Table Of Content

Heat Treatment Heat Treatment Principles and Techniques SECOND EDITION T.V. Rajan, C.P. Sharma and Ashok Sharma Department of Metallurgical and Materials Engineering Malaviya National Institute of Technology Jaipur New Delhi-110001 2011 HEAT TREATMENT: Principles and Techniques, Second Edition T.V. Rajan, C.P. Sharma, and Ashok Sharma © 2011 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-4095-4 The export rights of this book are vested solely with the publisher. Twenty-first Printing (Second Edition) ººººº ººººº January, 2011 Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Rajkamal Electric Press, Plot No. 2, Phase IV, HSIDC, Kundli-131028, Sonepat, Haryana. CCCCCooooonnnnnttttteeeeennnnntttttsssss Foreword xiii Preface xv Preface to the First Edition xvii 1. Introduction 1–5 Heat Treatment Process Variables 3 Questions 5 2. Nature of Metals and Alloys 6–28 Introduction 6 2.1 Crystal Structure 7 2.1.1 Cubic Crystal Structure 7 2.1.2 Hexagonal Close Packed Crystal Structure 8 2.2 Properties of Metals and Alloys—Role of Heat Treatment 8 2.3 Nature of Alloys 9 2.3.l When Two Metals show Complete Solid Solubility 12 2.3.2 When Two Metals have no Solid Solubility in Each Other 12 2.3.3 When Two Metals are Soluble to Limited Extent in Solid State 13 2.4 Phase Diagrams 13 2.4.l Isomorphous System 14 2.4.2 Eutectic System 14 2.4.3 Eutectoid System 16 2.4.4 Peritectic System 16 2.4.5 Phase Diagram Showing the Presence of Intermetallic Compounds 16 2.5 Metallography 17 2.5.1 Macroexamination 17 2.5.2 Microexamination 19 2.6 Metallurgical Microscope 20 2.7 Electron Microscope 23 2.8 Imperfections in Metal Crystals 24 2.8.1 Point Defects 24 2.8.2 Line Defects 26 2.8.3 Surface Defects 27 2.8.4 Volume Defects 28 2.9 Significance of Imperfections in Heat Treatment 28 Questions 28 v vi Contents 3. Iron-Cementite Phase Diagram 29–48 Introduction 29 3.1 Allotropic Changes in Iron 29 3.2 Transformation Temperatures: Notation 30 3.3 Recalescence 31 3.4 Iron-cementite Phase Diagram 31 3.4.1 Steels 32 3.4.2 Cast Irons 35 3.4.3 Lever Rule 38 3.4.4 Iron-Graphite Phase Diagram 40 3.4.5 Effect of Alloying Elements on the Fe-Fe C Phase Diagram 42 3 3.5 Classification of Steels 46 Questions 48 4. Principles of Heat Treatment of Steels 49–85 Introduction 49 4.1 Formation of Austenite on Heating 49 4.1.1 Kinetics of Formation of Austenite 52 4.2 Austenitic Grain Size 55 4.3 Determination of Austenitic Grain Size 58 4.4 Importance of Austenitic Grain Size 59 4.5 Decomposition of Austenite 60 4.6 Time-Temperature Transformation Curves 61 4.6.1 Effect of Alloying Elements on TTT Diagram 65 4.6.2 Continuous Cooling Transformation 66 4.7 Pearlitic Transformation 68 4.7.1 Mechanism of Transformation 68 4.7.2 Kinetics of Transformation 70 4.7.3 Effect of Alloying Elements on Transformation 71 4.7.4 Interlamellar Spacing 71 4.8 Bainitic Transformation 72 4.8.1 Mechanism of Bainitic Transformation 72 4.8.2 Bainitic Structures 73 4.9 Martensitic Transformation 76 4.9.1 Mechanism of Transformation 76 4.9.2 Kinetics of Transformation 77 4.9.3 M-M Temperatures 78 s f 4.9.4 Effect of Applied Stress on Transformation 79 4.9.5 Athermal and Isothermal Martensites 79 4.9.6 Reversibility of Transformation 80 4.9.7 Habit Planes 80 4.9.8 Bain Distortion Model 80 4.9.9 Hardness of Martensite 81 4.10 Retained Austenite 83 Questions 84 Contents vii 5. Heat Treatment Processes for Steels 86–107 Introduction 86 5.1 Stress Relieving 86 5.2 Annealing 87 5.2.1 Full Annealing 88 5.2.2 Isothermal Annealing 89 5.2.3 Diffusion Annealing 90 5.2.4 Partial Annealing 90 5.2.5 Recrystallization Annealing 91 5.2.6 Process Annealing 91 5.3 Spheroidizing 92 5.4 Normalizing 94 5.4.1 Normalizing Versus Annealing 95 5.5 Hardening 95 5.5.1 Factors Affecting Hardening Process 97 5.5.2 Hardening Methods 98 5.6 Tempering 100 5.6.1 Structural Changes during Tempering 100 5.6.2 Effect of Alloying Elements on Tempering 101 5.6.3 Temper Brittleness 102 5.6.4 Temper Colours 103 5.7 Austempering 104 5.8 Martempering 105 5.9 Sub-zero treatment 105 5.10 Patenting 106 Questions 107 6. Hardenability 108–122 Introduction 108 6.1 Significance of Hardenability 109 6.2 Relationship of Hardenability with Transformation Rates 109 6.3 Determination of Hardenability 111 6.3.1 Grossman’s Critical Diameter Method 111 6.3.2 Jominy End Quench Test 113 6.3.3 Estimation of Hardenability from Chemical Composition 117 6.3.4 Fracture Test 118 6.4 Factors Influencing Hardenability 118 6.4.1 Effect of Austenite Grain Size 118 6.4.2 Effect of Carbon Content on Hardenability 119 6.4.3 Effect of Alloying Elements 119 Questions 122 7. Quenchants 123–129 Introduction 123 7.1 Removal of Heat during Quenching 123 7.2 Characteristics of Quenchants 124 viii Contents 7.3 Quenching Media 125 7.3.1 Water 125 7.3.2 Aqueous Solutions 126 7.3.3 Oil 127 7.3.4 Air 128 7.3.5 Gases 128 7.3.6 Salt Baths 128 7.3.7 Synthetic Quenchants 129 Questions 129 8. Chemical Heat Treatment of Steels 130–144 Introduction 130 8.1 Carburizing 130 8.1.1 Pack Carburizing 131 8.1.2 Liquid Carburizing 132 8.1.3 Gas Carburizing 132 8.2 Vacuum Carburizing 133 8.3 Post-carburizing Heat Treatments 135 8.4 Cyaniding and Carbonitriding 137 8.4.1 Cyaniding 137 8.4.2 Carbonitriding 137 8.5 Nitriding 138 8.6 Plasma Nitriding (Ion Nitriding) 139 8.7 Salt Bath Nitrocarburizing 140 8.8 Boronizing 141 8.9 Chromizing 142 8.10 Toyota Diffusion (TD) Process 143 Questions 144 9. Surface Hardening 145–153 Introduction 145 9.1 Flame Hardening 145 9.2 Induction Hardening 146 9.3 Electron Beam Hardening 148 9.4 Laser Hardening 148 9.5 Case Depth Measurements in Steels 151 9.5.1 Hardness Method 152 9.5.2 Chemical Method 153 9.5.3 Macrostructure Method 153 9.5.4 Microscopic Method 153 Questions 153 10. Thermomechanical Treatment 154–163 Introduction 154 10.1 Classification 155 10.2 Controlled Rolling 156 Contents ix 10.3 Hot-cold Working 156 10.4 Ausforming 157 10.5 Isoforming 158 10.6 Marstraining 158 10.7 Cryoforming 159 10.8 Preliminary Thermomechanical Treatment 159 10.9 Thermomechanical Annealing 160 10.10 Thermomechanical Treatment of Non-ferrous Alloys 161 Questions 163 11. Heat Treatment Furnaces and Atmospheres 164–181 Introduction 164 11.1 Heat Treatment Furnaces 164 11.2 Classification of Heat Treatment Furnaces 164 11.3 Batch Furnaces 166 11.4 Continuous Furnaces 169 11.5 Salt Bath Furnaces 170 11.6 Controlled Atmospheres 171 11.6.1 Chemistry of Controlled Atmosphere Processes 172 11.6.2 Commercially Available Atmospheres 178 11.6.3 Control of Furnace Atmospheres 180 Questions 180 12. Temperature Measurement and Control 182–193 Introduction 182 12.1 Concept of Temperature 182 12.2 Measurement of Temperature 183 12.3 Thermometers 183 12.4 Thermo-couples 185 12.4.1 Choice of Thermo-couple Materials 185 12.4.2 Thermo-couple Materials 186 12.4.3 Temperature Measurement by Thermocouples 188 12.4.4 Calibration of Thermo-couples 188 12.5 Indirect Methods of Temperature Measurement 189 12.5.1 Optional Pyrometer 190 12.5.2 Radiation Pyrometer 191 12.6 Temperature Control 192 Questions 193 13. Possible Defects, Causes and Remedies in Heat Treatment 194–208 Introduction 194 13.1 Low Hardness and Strength after Hardening 194 13.2 Soft Spots 195 13.3 Oxidation and Decarburization 196 x Contents 13.4 Overheating and Burning of Steel 197 13.4.1 Detection of Overheating and Burning 198 13.4.2 Factors Responsible for Overheating and Burning 199 13.5 Quench Cracks 199 13.6 Distortion and Warping 200 13.6.1 Control of Distortion 201 13.6.2 Methods to Reduce Distortion During Main Heat Treatment 202 13.6.3 Treatments for Stabilizing Dimensions 204 Questions 208 14. Heat Treatment of Commercial Steels 209–254 Introduction 209 14.1 Plain Carbon Steels 209 14.1.1 Low Carbon Steels 209 14.1.2 Medium Carbon Steels 211 14.1.3 High Carbon Steels 211 14.2 Alloy Steels 213 14.2.1 Manganese Steels 213 14.2.2 Silicon Steels 215 14.2.3 Chromium Steels 217 14.2.4 Nickel Steels 221 14.2.5 Molybdenum Steels 222 14.2.6 Chromium-Molybdenum Steels 223 14.2.7 Nickel-Molybdenum Steels 224 14.2.8 Nickel-Chromium Steels 225 14.2.9 Nickel-Chromium-Molybdenum Steels 226 14.3 Structural and Tool Steels 230 14.3.1 Hot-rolled Steels 230 14.3.2 Cold-drawn and Cold-rolled Steels 230 14.3.3 Structural Steel Castings 231 14.3.4 Water Hardening Tool Steels 233 14.3.5 Shock-resisting Tool Steels 233 14.3.6 Cold Work Tool Steels 234 14.3.7 Hot Work Tool Steels 236 14.3.8 High Speed Steels 239 14.3.9 Stainless Steels 242 14.3.10 Maraging Steels 247 14.3.11 Spring Steels 248 14.3.12 Valve Steels 249 14.3.13 Heat Resisting Steels 250 Questions 253 15. Cast Irons and Their Heat Treatment 255–276 Introduction 255 15.1 Grey Cast Iron 255 15.2 White Cast Iron 259