Astm mnl 46 2007

Metallographic/Materialographic Preparation Part I is a description of sectioning, mounting, grinding, polishing, and etching ofspecimens for examination in reflected light, enabling the

Metallographic and Materialographic Specimen Preparation, Light Microscopy, Image Analysis and Hardness Testing

Printed in U.S.A

ASTM Stock No MNL46

Library of Congress Cataloging-in-Publication Data

Metallographic and materialographic specimen preparation, light microscopy,image analysis and hardness testing

Kay Geels; in collaboration with Daniel B Fowler, Wolf-Ulrich Kopp, and MichaelRückert

p cm.—共Manual; 46兲ASTM stock number: MNL 46

Includes bibliographical references

ISBN 978-0-8031-4265-7E-book ISBN 978-0-8031-5691-3

1 Metallography 2 Metallographic specimens I Title

TN690.G3785 2006

669⬘.95028—dc22 2006103391Copyright © 2007 ASTM International, West Conshohocken, PA All rights reserved.This material may not be reproduced or copied, in whole or in part, in any printed,mechanical, electronic, film, or other distribution and storage media, without thewritten consent of the publisher

Photocopy Rights Authorization to photocopy item for internal, personal, or educational classroom use, or the internal, personal, or educational classroom use of specific clients, is granted by ASTM International „ASTM… provided that the appropriate fee is paid to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923; Tel: 978-750-8400; online: http://

www.copyright.com/.

The Society is not responsible, as a body, for the statements and opinions expressed

in this publication

ASTM International does not endorse any products represented in this publication

Printed in City, StateMonth Year

This book is written both for the experienced and unexperienced metallographerterialographer兲 who wants specific advice and information It is also for persons seek-ing a broader knowledge of metallographic/materialographic specimen preparationand the examination methods, light microscopy, image analysis, and hardness testing.Special emphasis has been made on relations between ASTM standards andmetallography/materialography

共ma-The book will be useful for students in courses devoted to practical metallographyand materialography

The scope of the book is to give relevant information, in an efficient and clear way,covering the daily work in a metallographic/materialographic laboratory

Metallographic/Materialographic Preparation

Part I is a description of sectioning, mounting, grinding, polishing, and etching ofspecimens for examination in reflected light, enabling the reader to understand themechanisms of the entire preparation process This is combined with practical advice

on specimen preparation and an introduction to existing equipment and consumables.Part II is a “Hands-on” Manual guiding the metallographer/materialographer tothe correct preparation method, based on the material to be prepared and the purpose

of examination More than 150 methods are indicated covering practically all types ofmaterials, describing the preparation process from sectioning to etching This partalso includes a section on Trouble Shooting, covering all stages in the preparation pro-cess and artifacts developed during the preparation

Light Microscopy

Wolf-Ulrich Kopp

Part III is a description of the optical reflected-light microscope with photomicroscopygiving the reader both an introduction to the subject and a manual for the daily work.Also, a short introduction to electron microscopy and scanning probe microscopy can

be found in this part of the book

Quantitative Metallography/Materialography—Automatic Image Analysis

Daniel B Fowler

Part IV gives an introduction to quantitative microstructural analysis and automaticimage analysis, both theoretically and practically, with emphasis on the examinationsbased on ASTM standards and other types of commonly used analyses

iii

Hardness Testing

Wolf-Ulrich Kopp

Part V gives a description of the hardness testing methods, Brinell, Vickers, Rockwell,microhardness and instrumented共nano兲 indentation testing based on ASTM stan-dards, both theoretically and as a practical guide

The Metallographic/Materialographic Laboratory

Kay Geels

Part VI gives directions on how to establish and maintain a modern metallographic/materialographic laboratory The important rules and regulations covering occupa-tional safety are described and commented on

The authors of this book, representing more than 100 years’ experience with tical metallography and materialography, have tried to make this book a practical tooland helpful source of information to all who are involved in the noble art/science ofmetallography/materialography—Kay Geels

prac-Acknowledgments

The authors wish to acknowledge the four reviewers, who brought forward valuableinsight for improvement Special thanks to R C Nester, for his advice and suggestions

on extension and shortening of the chapters Thanks to G Petzow, F Mücklich and L

E Samuels for permission to use a number of illustrations, and to B Ottesen and W.Taylor for reading the manuscript and giving good advice A special acknowledgementgoes to fellow-metallographers/materialographers for support and advice through theyears and directly connected to the book The list includes U Täffner, S Glancy, E.Weidmann, A Z Jensen and A Guesnier A special thanks to L Bjerregaard for her veryimportant advice on many of the preparation methods, and to H Hellestad for her in-valuable support in making the illustrations Also, thanks go to W Taylor and StruersGmbH for providing important micrographs The authors acknowledge the followingcompanies for supply of information and illustrations, Buehler Ltd., Carl Zeiss AG,DoAll Company, Emco-Test GmbH, Leica Microsystems AG, Olympus Optical Co Inc.,and Struers A/S Particular thanks to G E Totten and K Dernoga at ASTM Interna-tional for establishing and maintaining the project of making this book Last but notleast, thanks to B Freiberg and J Hestehave for support and encouragement duringthe years of making the book

Abbreviations

AFM Atomic Force Microscope

BF Bright FieldCBN Cubic Boron Nitride

DF Dark FieldDIC Differential Interference Contrast

iv

EBSD Electron Backscatter DiffractionEDS Energy Dispersive SpectroscopyEPMA Electron Probe MicroanalyzerFIB Focused Ion Beam

MFM Magnetic Force MicroscopePCB Printed Circuit BoardPOL Polarized LightSEM Scanning Electron MicroscopeSPM Scanning Probe MicroscopeSTM Scanning Tunnel MicroscopeSTEM Scanning Transmission Electron MicroscopeTEM Transmission Electron Microscope

v

Part I: The Metallographic/Materialographic Preparation Process

1 Introduction

1.1 Metallographic/Materialographic Preparation—The True

Microstucture 5

1.1.1 Henry Clifton Sorby共1826–1908兲 5

1.2 The True Microstructure 6

1.3 Selection of Preparation Method 6

1.3.1 Artifacts 7

1.3.2 Preparation Methods 7

1.4 The Metallographic/Materialographic Specimen 7

1.4.1 “Specimen” or “Sample” 8

1.5 The Preparation Process 9

1.5.1 Sectioning 10

1.5.2 Mounting 11

1.5.3 Preparation of the Surface 11

1.5.4 Etching 13

2 Sectioning 2.1 Selection 14

2.1.1 General Studies or Routine Work 14

2.1.2 Study of Failures 14

2.1.3 Research Studies 14

2.1.4 Type of Section 14

2.1.5 Reporting of Locations 15

2.2 Sectioning 15

2.3 Wet Abrasive Cutting 15

2.3.1 The Cut-off Grinding Process 15

2.3.2 The Cut-off Wheel—Abrasives and Bond Materials 16

2.3.3 Grinding Mechanics 21

2.3.4 Mechanical Damage 22

2.3.5 Thermal Damage 23

2.3.6 Cut-off Wheel Wear 25

2.3.7 Cutting Fluids 26

2.3.8 The Metallographic/Materialographic Cutting Operation 29

2.4 Abrasive Cut-Off Wheels 32

2.4.1 Consumable Wheels 32

2.4.2 Slow Consumable Wheels 34

2.5 Abrasive Cut-off Machines 36

2.5.1 Design Principles of Wheel—Work Piece Contact 36

2.5.2 Machine Designs 39

2.6 Advice and Hints on Wet Abrasive Cutting 43

2.6.1 Cut-off Wheel Selection 44

2.7 Other Sectioning Methods 45

2.7.1 Fracturing 45

2.7.2 Sectioning by Melting 46

2.7.3 Shearing 46

2.7.4 Sawing—Table 2.1 47

2.7.5 Wire Cutting 52

3 Mounting 3.1 Purpose and Criteria 54

3.1.1 Purpose 54

3.1.2 Criteria for a Good Mount 54

vii

3.1.3 Surface Flatness—Edge Retention 54

3.2 Mounting Methods 57

3.2.1 Clamping 57

3.2.2 Hot Compression Mounting 58

3.2.3 Cold共Castable兲 Mounting 58

3.3 Hot Compression Mounting 58

3.3.1 Advantages of Hot Compression Mounting 59

3.3.2 Disadvantages of Hot Compression Mounting 59

3.3.3 MSDS共Material Safety Data Sheets兲 59

3.4 Hot Mounting Resins 60

3.4.1 Thermoplastic Resins 60

3.4.2 Thermosetting Resins 61

3.5 Mounting Presses 62

3.5.1 The Heating/Cooling Unit 62

3.5.2 The Hydraulic Press 63

3.5.3 The Air-operated Press 65

3.6 Advice and Hints on Hot Compression Mounting 65

3.6.1 Selection of Resins for Hot Compression Mounting 66

3.7 Cold共Castable兲 Mounting 67

3.7.1 Advantages of Cold共Castable兲 Mounting 68

3.7.2 Disadvantages of Cold共Castable兲 Mounting 68

3.7.3 MSDS共Material Safety Data Sheets兲 68

3.8 Cold Mounting Resins 68

3.8.1 Acrylics 68

3.8.2 Polyesters 69

3.8.3 Epoxies 69

3.9 Accessories for Cold共Castable兲 Mounting 70

3.9.1 Mounting Molds 70

3.9.2 Clips 71

3.10 Vacuum Impregnation 71

3.10.1 Dyes 72

3.11 Special Mounting Techniques 72

3.11.1 Taper Sectioning 73

3.11.2 Edge Protection 74

3.11.3 Mounting of Very Small Parts, Foils, and Wires 75

3.11.4 Mounting of Powders 76

3.11.5 Mounting of PCB Coupons 76

3.11.6 Conductive Mounts 77

3.12 Recovery of Mounted Specimen 77

3.13 Advice and Hints on Cold Mounting 78

3.13.1 Selection of Cold Mounting Materials 79

4 Marking—Storage—Preservation 4.1 Marking 80

4.1.1 Marking with Waterproof Ink 80

4.1.2 Identification Tag 80

4.1.3 Engraving 80

4.1.4 Stamping 80

4.2 Storage 81

4.3 Preservation 81

viii Metallographic and Materialographic Specimen Preparation

5 Cleaning and Cleanliness

5.1 Cleaning 82

5.1.1 Cleaning Before Start of Preparation 82

5.1.2 Cleaning During and After Preparation 82

5.2 Cleanliness 84

6 Mechanical Surface Preparation—Grinding 6.1 Grinding—A Basic Process 85

6.1.1 Plane Grinding共PG兲 85

6.1.2 Fine Grinding 86

6.2 Material Removal 86

6.2.1 Rake Angle 87

6.2.2 Grain Shape—Contacting Points 88

6.2.3 Grain Penetration 89

6.2.4 Force on Specimens 89

6.2.5 Grinding/Polishing Fluids 89

6.3 Deformation 89

6.3.1 Metals 89

6.3.2 Brittle Materials—Ceramics 92

6.4 Grinding Abrasives 93

6.4.1 Aluminum Oxide 93

6.4.2 Silicon Carbide 93

6.4.3 Diamond—Diamond Products 94

6.4.4 Cubic Boron Nitride共CBN兲 97

6.4.5 Boron Carbide 97

6.4.6 Hardness of Abrasives and Materials—Table 6.1 97

6.5 Grinding/Polishing Fluids—Lubricants 97

6.5.1 Water-Based Lubricant 97

6.5.2 Alcohol-Based Lubricant 97

6.5.3 Water-oil Based Lubricant 98

6.5.4 Oil-Based Lubricant 98

6.6 Traditional Grinding 99

6.6.1 Grinding Stones/Disks 99

6.6.2 SiC Wet Grinding Paper—Table 6.2 100

6.6.3 Alumina—Zirconia Alumina Wet Grinding Paper 105

6.7 Contemporary Grinding 106

6.7.1 Magnetic Fixation 106

6.7.2 Resin-Bonded Diamond Grinding Disks 107

6.7.3 Resin-Bonded SiC Grinding Disks 108

6.7.4 Metal-Bonded Diamond-Coated Disks 109

6.7.5 Diamond Pads 109

6.7.6 Diamond/CBN/ Al2 O3 /SiC Film 109

6.7.7 Rigid Composite Disks 109

6.7.8 Fine Grinding Cloths 116

6.8 Grinding/Polishing Equipment 117

6.8.1 Plane Grinding 117

6.8.2 Fine Grinding 119

7 Mechanical Surface Preparation—Polishing 7.1 Polishing: Producing the True Structure 120

7.1.1 Rough Polishing 120

7.1.2 Polishing 120

ix

7.2 Material Removal 120

7.2.1 Influence of Polishing Abrasive on Removal Rate 121

7.2.2 Force on Specimens 121

7.3 Deformation 122

7.3.1 The Beilby Layer 122

7.3.2 Influence of Polishing Abrasive, Cloth, and Fluid on Deformation 123

7.4 Polishing Cloths 124

7.4.1 Edge Retention—Relief 126

7.4.2 Cloths for Fine Grinding and Rough Polishing 126

7.4.3 Cloths for Polishing 127

7.5 Polishing Abrasives 129

7.5.1 Diamond Suspensions 129

7.5.2 Diamond Spray 129

7.5.3 Diamond Paste 130

7.5.4 Alumina 130

7.5.5 Silica 131

7.5.6 Other Oxides 132

7.6 Polishing Lubricants 132

7.7 The Metallographic/Materialographic Preparation Methods— Method Parameters 132

7.7.1 RPM of Grinding/Polishing Disk 133

7.7.2 RPM of Specimen Holder 133

7.7.3 Direction of Specimen Holder 134

7.7.4 Force on Specimens 134

7.7.5 Process Time 134

7.7.6 Stock Removal 134

7.8 Grinding/Polishing Equipment—Manual Preparation 135

7.9 Grinding/Polishing Equipment—Automatic Preparation 135

7.9.1 Machine Design 135

7.9.2 Polishing Dynamics 139

7.9.3 Semiautomatic and Fully Automatic Systems 140

7.10 Special Preparation Techniques 143

7.10.1 PCB Coupons 143

7.10.2 Microelectronic Materials—Nonencapsulated Cross Sections 143

7.10.3 Microelectronic Packages—Table 7.2—Target Preparation 147

7.10.4 EBSD 149

7.11 Field Metallography/Materialography—Nondestructive Mechanical Preparation 150

7.11.1 Portable Grinder/Polishers 150

7.11.2 Replication 150

7.12 Chemical Mechanical Polishing 151

7.12.1 Protection—Corrosion at CMP 152

7.13 Thin Sections 152

7.13.1 Thin Sections of Petrographic/Ceramic Materials 152

7.13.2 Thin Sections of Plastics/Polymers 153

7.14 Microtomy—Ultramilling 155

8 Electrolytic Polishing/Etching 8.1 The Electrolytic Polishing/Etching Process 156

8.1.1 The Polishing Cell 157

8.1.2 Smoothing and Brightening 157

x Metallographic and Materialographic Specimen Preparation

8.1.3 Electrolytic Etching 159

8.1.4 Advantages and Disadvantages 160

8.2 Electrolytes 163

8.3 Electropolishing in Practice 164

8.3.1 Factors Influencing Electrolytic Polishing 164

8.3.2 Example of Electrolytic Polishing/Etching 165

8.4 Electrolytic Polishing Equipment 165

8.4.1 Electropolishers for Laboratory Use 165

8.5 Field Metallography—Nondestructive Electropolishing 166

8.6 Electrolytic Thinning for TEM 167

8.7 Chemical Polishing 168

9 Etching 9.1 Microetching—Contrast 169

9.2 Contrast Without Surface Modifications—Microscope Techniques 169

9.2.1 Dark-Field Illumination共DF兲 169

9.2.2 Differential Interference Contrast共DIC兲 169

9.2.3 Polarized Light共POL兲 169

9.2.4 Fluorescence 170

9.3 Contrast with Surface Modification—Etching 170

9.3.1 Grain Contrast Etching 170

9.3.2 Grain Boundary Etching 171

9.3.3 Reproducibility 171

9.3.4 Safety Precautions 172

9.4 Classical Etching 172

9.4.1 Chemical Etching 172

9.4.2 Precipitation共Color兲 Etching 172

9.4.3 Heat Tinting 172

9.5 Electrolytic Etching 172

9.5.1 Anodic Etching 172

9.5.2 Anodizing 173

9.5.3 Potentiostatic Etching 173

9.6 Physical Etching 173

9.6.1 Relief Polishing 173

9.6.2 Ion Etching 173

9.6.3 Thermal Etching 174

9.6.4 Vapor Deposition 174

9.6.5 Sputtering 174

9.7 Macroetching 174

Part II: Metallographic/Materialographic Specimen Preparation—A Hands-On Manual 10 Introduction 10.1 Specimen Material 179

10.2 Purpose of Examination 179

10.3 Specimen Preparation 179

11 Specimen Material—Table 11.1 11.1 Classification of Materials 181

11.2 How to Use Table 11.1 181

11.3 Table 11.1—Materials/Methods 182

xi

12 Purpose of Examination

12.1 Purpose in General 188

12.2 Purpose: ASTM Standards 188

12.3 Table 12.1: Purpose/ASTM Standards 188

12.4 ASTM Standards—Metallography 188

12.4.1 Introduction 188

12.4.2 ASTM Standards in this Book 190

12.4.3 ASTM Standards—Document Summaries 193

12.5 Chemical Microetching—Table 12.2—Table 12.3 194

12.5.1 Etching Practice 194

12.5.2 Table 12.2—Numerical List of Etchants 195

12.5.3 Table 12.3—Etchant Names 217

13 Specimen Preparation 13.1 Introduction 218

13.2 Mechanical Preparation—The “Traditional” and “Contemporary” Methods 218

13.2.1 Material/Preparation Tables 218

13.2.2 Method Tables—Generic Methods—Parameters/Consumables— Table 13.1 219

13.2.3 Material/Preparation Tables—Methods C-01/T-01 to C-68/T-68 222

13.2.4 Manual Preparation 450

13.3 Electrolytic Polishing and Etching 453

13.3.1 Electropolishers 454

13.3.2 Electrolytes—Methods for Electropolishing—Table 13.2 454

13.3.3 Table 13.2—Electrolytes for Electropolishing/Etching 454

13.3.4 Mechanical Preparation for Electropolishing 456

13.3.5 Electropolishing—Method Tables 456

13.3.6 Electropolishing—Methods El-01 To El-25 456

13.4 Field Metallography/Materialography—Nondestructive Preparation 475

13.4.1 Mechanical Preparation 475

13.4.2 Electrolytic Polishing 475

13.4.3 Replication 475

13.5 Trouble Shooting—How to Improve Preparation Results 476

13.5.1 Sectioning 477

13.5.2 Mounting 479

13.5.3 Mechanical Preparation 482

13.5.4 Electrolytic Polishing 483

13.5.5 General Rules—“The Metallographer’s Rule of Thumb” 483

13.6 Trouble Shooting—How to Overcome Preparation Artifacts 484

13.6.1 Preparation Artifacts—Flow Charts 484

13.6.2 Sectioning—General Problems—Flow Charts 485

13.6.3 Mounting—General Problems—Artifacts 495

13.6.4 Grinding and Mechanical Polishing—Flow Charts 498

13.6.5 Electropolishing—General Problems—Artifacts 521

Part III: Light Microscopy 14 Introduction 14.1 Visible Light–Table 14.1–Table 14.2 525

14.2 The Human Eye 526

xii Metallographic and Materialographic Specimen Preparation

14.3 Magnifying Lens and Microscope 527

14.4 Magnification 527

15 The Optical Reflected Light Microscope 15.1 The Path of Light Rays 528

15.2 The Objective 528

15.2.1 Numerical Aperture—Resolution-Magnification–Table 15.1–Table 15.2 528

15.2.2 Aberrations in Image-Formation 532

15.2.3 Available Objectives 533

15.3 Eyepieces 535

15.4 Illumination 536

15.4.1 Koehler’s Illumination System 536

15.5 Microscope Options 537

15.6 The Reflected-Light Microscope 538

15.6.1 Upright Type of Reflected-Light Microscope 538

15.6.2 Inverted Type of Reflected-Light Microscope 538

15.7 Optical Examination Methods 540

15.7.1 Bright-Field共BF兲 Illumination 541

15.7.2 Dark-Field共DF兲 Illumination 541

15.7.3 Polarization Contrasting共POL兲 542

15.7.4 Differential Interference Contrasting共DIC兲 544

15.7.5 Fluorescence in Reflected Light 545

15.8 Practical Use of the Microscope 546

15.8.1 Setting up the Microscope 546

15.8.2 Working with the Microscope 547

15.8.3 Correct Adjustment of the Microscope 548

15.8.4 Focusing and Practical Use 548

15.8.5 Measurements of Length 549

15.8.6 Measurements of Height Differences 550

15.8.7 Maintenance of the Microscope 550

15.9 Documentation 550

15.10 The Confocal Laser Scan Microscope 552

15.10.1 Function of Confocal Laser Scan Microscope 552

15.10.2 Applications of Confocal Laser Scan Microscope 554

15.11 Stereo Microscopy 555

16 Electron Microscopy—Scanning Probe Microscopy 16.1 The Transmission Electron Microscope共TEM兲 558

16.1.1 The Scanning Transmission Electron Microscope共STEM兲 558

16.2 The Scanning Electron Microscope共SEM兲 558

16.2.1 Energy Dispersive Spectroscopy共EDS兲 559

16.2.2 Electron Backscatter Diffraction共EBSD兲 559

16.2.3 The Electron Probe Microanalyzer共EPMA兲 560

16.3 Focused Ion Beam共FIB兲 560

16.4 Scanning Probe Microscopes共SPM兲 560

Part IV: Quantitative Metallography/Materialography— Automatic Image Analysis 17 Quantitative Metallography/Materialography—An Introduction 17.1 Quantitative Metallography/Materialography 565

17.1.1 Stereology–Table 17.1 565

xiii

17.1.2 Specimen Preparation 567

17.1.3 Calibration 568

17.1.4 Field Selection—Bias 568

17.2 Volume Fraction—Point Count 569

17.2.1 ASTM Test Method for Determining Volume Fraction by Systematic Manual Point Count共E 562兲 569

17.3 Inclusion Rating 570

17.3.1 ASTM Standard Test Method For Determining the Inclusion Content of Steel共E 45兲 570

17.3.2 ASTM Practice for Obtaining JK Inclusion Ratings Using Automatic Image Analysis共E 1122兲 共withdrawn 2006, replaced by E 45兲 570

17.3.3 ASTM Practice for Determining the Inclusion or Second-Phase Constituent Content of Metals by Automatic Image Analysis 共E 1245兲 570

17.4 Grain Size 571

17.4.1 ASTM Test Methods for Determining Average Grain Size共E 112兲 571

17.4.2 ASTM Test Methods for Estimating the Largest Grain Observed in a Metallographic Section共ALA Grain Size兲 共E 930兲 573

17.4.3 ASTM Test Methods for Characterizing Duplex Grain Sizes共E 1181兲 573

17.4.4 ASTM Test Methods for Determining Average Grain Size Using Semiautomatic and Automatic Image Analysis共E 1382兲 573

17.5 Banding 574

17.5.1 ASTM Practice for Assessing the Degree of Banding or Orientation of Microstructures共E 1268兲 574

17.6 Porosity in Thermal Spray Coatings 574

17.6.1 ASTM Test Methods for Determining Area Percentage Porosity in Thermal Sprayed Coatings共E 2109兲 574

17.7 Decarburization—Case Depth—Coatings 575

17.7.1 Specimen Preparation 575

17.7.2 ASTM Test Methods for Estimating the Depth of Decarburization of Steel Specimens共E 1077兲 575

17.7.3 Case Depth 575

17.7.4 ASTM Test Method for Measurement of Metal and Oxide Coating Thickness by Microscopical Examination of a Cross Section共B 487兲 576

17.7.5 ASTM Test Methods for Thickness of Diffusion Coating共C 664兲 576

17.8 Other ASTM Standards for Quantitative Materialography 576

18 Automatic Image Analysis 18.1 Introduction 577

18.2 Qualitative and Quantitative Metallography/Materialography 577

18.2.1 The Transition to Quantitative Standards 577

18.2.2 Structure, Stereology, and Statistics 578

18.3 Principles of Digital Imaging 579

18.3.1 What is Digital Image Analysis? 579

18.3.2 Image Acquisition 579

18.3.3 Image Digitization—Gray Scale 580

18.3.4 The Histogram 581

18.3.5 The Effects of Brightness and Contrast on Illumination Distribution 581

18.3.6 Image Processing and True Microstructure 586

18.3.7 Image Calibration 595

xiv Metallographic and Materialographic Specimen Preparation

18.4 Image Measurement 598

18.4.1 Manual Measurements共Operator Defines Points, Lines, or Areas兲 599

18.4.2 Automatic Measurements共Objects Defined by Image Segmentation兲 600

18.5 Digital Imaging Applied to Quantitative Materialography 602

18.5.1 Percent Area共Volume Fraction兲 602

18.5.2 Inclusion Rating 603

18.5.3 Grain Size 606

18.5.4 Degree of Banding 608

18.5.5 Depth or Thickness Measurements 608

18.5.6 Graphite in Iron Castings 610

18.6 Digital Imaging Technology 613

18.6.1 Hardware 613

18.6.2 Software 616

18.7 Digital Imaging System Implementation 617

19 Digital Image Management „Archiving… Part V: Hardness Testing 20 Introduction 20.1 Indentation Hardness 623

20.2 ASTM Standards 625

21 Static Hardness Testing Procedures 21.1 Brinell Hardness Testing 626

21.1.1 Calculations and Procedures 626

21.1.2 Brinell Hardness Testers 628

21.2 Vickers Hardness Testers 628

21.2.1 Calculations and Procedures 628

21.2.2 Vickers Hardness Tester 632

21.3 Knoop Hardness Testing 633

21.3.1 Calculations and Procedures 633

21.4 Rockwell Hardness Testing 634

21.4.1 Calculations and Procedures 634

21.4.2 Rockwell Hardness Testers 636

21.5 Microindentation Hardness 636

21.5.1 Methods 636

21.5.2 Specimen Preparation 637

21.5.3 Taking the Measurements 638

21.5.4 Microindentation Hardness Testers 639

21.5.5 Examples of Indentations 639

21.6 Universal Hardness—Martens Hardness—Instrumented Indentation Testing—Nano Indentation 639

21.6.1 Instrumented Indentation Testing—Nano Indentation 641

21.7 Precision of Hardness Values 642

21.8 Conversion of Hardness Values 642

22 Dynamic Hardness Testing Procedures

23 Special Methods for Hardness Testing

xv

Part VI: The Metallographic/Materialographic Laboratory

24 Introduction

24.1 Establishing a Metallographic/Materialographic Laboratory 649

24.2 Running a Metallographic/Materialographic Laboratory 649

24.3 Occupational Safety and Health 649

25 How to Build a Metallographic/Materialographic Laboratory 25.1 Purpose 650

25.1.1 Quality Control共QC兲 650

25.1.2 Research and Education 651

25.1.3 Testing and Inspection Laboratories—Failure Analysis 651

25.2 Rationalization and Automation 651

25.2.1 Reproducibility—Standards—Occupational Safety 652

25.2.2 Productivity—Cost Per Specimen 653

25.3 Planning the Metallographic/Materialographic Laboratory 654

25.3.1 Basic Planning 654

25.3.2 Detailed Planning 655

25.4 Equipment and Laboratory Layout 656

25.4.1 Equipment—Table 25.1 656

25.4.2 Layout—Furniture—Installations 660

25.5 Maintenance 662

25.5.1 Organizing 662

25.5.2 Cleaning 662

25.5.3 Servicing 663

26 Occupational Safety and Health in the Metallographic/ Materialographic Laboratory 26.1 Dangers in the Metallographic/Materialographic Laboratory 664

26.1.1 Sectioning 664

26.1.2 Mounting 664

26.1.3 Mechanical Preparation 665

26.1.4 Electrolytic Polishing/Etching 665

26.1.5 Etching—Etchants—Electrolytes 665

26.1.6 Dust 667

26.1.7 Cold共Castable兲 Mounting Resins 667

26.1.8 Standard Guide on Metallographic Laboratory Safety共E 2014兲 668

26.2 Safety Information 668

26.2.1 Identification 668

26.2.2 Material Safety Data Sheet共MSDS兲 670

26.2.3 Standard Operating Procedure共SOP兲 672

26.2.4 Job Safety Analysis共JSA兲 672

26.3 Disposal of Chemicals 672

26.4 Occupational Safety in General 673

26.4.1 Standards 673

26.4.2 Training 673

26.4.3 Maintenance and Service 673

26.5 Standards and Regulations—Organizations 673

26.5.1 Designations and Abbreviations Used to Describe a Chemical Substance 673

26.5.2 ASTM Standard 674

26.5.3 OSHA—OSHA Standards 674

xvi Metallographic and Materialographic Specimen Preparation

26.5.4 National Institute for Occupational Safety and Health共NIOSH兲 681

26.5.5 International Chemical Safety Cards共ICSCS兲 682

26.5.6 Environmental Protection Agency共EPA兲 683

26.5.7 National Technical Information Service共NTIS兲 683

26.5.8 American Conference of Government Industrial Hygienists 共ACGIH兲 683

26.5.9 National Toxicology Program共NTP兲 683

26.5.10 Agency for Toxic Substance and Disease Registry共ATSDR兲 683

26.5.11 National Fire Protection Association共NFPA兲 684

26.5.12 National Paint and Coatings Association共NPCA兲—HMIS 684

26.5.13 BSI—ISO 684

26.5.14 EU 684

26.6 Literature on Laboratory Safety 684

27 Literature 27.1 Books 685

27.2 Periodicals 686

Appendixes Appendix I: Other Standards on Metallography/Materialography 686

Appendix II: Other Standards on Hardness Testing 691

Appendix III: Hardness Conversion Tables for Metals共E140兲 694

Appendix IV: SI Quick Reference Guide: International System of Units共SI兲 694

Glossary 695

Index 727

xvii

Part I:

The Metallographic/Materialographic Preparation Process

Introduction

“METALLOGRAPHY” or “MATERIALOGRAPHY”? IN MODERN

TECHNOL-ogy and Materials Science we are examining the microstructure of all solid materials;

therefore, materialography seems to be the correct word instead of the traditional allography In 1968, Crowther and Spanholtz1suggested this and it now seems appro-priate to use the word “materialography” to cover the examination of the infinite num-ber of existing and future materials Also, the term “metallographer” should bechanged to “materialographer.” Changes of this kind, however, take time, and thereforethe terms “metallography” and “metallographer” are used in this book, except in con-texts where materials other than metals are discussed

met-G Petzow2defines Materialography共metallography兲 as “an investigative method

of materials science It encompasses the optical examination of microstructures, andits goal is a qualitative and quantitative description of the microstructure.”

The term materialography includes ceramography 共ceramics兲, metallography共metals兲, plastography 共polymers兲, and mineralogy 共minerals兲, in this way covering themicrostructural examination of most materials

Metallography/materialography includes a wide field in material investigation; itbridges the gap between science in new and existing materials and engineering usingthe materials in modern technology Figure 1.13shows how materialography coversthe examination of parts from the centimetre and metre共in and ft兲 range to atomicdimensions in the nm and sub nm range

The microstructure is characterized through size, shape, arrangement, amount,type, and orientation of the phases and the defects of these phases, as schematically

Fig 1.1—Metallography/materialography can be described as a bridge between engineering

and science, covering the examination of the part in cm and m to the examination of the single atom in Å.

3

shown in Fig 1.23 Each material contains many millions of microstructural featuresper cubic centimetre and these features strongly influence many of the properties ofthe material As seen in Fig 1.1, the microstructural features can exist in sizes of atleast ten orders of magnitude There are many instruments today that visualize nearlyall of the features across this range.

The image we see in the typical microscope is two-dimensional, but we should notlose sight of the fact that the constituents in a material are three dimensionally ar-ranged

A photomontage shows the prepared surface of a silicon nitride alloy posed on a pile of silicon nitride crystals共see Fig 1.3兲.3It shows that the true size of thecrystals cannot be deduced directly from the microstructure A statistical extrapola-tion of the two-dimensional surface shows that approximately 80 % of the crystals arerelatively short and have an equiaxial shape Stereological calculations, however, show

superim-a much higher vsuperim-arisuperim-ation in crystsuperim-al length The superim-aversuperim-age crystsuperim-al length is lsuperim-arger, sponding to the three-dimensional characteristics shown in Fig 1.3

corre-It can be concluded that the analysis of the microstructure plays an important role

in modern materials science and engineering, and consequently, the metallographic/materialographic preparation It is important to secure the true microstructure be-cause without this the best examinations and inspired interpretations will be of noavail

As stated in the Preface, this book concentrates on metallographic/materialographic preparation and the most commonly used examination methods.For a comprehensive, in-depth coverage of metallurgy and microstructures, includinginterpretation of the microstructures, ASM Handbook, Volume 9, Metallography andMicrostructures,4is recommended

This part of the book concentrates on the preparation of the specimen surface forexamination in the reflected-light optical microscope This preparation can also beused frequently for the scanning electron microscope共SEM兲 The mechanical removal

Fig 1.2—The constituents of a microstructure and the factors affecting them.

4 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

of material will be described and discussed rather intensively because it is the centralprocess in abrasive cutting, sawing, plane/fine grinding, and polishing, as will the prob-lems involved in obtaining the true microstructure The machines and consumablesavailable will also be described and discussed.

Etching, often performed after the specimen preparation process to obtain a trast to highlight or clearly reveal certain features, will be described in theory and prac-tice

con-1.1 Metallographic/Materialographic Preparation—The True Microstructure

The goal of the metallographic/materialographic preparation is to obtain the true crostructure or “The True Structure,” meaning an undisturbed material surface, whichcan be analyzed in an optical共light兲 microscope or an SEM

mi-The basic problem for a metallographer preparing a specimen is that the tion process itself modifies the specimen surface and, theoretically, a “true structure”completely without artifacts can never be obtained Consequently, a preparation pro-cess should be used that creates the smallest amount of artifacts, making it possible, inpractice, to analyze a microstructure in a satisfactory way

In the 1860s, because he understood that to obtain a “true structure” he had to removethe irregularities of the material surface, H C Sorby was able to produce what is con-

Fig 1.3—Photomontage of a microsection of silicon nitride alloy superimposed upon a pile of

silicon nitride crystallites.

Chapter 1 Introduction 5

sidered the first true microstructure In 1863 he prepared a specimen of Bessemer steel

by using a preparation method with several steps, a method similar to the mechanicalpreparation used today Figure 1.45shows the microstructure, which was prepared inseveral steps, a rough polishing step and a fine polishing step

1.2 The True Microstructure

Based on studies by Vilella and Samuels,6–8the true structure can be defined as:

No deformation—The plastically deformed layer created by the preparation should

be removed or be negligible

No scratches—Scratches normally indicate a surface that is not yet sufficiently

pre-pared, but small scratches might be allowed if they do not disturb the examination

No pull-outs—Especially in brittle materials, particles can be pulled out of the

sur-face leaving cavities that can be taken for porosity

No introduction of foreign elements—During the preparation process, abrasive

grains can be embedded in the surface

No smearing—With certain materials, the matrix or one of the phases might smear

共flow兲, resulting in a false structure or covering of structure details, or both

No relief or rounding of edges—Relief can develop between different constituents of

the surface, caused by different hardness or other condition Edge retention is tant if the edge has to be examined

impor-1.3 Selection of Preparation Method

The preparation process will always influence the prepared surface, creating artifacts.Artifacts are defined as false structural details introduced during the preparation

Fig 1.4—Original specimen prepared by H C Sorby, 1863, Bessemer steel 0.2 % carbon BF,

450:1 Preparation Method—Rough grinding: Emery paper from coarse to fine Fine grinding:

“Fine grained” water-of-Ayr stone Rough polishing: “Finest grained” crocus 共Fe 2 O3used for industrial polishing 兲 Polishing: “Very best and finest washed” rouge 共Fe 2 O3, jeweler’s rouge 兲.

6 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

The choice of preparation is usually between using mechanical or electrolytic ishing, but chemical and chemical-mechanical polishing are also used.

pol-1.3.1 Artifacts

A number of artifacts are already stated above under the true structure, but a few morecan be added Microcracks, comet tails, pitting, contamination, and lapping tracks areall caused by the preparation process Artifacts can also be introduced during chemicaletching of the surface Most of these artifacts can be readily observed under the micro-scope In some cases, artifacts can be accepted and the metallographer can decidewhether, for example, a scratch is acceptable as it does not disturb the structural analy-sis, or whether the specimen surface should be reprepared

In some cases it can be very difficult to establish the true structure, e.g., a smearedlayer can cover pores It is important that the metallographer pay attention to this pos-sibility when analyzing a structure共see Section 13.5兲

Artifacts of Mechanical Polishing

With mechanical polishing, it is possible to obtain an approximate true structure whenthe correct procedures are followed, even with very heterogeneous materials Figure1.5 shows the following most common artifacts: relief between phases caused by differ-ence in hardness; embedded abrasive grain; inclusion protruding共it could also bemissing兲; pull-out looking like a pore; rounding of the edge; and deformation of thematrix

Artifacts of Electrolytic Polishing

With electrolytic polishing, the electrolysis might create problems if more than onephase is present in the structure Figure 1.6 shows the most common artifacts Reliefbetween phases caused by a difference in electrochemical potential: in some cases onephase will be removed much faster than another phase, in other cases a phase mightnot be electrically conductive and, as such, will not take part in the polishing process

Inclusions might react in the same way; they will often be dug out during the process.

Pitting might develop if the electrolytic process is not controlled correctly Also, a nounced rounding of the edge will take place because the current density is alwaysstronger at the edge

pro-1.3.2 Preparation Methods

Because most materials are heterogeneous共or even nonconductive兲, the conclusionmust be that mechanical polishing is by far the most commonly used method For cer-tain materials, however, electrolytic polishing gives very good results

Alternatives to the above-mentioned methods are chemical polishing andchemical-mechanical polishing Chemical polishing is not used much, although reci-pes for polishing of a number of materials are developed Chemical mechanical polish-ing or attack polishing can be seen as an extension of mechanical polishing and, whenrelevant, recipes will be stated in connection with the specific material

For recipes on chemical and chemical mechanical polishing, see Refs 2, 4, and 9

1.4 The Metallographic/Materialographic Specimen

In practice, the total work piece normally cannot be prepared and examined For thisreason, a small part of the work piece, the sample共specimen兲 must be extracted For

Chapter 1 Introduction 7

both specimen preparation and examination, using an optical microscope or an SEM,the ideal specimen size is 12– 40 mm共0.5–1.5 in兲 square or cylindrical, with a height of12– 30 mm共0.5–1.2 in兲 There are, of course, exceptions like welds, where larger speci-mens have to be prepared.

With the specimen being only a small part of the material to be examined, if theinterpretation is to be valuable, it is very important that the specimen be representative

of the material to be studied This usually happens by cutting out the specimen from acorrect location and in the correct direction共see Section 2.1兲 Most ASTM standardscovering examination of a metallographic/materialographic specimen offer guidance

in selection and sectioning of specimens共see Section 12.4兲 The preparation can be formed once the specimen is established

per-1.4.1 “Specimen” or “Sample”

The two words are often used indiscriminately, describing the object prepared and amined The “sample” can be defined as the piece of material in its “raw” state, as taken

ex-Fig 1.5—Mechanical polishing: the most common artifacts shown schematically.

8 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

from the original material共work piece兲 As soon as the “sample” is treated 共prepared兲and described, it turns into a “specimen,” and for this reason only the word “specimen”

is used in this book, except in a few cases where “sample” is the correct description

1.5 The Preparation Process

As mentioned above, several polishing methods are available, but in the diagram, Fig.1.7, only the two methods used for almost all preparation, mechanical and electrolytic,are shown The diagram gives an overview of the total process, of which each step will

be discussed further in this part of the book

Fig 1.6—Electrolytic polishing: the most common artifacts shown schematically.

Chapter 1 Introduction 9

1.5.1 Sectioning

To obtain a specimen, some kind of sectioning from the basic material共work piece兲 isnecessary If this sectioning could take place without disturbing the specimen surface,the specimen could be examined without further work, but unfortunately all theknown sectioning methods will leave some kind of irregularities on the surface Abra-sive wet cutting using a precision cut-off machine is considered as a sectioning methodgiving a low deformation of the specimen surface Figure 1.8 shows a surface from aspecimen cut on a precision cutter and measured with an atomic force microscope共AFM兲, and the irregularities of the surface are evident

Abrasive wet cutting is the most frequently used sectioning method, but other

Fig 1.7—Diagram showing the total preparation process based on mechanical and electrolytic

preparation.

10 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

methods, such as shearing, sawing, and punching are used as well共see Section 2.7兲.

1.5.2 Mounting

In some cases, the sample taken from the base material can be handled and treateddirectly as a specimen, but often a mount must be made to secure the handling and asatisfactory preparation The mounting can be made by clamping the specimen be-tween two pieces of a material compatible to the specimen material This way ofmounting has a number of drawbacks共see Section 3.2.1兲; therefore mounting mainlytakes place as hot compression or cold共castable兲 mounting in a mounting plastic共resin兲 Figure 1.9共a兲 shows three mounts made with hot mounting, giving mounts withvery precise dimensions Figure 1.9共b兲 shows three mounts made with cold mounting;these mounts, made in molds, are less exact than the hot mounts

1.5.3 Preparation of the Surface

The goal of the preparation is to obtain the true microstructure or at least a ture in a condition that makes a satisfactory examination possible This means that thenumber of irregularities共artifacts兲 in the surface must be kept at a minimum

microstruc-The preparation is done through a number of steps, either mechanical or lytical共see Fig 1.7兲

electro-Fig 1.8—Surface cut with a precision cut-off machine in a very careful way to avoid

irregularities in the cut surface Measurements with an atomic force microscope 共AFM兲 give the peak-to-valley value of irregularities: higher than 1000 nm 共1␮m 兲 This shows that even with the most gentle sectioning technique, the cut surface will have deformations which have to be removed in the following preparation steps.

Chapter 1 Introduction 11

A mechanical preparation method will normally contain a plane grinding step, one

or more fine grinding steps, and one or more polishing steps

Electrolytic polishing usually takes place as one electrolytic step, performed on amechanically ground or polished surface

Fig 1.9—Mounts made with hot compression mounting共a兲 and cold 共castable兲 mounting 共b兲.

12 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

1.5.4 Etching

The prepared surface often reacts as a mirror when examined in the microscope, notshowing all phases of the microstructure For this purpose, the surface can be etchedchemically or electrolytically or treated in other ways to discriminate between phases,grains, grain boundaries, and other details Figure 1.10 shows a copper specimen共a兲 in

an unetched condition, giving very little information; and共b兲 one that is etched, ing the microstructure

show-Fig 1.10—Copper unetched 共a兲 showing a bright, reflecting surface and color etched with Klemm III45共b兲, revealing the microstructure.

Chapter 1 Introduction 13

Sectioning 2.1 Selection

IT IS VERY IMPORTANT THAT THE SPECIMEN IS SELECTED CORRECTLY SO

that the specimen material is representative of the material to be studied The intent orpurpose of the examination will usually dictate the location of the specimen

With respect to the purpose of the study, metallographic examination may be vided into three classifications, as stated in ASTM Practice for Preparation of Metallo-graphic Specimens共E 3兲 共see Section 12.4兲

di-2.1.1 General Studies or Routine Work

Specimens should be chosen from locations that are most likely to show the maximumvarieties within the material being studied For example, specimens should be takenfrom a casting in the zones wherein maximum segregation should occur, as well asspecimens from sections where segregation should be at a minimum In the examina-tion of strip or wire, test specimens should be taken from each end of the coils

2.1.2 Study of Failures

Specimens should be taken as closely as possible to the fracture or to the initiation ofthe failure Before taking the specimens, study of the fracture surface should be com-plete, or, at the very least, the fracture surface should be documented In many cases,specimens should be taken from a sound area for a comparison of structures andproperties

be studied Special investigations may at times require specimens with surfaces pared parallel to the original surface of the product In the case of wire and smallrounds, a longitudinal section through the center of the specimen proves advantageouswhen studied in conjunction with the transverse section

pre-Cross sections or transverse sections taken perpendicular to the main axis of thematerial are more suitable for revealing the following information:

• Variations in structure from center to surface

• Distribution of nonmetallic impurities across the section

• Decarburization at the surface of a ferrous material, see ASTM Test Methods for

14

Estimating the Depth of Decarburization of Steel Specimens共E 1077兲, Section12.4.

• Depth of surface imperfections

• Depth of corrosion

• Thickness of protective coatings and structure of protective coating

Longitudinal sections taken parallel to the main axis of the material are more able for revealing the following information:

suit-• Inclusion content of steel, see ASTM Test Methods for Determining the InclusionContent of Steel共E 45兲 and other ASTM standards, Sections 12.4 and 17.2

• Degree of plastic deformation, as shown by grain distortion

• Presence or absence of banding in the structure, see ASTM Practice for Assessingthe Degree of Banding or Orientation of Microstructures共E 1268兲, Sections 12.4and 17.5

• The quality attained with any heat treatment

2.1.5 Reporting of Locations

The locations of surfaces examined should always be given when reporting results and

in any illustrative micrographs A suitable method of indicating surface locations is tomake a sketch of the work piece with an indication of the location

2.2 Sectioning

The goal is to extract the specimen to be prepared from the material to be studied共workpiece兲 This should be done so that the specimen is representative of the work piecematerial and it should be done with a minimum amount of damage to the surface that

is to be prepared

In principle, all methods, including sawing with a hacksaw, shearing, flame ting, fracturing, etc., can be used to separate a specimen from the work piece It is,however, important that the surface being prepared is only influenced mechanically or

cut-by heat to a degree that is suitable for a rational preparation that follows This limitsthe sectioning methods to wet abrasive cutting and a few other methods that will bedescribed in the following sections

2.3 Wet Abrasive Cutting

Abrasive cutting is a cut-off grinding process

2.3.1 The Cut-off Grinding Process

The cut-off grinding共abrasive cutting兲 is a special operation following the general ciples of the machining process, grinding

prin-Within the spectrum of machining processes, the uniqueness of grinding is found

in its cutting tool Grinding wheels are generally composed of two materials: abrasiveparticles called grains that do the cutting and a softer bonding agent to hold the count-less abrasive grains together in a solid mass

During most grinding processes the surface of the work piece is treated to obtain agiven accuracy or surface finish In cut-off grinding, a very thin grinding wheel共nor-mally the thickness of the wheel is 1 / 100 of the wheel diameter, or less兲 grinds its waythrough a work piece In metallographic/materialographic cutting, this is to separate a

Chapter 2 Sectioning 15

sample suited for further preparation from the work piece Although there isn’t a mand for high accuracy, the surface quality concerning mechanical damage, thermaldamage, and planeness is important.

de-Cut-off wheels are made by cementing together abrasive grains with a suitablebonding material Each grain is a potential microscopic cutting tool The grinding pro-cess uses thousands of abrasive points simultaneously and millions continually

By choosing a cut-off wheel with the correct abrasive and bond and using it on asuitable machine, both the mechanical and thermal damage and the planeness can bekept inside narrow limits This will shorten and facilitate the following preparationprocess

Figure 2.1 shows the surface roughness of mild steel after cutting, after grit P220SiC grinding paper, and after P320 grinding paper It can be seen that the irregularscratches from the cut-off are removed by the grinding papers, and for most materials agrinding with grit P220 after cutting will give a satisfactory surface for further prepara-tion; this will be discussed further below For certain materials P320 paper can be used

as the first step after cut-off, omitting plane grinding with grit P220

2.3.2 The Cut-off Wheel—Abrasives and Bond Materials

The cut-off wheels belong to the category of “bonded abrasive tools.” Such tools consist

of hard abrasive grains held in a weaker bonding matrix Depending on the particulartype of bond, the space between the abrasive particles may only be partially filled, leav-ing voids and porosity, resulting in an open bond A dense bond is the result of com-pletely filled spaces between the grains Aside from abrasive and bond material, fillersand grinding-aid material may also be added The correct combination of abrasive andbond is important to ensure the right cut-off process

Every abrasive particle has a number of cutting points with each removing a tinychip from the work piece Eventually the cutting edge becomes blunt and it must bear alarger force in order to remove the chip from the work piece The force rises until itcauses the grain to fracture and present a new, sharp edge to the work piece In this waythe grain reduces its size until finally the cutting force共see Section 2.3.3兲 causes it to becompletely torn out of the wheel, exposing new grains This “self-sharpening” process

is highly controlled by the combination of abrasive material and bond material共seeFig 2.2兲 that schematically shows the abrasive grains in the bond with voids 共pores兲 inbetween

Depending on how the wheel is breaking down, the wheels are defined as either

“Consumable Wheels” or “Slow Consumable Wheels”共see Section 2.4兲

Cut-off Wheel Specifications

The basic specification of a consumable cut-off wheel defines the following eters:

param-1 The type of abrasive, expressed with a number and a letter共aluminum oxide: A,silicon carbide: S兲

2 The size of abrasive grains, expressed with grit number共see Section 6.6.2兲

3 The grade共hard/soft兲 of wheel bond expressed with a letter

4 The wheel’s structure expressed with numbers

5 The bond material expressed with a letter

6 A code in numbers to express the maker’s details of manufacture

In the following sections parameters 1–5 will be described in detail

16 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

Fig 2.1—共a兲 Steel after wet abrasive cutting An abrasive grain 共arrow兲, embedded in the surface during the cutting, can be seen, 共b兲 after grinding with grit P220 SiC paper, and 共c兲 after grinding with P320 grinding paper.

Chapter 2 Sectioning 17

Type of Abrasive

For cut-off wheels four types of synthetic abrasives, aluminum oxide共Al2O3兲, siliconcarbide共SiC兲, cubic boron nitride 共CBN兲, and diamond are used 共see Table 6.1 and alsoSection 6.4兲

Al 2 O 3—Although this is the softest of the abrasive materials, it is the abrasive used

in most cut-off wheels This is due to the fact that Al2O3is best suited for ferrous rials, from mild steel to high-strength materials, i.e., alloy steels Al2O3is not suited forcast iron共see SiC below兲

mate-Al2O3is made synthetically in different types with varying hardness and friability,and is used for cutting of different materials It is used in consumable cut-off wheels

SiC—This synthetic material is harder and tougher than Al2O3, but dulls andglazes rapidly when used with steels It is well suited for cutting of softer materials likenonferrous metals, and it is also suited for cast iron SiC is made in two varieties, blackand green; normally the black type is used in cut-off wheels It is used in consumablewheels

CBN—This very hard, synthetic abrasive共superabrasive兲 is used for cutting ofhard materials that are not to be cut with Al2O3and SiC CBN is rather expensive; theprice is comparable to the price of diamond, but CBN has the advantage that it cutsferrous materials that cannot be cut with diamond CBN has a very high thermal stabil-ity and will work for a very long time before getting dull and needs little dressing共seeSection 2.3.6兲 It is used in slow consumable wheels where the wheel consists of a metalbody, and CBN is only part of the rim in a very stable bond共see Section 2.4兲 CBN grainstend to be blocky shaped with sharp edges and smooth faces, which makes bondingdifficult Therefore CBN, as diamond, normally is coated before being used in a resinbonded cut-off wheel

Diamond—Diamond is the hardest abrasive共see Table 6.1兲 and is used for cutting

of the hardest materials In spite of its extreme hardness, diamond has been found to beunsuitable for cutting ferrous materials This is due to graphitization and carbon diffu-sion into the iron causing excessive diamond wear.10Diamond is found as natural dia-monds, but mostly synthetic diamonds are used in cut-off wheels The diamond grains

Fig 2.2—Schematic drawing of cut-off wheel showing abrasive grains and bond material with

voids 共pores兲.

18 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

are normally coated to improve the fixation of the grain in the bond Diamond is onlyused in slow consumable wheels as described under CBN above共see Section 2.4兲.

Grain Size

The grain size is expressed as a grit number共#兲 This number refers to the number ofopenings per linear inch in a mesh screen through which the grain is just able to pass.The grit sizes are standardized by ANSI共American兲 and FEPA 共European兲 共see Table6.2 and Section 6.6兲 For cut-off wheels, grit sizes between 50 共336␮m兲 and 120共125␮m兲 are normally used

Generally speaking, large grains will have a higher material removal rate, but arougher finish

Large grains also allow for a more open bond structure because the pores共voids兲between the grains can be relatively large共see Fig 2.2 and Structure below兲 An open

bond structure allows room for the chips created during the cutting process so thatthey can be removed without disturbing the process For this reason cut-off wheelswith large grains, which enable an open structure with large pores, are suited for cut-ting of large work pieces This creates a large contact area共arc of contact兲 between cut-off wheel and work piece共see l in Fig 2.3兲 In the long contact area, chips and bondparticles will be accumulated before they are removed from the wheel outside the workpiece, and this accumulation takes place in the large pores If not accumulated in thepores, the chips and particles will take room in the interface between wheel and workpiece, reducing the cutting action and creating heat

Fine grains have a lower removal rate, but a better surface finish Fine grains willgive a relatively dense bond structure共small pores兲, and therefore a fine-grained cut-offwheel is most suited for brittle materials共very small chips兲 and smaller work pieces

Fig 2.3—Schematic drawing of the cut-off process The rotating cut-off wheel is cutting into

the fixed work piece.

Chapter 2 Sectioning 19

with a short arc of contact.

Grade

The grade expresses the degree of retaining grip exerted on each grain by the bonding

material that corresponds to the cutting force needed to dislodge the grain Figure 2.2shows the grains bonded together with voids共pores兲 in between When the cuttingforce has increased to a certain point, the grain will be dislodged from the bond.Wheel grades are expressed with letters from E共very soft兲 to X 共very hard兲 Cut-offwheels are mostly in the range K to R

A soft grade of bond has a weak hold in the abrasive grain Blunt grains will be tornaway easily, thus the self-sharpening action will be pronounced This is desirable when

cutting hard materials expressed in the rule: Hard Material—Soft Wheel.

A relatively soft wheel is used if the arc of contact is very large because the long arcwill normally reduce the force per grain共see Sections 2.3.3 and 2.3.8兲

If the wheel is too soft for a given material it will in most cases cut very well, but thewheel wear will be excessive causing a bad economy In principle, the hardest possiblewheel for a given material should be used to secure the most economical sectioning

A hard grade has a stronger hold in the abrasive grain, making it suited for softer

materials expressed in the rule: Soft Material—Hard Wheel.

A hard wheel is also used with a short arc of contact共see Section 2.3.6兲

A hard bond gives a longer wheel life, but if it is too hard the blunt grains may beretained for too long, leading to a condition called glazing of the edge of the wheel Inthis condition the wheel might stop cutting completely and will only generate heat

A wheel may be made to act harder or softer by varying the forces acting on thegrains Decreasing the wheel speed or raising the feed speed will increase the cuttingforces This will cause the wheel to shed grains and wear quicker so it will appear to beacting as a softer grade of wheel Increasing the wheel speed or reducing the feed speedwill decrease the cutting forces and the wheel will act as a harder wheel

This can be used in cut-off machines with variable speeds共see Sections 2.3.3 and2.3.8兲

The porosity, the voids deliberately built into the wheel共see Fig 2.2兲, are designed

to take the chips away, to avoid clogging the wheel edge共glazing兲, and to allow grains tocut efficiently

A dense structure has closely spaced, relatively small grains and small pores sothat only a small amount of material is removed An open structure with larger grains

and larger pores can cope with higher rates of material removal as described in Grain

Size above.

Bond Material

The bond material keeps the abrasive grains together In general, the bond must bestrong enough to withstand grinding forces, high temperatures, and centrifugal force.Consumable 共abradable兲 cut-off wheels most often have a phenolic 共bakelite兲

20 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN

bond It is produced by mixing abrasive grains with phenolic thermosetting resin andplasticizers, molding to shape and baking共curing兲 at 150–200°C 共300–400°F兲 Thebond hardness and porosity are varied by controlling the amount of plasticizer and byadding fillers.

Phenolics are also used for cut-off wheels of the slow consumable type, using CBNand diamond These wheels are soft compared to the metal bonded wheels共see below兲and will give a smooth cut on very hard materials, but the wheel wear will be relativelyhigh

Bakelite wheels are sensitive to prolonged exposure to cutting fluids The fluid ers the strength of the wheel so that it wears quicker; therefore cut-off wheels must bekept out of the fluid when not in use and stored in a dry place

low-Rubber bonds consist of vulcanized natural or synthetic rubber They are strongerthan phenolics and are often used for extra thin cut-off wheels Bakelite rubber bond is

a mixture giving a stronger bond than pure bakelite that allows for a thinner wheel Thedisadvantage with rubber as part of the bond is a strong smell, even with an efficientcooling during the cutting process

Metal bonds are used for CBN and diamond wheels The most common metalbond is sintered bronze that is produced by powder metallurgy methods Other metalbonds that are generally stronger include iron and nickel A low-cost diamond wheel ismade with the diamond grains fixed through an electroplating process Metal bondsand electroplating are used for slow consumable wheels共see Section 2.4.2兲

2.3.3 Grinding Mechanics

Abrasive cutting is a grinding process where the material removal takes place when theabrasive grains interact with the work piece The mechanics of the process highly influ-ence the result and the economy of the cut-off process; the most important parameterswill be discussed below

Grinding forces, power, and specific energy forces are developed between thewheel and the work piece共see Fig 2.3兲 The total force against the wheel, F, can be sepa-rated into a tangential component Ftand a normal共radial兲 component Fn.10–12

The grinding power P associated with the force components in Fig 2.3 can be ten as:

whereFtis the tangential force and v is the wheel velocity

An important parameter is the energy per unit volume of material removalcific energy兲, u

where d is down feed rate (feed speed), l is length of cut (arc of contact), and b is width ofcut (width of wheel)

The mean force per grain Ft⬘is another important parameter since it determines

the tendency to cause grain fracture and therefore plays a major role in relation towheel wear共self sharpening兲

Ft⬘= u · d · l · b/v · l · b · C =共d/v兲u/C 共3兲where C is the number of active cutting points per square mm/in of the wheel surface

Chapter 2 Sectioning 21

It can be seen from Eq共3兲 that the ratio 共d/v兲, feed speed, and wheel velocity plays amajor role At a higher force per grain, Ft⬘a given wheel should wear faster It can be

expected that Ft⬘ in a given cut-off operation will increase until the grain fracture

strength is reached, then the worn grain will either be sharpened共fractured兲 or forcedfrom the wheel共see Section 2.3.6兲

Chips, Sliding, and Plowing

Some of the energy used in the grinding process is used for creating chips These verysmall chips are comparable to chips made by other cutting processes such as turningand milling The grinding chips are irregular, probably because of the variation in abra-sive grains and the negative rake angles共see below and Section 6.2.1兲

Part of the energy is expended by mechanisms other than chip formation Onesuch mechanism could be flattened parts of the abrasive grain sliding against the workpiece surface without removing any material, as shown in Fig 2.4 Another part of theenergy will be used for plowing, only displacing the material without cutting共see Fig.6.3, Section 6.2兲

The high energy used for grinding compared to other cutting processes can be plained with the energy used for sliding and plowing The specific energy used forgrinding is virtually the same as the melting energy of the removed material.10

ex-2.3.4 Mechanical Damage

Abrasive cutting generates a surface with scratches that are produced by interaction ofabrasive cutting points with the work piece, as shown in Figs 2.4 and 2.5 Both figuresshow chips being removed from the surface of the work piece Both the making of chipsand the plowing will create deformation in the specimen surface共see Section 6.3兲 andthe depth of the deformed layer will depend on the material, cut-off wheel, feed speed,and other factors

According to the literature7,13–17the general deformation depth at wet abrasivecutting will be so that it is easily removed by plane grinding with grit 220 SiC grindingpaper For annealed polycrystalline 70:30, brass, the damage depth has been measured

to 700␮m 共maximum depth Dd, see Section 6.3兲 and significant deformation depth to

16␮m.7For carbon steel the damage depth has been measured to 125␮m and for trolytic copper 250␮m.14In case of precision cutting with very thin wheels, a low force

elec-on the wheel and lower cutting speeds, the damage is lower, in the range of 50␮m.14

For annealed steel共AISI/SAE 4130兲 deformation of below 10␮m has been measured atconventional wet cutting and less than 2␮m at precision cutting.16

Very often the unplaneness of the cut surface will be in the range of 300– 500␮m

Fig 2.4—Schematic drawing of an abrasive grain producing a chip from a metal work piece.

22 METALLOGRAPHIC AND MATERIALOGRAPHIC SPECIMEN