Machinability of Powder Metallurgy Steels

MACHINABILITY OF POWDER METALLURGY STEELS

MACHINABILITY OF POWDER METALLURGY STEELS

A. Šalak, M. Selecká and H. Danninger

CAMBRIDGE INTERNATIONAL SCIENCE PUBLISHING

Published by Cambridge International Science Publishing 7 Meadow Walk, Great Abington, Cambridge CB1 6AZ, UK http://www.cisp-publishing.com

First published 2005

© A. Šalak, M. Selecká and H. Danninger

Conditions of sale All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library

ISBN 1-898326-82-7 Printed in the UK at the University Press, Cambridge

Acknowledgements The authors wish to express their gratitude to all persons and institutions that have contributed to this book in some or other way. Our first thanks go to our families for their patience and understanding; there were many evenings and weekends we were not available to them. Secondly, we want to thank the staff members of our respective institutes, i.e. the Institute of Materials Research, Slovak Academy of Sciences, Košice, Slovakia, and the Institute of Chemical Technologies and Analytics, Vienna University of Technology, Vienna, Austria. From both institutes came numerous contributions to this book such as micrographs, photographs, chemical analyses, data, process description and sample preparation. We thank also the colleagues from other universities and institutes who have helped us through information and advice; their names are listed in the references. The authors are particularly grateful to the companies that have either directly contributed or granted permission to publish illustrations and data. Höganäs AB, Höganäs, Sweden; MIBA Sinter Austria GmbH, Vorchdorf, Austria; Chemetall GmbH, Arnoldstein, Austria; Boehlerit GmbH, Kapfenberg, Austria; Mashad Powder Metallurgy Co., Mashad, Iran, and Metalsint a.s., Dolný Kubín, Slovakia must be specially mentioned. Finally, we want to thank our publisher, Cambridge International Science Publishing, for the excellent cooperation and the high technical quality of the book.

Andrej Šalak, Marcela Selecká Košice, Slovakia, December 2004 Herbert Danninger Vienna, Austria, December 2004

v

vi

Contents 1.

Introduction................................................................ 1

2.

Powder Metallurgy Processes and Materials ....... 5

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5

METAL POWDER PRODUCTION ................................................................ 5 Reduction of iron oxides .................................................................................. 6 Water atomisation of metal powders ............................................................... 9 Water atomisation of prealloyed powders ..................................................... 12 Oil atomisation .............................................................................................. 14 Inert gas atomisation .................................................................................... 14

2.2

CHEMICAL, PHYSICAL AND TECHNOLOGICAL CHARACTERISTICS ... OF METAL POWDERS ................................................................................ 15 2.2.1 Basic characteristics of some iron and prealloyed powders ........................ 15 2.2.2 Impurities ...................................................................................................... 17 2.3 2.3.1 2.3.2 2.3.3

MIXING AND COMPACTION ..................................................................... 20 Cold die compaction ...................................................................................... 21 Warm compaction ......................................................................................... 23 New and developing compaction processes ................................................... 24

2.3.3.1 Cold die pressing with cross holes ............................................................................. 24 2.3.3.2. Warm flow compaction .............................................................................................. 24 2.3.3.3 High energy compaction ............................................................................................ 26

2.3.4. Powder forging and cold forging .................................................................. 27 2.3.4.1 Powder forging ........................................................................................................... 28 2.3.4.2 Cold forging ............................................................................................................... 29

2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5

SINTERING ................................................................................................... 30 Sintering temperature and time ................................................................... 32 Sintering atmosphere ................................................................................... 33 Carbon ........................................................................................................... 42 Sintering under getter .................................................................................. 44 Vacuum sintering ......................................................................................... 44

2.5 ALLOYING METHODS AND ALLOYING ELEMENTS .............................. 45 2.5.1 Alloying methods .......................................................................................... 45 2.5.1.1. Mixed alloy steels ....................................................................................................... 46 2.5.1.2 Diffusion alloyed steels .............................................................................................. 48 2.5.1.3 Prealloying ................................................................................................................. 50 2.5.1.4 Hybrid systems ............................................................................................................. 51 2.5.1.5 Coating and infiltration ............................................................................................ 52

2.5.2 Alloying elements ......................................................................................... 53 2.5.2.1 Alloying elements with low oxygen affinity ............................................................. 53

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2.5.2.2 Alloying elements with high oxygen affinity ........................................................... 58

2.6 SECONDARY OPERATIONS ...................................................................... 63 2.6.1 Mechanical .................................................................................................... 63 2.6.2 Sintered gears from manganese steel without surface densification for high dynamic loading ............................................................................................ 67 2.6.3 Heat treatment and surface hardening ......................................................... 68 2.6.3.1 Through hardening ................................................................................................... 70 2.6.3.2 Sinter hardening ........................................................................................................ 71 2.6.3.3 Case (surface) hardening ........................................................................................... 73

2.6.4 Hardfacing ..................................................................................................... 73 2.6.5 Steam treatment and coating ........................................................................ 73 2.7

POROSITY AND MECHANICAL PROPERTIES OF SINTERED IRON AND STEEL ........................................................................................................... 74 2.7.1 Sintered iron ................................................................................................. 75 2.7.2 Iron–carbon, iron–copper steel .................................................................... 77

3.

Principles of Machining of Steel ........................... 80

3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.1.7 3.1.8

MACHINING PROCESSES .......................................................................... 81 Drilling ......................................................................................................... 83 Turning ......................................................................................................... 83 Milling ........................................................................................................... 84 Tapping .......................................................................................................... 84 Grinding ........................................................................................................ 85 Honing ........................................................................................................... 86 Lapping .......................................................................................................... 87 Other processes ............................................................................................ 88

3.2 CHARACTERISATION OF MACHINING PROCESSES ............................ 89 3.2.1 Motions in cutting shaping of metals ........................................................... 89 3.2.2 Cutting tool characteristics .......................................................................... 90 3.2.2.1 Cutting edge nomenclature and base characteristics ................................................ 92

3.2.3 Cutting tool characteristics and nomenclature ............................................ 94 3.2.2.1 Drilling tools .............................................................................................................. 94 3.2.3.2 Tapping ....................................................................................................................... 97 3.2.2.3 Turning ...................................................................................................................... 97

3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.3.6 3.3.7

ANALYSIS OF THE MACHINING PROCESS ........................................... 100 Cutting process ........................................................................................... 101 Cutting forces ............................................................................................. 102 Formation of a chip ...................................................................................... 103 Plasticity during turning ............................................................................ 105 Heat factors and cutting process ................................................................ 107 Built-up edge ............................................................................................... 111 Tool wear and tool life ................................................................................. 112

viii

3.3.7.1 Tool wear and speed and feed rates ........................................................................... 118 3.3.7.2 Surface finish and surface integrity ........................................................................ 120

3.4 MACHINABILITY TESTING ...................................................................... 122 3.4.1 Measurement of machinability ................................................................... 123 3.4.2 Machinability testing .................................................................................. 126 3.4.2.1 Drilling test .............................................................................................................. 127 3.4.2.2 Turning test .............................................................................................................. 131 3.4.2.3 Face milling test ....................................................................................................... 133

4.

Cutting Tools .......................................................... 135

4.1 CUTTING TOOL MATERIALS .................................................................. 137 4.1.1 High speed steels ........................................................................................ 139 4.1.1.1 Ingot metallurgy high speed steels ........................................................................... 140 4.1.1.2 Powder metallurgy high speed steels ........................................................................ 142

4.1.2 4.1.3 4.1.4 4.1.5 4.1.6 4.1.7

Hardmetals .................................................................................................. 149 Stellite ......................................................................................................... 153 Cermets ....................................................................................................... 153 Ceramics ..................................................................................................... 155 Cubic boron nitride ..................................................................................... 156 Polycrystalline diamond (PCD) .................................................................. 159

4.2 4.2.1 4.2.2 4.2.3

TOOL COATINGS AND HARDENING PROCESSES .............................. 159 Tool coatings ............................................................................................... 159 Heat and surface hardening treatment processes ...................................... 164 Residual stresses ........................................................................................ 165

4.3 CUTTING TOOL WEAR AND TOOL LIFE ............................................... 166 4.3.1 Effect of cutting tool type in PM turning .................................................... 167 4.3.2.1 Effect of cutting edge ............................................................................................... 170 4.3.2.2 Effect of drill point angle ........................................................................................ 171

4.4

CUTTING TOOLS, WORKPIECE MATERIAL AND SURFACE INTEGRITY ................................................................................................. 171 4.4.1 Effect of drill geometry on surface integrity .............................................. 172 4.4.2 Effect of metallurgy factors on surface finish ............................................ 173

5. Factors Influencing the Machinability of PM Steels ....................................................................... 175 5.1

EFFECT OF PROCESSING CHARACTERISTICS ON MACHINABILITY .... ..................................................................................................................... 180 5.1.1 Compaction .................................................................................................. 180 5.1.2 Sintering ..................................................................................................... 180 5.1.3 Sintering atmosphere ................................................................................. 181 5.2 EFFECT OF MATERIALS CHARACTERISTICS ON MACHINABILITY 186

ix

5.2.1 Effect of porosity on machinability .............................................................. 186 5.2.1.1 Interrupted cutting theory ........................................................................................ 188 5.2.1.2 Deformation cutting theory ...................................................................................... 190

5.2.2 5.2.3 5.2.4 5.2.5 5.2.6

Effect of porosity on thermal conductivity ................................................... 198 Effect of porosity on tool wear and cutting force ......................................... 200 Effect of porosity on surface finish ............................................................. 202 Effect of porosity on chip formation ............................................................ 203 Effect of composition on machinability ....................................................... 204

5.2.2.1 Effect of carbon on machinability .......................................................................... 207 5.2.6.2 Effect of copper on machinability ........................................................................... 212 5.2.6.3 Effect of manganese ................................................................................................. 213

5.2.3 Effect of microstructure on machinability .................................................. 214

6.

Measures to Improve the Machinability of PM Steels ............................................................... 219

6.1 MACHINING AIDS IN POWDER METALLURGY .................................... 221 6.1.1 Role of machining aids in machining process ............................................ 223 6.1.1.1 Effect of machining aids on compressibility ........................................................... 228 6.1.1.2 Machining aids – base characteristics ..................................................................... 228

6.1.2 Basic metal sulphide characteristics and effects ....................................... 230 6.1.2.1 Effect of sulphur ...................................................................................................... 230 6.1.2.2 Thermodynamic stability of sulphides in ferrous matrices ..................................... 234

6.1.3 Manganese sulphide characteristics .......................................................... 236 6.1.3.1 Low purity manganese sulphide .............................................................................. 239 6.1.3.2 High purity manganese sulphide ............................................................................ 240 6.1.3.3 Surface coated manganese sulphide ........................................................................ 241

6.1.4 Sulphides of other elements ........................................................................ 242 6.1.4.1 Molybdenum disulphide .......................................................................................... 242 6.1.4.2 Other sulphides ........................................................................................................ 245

6.1.5 Characteristics of bismuth, selenium, tellurium and lead as machining aids .............................................................................................................. 248 6.1.5.1 6.1.5.2 6.1.5.3 6.1.5.4

Effect of bismuth ...................................................................................................... Effect of tellurium .................................................................................................... Effect of selenium .................................................................................................... Effect of lead ............................................................................................................

248 249 251 251

6.1.6 Boron nitride, MnX and resin impregnation as machining enhancers ..... 253 6.1.6.1 Boron nitride ............................................................................................................ 253 6.1.6.2 MnX .......................................................................................................................... 254 6.1.6.3 Resin impregnation and infiltration ....................................................................... 254

6.2 PRODUCTION PROCESSES ..................................................................... 257 6.2.1 Microstructure modification ...................................................................... 257 6.2.1.1 Controlled cooling ................................................................................................... 260

6.2.2 Green machining ........................................................................................ 261 6.3

EFFECT OF MACHINING OPERATIONS ................................................. 262

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6.3.1 Tool coating ................................................................................................. 262

7.

Machining of Sintered Steels – State of the Art 263

7.1 PLAIN IRON AND IRON–CARBON STEELS .......................................... 264 7.1.1 Effect of manganese and carbon .................................................................. 265 7.1.2 Effect of S, MnS and MoS2 .......................................................................... 267 7.1.2.1 Effect of S, MnS and MoS2 on mechanical properties .......................................... 267 7.1.2.2 Effect of S, MnS, and MoS2 on machinability ...................................................... 269

7.1.3 Effect of sulphur prealloying ...................................................................... 274 7.1.4 Effect of drilling conditions for steels prepared from different iron powder ... grades .......................................................................................................... 276 7.1.5 Effect of non-sulphide machining aids ........................................................ 277 7.1.5.1 Effect of lead, bismuth and boron nitride ............................................................... 277 7.1.5.2 Effect of resin and oil impregnation ....................................................................... 278

7.1.6 Effect of machining aids on surface finish and tool wear ........................... 281 7.1.6.1 Effect of machining aids on surface finish ............................................................ 281 7.1.6.2 Effect of carbon, machining aids, and machining conditions on tool wear ......... 285

7.1.7 Effect of machining aids on chip formation ................................................ 286 7.1.8 Summary ..................................................................................................... 287 7.2 IRON–COPPER–CARBON STEEL ........................................................... 290 7.2.1 Effect of copper, manganese, and carbon ..................................................... 290 7.2.2 Effect of S, MnS and MoS2 .......................................................................... 293 7.2.2.1 Effect of S, MnS and MoS2 on mechanical properties of Fe–Cu–C steels ........... 293 7.2.2.2 Effect of MnS and MoS2 on machinability of Fe–Cu–C steels ............................ 295 7.2.2.3 Effect of MnS and MnX .......................................................................................... 297

7.2.3 Effect of sulphur prealloying ...................................................................... 298 7.2.4 Effect of MnS and cutting conditions .......................................................... 299 7.2.4.1 Effect of drill speed .................................................................................................. 299 7.2.4.2 Effect of feed rate ...................................................................................................... 300

7.2.5 Effect of non-sulphide machining aids ........................................................ 301 7.2.5.1 7.2.5.2 7.2.5.3 7.2.5.4 7.2.5.5

Effect of boron nitride .............................................................................................. 301 Effect of retained graphite ....................................................................................... 304 Effect of enstatite ...................................................................................................... 307 Effect of resin impregnation .................................................................................... 307 Effect of calcium sulphide ....................................................................................... 309

7.2.6 Summary ..................................................................................................... 309 7.3 NICKEL ALLOYED STEELS ...................................................................... 311 7.3.1 Effect of machining aids .............................................................................. 312 7.3.1.1 Effect of MnS on mechanical properties ................................................................. 312 7.3.1.2 Effect of MnS on machinability ............................................................................. 314 7.3.1.3 Effect of resin impregnation .................................................................................... 315

7.3.2 Dual-phase nickel steel .............................................................................. 316 7.3.3 Summary ..................................................................................................... 317 7.4

DIFFUSION ALLOYED STEELS ................................................................ 318

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7.4.1 Effect of machining aids on properties and machinability of Distaloy SA and AB steels .............................................................................................. 320 7.4.1.1 Effect of MnS ........................................................................................................... 320 7.4.1.2 Effect of various machining aids on mechanical properties and machinability .. 320 7.4.1.3 Effect of copper and MnS addition on machinability of Distaloy SA and AB steels .......................................................................................................................... 322

7.4.2 Effect of machining aids on properties and machinability of Distaloy SE and Distaloy AE steels ................................................................................. 326 7.4.2.1 Effect of MnS ........................................................................................................... 326 7.4.2.2 Effect of ‘new’ machining aid ................................................................................ 328 7.4.2.4 Effect of MnS and MnX on properties and machinability of hybrid steels based on prealloyed Fe–0.85Mo powder ........................................................................... 330

7.4.3 Summary ..................................................................................................... 331 7.5

CHROMIUM, MANGANESE, CHROMIUM–MANGANESE, AND SILICON . ALLOYED STEELS .................................................................................... 334 7.5.1 Chromium steels ......................................................................................... 336 7.5.1.1 Effect of sulphur on machinability of Cr prealloyed sintered steel ........................ 336

7.5.2 7.5.3 7.5.4 7.5.5

Manganese steel .......................................................................................... 338 Chromium-manganese steel ....................................................................... 340 Iron-silicon steel ......................................................................................... 341 Summary ..................................................................................................... 341

7.6 7.6.1 7.6.2 7.7.1

IRON–PHOSPHORUS STEEL ................................................................... 342 Effect of copper and nickel .......................................................................... 343 Effect of manganese sulphide ...................................................................... 346 Austenitic stainless steel ........................................................................... 352

7.7.1.1 7.7.1.2 7.7.1.3 7.7.1.4 7.7.1.5

Effect of density and sintering temperature and atmosphere .................................. Effect of MnS and sintering atmosphere ................................................................. Effect of MnS, BN and sintering atmosphere ......................................................... Effect of sulphurisation and resin impregnation ................................................... Effect of waterglass, boron nitride and talc .............................................................

352 354 355 357 358

7.7.2 Ferritic stainless steel ................................................................................ 360 7.7.2.1 Effect of MnS and sintering atmosphere ................................................................. 361

7.7.3 7.7.4 steels 7.7.5

Effect of carbon and nitrogen ...................................................................... 362 Effect of drill material and geometry on machining of wrought stainless 363 Summary ..................................................................................................... 364

7.8

COMPARING THE MACHINABILITY OF VARIOUS STEELS UNDER ......... DIFFERENT CUTTING CONDITIONS ...................................................... 365 7.8.1 Drilling test with constant thrust force ...................................................... 365 7.8.1.1 Basic properties and machinability ......................................................................... 366 7.8.1.2 Microstructure and fracture ..................................................................................... 368 7.8.1.3 Effect of drilling speed at constant thrust force on machinability of Distaloy SA steel ..................................................................................................................... 371

7.8.2 Effect of some machining aids on machinability of various alloys ............. 374 7.8.3 Machinability vs. mechanical and other characteristics of PM steels ....... 376

xii

7.8.3.1 Machinability vs. hardness ...................................................................................... 376 7.8.3.2 Machinability vs. wear ............................................................................................. 382

7.8.4 High machinability rating PM steels and conditions ................................. 385 7.8.5 Summary ..................................................................................................... 386 7.9

STANDARDIZING THE MACHINABILITY OF PM STEELS ................... 387

7.10

SPECIAL PROCESSING AND MACHINING ROUTES FOR HIGH ............... STRENGTH - HARDNESS PM STEELS .................................................... 391 7.10.1 Green machining ........................................................................................ 393 7.10.1.1 Green machining of warm compacted materials ................................................... 395 7.10.1.2 Green machining of parts with improved bonding strength ................................. 397

7.10.2 Heat treatment processes and machinability .............................................. 399 7.10.2.1 Soft annealing and presintering ............................................................................ 399 7.10.2.2 Hard turning of sinter hardenable steels ................................................................ 403 7.10.2.3 Machinability of hybrid warm compacted steels based on prealloyed Fe–1.5Mo powder ........................................................................................................................... 407 7.10.2.4 Hard turning of through hardened and tempered steels ........................................ 408

7.10.3 Summary ..................................................................................................... 412 7.11

MACHINING OF POWDER FORGED STEELS ........................................ 415

7.11.1 Mechanical properties of powder forged Cr-prealloyed steel .................................. 418 7.11.2 Machining of powder forged steel in general ......................................................... 421 7.11.2.1 Effect of porosity and inclusions ............................................................................ 424 7.11.2.2 Influence of heat treatment ..................................................................................... 428 7.11.2.3 Effect of machining aids ........................................................................................ 430 7.11.3 Machining of Ni- and Cr-alloyed powder forged parts .......................................... 432 7.11.4 Some recommendations for machining of other powder forged steels .................... 434 7.11.5 Summary ................................................................................................................... 436

8.

Recommendations for Machining of PM Steels 438

8.1 RECOMMENDATIONS FOR DRILLING ................................................... 440 8.1.1 Selection of drill type ................................................................................. 440 8.1.1.1 Drill types ................................................................................................................. 441 8.1.1.2 Drill dimensions and geometry ................................................................................ 442

8.1.2 Drill recommendations and cutting data for structural steels .................. 444 8.1.3 Recommendation for drilling stainless steel ............................................. 450 8.1.4 Burrs ........................................................................................................... 451 8.2 8.2.1 8.2.2 8.2.3 8.2.4

RECOMMENDATIONS FOR TURNING .................................................... 452 Cutting tools and general cutting conditions ............................................ 452 Specific recommendations and cutting data .............................................. 454 Surface finish ............................................................................................. 459 Recommendation for turning with liquid nitrogen cooled cutting tool ...... 459

8.3 TAPPING AND THREADING ..................................................................... 461 8.3.1 Base characteristics for tapping ................................................................ 461 8.3.1.1 Selection and characterisation of the taps ............................................................... 463

xiii

8.3.2 Threading .................................................................................................... 466 8.4 8.4.1 8.4.2 8.4.3 8.4.4

RECOMMENDATIONS AND CUTTING DATA FOR MILLING, REAMING, . BROACHING AND GREEN MACHINING ................................................. 467 Milling ......................................................................................................... 467 Reaming ...................................................................................................... 468 Broaching ................................................................................................... 468 Green machining ........................................................................................ 469

8.5

RECOMMENDATIONS FOR MACHINING WITH GEOMETRICALLY NOT DEFINEDCUTTINGEDGE ......................................................................... 470 8.5.1 Grinding ...................................................................................................... 470 8.5.2 Honing and lapping ..................................................................................... 472 8.5.3 Burnishing .................................................................................................. 473 8.6 COOL-LUBRICATION IN STEEL MACHINING ...................................... 475 8.6.1 Cool-lubrication methods and matters ........................................................ 475 8.6.1.1 Liquid cool-lubricants ............................................................................................. 476 8.6.1.2 Gaseous cooling matter ............................................................................................ 478 8.6.1.3 Air cooling ............................................................................................................... 478

8.6.2 Cool-lubrication in PM machining ............................................................. 479 8.7 8.7.1 8.7.2 8.7.3 8.7.5 8.7.6 8.7.6

PARAMETERS FOR OPTIMISING THE MACHINING OF PM PARTS ... 482 Material characteristics ............................................................................. 484 Machining aids ............................................................................................ 486 Microstructure and heat treatment ............................................................ 488 Testing ......................................................................................................... 489 Tools ............................................................................................................ 490 Best results ................................................................................................. 491

9.

APPENDIX .............................................................................492

9.1

RELATIONSHIP BETWEEN HARDNESS VALUES DETERMINED BY ......... VICKERS AND ROCKWELL METHODS ................................................. 492 CHEMICAL COMPOSITION AND DESIGNATION OF POWDER METALLURGY STEELS ............................................................................. 492 TRADE DESIGNATION AND BASE CHARACTERISTICS OF PLAIN IRON AND ALLOY POWDER PRODUCTS (FOR PM PARTS PRODUCTION) .... SUPPLIED BY HÖGANÄS AB, HOEGANAES CORP. AND QMP ........... 497 HÖGANÄS AB [11] .................................................................................... 497 HOEGANAES Corp. [357,358] ................................................................... 498 QMP (ATOMET Powder Metallurgy Products) [359] ................................ 500 Wear kinds of inserts in turning ............................................................... 501 Wear kinds of inserts in milling ................................................................ 504

9.2 9.3

9.3.1 9.3.2 9.3.3 9.4.1. 9.4.2.

REFERENCES ................................................................................509 INDEX .............................................................................................533

xiv

Introduction

1 Introduction Ferrous and nonferrous structural precision parts comprise about 80% in tonnage of powder metallurgy mass products. Of these parts, roughly 75% are used for transportation, primarily in the automotive industry, in which case particularly high requirements towards mechanical and functional properties, shape precision, and surface finish have to be met. In this area, powder metallurgy mass production is highly competitive to conventional metalworking techniques. Here however it should be kept in mind that the production of components from wrought steels, cast iron, and various nonferrous alloys is done mostly by well established machining techniques with geometrically defined, in part also undefined, cutting edges. These machining processes have been revolutionized by the introduction of hardmetals, a special PM product, and of related, still harder cutting materials. This group of materials, for which PM is the only feasible manufacturing route, in production value even exceeds the PM precision parts, underlining the technical and economical importance of powder metallurgy products in general. The competitiveness of PM precision components is enhanced by secondary operations which improve mechanical, functional, and geometrical properties. Among those operations, machining seems to be the most complicated and least understood and at the present time is not sufficiently managed. Machining of PM parts, employing different cutting methods, extends the range of shapes and thus also the range of applications without lowering the standard for static and especially dynamic mechanical properties, shape, geometrical precision, and surface finish. This latter precondition is an essential point when studying PM parts machining. As mentioned above, the success of machining conventional materials was in part achieved by the use of new cutting tool 1

Machinability of PM steels

materials supplied by powder metallurgy. The group of PM tool materials can be expected to boost also machining of PM precision parts, both with regard to increased productivity (e.g. through higher cutting speeds) and decrease of overhead cost. Generally, the degree of machining and the amount of material this removed are drastically lower for PM parts compared to those from wrought steels which results in minimum solid and fluid waste generated and thus gives PM a clear advantage from the environmental viewpoint. The reason for machining operations performed on PM precisions parts is that the standard PM shaping process by uniaxial die compaction, although very economical, cannot deliver all geometrical features; e.g. threads, cross-holes, undercuts, etc. cannot be pressed but have to be machined. Furthermore, it may be more economical or technically easier to introduce some geometrical features into sintered parts through machining rather than pressing even if the latter would be technically possible. Machining is increasingly also done to restore or improve the geometrical precision of components such as heat treated gears, powder forged heavy duty parts, and many others. This latter role of machining can be expected to become more and more important with increasingly higher requirements regarding both geometrical tolerances and mechanical properties (which frequently means heat treatment). It is estimated that at least for the European market about 40–50% of all PM ferrous parts undergo some machining process from these reasons [Beiss, P.: personal communication, 2004]. It is well known that machining of sintered steels is a difficult job, although the reasons for the problems encountered are not quite clear, and there are different explanations given. Many basic rules from machining of wrought steels can be transferred to machining of sintered steels; there are however peculiarities that are in part attributed to the porosity, in part to the specific microstructure of sintered steels, although the understanding of the various effects is still rather missing. It can be stated generally that the problems in machining of PM steel parts can be attributed to their microstructure when pores are regarded as microstructural constituents of zero Young’s modulus; also in PM machining the result of a cutting process is determined by the interaction between the workpiece materials and the cutting tool under the given conditions. Unfortunately the literature about machining of sintered steels is very scattered and incomplete, and due to the large variety of materials investigated and machining processes and conditions 2

Introduction

selected it is extremely difficult to identify general outlines and give general rules. Furthermore, the experience that has been collected by the PM parts manufacturers is commonly kept confidential, due to competition reasons. Therefore, quite a large proportion of the published literature and data about machinability comes from powder manufacturers rather than from those companies who actually do the machining, and there are hardly any data from the cutting tool manufacturers, quite in contrast to the extensive information given for conventional fully dense materials in the catalogues. In this book it has been attempted to collect and structure as much as possible the information available from the viewpoint of the metallurgist. It was intended to describe all factors and parameters that are relevant for the formation of a solid PM material with defined physical and mechanical properties and with a characteristic microstructure since at the end all this affects the machinability. Thus the very important relationships between the ‘history’ of a PM component, which is affected by significantly more parameters than in case of wrought steel parts, and its machinability are described by giving an overview of powder metallurgy processes and materials, the principles of steel machining, cutting tools available, the various factors that influence – and the ways to improve - the machinability of PM steels, measures to improve machining of selected materials groups, and finally recommendations for machining sintered steel components. Recommendations for the machining shop are already available, even as standards; in this book, it has been tried to link the machinability, in particular the optimum machining parameters – usually obtained empirically – , to the material, its properties, and its microstructure and thus ultimately to its manufacturing history. Due to the very scattered literature, the reader will find quite contradicting statements and also sizeable gaps in the knowledge base. In the following, the findings from the literature are given as published, with only some comments, in order to stimulate the reader to draw her/his own conclusions. It is hoped that this book, by showing the deficiencies in current knowledge and practices, will also motivate the PM community to put more effort on machinability – at best in cooperation with cutting tool producers –, in particular on testing routines and standardization, in order to obtain a more reliable knowledge base about this frequently neglected but essential part of the PM production process. Machining should be regarded not as a mere ‘secondary’ operation but as an effective tool for enhancing the 3

Machinability of PM steels

precision and extending the shape complexicity range of PM components. The aim of this book is to contribute to recognition of this potential.

4

Powder Metallurgy Processes and Materials

2 Powder Metallurgy Processes and Materials The powder metallurgy processes are more competitive than other fabrication methods for numerous materials and finished parts, in part due to their high flexibility regarding the manufacturing processes involved. PM offers chances for manufacturing of materials than cannot be obtained by classical metallurgical techniques, such as cemented carbides or very high alloy tool steels with an isotropic microstructure. In production of structural parts, PM offers higher precision, eliminating most of the machining operations required for castings and wrought semi-products. It is thus more cost effective and environmentally friendly, resulting in high material utilization. Figure 2.1 shows the basic steps of the powder metallugy process as used for manufacturing of structural parts and semi-finished products which may be subjected to supplementary machining. Each of these main steps contains some particularities and introduces specific physical, chemical and technological factors that affect the processing and final properties of a sintered material and by this also of each structural component. 2.1 METAL POWDER PRODUCTION At present, there are two main methods for production of iron and steel powders. Together, the production of sponge iron powders by reduction of iron oxides and by water atomisation of iron and of low alloyed steel powders accounts for more than 90% of iron and steel powders produced around the world. (The production of powders by gas atomisation is limited to high alloy and some special powders). The main physical, chemical and technological properties of these two groups of iron-based powders are extensively documented by manufacturers and in the literature. 5

Machinability of Powder Metallurgy Steels Metal Powder Production Reduction

Water Atomisation

Lubricant

Blending

Additives

Compaction Warm Compaction

Cold Die Compaction

High Velocity Compaction

Sintering Secondary Operations

Powder Forging

Sizing / Coining

Annealing

Heat Treatment

Machining

Machining

Heat Treatment

Others

Grinding

Finished Product

Fig.2.1 Basic steps of the powder metallurgy process.

Most of the chemical, physical, and technological properties of iron powder grades resulting from raw stock, manufacturing, and preparation methods have a pronounced impact on the green and final properties of sintered parts. Their effects are not fully suppressed by the following processing steps to the final sintered material. Therefore, the selection of the starting powder(s) is a critical step in PM parts production. 2.1.1 Reduction of iron oxides The production of sponge iron powders by reduction of iron oxides is basically a chemical process. By using different iron oxide grades and processing parameters, it is possible to produce sponge iron powders with different physical and technological properties. A typical example of this production method is the Höganäs process, which has been the first mass production route for plain iron powder 6

Powder Metallurgy Processes and Materials

grades. The process is based on the use of quite pure magnetite (Fe 3 O 4 ) ores. The milled and magnetically separated iron ore powder is reduced with a carbonaceous material in a tunnel kiln furnace (typically indirect reduction with coke with addition of limestone to bind any sulphur). After crushing and milling the powder cake, the resulting powder is carefully magnetically separated. The characteristic feature of the reduced iron powder is a spongy morphology with a higher specific surface area compared to smooth surface and full dense powder particles obtained by atomisation (see below). Mill scale is another iron oxide suitable for production of sponge iron powder. ‘Pyron iron powder’ is produced from mill scale by reduction with hydrogen at moderate temperatures. The porosity of these iron powder particles is finer than that of powders produced from iron ore. The reduction of low-carbon steel mill scale following the Höganäs process produced a sponge iron powder with an apparent density ~2.1 g/cm 3 and a larger specific area. By additional milling it was possible to increase the apparent density of this powder to ~4.0 g/cm 3 for use in production of welding electrodes [1]. In another process, the very fine mill scale was reduced by combined reducing agents. Soot was added to the milled scale, and the admixture was compacted to briquettes, which were then reduced in converted natural gas [2]. The required low carbon and oxygen content of the reduced iron powders is obtained by subsequent annealing steps in a hydrogencontaining atmosphere. The final particle shape, particle size distribution, apparent and tap density and flowability are adjusted by separate preparation steps to attain the optimum specific properties of powders for the given application. The microstructure of plain sponge iron powder particles is ferritic with different grain sizes, depending on the production method and final annealing temperature. In some cases, the reduced iron particles are very coarse-grained, it means one particle = one metallographic grain. The characteristic microstructure of sponge iron powder particles is shown in Figs.2.2 and 2.3. The spongy character of the reduced powder is more pronounced in the iron powder reduced from mill scale as shown in Fig.2.4. The internal pores are more evident in iron powder NC100.24 compared to SC100.26 iron powder. The microhardness of particles is one of the characteristic properties of the base powders; for these iron powder grades, it was in the range of 110–123 HV 0.02. Figure 2.5 shows the morphology of a sponge iron powder 7

Machinability of Powder Metallurgy Steels

Fig.2.2 Microstructure of a green sample from sponge iron powder NC100.24 (Höganäs) compacted at 400 MPa. Optical micrograph. Nital etched. Fig.2.3 (right) Microstructure of a green sample from sponge iron powder SC100.26 (Höganäs) compacted at 400 MPa. Optical micrograph. Nital etched.

Fig.2.4 Cross-section of loose iron powder particles reduced from mill scale. Optical micrograph. Unetched. Fig.2.5 Morphology of one particle of sponge iron powder (NC100.24, Höganäs). SEM.

particle in the as-received condition, e.g. after the standard milling, annealing, and screening operations. Comparison of Figs.2.5 and 2.6 shows large differences between the sponge iron powder grades which can affect the properties of intered iron. The morphology of the iron powder particles, characterised by the specific surface area, is a physical property which clearly demonstrates the difference in the surface character caused by the starting raw material type and by the production method used. Compressibility, green strength, and interparticle contact formation 8

Powder Metallurgy Processes and Materials

Fig.2.6 Morphology of a sponge iron powder particle (SC100.26, Höganäs), SEM.

during the initial sintering stages are affected by the particle morphology. Another characteristic of the reduced iron powders is the absence of segregation effects, i.e. nonreducible (slag) impurities remain in the powder (unless they can be removed by magnetic separation) and, therefore, also in the sintered product. 2.1.2 Water atomisation of metal powders Atomisation of molten metal, low in Al and Si, by high-pressure water jets has become one of the main methods of producing iron and some low alloyed (prealloyed) powders. The main advantage of water atomisation is the possibility of producing chemically much cleaner metal powders. The water used for atomisation should be soft, without nitrates. Water atomisation is the standard production route for plain iron powder, some low alloyed prealloy powders and some special powders, such as stainless steel or tool steel powders. The starting material used for iron powder production by atomisation is carefuly selected low carbon steel scrap, as best deep drawing steel sheet scrap or a pure and stable ilmenite ore body which afford the necessary high chemical cleanliness. Crushed reduced sponge iron billets and oversize iron particles are also suitable ‘scrap’. The solidified droplets are superficially oxidised and can be hard with the non-equlibrium structure of the particles due to high cooling rate when the carbon for deoxidation was added or when the scrap with a higher carbon content was used. The cooling rate increases with the reduction of the droplet size, with the corresponding effect on the microstructure especially of alloyed materials. This effect is very sensitive in water atomised HSS powders. 9

Machinability of Powder Metallurgy Steels

The base particle size depends on the water pressure and, therefore, on the water jet velocity. The bulk in the iron powder production is targeted towards an average particle size near 150 µm. After drying, the powder is magnetically separated from slag particles, screened to various particle size fractions and homogenized, i.e. from the fractions a ‘synthetic’ powder with a given particle size distribution is mixed in order to ensure the necessary consistency of the powder properties. After drying, screening and mixing, the final chemical and processing properties of water atomised iron powders are also established by low-temperature reducing annealing usually done at temperatures