubricated bearings, a very close clearance is required to induce film lubrication owing to the very low viscosity of the lubricant. The shaft will expand more than the bearing when heated and cause loss of clearance. Clearance can be kept constant at operating temperatures by shrink fitting the carbon graphite bearing in as steel sleeve and pressing the sleeved bearing into the steel housing. The carbon graphite will be under compression in the sleeve and as the steel housing expands during heating, the carbon graphite will press out against the confining wall and follow it as it expands. In this way, a nearly constant clearance can be maintained in the bearing. Because o f the very thin fluid film generated in water lubricated bearings, the surfaces of the bearing and shaft must be finished to a polished condition. It is especially important that the steel shaft surface be smooth since it will wear the carbon graphite when it comes into contact with it. For successful bearing operation, the shaft surface should be in the neighborhood of 5 to 10 microinches cla. Shrink fitting carbon graphite bearings in metal housings is also useful to reduce fracture tendency under impact loading and to prevent rotation of the bearing in the housing. For unlubricated bearings, wear and heat generation are two important criteria to consider when selecting materials. The use of PV criterion for estimating carbon graphite performance can be helpful. Shobert recommends the PV levels shown in Table 5.7. Table 5.7. PV Values for Carbon Graphite - Steel Combinations Lubrication

Oil Moist Air Dry air Hi Temp ( > 500C) *P= psi : V= fpm

PV*

150,000 15,000 15,000 15,000

Remarks

Adjuvants required Ox idat ion inh i b i t ion required

126

It must be remembered that the PV level is a measure of heat generation and that the friction coefficient will influence this value as much as the load and sliding velocity. In conditions where the upper limit of PV is approached, frictional heating can el iminate resin impregnated carbon graphites and materials with minimum friction (containing significant graphite) should be considered. Carbon graphite bearings can be used over a wide range of temperatures from cryogenic to 500'C - with adequate attention to either adjuvants for supplementing water vapor or protection from oxidation. As long as they are not exposed to excessive abrasive or impact conditions, performance is reliable. Since the material is inert, it can be used in special applications such as food processing equipment,corrosive fluid pumping, solvents hand1 ing, or where dimensional stability, low friction or resistance to thermal shock is required. Other unique properties include low thermal expansion, low density, high electrical and thermal conductivity. Properties for various grades of carbon graphite are listed in table 5.1. THRUST BEARINGS AND VANES

Carbon graphite performs very well as self lubricating thrust bearings and as sliding vanes in vane pumps. Under these conditions, the material operates under boundary lubrication if a process fluid is present and therefore, the self lubricating capability of the material is important to endurance and re1 iabil ity. Flanges are not recommended for sleeve bearing thrust surfaces. Instead, allowing the thrust to be carried on the end of the bearing is usually satisfactory. A small shoulder on the end of the bearing for thrust is appropriate. If larger thrust surfaces are needed, a separate thrust ring is recomended.These can be metal jacketed to reduce chances of edge chipping and cracking.

127

Vane pumps f o r b o t h gases and l i q u i d s o f t e n use carbon g r a p h i t e vanes. Common examples where t h e y a r e used a r e a i r blowers, and pumps f o r g a s o l i n e , r e f r i g e r a n t s , s o l v e n t s and w a t e r . For gases, hard a b r a s i o n r e s i s t a n t grades o f carbon g r a p h i t e a r e used f o r vane m a t e r i a l . The pump housing i s made o f a hardened s t a i n l e s s s t e e l coated w i t h hard chromium p l a t e o r f l a m e sprayed m a t e r i a l which has been p o l i s h e d t o a 4 t o 6 m i c r o i n c h f i n i s h . For l i q u i d pumps, c o n s i d e r a t i o n must be g i v e n t o t h e tendency o f t h e carbon g r a p h i t e t o s w e l l i n water o r o t h e r p o l a r l i q u i d s , The i n c r e a s e i n dimension can be as much as .05%. To p r e v e n t t h i s , t h e carbon g r a p h i t e must be f i r e d t o above 1100'C t o reduce t h e micropore s t r u c t u r e . One added advantage o f carbon g r a p h i t e f o r vane pump use i s i t s h i g h f a t i g u e s t r e n g t h i n bending. Paxton mentions one grade o f carbon g r a p h i t e having an endurance l i m i t s i m i l a r t o s t e e l . A r e d u c t i o n i n s t r e n g t h o f about 30% was found a t t h e endurance l i m i t .

C A R B O N BRUSHES Carbon and carbon g r a p h i t e m a t e r i a l s have l o n g been used as b r u s h m a t e r i a l s i n r o t a t i n g e l e c t r i c a l machinery because o f t h e i r good e l e c t r i c a l c o n d u c t i v i t y , l o w wear, l o w c o n t a c t r e s i s t a n c e and l o w f r i c t i o n . The s e l e c t i o n and d e s i g n o f carbon brushes i s a h i g h l y s p e c i a l i z e d f i e l d . A number o f t e x t s e x i s t on t h i s s u b j e c t alone [80,81,82]. There a r e a number o f grades used f o r e l e c t r i c brush a p p l i c a t i o n s . These m a t e r i a l s i n c l u d e carbon g r a p h i t e , e l e c t r o g r a p h i t e (carbon p r o d u c t heat t r e a t e d a t a temperature above 2525'C), metal impregnated g r a p h i t e and a b r a s i v e impregnated carbon g r a p h i t e f o r keeping t h e commutator c l e a n . Table 5.8 shows t y p i c a l brush a p p l i c a t i o n s and t h e carbon grades used.

128

Table 5.8 Electric brush applications for carbon.*

Appl ication

Type

Pressure Velocity Hardness 2 KPa m/sec MN/m

Wear Coeff 7 x 10

Subfract HP Metal-Graphite Motors

23

1.9

80

Fract HP Motors

Carbon Graphite

63

17.5

100

Automotive

Metal-Graphite

48

4.2

40

7.3

98

11.8

30

0.64

162

7.1

180

A1 ternators Elec-Graphite Starters

Metal -Graphite

7.2

111

134

Diesel-Elec Elec-Graphite

55

45

90

0.85

Aircraft

68

23.5

90

2.4

Elec-G

t

BaF 2

* Brushes running against copper. Moisture and oxygen are important components in a high endurance electrical brush system. When the dew point drops below -1O0C,untreated brush wear will increase from about 100 microns per hour to 10 cm/hr with a release of a large cloud of dust. High altitude, arctic climate and space environment can raise this problem. For this purpose, carbon graphite impregnated with barium fluoride, molybdenum disulfide and other additives are used. These additives do the job that water vapor does and prevent rapid dusting wear. Some gaseous environments can be destructive to carbon brush operation owing to chemical reactions. Those contaminants to be wary of when using carbon brushes include chlorine, hydrogen sulfide, HC1, sulfur dioxide and ammonia.

129

OTHER

USES

Carbon graphite has found uses in other Tribological applications. Some successful ones include meter bearings, balls seats, jet engine exhaust nozzle roller bearings, rupture disks, flow meter rotors, chemical metering pumps and self a1 igning spherical seat bearings.

130

CHAPTER

6 - C E R A M I C S AND

SPECIAL

MELTING POINTS HARDNESS TENSILE STRENGTH YOUNG'S MODULUS DENSITY THERMAL EXPANSION THERMAL CONDUCTIVITY FRACTURE TOUGHNESS, K I c

ALLOYS

1275'C - 3600'C 157 DPH - 3500 DPH 517 MPa - 2400 MPa 150 - 550 GPa 2.2Et3 - 1.7E+4 Kg/m3 2.3E-6 - 1.78E-5 m/m 1.6 - 176 W/m 0.7 - 12 MPa m4

TYPES C ERAMI CS CERMETS GLASSES REFRACTORY METALS SUPER ALLOYS HARD FACING ALLOYS

APPLICATIONS Bearings & Seals i n S l u r r y Pumps Bearing & Seals i n L i q u i d Metal Pumps Rocket Bearings & Seals C o n t r o l Rod Bearings i n Reactors Gas T u r b i n e P a r t s Low Heat Loss D i e s e l Engine C y l i n d e r L i n e r s & Valves S t e r l i n g Engine P i s t o n s Exhaust Valves i n I n t e r n a l Combust i o n Engines M i n i n g Equipment Rock D r i l l i n g B i t s H i g h Speed Tools High Speed Angular Contact B a l l Bearings

131

INTRODUCTION

The materials discussed in this chapter are used mainly in tribological applications involving high temperatures and/or highly corrosive environments. Although these materials are very wear resistant, economics usually precludes their use solely for wear reduction. There are three classes of materials to be discussed: Ceramics and Cermets Cobalt and nickel base superalloys Refractory metals (molybdenum,tungsten,zirconium,titanium) High temperature bearings, seals and gearing have seen two periods of development in the U . S . during the last 50 years. The first period was in the 50s and early 60s when 315'C (600'F) pressurized water and liquid metal heat transfer systems were being developed for nuclear reactors and development of rocket engines was under way. Considerable testing o f the wear properties of cobalt and nickel based alloys and ceramics 1 ike silicon carbide, aluminum dioxide and silicon nitride was carried out at high temperatures and in unusual environments. Much good information was generated and is still available in publications from that period [83]. The second period started in 1978 with the large scale commitment to the development of the "adiabatic" diesel engine as part of an energy conservation program [84]. This development period has extended through the decade of the eighties and been concerned mostly with the use of ceramics in internal combustion engines with very high combustion chamber temperatures. In addition to the diesel engine applications, there has been a continual striving t o use ceramic materials in high performance gas turbines and in other heat engines such as the Sterling engine. Nickel base and iron base superalloys received considerable attention during the development of nuclear power generating stations. Wear and corrosion testing was done in 316'C pressurized water, liquid sodium, liquid NaK and liquid salts. The results of those tests are available to the engineering community. Data has been extracted from these sources and included in this chapter.

132

Other uses for ceramics and special alloys include high speed cutting tools, rock drill s, coal gasification components, high temperature water pumps, valves and heat exchanger parts, pump parts for liquid slurry pipe lines, high speed rolling contact bearings and high speed tape recording systems. Ceramics are costly not only in their manufacture but also in fabricating parts from them. Quality control is difficult and very important in the consistency of results in application. Ceramic parts have to be taken from their molded or sintered shape to the final part by time consuming and expensive grinding operations. Special techniques are required to eliminate imbedded debris from grinding which can have disastrous results when it becomes loose in the operating system. Because of the above problems associated with fabrication of ceramic parts, ceramic coatings are finding greater usage as they are developed. In this way, the bulk of a part can be made of a relatively inexpensive metal alloy and the wear surface then coated with the appropriate ceramic material. The same procedure is used for special alloys, too. Ceramics have been used by man for many thousands of years. They are found in the ruins of ancient cities in the forms of pottery, bricks and decorative tiles. It has been only relatively recently that ceramics have been used in machinery. The ceramics used for wear resistance constitute a small percentage of the uses of ceramics today. The materials selected for review in this chapter represent the grades which have been appl ied to tribological problems and found to be effective. These materials are very pure compared to bricks and t i 1 es .

CERAMICS

Engineering ceramics can be divided into the following groups: Borides Carbides Nitrides Single oxides Mixed oxides Sil icides G1 asses

133

The range of properties t h a t these groups cover i s shown i n f i g u r e s 6.1 t o 6.5 (found i n t h e excellent handbook by Lynch, Ruderer and Duckworth [ 8 5 ] ) . Note t h a t t h e r e a r e some exceptions t o t h e general t r e n d s a s , f o r instance, t h e very low e l a s t i c modulus f o r boron n i t r i d e ( B N ) and t h e very high e l a s t i c modulus f o r tungsten carbide ( W C ) . The very high thermal c o n d u c t i v i t i e s of s i l i c o n carbide (SIC) and b e r y l l i a (BeO) are notable a l s o . Note, a l s o , t h a t t h e thermal conductivity o f Be0 drops more than an order of magnitude a t elevated temperature. I t can be seen t h a t t h e r e i s a l a r g e range of possible combinations of p r o p e r t i e s afforded by t h e d i f f e r e n t c l a s s e s of engineering ceramics. The mechanical and physical p r o p e r t i e s of s e l e c t e d grades o f ceramics a r e presented i n t a b l e s 6.1.

Table 6 . 1 P r o p e r t i e s o f E n g i n e e r i n g Ceramics

MATERIAL

HARDNESS TENSILE VICKERS STRENGTH OPH MPa

ALUMINA BERYLLIA BORON CARBIOE CHROMIUM CARBIDE GLASS, QUARTZ GLASS, SODA HP TIC K 1626 PSZ 1027 PSZ 2016 PSZ MS P S Z TZ3Y PSZ 2191 SiAlON SILICA SILICON CARBIDE SILICON NITRIDE SPK SN80 TITANIA TITANIUM TITANIUM T I TAN I UM TUNGSTEN ZYS2Y20A

1500 3200 2600

3000 1400

1600 1158 286 1780 853 2700 1300

Zr02 CARBIDE OIBORIOE N I T R I OE CARBIDE Zr02

900 3000 3500

262 103 172 262 110

YOUNGS MODULUS MPa

KIc MPa

mf

3.00

OENSITY

1760

3.88E+03 2 . 2 1E+03 6,64E+03 6.64Ec03 2.49E+03 2.49E+03 5.54E+03 6.09E+03

6.00 0.68 0.70

1649

345 276 172

10.00

689

9.00

1172 1020 450 103 103 524 545

6 50

io.oa 8.50 7.70 0.60 4.60 4.00 5.30 2.70

896 7.00

zoo0 1500 1400

MAX OP TEMP 'C

896 2399

12.00 10.00

THERM CON THERM EXP HEAT m/m CAPACITY

Kg/ WATT/m Cu Meter K

34.60 1.73 19.03 19 03 1.56 17.30 19.03

1371 1371

1482 1400 2399 1649 1482

5.54E+03 6,09E+03 5.81E+03 3.24~+03 3.04E+03 3.04~+03 3.04E+03 4.15E+03 4.15E+03 4,43E+03 4.43E+03 5.54E+03 3,04E+03 5.54Ec03

1.73 2.94 2.94 21.30 1E4.70 147.05 30.00

25.09 5.02 25.95 25.95 65.74 1.56

5.02

I'C

7. i a ~ - o 6 3.60E-05 2.50E-06 9.80E-06

1. O ~ E - O ~ 8.60E-06 9.50E-06 i.98~-05 i.00~-05 1. O ~ E - O ~ 1.00E-05 3.04E-06 1.30E-05 4.00E-06 2.30E-06 8.10E-06 9.4UE-06 8.00E-06 8.10E-06 8.00E-06 6.00E-06 1 .o w 0 5

JIgK

0.13 2.09 1 67 0.84 1.26 1 26

1.05 0.67

0.50 0.84 0.67 1.05 0.85 0.84 0.84 1.05 0.84

0.25

134

Figure 6.1 Theoretical Density and Melting or Decomposition Temperatures for Classes o f Ceramics

Aeltina Point or Decomposition Temperature, F

Theore tical Density. g/cm3

Material Class

P

1 BORIDES

I

I

r

n

I

-

N

-

(

N

n

0

I

--a s

CARBIDES

O”0

NITRIDES

-02

SINGLE OXIDES

0

M I X E D OXIDES

SULFIDES

SlLlClDES

I

1

METALLOID ELEM E N T S (B.Ge,Si) MISC. METALLOID

n lca N

W

NTERMETALLICS

-

135

Mater'al Class

10

0

20

30

40

50

60

70

80

BORIDES

CARBIDES

Legend NITRIDES

OC

+

70 F 20OC

IN SINGLE OXIDES

MIXED OXIDES

T - -.-

--OMg0*A1203

SULFIDES

SlLlClDES

lo6 psi

0

I0

20

30

40

50

60

70

80

YOUNG'S MODULUS

F i g u r e 6.2 Range of Young's Modulus Values f o r Various Classes o f Ceramics ( f r o m Lynch, Ruderer & Duckworth)

136

Class

110

0.2

0.4

0.6

0.8

1.0

1.2

BORIDES

1.4

1.6

Legend

CARBIDES

NITRIDES

SINGLE OXIDES

M I X E D OXIDES

SULFIDES

SlLlClDES

Percent

0

0.2

0.4

0.6

0.8

1.0

12

1.4

1.6

LINEAR THERMAL EXPANSION

Figure 6.3 Range of Linear Thermal Expansion for Various Classes of Ceramics (from Lynch, Ruderer & Lynch)

137

'm I Legend

Sic

--a7 0 F 2000F

I

16 11 I

NITRIDES

I

I

............ ............ ........... .......................

J-

-

1

1

1Btu/(hr)(ft)(F)lO

10

20

30

40

50 FIdlOO

I20

140

- J T H E R M A L CONDUCTIVITY

F i g u r e 6.4 Range o f Thermal C o n d u c t i v i t y Values f o r V a r i o u s C l a s s e s o f Ceramics ( f r o m Lynch, R u d e r e r & D u c k w o r t h )

138

Class

Microhardness, lo3 Kg/mm2

BORIDES

CARBIDES

N l T R IDES SINGLE OXIDES

M I X E D OXIDES

SULFIDES

I

SlLlClDES

M IC RO H AR DNESS F i g u r e 6.5 Range o f Microhardness Values f o r c l a s s e s o f Ceramics (from Lynch, Ruderer & Duckworth)

139

STRUCTURE AND PROPERTIES OF CERAMICS

Ceramics differ from metal alloys in that they are ionic or covalent bonded crystals while the metallic bond is by free electrons. Metals can be melted and allowed to crystalize in a poured shape as they cool. Ceramics require the bonding o f solid crystalline powders to produce bulk shapes. Ceramics are held together by relatively low melting point glassy binders or by self bonding of crystals compressed and sintered at high temperature. The bonds between atoms in covalently bonded ceramic materials are very strong and highly directional. Thus bonds are highly stable and the result is high melting points. Hafnium carbide has perhaps the highest melting point, 4150’C (7500’F). These materials also have very low Poisson’s ratios ( as low as 0.10). Silicon is a good example of a covalent ceramic. Metallic bonds are highly nondirectional and therefore dislocations can move under much lower applied stress. Ionic bonded crystals are held together by the attraction of arrays of positive and negative ions. In single crystals, the resistance to slip is relatively low. Thus, single crystal MgO and NaCl have mechanical properties similar to metals and have been used to model metals in dislocation studies. However, the slip systems in ionic materials are more limited than in metals and when ionic crystals are combined to make polycrystall ine materials, deformation will be inhibited by the inability of individual crystals to accommodate to deformation changes in neighboring crystals. Thus cracks will develop along grain boundaries. Some ceramics like alumina have complex sl ip systems because positive and negative ions are located on parallel slip planes and dislocations must extend normal to the slip plane in order to move through the crystal without changing the force fields in the lattice. This makes slip in polycrystalline alumina very difficult and hence increases the brittleness of this material. Because of the way in which ceramics are made, it is almost impossible to achieve theoretical density in a fabricated part. That is, ceramics are porous. Because of their porosity and the presence of microcracks, ceramics are brittle. Because it is difficult to control the porosity of ceramic bodies, Weibull statistics are sometimes used in reporting strength values from multiple tests. This practice is not used very much for very dense ceramics.

140

... Ceramics are generally used at high temperatures or in situations where frictional heating generates high surface temperatures. Therefore, resistance to thermal shock is essential in these applications. Rapid cooling is more detrimental than rapid heating because it puts the surface in tension. Therefore the design must minimize quenching conditions. High thermal gradients are built up in ceramics as a result of their low thermal conductivities and small existing cracks grow into fractures owing to the low fracture toughness of ceramics.

Thermal shock

It has been suggested that thermal shock resistance is a function of thermal expansion rate, elastic modulus and thermal conductivity. The relation is: TSR

- ok/Ea

Where

a is the fracture stress k is the thermal conductivity E is the elastic modulus a is the thermal expansion coefficient

The TSR values of some ceramic materials and hard metals are compared in table 6.2

141

Table 6.2 Thermal Shock Resistance of Ceramics

Material

Beryl 1 ia Tungsten Carbide PSZ MS Chromium Carbide HP Tic PSZ 2191 Boron Carbide Alumina Sil ica Titanium Carbide Silicon Carbide SiALON Silicon Nitride Tool Steel Carbon Graphite S95

Thermal Therma1 Expansion Conductivity

36 6 10 9.8 8.6 10 2.5 7.1 12.96 8

4 3.04 2.3 10

4

1.7 1.56 1.7 19 17.3 2.9 19 34.6 164.7 26 147 21.3 30 38 138.4

TSR

0.01 0.42 0.6 1.4 1.7

1.5 3 3.4 3.5 6 10.3 10.95 22 50 244

Conversion factors: x 0.176 = BTU/ft3/hr/"F

W/m/K

Note the very low thermal stress resistance value for beryllia. This material has a high elastic modulus, high thermal expansion coefficient and low fracture tensile strength. Note, also, the larger thermal stress resistance for silicon nitride as compared with silicon carbide. Silicon nitride has a larger tensile strength than silicon carbide. The high conductivity carbon graphite grade shown in the table shows an exceptionally high thermal shock resistance. The very low elastic modulus for carbon graphite is responsible for this unusual value. Even though PSZ exhibits high fracture toughness for a ceramic material, its thermal shock resistance is low owing to its poor thermal conductivity and high thermal expansion coefficient. This is a significant factor in its performance under high velocity sliding conditions along with its poor thermal mechanical stress properties, explained below. The above thermal shock relationship is only an indicator - it should not be used as a precise measure for fracture.

142

Other approaches t o e s t i m a t i n g m a t e r i a l s p r o p e n s i t y t o crack under thermal shock have been suggested, i n c l u d i n g use o f t h e G r i f f i t h c r i t e r i o n f o r b r i t t l e f r a c t u r e [86].

.

D u r i n g h i g h speed s l i d i n g on d r y surfaces, Thermal Mechanical I n s t a b i l i t y . . m a t e r i a l s can experience t r a n s i e n t h o t spots. The process r e s p o n s i b l e f o r t h i s phenomenon i s known as thermal mechanical i n s t a b i l i t y ( T M I ) . It has a l s o been c a l l e d thermal e l a s t i c i n s t a b i l i t y . Ceramics a r e s u s c e p t i b l e t o TMI because o f t h e i r poor thermal d i f f u s i v i t y p r o p e r t i e s . When TMI occurs, r e d h o t t o w h i t e h o t s t r e a k s can be seen on t h e s l i d i n g s u r f a c e exposed t o view. These s t r e a k s o f t e n t e n d t o move back and f o r t h across t h e wear p a t h i n a p e r i o d i c f a s h i o n . An example o f c l a s s i c TMI i s shown i n f i g u r e 6.6. T h i s was developed i n a b u t t o n vs d i s k c o n f i g u r a t i o n . The d i s k was r o t a t e d a t h i g h speed and t h e s t r e a k s were photographed i s s u i n g f r o m t h e t r a i l i n g edges o f t h e b u t t o n s .

F i g u r e 6.6 H i g h temperature s t r e a k s on a h i g h speed b u t t o n - d i s k ceramic m a t e r i a l t e s t e r . Streak temperatures e s t i m a t e d a t 880'C [87]

143

Thermal distortion of the sliding surfaces is responsible for the development of hot spots. The process is self energizing, in that a few asperity contacts produce localized rapidly increasing temperatures. Owing to the inability of the material to dissipate heat the asperity expands and increases in height above the surface. This point, then takes most of the contact load and the heat input increases. Temperature and expans ion increase at an exponential rate until the asperity softens or vaporizes and another asperity takes up the contact support and begins the same process. Rapidly changing high temperature locations on the surface result. A diagram o f this process i s shown in figure 6.7. For a material with poor thermal shock properties, TMI can be devastating. Hot spots jumping all over the surface will produce spalling and accelerated wear.

TMI has been developed [88]. The onset of TMI can be predicted as a critical velocity for a given material and sliding velocity. The critical velocity, Vcr can be derived from the following relationship:

A mathematical model of

Vcr

=

4kz/(pE)znr]z

(89)

Where: Vcr k P a E rl

d C Z

critical sliding velocity thermal conductivity = coefficient of friction = coefficient of thermal expansion = Young's modulus = thermal diffusivity (k/dc) = density = specific heat = width of slider = =

Critical velocity is the approximate sliding velocity above which thermal instability sets in. Note that contact load does not influence the onset of thermal instability. Load will influence the amount of frictional heat generated.

144

r T h e r r n a l stress crack

Figure 6.7 Diagram of TMI process

There is a significant difference between critical velocity values for different ceramics - depending on their thermal and mechanical properties and the coefficient of friction at the sliding interface. Some materials are compared in table 6.3.

Table 6.3 Critical Velocities for TMI for Several Ceramics (From Dufrane [ g o ] ) Material

Coefficient of Friction (estimated)

PSZ (Yittria stabilized)

.12 (Lubricated)

ATTZ

.12 (Lubricated)

Si3N4

Sic

"c r cm/sec (fpm) 8.9

(17.6)

15

(29.7)

.08

538

(1065)

.07

1100

(2178)

145

The l a r g e d i f f e r e n c e s i n c r i t i c a l v e l o c i t y f o r t h e m a t e r i a l s shown i n t a b l e 6.3 can be seen and were proven b y Dufrane i n s i m u l a t e d engine experiments [go].

Heat treatable ceramics .... T r a n s f o r m a t i o n toughened ceramics have been a r e c e n t development i n ceramic technology. By m o d i f y i n g t h e s t r u c t u r e w i t h small amounts o f a d d i t i v e s , toughening can be achieved b y t h e development o f b e n e f i c i a l r e s i d u a l s t r e s s e s . Z i r c o n i a toughened alumina (ZTA) i s a f i n e g r a i n e d alumina c o n t a i n i n g 10 t o 20 % z i r c o n i a . T h i s m o d i f i c a t i o n o f alumina produces an i n c r e a s e i n s t r e n g t h and e l a s t i c modulus p l u s a two t o t h r e e f o l d i n c r e a s e i n f r a c t u r e toughness w i t h o u t an increase i n d e n s i t y . ZTA cannot be used a t temperatures above 900'C because o f much i n c r e a s e d creep. Other ceramics have been toughened i n a l i k e manner - s i l i c o n n i t r i d e - z i r c o n i a , f o r instance. Z i r c o n i a i t s e l f can be toughened b y a d d i t i v e s and h e a t t r e a t i n g . Oxides o f calcium, magnesium and y t t r i u m have been s u c c e s s f u l l y used i n t h i s process. O r i g i n a l l y , these a d d i t i v e s were used t o s t a b i l i z e t h e c u b i c s t r u c t u r e z i r c o n i a achieves a t s i n t e r i n g temperatures. When p u r e s i n t e r e d z i r c o n i a i s c o o l e d t o room temperature, t h e c u b i c phase t r a n s f o r m s t o a m o n o c l i n i c phase w i t h an accompanying l a r g e volume change. The r e s u l t i n g t e n s i l e s t r e s s e s w i l l cause e x t e n s i v e c r a c k i n g i n t h e s o l i d body. F u l l y s t a b i l i z e d z i r c o n i a remains c u b i c b u t has a h i g h thermal expansion c o e f f i c i e n t and t o g e t h e r w i t h l o w thermal expansion r a t e , r e s u l t s i n v e r y poor thermal shock r e s i s t a n c e .

I f t h e amount o f s t a b i l i z e r i s reduced, a p a r t i a l l y s t a b i l i z e d z i r c o n i a (PSZ) i s produced. R e f e r r i n g t o t h e phase diagram f o r z i r c o n i a - m a g n e s i a shown i n f i g u r e 6.8, i t can be seen t h a t t h e a d d i t i o n o f magnesia reduces t h e temperature o f t h e c u b i c t o t e t r a g o n a l phase change. S t a r t i n g w i t h t h e 8 t o 11 % Mgo c o n t e n t used i n PSZ, one can s i n t e r a t a reasonable temperature.Since phase changes i n t h i s m a t e r i a l a r e s l u g g i s h , one ends up w i t h a c u b i c m a t e r i a l c o n t a i n i n g a c e r t a i n amount o f t e t r a g o n a l phase. T h i s m a t e r i a l has a much l o w e r thermal expansion c o e f f i c i e n t t h a n f u l l y s t a b i l i z e d z i r c o n i a because o f t h e volume expansion on c o o l i n g . The r e s u l t i n g m a t e r i a l , however, i s l o w i n s t r e n g t h . I f , however, t h e m a t e r i a l i s s i n t e r e d i n t h e c u b i c r e g i o n and aged i n t h e c u b i c t t e t r a g o n a l r e g i o n a f i n e p r e c i p i t a t e o f t e t r a g o n a l phase r e s u l t s . T h i s phase tends t o t r a n s f o r m t o m o n o c l i n i c when s t r e s s e d causing l o c a l areas t o go i n t o compression and t h u s i n h i b i t i n g crack p r o p a g a t i o n [91]. The r e s u l t i n g f r a c t u r e toughness i s almost t w i c e t h a t o f s i l i c o n c a r b i d e and y e t , t h e hardness and a b r a s i o n r e s i s t a n c e are retained.

146

Figure 6.8 Phase Diagram f o r zirconia - MgO system [ 9 2 ]

147

An a l l t e t r a g o n a l z i r c o n i a (TZP) i s a v a i l a b l e t h r o u g h t h e s i n t e r i n g o f submicron z i r c o n i a powder, t h u s suppressing t h e t r a n s i t i o n t o m o n o c l i n i c . TZP has e x c e p t i o n a l l y h i g h h o t hardness and toughness. The d i f f e r e n c e between PSZ and TZP i s shown i n t a b l e 7.4.

Table 6.4 Mechanical and P h y s i c a l P r o p e r t i e s o f M o d i f i e d Z i r c o n i a s

Density Flex Strength Hardness Youngs Mod F r a c t . Tough. Max Temp.

PSZ

Z TA

5.2 - 5.9

3.9

600

800

900

330

1300

2000

1300

1900

200

450

200

380

7-12

2 - 5

6 - 8 900

Therm Cond

1.3 - 1.5

Expans Coef

8 - 12

Conversion f a c t o r s : MPa 0.145 = k s i 3 kg/m x 3.613E-5 = l b / s u i n : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

6 -10

TZP

5.6

900

900

3.8

1300

1.7 - 3.5

26

7 - 15

8.0

8.0

4

=

Alumina

gm/cc (SG)

148

These advanced ceramics are being used increasingly in difficult wear problems. These include tool inserts, cam lobes, high temperature seals, high temperature water pump bearings etc. The PSZ materials have been considered for use in low heat loss diesel engines for cyl inder 1 iners and valve seats. Experimental work has shown, however, even though the material has good fracture toughness and strength at elevated temperature, the critical velocity for thermal mechanical instability is below the normal piston velocities expected in these advanced engines. The result is thermal shock induced spalling caused by thermal spikes [ 9 3 ] . CERAMIC TOOLS

Ceramics are finding application as tool materials for high speed finishing, high removal rates and machining difficult to machine materials. The advantage ceramics provide is hot hardness, high wear resistance and resistance to corrosion. Two classes of ceramics are generally used for metal cutting: alumina and silicon nitride. Because of brittleness, these materials are modified to provide a tool which will stand up under machining conditions. Alumina is modified with titanium carbide, zirconia and silicon carbide whiskers. One of the first modifications used was "black ceramic" or alumina with 25 to 40% Tic. This increases the hardness and the thermal conductivity of the alumina. Zirconia transformation toughened alumina (ZTA) is obtained by the addition of 10 -20% zirconia. The differences in properties of ZTA and pure alumina are shown in table 6.4. Alumina with higher zirconia content and traces of tungsten carbide are also in use for tool materials as well as silicon carbide whisker reinforced alumina [94]. Silicon nitride has the desired combination of high temperature mechanical properties, resistance to scaling and good thermal shock resistance. The thermal conductivity is almost twice that of alumina-Tic and the thermal expansion of about half. Thus, as has been pointed out earlier, thermal shock is proportional to thermal conductivity and inversely proportional to thermal expansion. Note in table 6.2 that thermal shock resistance of silicon nitride is 22 while that of alumina is 3 . 4 . Silicon nitride comes in many forms because of the different additives that are required to promote sintering. Alumina and silica are the most common additives used.

149

The SiALONs are a series of compositions combining silicon nitride and alumina. One benefit derived from the use o f alumina is its inertness. The presence of alumina inhibits cratering wear associated with chemical reaction between the tool and the workpiece. Although SiALON is not as hard as titanium carbide modified alumina, its wear resistance and hot hardness is impressive. The hot hardness of a number of tool materials is shown in the chart in figure 6.9. Note that Sialon maintains its hardness better than the other materials from 6OO0C to 1000°C. It should be pointed out that silicon nitride will react with steel at elevated temperatures so that Sialons are not usually used for cutting steels. Alumina-Tic is more appropriate. For cutting superalloys, sialons and Sic whisker reinforced alumina have been used. Temperature.

O F

750 1100 1500 1800 2200 2550

32

400

0

200 400 600 800 1000 1200 1400 Temperature. a C

Figure 6.9 Hot Hardness o f Some Ceramic Tool Materials [95]

150

CERAMIC ROLLING CONTACT BEARINGS

Silicon nitride has been evaluated extensively for use as a rolling element in ball bearings. The interest stems from requirements for advanced gas turbines for aircraft. Its advantages for that application include low mass (only 40% that of M50 tool steel) which reduces the centrifugal force loading of balls against the races at high speeds, high hot hardness, hence lower contact area and less heat generation, and better corrosion resistance. Low mass balls in angular contact bearings will not be forced up the race at high speed, thus requiring less thrust load to insure traction. Very smooth finish and close to theoretical density for the silicon nitride balls is required for effective performance. Rolling contact experiments with silicon nitride running against M-50 have demonstrated contact fatigue lives comparable to AISI 52100 and M-50 steels [96, 971. Silicon nitride rolling elements perform well in turbine bearing 1ubricants.The relative rolling contact fatigue lives of silicon nitride, M-50 tool steel and AISI 52100 bearing grade steel are shown in the stress -life plots in figure 6.10.0ther ceramics such as alumina and silicon carbide have been evaluated as possible low mass rolling elements but have not been as effective as silicon nitride.

6.5

............

\ El:

N

E

In-

6 :

In w

g

5.5

:

r

% I

5 :

.A

-.-.

3

4.5

: I

4

\..

-.

3

I

.........

."".I

--*-HS-l

I 0 SIN

- 6 N C -

I 3 2 SIN

\..

: : 'L : -.-A : . . . . . . . . . . . . ..-

-.

x...

\..

from R.J. Parker 8 E.V. Zaretsky Trans ASK (JOLT) July 1915, 350

I

Figure 6.10 Rolling Contact Fatigue Life of Hot-Pressed Silicon Nitride Balls and Steel Balls from 5 Ball Fatigue Tester .

151

C E RMETS Cermets are aggregates of small ceramic particles in a matrix of metal. The ceramic constituent provides the high hardness and resistance to wear while the metal provides the fracture toughness and tensile strength and acts as a binder for the ceramic particles. Probably tungsten carbide, cobalt bonded, is the most familiar engineering cermet. Other engineering cermets include: Tic-Ni A1 umina-Cr Tungsten Carbide Cobalt bonded tungsten carbide has long been used for machine tools and in rock drilling bits. The cermet is produced by liquid phase sintering of cobalt and WC. The milled WC particles and cobalt powder are thoroughly mixed and formed into the part by high pressure. The green part i s then heated to about 1400 'C to melt the cobalt phase.

152

I600

I I

LlOUlD

-

I too

!

I

I

I

1000

0

CO

I I

10

2 0 30 4 0 SO 60 70 80 TUNGSTEN C A R B I D E (WC), Wt%

90

1

10

wc

Figure 6.11 Tungsten carbide - Cobalt Phase Diagram [98]

153

Referring to the phase diagram in figure 6.11 and assuming a common grade of tungsten carbide with 94% WC and 6% Co by weight, as the temperature of the mixture is raised slowly, WC dissolves into the cobalt by solid state diffusion. (The solubility of WC in cobalt at 1000°C is 2%). When the temperature reaches 1400'C and is held, a liquid having the composition as shown by C will form. The liquid then wets the WC crystals and is drawn into the interstices by capillary action. By this means, a high density body is produced. Depending on the percentage of cobalt present, the cermet will either be a three dimensional network of WC with cobalt filling the voids in the network, or a matrix of cobalt with WC imbedded in it. In some cases, WC particles may be separated by a very thin film of cobalt.Since the solubility of WC in cobalt decreases as the temperature is lowered. Fine WC crystals precipitate from the cobalt matrix as the part is cooled after sintering. This precipitation process tends to strengthen the cobalt binder and make the cermet stronger. Additions of Tic and TaC to tungsten carbide cermets are used to reduce cratering of tools in machining of steel.

Tic cermets with cobalt, nickel and mixtures of Co, Ni and Cr are used for high temperature wear appl ications since they provide superior oxidation resistance, thermal shock resistance and high strength at elevated temperature as compared with WC cermets. Chromium-alumina cermets provide the extreme inertness of alumina and the increased toughness of the metal binder. These materials are extremely resistant to high temperature oxidation. Impact resistance is not as good as the WC and Tic cermets. The alumina cermets resist attack by liquid metals and have been used for bearing and seal parts in liquid metal pumping systems in liquid sodium heat transfer systems. Properties of some of the above cermets are listed in table 6.5

154

Table 6.5 Properties of Some Cermets Cermet

Hardness HV

Tensile Strength MPa

Young‘s Modulus GPa

3%CO WC 6%Co WC 1O%Co wc 25%Co WC 20%Ni Tic K162B*

1400 1300 1100 860 1300 513

4240 4233 4137 3068 524 712

0.72 0.72 0.60 0.48

LT-lB**

0.4 0.26

Poisson‘s Thermal Thermal Ratio Conduct. Expans. W/m/K OC x .24 .28 .25 .25

121 100 71 19

5.5 5.8 7.0 11.1 9.5 8.5

* 25% Ni, 5% Mo, 70% Tic **19% A1203, 2% Ti02, 59% Cr, 20% Mo GLASSES

Glasses are non crystalline sol id metal oxides. Glass is not a supercooled liquid because it has short range order. When glass is cooled from the liquid state,its viscosity increases and there is no crystal1 ization and no sudden change in density and mechanical properties as it solidifies to the glassy state. A glass transition temperature does exist, however, for the thermal expansion coefficient. That is, the expansion coefficient is much less in the glassy state as compared with the liquid. The temperature-viscosity properties for several different glasses are shown in figure 6.12.

155

I5 13

II h

"E

93;Z v

-

.-x

7 1 ;

.-5(>

5 "

2

s

3 I

200

400

600

XOO

loo0

1200

1400

1600

Temperature ( " C )

Figure 6.12 Viscosity-temperature curves for various glasses. [99]

156

The principal glass forming oxides include Si02, B203 & P 0 . Other glass 2 5 formers include oxides of arsenic and germanium, lead fluoride and selenium and sulphur. All other oxides are found as additives to these glass formers. Thus, during high temperature sliding, materials which will oxidize to the glass forming oxides can form glassy surface films. Some glasses used in engineering applications are listed in table 6.6. Table 6.6 Properties o f some gl asses Glass

Densiiy Mg/m

Silica

2.2

700

100-120

72

0.17

5.5

1.45

Soda Lime 2.5

550

80-100

69

0.21

87

1 .o

52

0.23

91

62

0.20

33

87

0.25

42

120

0.24

57

Hi Lead

Hardness Tensile Young's DPN Strength Mod, MPa GPa

4.3

290-340

Borosilic. 2 . 2

550-600

Alum.si1 ic.2.5

580-630

Pyroceram 2.6

620-640

80-100

185

Conversion factors: MPa 0.145 = ksi 3 kg/m x 3.613E-5 = lb/$uin : kg/m x 0.001 W/m/K x 0.176 = BTU/ft /hr/'F

Poisson's Ratio

4

=

gm/cc or Mg/m

Expansion Thermal Coeff. Comd. 'C x W/m/'c

3

(SG)

1.2

3.6

157

Glasses find a limited use in tribological applications owing to their brittleness. Any appl ication that involves concentrated contact can lead to local spalling and chipping. Glass has been used for thread guides in yarn spinning and fabric making machines. Glass fibers are used as reinforcement for plastic bearing materials. Glass reinforced PTFE makes a good self lubricating sleeve bearing. (See chapter 7 ) Because of the fine surface finish possible in glass, it has been used for self acting air bearings in lightly loaded fractional horsepower electric motors [loo]. Glass has been used in transparent bearings to study lubricant cavitation and EHD films [ l o l l . Note the wide range of thermal expansion properties and elastic moduli in table 6.6. Pyroceram, shown in the table is a crystallized glass. It is made by precipitating small amounts of metal crystals in the glass. Heat treating will cause glass to crystallize around the metal nuclei and produce a fine grained very hard and abrasion r sistant glass-ceramic. Pyroceram is translucent owing to its crystal ized state. It has high strength and high thermal shock resistance, making it useful in bearing and seal applications. It can be bonded to metals readi y.

REFRACTORY METALS AND ALLOYS

For very high temperature sl iding and rolling contact conditions such as reactor cool ing systems, rocket seals, reentry vehicle control surfaces, hot working machines etc, metals with structural strength and fracture toughness are available. Some of the properties required for such systems include high me1 ting temperature, high hot hardness, dimensional stability, viable oxide layers, corrosion resistance and resistance to galling and seizure. Dimensional stability involves the ability of a material to resist permanent changes in size at stresses below the elastic limit. Dimensional instability at elevated temperature is usually the result of solid state reactions such as transformation or decomposition of unstable phases and relaxation of internal stresses. For instance, in cobalt base alloys, the cobalt will change from c.p.h. structure to f.c.c above 430'C and the change in volume resulting from this phase change can distort the part. High melting point is an indicator of the maximum temperature limit at which an alloy can be used. Those materials with high melting points are usually those with high temperature strength.

158

Oxidation characteristics will strongly influence the maximum service temperature of an alloy. If the alloy forms a strong, tough oxide scale, it generally will resist galling and excessive wear. Phase changes in the alloy at elevated temperatures can change the oxidation characteristics to undesirable products. Some metal oxides change their oxidation rate drastically at a given critical temperature. For instance, molybdenum produces a tight protective oxide to 760'C. Above this temperature, so called catastrophic oxidation takes place in which the Moo3 vaporizes and no longer protects the metal surface. Alloys containing glass forming constituents such as silicon will often develop a glassy oxide when subjected to high temperature sliding. This oxide tends to be protective and reduces the friction.

Refractory Metals.. . Refractory metals are those metals having melting points at least as high as chromium (1875'C). Metals which fall into this category and have been used in tribological appl ications include chromium, molybdenum, tantalum and tungsten. For high temperature use, these metals are usually used in alloys with improved oxidation resistance. Tantalum is used as a metal matrix binder in an MoS2 base compact. Molybdenum has been used in two alloy forms, TZM and %Ti for high temperature sliding contacts where nonoxidizing conditions exist. Examples include inert gas lubricated bearings, liquid sodium or NaK bearings and high vacuum conditions. TZM alloy can be used at temperatures as high as 870'C in non oxidizing environment. Molybdenum has a high elastic modulus and low thermal expansion coefficient. Therefore it is attractive for high temperature shafting. Molybdenum shows excellent compatibility with molybdenum disulfide sol id 1 ubricant. When molybdenum is used in 1 iquid sodium, boundary lubrication conditions can be produced when small amounts of oxygen are present. The surface oxide of molybdenum will react with sodium to form sodium molybdate, a renewable solid 1 ubr icant .

159

SUPER ALLOYS

Super alloys include iron, cobalt and nickel base alloys capable of service above 650'C. Of these three, probably the iron base alloys are the least oxidation resistant and least wear resistant. However, the iron base chromiummolybdenum-nickel alloy 16-25-6in the nitrided condition will form a tight spinel type oxide scale when lubricated with a solid lubricant at elevated temperature. The Inconels and Hastelloys have excellent oxidation resistance at temperatures up to 870'C. However, they require lubrication to prevent adhesion and galling. Inconel X has performed well with a calcium fluoride base lubricant over the temperature range 100' - 815'C [102]. Super alloys have been evaluated in dry sliding at 870'C and their relative wear damage observed. The results are 1 isted in table 6.7, below.

Table 6.7 Wear of Superalloys Sliding Against Inconel X at 815'C (1600'F) (From Amateau & Glaeser [lo21 Load 1.9 kg; Velocity 0.02 m/s Material

Renee 41 Hastelloy C M-252 Incoloy 901 J 1570 Inconel 700 Inconel X A-286*

Friction Coeff. Initial Final

0.47 0.34 0.31 0.44 0.42 0.66 0.85 0.88

0.25 0.29 0.27 0.28 0.30 0.35 0.35 0.32

Order of Increasing Damage 1

2 3 4

5 6 7 8

* Age hardenable iron base superalloy used in gas turbines

160

Nickel Base Alloys ... High nickel content alloys (40 to 50 % Ni) provide resistance to oxidat ion at elevated temperatures and good high temperature strength. Since nickel alone is relatively soft and adhesive, nickel alloys containing chromium, aluminum and titanium for hardening are used for wear resistance. Lubrication, however, is almost a must in most applications. Oxidation of nickel alloys aids in the reduction of adhesion during sliding contact. Generally, a significant reduction in friction is seen at temperatures above 540'C. This is often a reversible process - as the temperature drops, the friction rises after reaching a critical temperature. Cobalt Base Alloys ... Many hard facing alloys are cobalt base materials. The Stellites, Colmonoys, etc. are the more familiar cobalt alloys. These alloys have excellent high temperature oxidation resistance. Their high temperature strength is not as good as nickel base alloys. Cobalt reacts differently to high temperature than nickel. A plot of friction versus temperature is shown in figure 6.13. The plot shows that cobalt exhibits its lowest friction level at room temperature. As the temperature increases, friction gradually rises and in the range between 480'and 59O0C,the friction rises rapidly to a high level. In this range, surface damage increases also. A decrease in friction is seen at higher temperatures. These effects are attributed to complex oxidation processes influenced by temperature 1 eve1 . Cobalt also transforms from c.p.h. to f.c.c at about 430'C change has been associated with frictional changes.

and this phase

Tribaloys ... A relatively recent development in high temperature alloys for wear resistance has been the Tribaloy series. These alloys are either cobalt base or nickel base and contain molybdenum , chromium and silicon. The alloy combination results in a hard intermetallic phase, known as Laves phase. This produces a hardened superalloy with high hardness and wear resistance at elevated temperature. These materials have excellent abrasion resistance. The abrasion resistance of Tribaloys is compared with other cobalt base materials in figure 6.14. Properties and composition o f super alloys are listed in tables 6.8 - 6.10

161

TEMPERATURE O F

F i g u r e 6.13 E f f e c t o f Temperature on C o b a l t F r i c t i o n [lo31

162

Table 6.8 Mechanical P r o p e r t i e s o f High Temperature A l l o y s

MATER IAL

FORM WORKED CAST

TEMPER HEAT TREAT

HARDNESS VICKERS DPH

TENSILE YOUNGS MAX OP STRENGTH MODULUS TFMP MPa MPa C ~

PLATED Heat 400C Electroless N i PLATED As Depos. Electroless N i HASTELLOY C CAST ANNEALED INCONEL 718 CAST Aged MOLYBDENUM, 0.5% T i ARC CAST MOLYBDENUM TZM ARC CAST MONEL K-500 CAST Rene 41 CAST STELLITE 1 CAST STELLITE 3 C h i l l Cast STELLITE 6 CAST STELLITE F CAST STELLITE STAR J CAST STOODY 6 CAST T i t a n i u m 6A1-4V Boronized HT T i t a n i u m 6A1-4V CAST Annealed TRIBALOY T-400 TRIBALOY T-700 TRIBALOY T-800 TUNGSTEN PM SINTER DRAWN VANASIL 77 CAST Age Hdnd WASPALOY Annealed WASPALOY HT, PH WAUKESHA WAUK. 54 ZIRCALLOY 2 CAST ANNEALED

689 55 1 1379 689 790 689 1419

2.00Et05 2.00Et05 1.97Et05 2.00Et05 3.24Et05 3.17Et05 1.79E+05 2.13Et05 2.41Et05

393

834

2.10Et05

675 526 1700 301 655 485 739 257 140 157 373 165 220 185

517 689

2.55Et05 2.07Et05 1.10Et05 1.10Et05 2.69Et05 2.14Et05 2.41Et05 2.76Et05 9.30Et04 2.11Et05 2.11Et05 1.52Et05 1.59Et05 9.66Et04

1050 500 216 410 264 325 320 562

896 689 689 276 827 275 517 586 517

320 320 649 538 649 538

649 400 400 704 649 704 260 87 1 87 1 649 400

163

Tab1e 6.9 P h y s i c a l P r o p e r t i e s o f H i g h Temperature A1 1oys

MATERIAL

DENSITY THERM CON KG/cu m WATT/m K 4.18 135.70 13.84 131.50 128.02 13.84 20.20

Electroless N i 8.03Et03 HASTELLOY C 8.86Et03 INCONEL 718 8.03Et03 MOLYBDENUM TZM 1.02Et04 MOLYBDENUM, 0.5% T i 1.02Et04 MONEL K-500 8.30Et03 Ren6 41 8.30Et03 STELLITE 1 9.13Et03 STELLITE 3 STELLITE 6 8.30Et03 STELLITE F STELLITE STAR J 8.86Et03 STOODY 6 8.30Et03 T i t a n i u m 6A1-4V 4.43Et03 T i t a n i u m 6A1-4V 4.43Et03 TRIBALOY T-400 9.13Et03 TRIBALOY T-700 8.86Et03 TRIBALOY T-800 8.66Et03 TUNGSTEN 1.70Et04 VANASIL 77 2.77Et03 WAS PALOY 8.14Et03 WASPALOY 8.14Et03 WAUKESHA 8.86Et03 WAUK. 54 8.86Et03 ZIRCALLOY 2 6.70Et03

THERM EXP

m/v

/ c

1.20E-05 1.25E-05 1.31E-05 5.40E-06 5.40E -06 1.40E-05 1.57-05

1304 1370 2610 2610 1316

1275

176.46

7.27 7.27 176.46 136.67 171.27 167.00 126.29 12.11 12.11 27.68 25.95 14.50

MELT PQINT C

1.44E-05 1.44E-05 9.44E-06 9.44E-06 1.78E-05 1.85E-05 7.00E-06 1.62E-05 1.39E-05 1.39E-05 3.24E-06 4.50E-06 6.50E-06

3

x 3.613E-5

W/m/K x 0.578 J/kg/K

=

=

3

x 0.001

BTU/sqft/ft/hr/*F

x 2.388E-4

m/m/"C x 0.55

l b / c u i n : kg/m

=

=

BTU/lb/'F

in/in/'F

o r cal/g/*C

=

90.00 140.00 13.00 50.00

1288 1243 1288 3410 538

gm/cc (SG)

HEAT CAPACITY J/kg/'K

3.85Et02 4.60Et02 2.51Et02 2.93Et02 2.93E+02 4.50Et02

104.00 104.00 91.00

171.00

Conversion f a c t o r s : kg/m

RESIS micro ohm-cm

5.86Et02 5.86Et02

5.65 8.37Et02 5.23Et02 74.00

164

Table 6.10 Chemical Composition of High Temperature A l l o y s MATER IAL

Electroless N i HASTELLOY C INCONEL 718 MOLYBDENUM, 0.5% T i MOLYBDENUM TZM MONEL K-500 Rene 41 STELLITE 1 STELLITE 3 STELLITE 6 STELLITE F STELLITE STAR J STOODY 6 T i t a n i u m 6A1-4V TRIBALOY T-400 TRIBALOY T-700 TRIBALOY T-800 TUNGSTEN VANASIL 77 WASPALOY WAUKESHA WAUK. 54 ZIRCALLOY 2

COMPOSITION

P C C C

1-12,Ni 88-99 .15,W 4,Fe 6,Cr 16,Mo 17,Va .3,Mn 1, S i 1, N i 54.6 . 1 , S i .75,Mn .5,Cu .75,Ni 50,Cr 18,Co 5,Mo3,A1.8,Til,Fe .02,Ti .5,Mo 99.4 T i 0.5,Zr 0.1,C 0.02,Mo 99.25 N i 65,Al 3,Fe 2,Mn 1.5,C .25,Si 1.0,Ti .5,Cu 27 C r 19,Co 11,Mo 10,Fe 5,C 0.09,Ti 3,Al 1.5,Ni 50 C r 30,C 2.5,Si 1,Mo 1,Fe 3,Ni 3,W 12,Co 47.5 N i 3,Si l,Mn l , C r 30,W 12,C 2.5,Co 50 C 1.1,Cr 28,W 4,Co 67 C 2, S i 1, Cr 26, N i 23, W 12, Fe 1, Co 35 C 2.5, Mn 1, S I 1, Fe 3,Ni 2.5,Cr 32,W 17,Co 41 C 1.2,Si 1.2,Fe l , N i l , C r 30,W 5,CO 60.5 C . l , A l 6,V 4, T i 90 Co 62,Mo 28,Cr 8,Si 2,C .08 N i 50,Mo 32,Cr 15,Si 3,C .08 Co 52,Mo 28,Cr 17,Si 3,C .08 N i 7,Cu 3,W 90 S i 22,Zn .l,Cu 1.5,Fe .75,Ti .15,Mn .1,Ni 2.2,Mgl,Va.l,Al C .1,Mn .5,Si .75,Cr 20,Ni 57,Mo 4,Co 13,Ti 3,Al l , Z r .1 C .05,Si .3,Mn 8,Sn 4,Bi 4,Mo 3,Cr 12,Ni 68 N i 80,Pb 4,Sn 8,Zn 7,Mn 1 Sn 1.5,Fe . l , C r .1,Ni .05,Zr 98.4

165

I

I

/ ASTM Rubber Wheel Abrasion Test / / 2000 revolutions, 13.6 kg load I

; dry sand

I

I

II

Figure 6.14 Abrasive wear properties o f cobalt base alloys

166

MATERIALS FOR NUCLEAR REACTORS

Since the early 1950s there has been considerable work done in the U . S . , Great Britain and France on the study of corrosion and wear of high temperature materials in liquid sodium and high temperature water.The applications include coolant pump bearings, control rod guides, fuel rod separators and heat exchange tube and sheet assemblies. Liquid sodium is used as a heat exchange fluid operating at temperatures up to 659'C. It is used because it remains liquid over a large temperature range at relatively low pressure. Reactor grade water is used at temperatures to 343'C and is pressurized to high pressure to keep it in the liquid phase. Boiling water cooled reactors use water at lower temperatures and much lower pressure. Both of these heat exchange media are extremely corrosive and the variety of materials which are resistant to them and not susceptible to radiation damage is limited. Liquid Sodium: Material combinations from tests made in 659'C high purity sodium at the Liquid Metal Engineering Center [lo51 which showed promise in slow speed sl iding contact include: Udimet 630 LT-2 Haynes 273 Stell ite 1 K-1626 K- 1628

vs vs vs vs vs vs

K-95 LT-2 Stel 1 ite 1016 K-95 K-95 Hastelloy C

Some material combinations which proved to be unsatisfactory included: M-1 Tool Steel vs TZM vs Molybdenum vs

M-1 Tool Steel Hastelloy C Inconel 718

167

Compositions of the above listed materials are included in the table below:

Mater i a1 Udimet 630 Hastelloy C Haynes 273 Stel 1 ite 1016 Stell ite 1 K- 1628 K-95 LT-2

Composition

Ni 53, Cr 17, Fe 18, Mo 3, Nb 6, A1 .5, C .04, Ti 1 54, Co 2.5, Cr 15, Mo 16, W 4, Fe 6, C .08, V .35 57, Cr 16, Mo 17, W 4.5, Fe 5.3, C 0.1 48.5, Cr 32, W 17, C 2.5 55.5, Cr 30, W 12, C 2.5 Tic 63, Ni 25, Mo 5, Nb 5.3 wc 91, co 9 W 60, Cr 25, Alumina 15 Ni Ni Co Co

Sodium-metal -oxide 1 ubrication As has been pointed out, it is possible to achieve boundary lubrication conditions in liquid sodium with certain alloys. Early work at Battelle [lo51 revealed that the friction coefficients of alloys containing molybdenum, tungsten or chromium are lower in sodium containing minute amounts of oxygen than in inert gaseous environment at the same temperature. Roberts [lo61 then showed that molybdenum, tungsten and chromium oxides are stable below 427'C and hence alloys containing them should form complex double metal oxides which behave like solid lubricants. (Sodium molybdate was known to be a solid lubricant). Further, there was a combined effect o f temperature and oxygen level in the sodium which encouraged formation of boundary films. The Liquid Metal Engineering Center developed a sodium boundary lubrication map from plots of free energy curves [107]. This map i s shown in figure 6.15.

168

HIGH TEMPERATURE PROPERTIES OF SUPER ALLOYS

High temperature materials are generally used in bearings, seals and gears because they have hot hardness and resist rapid oxidation. Elevated temperature changes other room temperature physical and mechanical properties which may be ignored during the design process. For instance, thermal shock resistance factor, discussed on page 5, is a function o f thermal conductivity, elastic modulus and thermal expansion coefficient. This factor will differ depending on the temperature the component is required to operate at. The thermal expansion coefficient generally increases by an order of magnitude with an increase in temperature of 100O'C. Some materials, like graphite, for instance, show relatively small increases in expansion coefficient with large temperature increases.Therma1 conductivity increases with temperature for some materials and decreases with temperature with others. These effects must be considered when trying to predict performance involving multi-factor relationships at elevated temperatures. Charts showing the change in properties of a number of high temperature materials with increasing temperature are shown in figures 6.16 - 6.23. These charts were produced by Sibley, Mace, Greiser and Allen in 1960 [88].

169

5 PPm

1 Oxygen, P P ~ Figure 6.15 Temperature-oxygen map for boundary lubrication o f austenitic stainless steel in liquid sodium.

170

Temprroture. F

0 .L

3.0

0 .f 2.0 0.4

I .o

02

0.8 0.6

0.s 0.

0.4

0.08

-

0.3

g

0.2

E

-

z "

a 0.w -Y

-

-E a04

-E

0

-P

\ Y)

\

O.'

aoe

9

0.02

am 0.05 0.04

0.03

0.01 0.008

0.006 0.02 0.004

0.01

0.002

Figure 6.16 Effect of temperature on the thermal conductivity of high temperature materials.

171

30

Temperature, C

Figure 6.17 Effect of Temperature on Average Short Time Tensile Strength

172

Temperature, F 5 00

I

\I

I

or AI,03 - C r ( 7 0 : 3 0 )

~ A I , O , porcelain Y

-- - - -- -\-

Sic-Si, N

,

A

m--I

I

I

I

I

I

I

2 00

400

600

S ~ O , gloss

Graphite

,

aoo

loo0

1200

Temperature, C

Figure 6.18 Elastic Modulus as a Function o f Temperature

1400

173

2.:

2.c

steel

/

iC-Ni-Mo

I .I

01 C

k a C

-

1 .:

.-0

porcelain

e 0

a x

W

b

.-c

-I

-

0

0.f

$

7Grophite

O.d

.-

I

Si 0, gloss

C

200

400

600

000

1000

1200

1400

Temperature, C

Figure 6.19 Thermal Expansion o f High Temperature M a t e r i a l s

1600

174

I .a

ae 0.6

0.4

0.3

0.2

0) 0

2

0.1

0

0.08

E

0.02

0.01

0.008

0.004 0

1

1

200

400

1

1

600

800

1000

I 200

140

Temperature, C

Figure 6.20 Thermal Diffusivity as a Function o f Temperature

175

Temperature, F I001

I

T Srophite

/ 10

I

f

I 200

400

600

800

loo0

I200

Temperature, C

Figure 6.21 Transient Thermal Stress Resistance Factor as a Function o f Temperature

1400

176

Temperoture,

F

5 00 0.7

0.6

0.5

-L ! Y

F

-

LL

c

n

04c

\

a

1

0.4 L

uu0

--. 0

\

0

0.3

0

I

.-U c .U

a v)

0.2

//.-l----

alloy