1. BRIEF INTRODUCTION TO “PRACTICAL CARBURISING TECHNOLOGY GAS CARBURISATION
AND MATERIAL STRENGTHENING METHODS”
THIRD EDITION
BY
Dr TAKESHI NAITO
THE COMPILATION OF THE “THIRD EDITION “
WAS SUPERVISED
PRATAP GHORPADE
SYNOPSIS
This book (Third Edition) about carburisation and Heat treatment was written by the author Dr
Takeshi Naito over 30 years back.
That is why the heat-treatment experts recognise this book as the “Bible of the carburization
technology.”
This is the up dated Third edition of the original publication. (It is available in English version.)
The book begins with surface hardening methods using carburization operation; the heat-treatment
of the carburizing of steel,
It leads to heat-treatment furnace design, different types and methods of carburising and theory of
carburisation.
The book covers the inspection of carburized parts, failure analysis and analytical techniques for
gears and the strengthening methods of materials by the carburizing heat-treatment.
It also covers maintenance and safety features for furnaces and gas generators.
The author primarily wrote this book for the engineers in the shop floor.
We hope this book will help in understanding the carburising process a little better.

2. FOREWORD
Dr. Ryohei Tanaka: Professor Emeritus of Tokyo Institute of Technology
He said the feeling of the end of the 20th
century is running deep. The book that Dr. Naito wrote
twenty years ago has been completely revised. In thoroughly incorporating the new technology
trends since the book was originally written, the generalizations of the foundations and facts of 20th
century carburization technology are written. In publishing this second edition, I have reason to
highly recommend this to engineers and researchers in related fields, in addition to complimenting
Dr. Naito’s work.
January 1999 (Second Edition)
ACKNOWLEDGEMENT
On the Preparation of this Text
The third edition of this book was originally published in Japanese by The Nikkan Kogyo Shimbun, LTD
in 1999. This English edition was published, and intended to be used as teaching materials for the
author’s lectures and seminars. This publication is now out of print at The Nikkan Kogyo Shimbun,
LTD.
The Japanese edition is now published from a company, “Speak man” bookstore.
(SPEAKMAN.CO.JP) In addition, this book is self-publishing.
I have received tremendous cooperation and supervision in the preparation of this text from
Mr Pratap Ghorpade of Bangalore, INDIA. I wish to record my appreciation and thank him from the
bottom of my heart..
March 1, 2013

3. Author and Supervising Editors Profile
Dr. Takeshi Naito (Author
Dr. Takeshi Naito graduated in Metallurgical Engineering course of IWATE University Japan in
1959. After graduation, he joined Komatsu Manufacturing Company and continued his education,
receiving his Doctorate in Engineering from Tokyo Institute of Technology, Tokyo Japan in 1972.
He served as Plant Manager and Chief Metallurgist of Komatsu Research Centre before retiring
in 1986. After retiring from Komatsu Co., Ltd, he joined Dowa Mining Co., Ltd as Division Director and
finally he became a Supreme Engineer of Dowa Mining Co., Ltd.
He has authored numerous technical papers, books and patents. He has also received the “Arch T.
Colwell Merit Award” in 1983 by Society of Automotive Engineer (SAE) USA. He has also received
the honourable “Hayashi Award” in 1995 for the article on “Study on Carburizing” by The Japan
Society for Heat-Treatment.
He is a member of The Japan Society for Heat-Treatment, and also a honourable member of
Society of Shot Peening Technology of Japan. He was former member of The Iron and Steel Institute
of Japan and ASM International.
He retired from Dowa Mining Co., Ltd on reaching retirement age. Now he is active as a Technical
Adviser.
Address:
Editors18-34 Sodegahama Hiratuka-City Kanagawa JAPAN 254-0813
Tel. & Fax, : +81-463-22-1552, e-mail : naito36@mb.scn-net.ne.jp

4. Pratap Ghorpade (Supervising Editor)
Pratap Ghorpade did his schooling at Clarence High School in Bangalore, India. He graduated with a
B.Tech (Hons) degree in Metallurgical Engineering from the prestigious Indian Institute of Technology
(IIT) , Kharagpur, India.
He has worked in the Manufacturing and Engineering sectors in various Companies and held
positions in Production, Quality Assurance, Design Engineering, Metallurgy and Sales & Marketing
.His expertise is in the field of Metallurgy and Heat Treatment of Metals. He is currently a Consultant
Advisor and Director -Sales and Technology at Hightemp Furnaces Ltd, India (A subsidiary of Dowa
Thermotech Co., Ltd, Japan)
He has travelled extensively on assignments in the course of his work to USA, Canada, UK, France, Italy,
Germany, Singapore, Malaysia, Thailand, China and Japan.
He is a member of the American Society of Metals (ASM International) and former member of the Heat
Treating Society.
He has authored several papers mainly in the areas of Heat Treatment of Metals.
He was actively involved in bringing in new technologies, collaborations and license agreements to
Hightemp Furnaces from various parts of the world, such as the:
“Nitrotec” surface engineering process from TTI, Ltd, UK;
;” Intensive Quench Technology” from IQT International, USA;
“Continuous Atmosphere Aluminium Brazing (CAAB)” Equipment and process from AFC-Holcroft, USA;
“Rapid Tempering” of Steels Equipment and process from Pyromaitre , Canada, “Austempering of
Ductile Iron” (ADI) process from Applied Process, USA
“Vacuum Heat Treatment and Brazing Furnaces and processes” from VacAero International, Canada
He can be contacted at pratap.ghorpade@hightemp-furnaces.com and pratapg@usa.net

5. ___________________________________________________________________________
TABLE OF CONTENTS
Chapter 1 Carburizing Operation
1.1 Surface Hardening Methods Using Carburization
1.1.0 Carburization Treatment for Surface Hardness 1
1.1.1 History of Carburization 1
1.1.2 Types and Characteristics of Carburization 2
1.2 Gas Generating Furnaces 5
1.2.1 Gas Generating Furnaces 5
1.2.2 Raw Gases 5
1.2.3 Construction of Gas Generating Furnaces 6
1.2.4 Carrier Gas Processes8
1.2.5 Gas Reactions in Generating Furnaces 9
1.2.6 Properties of Generating Furnace Gases 11
1.2.7 Automatic Atmosphere Control in Generating Furnaces 19
1.3 Batch Furnace Gas Carburization 25
1.3.1 Types of Batch Gas Carburizing Furnaces 25
1.3.2 Reactions in the Carburizing Furnaces 26
1.3.3 Carbon Diffusion 28
1.3.4 Gas Equilibrium in Batch Gas Carburizing Furnaces 32
1.3.5 Operating Methods of Batch Gas Carburizing Furnaces 36
1.3.6 Automatic Atmosphere Control in Batch Furnaces 53
1.4 Continuous Gas Carburizing Furnaces 59
1.4.1 Summary 59
1.4.2 Construction of Continuous Gas Carburizing Furnaces 59
1.4.3 Carburizing Operation 62
1.5 Drip Feed Carburization 63
1.5.1 Drip Feed Carburization Background 63
1.5.2 Drip Feed Carburizing Furnaces 64
1.5.3 Construction of Drip Feed Carburizing Furnaces 65
1.5.4 Gases Which Are Used in Drip Feed Carburization 65
1.5.5 Carburizing Reaction and Carbon Potential 66
1.5.6 Atmosphere Control 68
1.5.7 Operational Illustration 70
1.5.8 Operational Warnings 74

6. 1.6 Vacuum Carburization 75
1.6.1 Vacuum Carburization (Low-Pressure Carburization) 75
1.6.2 Construction of Vacuum Carburizing Furnaces 75
1.6.3 Vacuum Carburization Theory 77
1.6.4 Vacuum Carburization Characteristics 79
1.6.5 Mechanical Properties of Vacuum-Carburized Work 82
1.7 High Carbon Carburization 86
1.7.1 High Carbon Carburization Background 87
1.7.2 High Carbon Carburization Thoughts 88
1.7.3 Formation of Carbide 89
1.7.4 Mechanical Characteristics of High Carbon Carburized Parts 94
1.7.5 Applied Parts 97
1.8 Plasma Carburization 97
1.8.1 Complications of Plasma Carburization Growth 97
1.8.2 Plasma Carburization Theory 98
1.8.3 Plasma Carburizing Furnaces 99
1.9 Direct Carburization 99
1.9.1 Direct Carburization Development Complications 99
1.9.2 Special Characteristics of the Direct Carburizing Method 99
1.9.3 Problems with the Direct Carburizing Method 100
1.9.4 Comparison testing Between Endothermic Carburization and Direct Carburization
100
1.9.5 Direct Carburization Mechanisms 113
1.9.6 Development of Direct Carburization and Remaining Problems 114
Chapter 2 Heat Treatment of Carburizing Steel
2.1 Steel for Carburization 118
2.1.1 Standards for Carburization Steel 123
2.1.2 Fundamental Properties of Raw Materials 124
2.2 Special Heat Treatment Characteristics of Carburization 135
2.2.1 Transformation Phenomenon of Carburized layer 135
2.2.2 Hardness in Carburization 139
2.2.3 Carburization Microstructure 143
2.2.4 Tempering Hardness 146
2.2.5 Direct Hardening, Re-Heating Hardening 150
2.2.6 First and Second Hardening 155

7. 2.3 Retained Austenite in Carburization 157
2.3.1 Advantages of Retained Austenite 157
2.3.2 Obstructions 159
2.3.3 Usage Conditions 159
2.3.4 Characteristics of Carbonitrided R 160
2.3.5 R Summary 164
2.4 Rapid Heat Treatment of Carburized Steel 165
2.4.1 Theory of Rapid Heat Treatment 165
2.4.2 Special Advantages of Rapid Heat Treatment 168
2.4.3 Rapid Heat Treatment Methods 169
2.4.4 Mechanical Characteristics of Rapid Heat Treatment Work 170
2.5 Intergranular Oxidation of Carburized Steel 172
2.5.1 Intergranular Oxidation 172
2.5.2 Alloy Elements in Surface Area 175
2.5.3 Intergranular Oxidation Hardness 175
2.5.4 Formation Mechanism of Intergranular Oxidation 176
2.5.5 Cooling Speed and Hardness 180
2.5.6 Mechanical Characteristics 181
2.5.7 Residual Stress 182
2.5.8 Study of Preventing Problems Due to Intergranular Oxidation 183
2.6 Heat Treatment of High Carbon Carburized Steel 187
2.6.1 Heat Treatment Theory 187
2.6.2 Heat Treatment Quality 189
2.6.3 Mechanical Characteristics 192
2.7 Tempering of Carburized Steel 193
2.7.1 Tempering Mechanism194
2.7.2 First Stage Research 194
2.7.3 Industrial Significance of Tempering 195
2.7.4 Special Characteristics of Low Temperature Tempering 196
2.7.5 Summary 201
Chapter 3 Heat Treatment Planning of Carburizing Steel
3.1 Heat Treatment Planning 204
3.1.1 Heat Treatment Planning Theory 204
3.1.2 Carburizing Steel selection 204
3.1.3 Matrix Hardness 204

8. 3.1.4 Carburization Hardness 205
3.1.5 Estimating Carburization Hardness 208
3.2 Heat Treatment Planning of Ring Gears 211
3.2.1 Theory of Heat Treatment Planning for Gears 211
3.2.2 Damage Classifications of Gears
3.2.3 Required Special Characteristics 212
3.2.4 Pitching and Spalling 212
3.2.5 Stress Distribution of Surface Contact 215
3.2.6 Formation Mechanism of Spalling and Its Countermeasures 217
3.2.7 Formation Mechanism of Pitting
3.2.8 Various Factors and Countermeasures of Pitting Damage 227
3.2.9 Bending Strength of Tooth Root 230
3.2.10 Summary 232
Chapter 4 Shot Peening Carburized Steel
4.1 Development of Shot Peening Technology 234
4.1.1 Development Complications 234
4.1.2 Development Background 234
4.2 Shot Peening Theory 235
4.3 Shot Peening Equipment Types and Characteristics 236
4.3.1 Wheel Type 236
4.3.2 Pressure Type 237
4.3.3 Suction Type
4.4 Shot Peening Conditions of Compressive Residual Stress Value 241
4.4.1 Projection Time 242
4.4.2 Distance Between the Nozzle and Treatment Materials 243
4.4.3 Shot Diameter and Hardness 243
4.4.4 Projection Angles of Residual stress Distribution 244
4.5 Surface Hardness of Carburized Steel From Shot Peening 247
4.6 Carburized Steel Structure After Shot Peening 247
4.7 Summary 249

9. Chapter 5 Durability of Carburized Steel
5.1 Carburized Steel Durability Transition 251
5.1.1 Carburization Technology Progress 251
5.1.2 Changes in Strength methods of Carburized Steel 252
5.2 Strengthening Carburized Steel by Shot Peening 252
5.2.1 Bending strength of Carburized Steel 253
5.2.2 Pitting Resistance Durability of Carburized steel 255
5.3 Wear Characteristics of Carburized Steel from Shot Peening 261
5.4 Bending and surface Fatigue of carburized Steel 262
5.4.1 Bending Durability of Carburized steel 262
5.4.2 Surface Durability of Carburized Steel 263
Chapter 6 Inspection of Carburized Steel
6.1 Inspection Purposes 267
6.2 Factors of Carburized Steel Durability 267
6.2.1 Carbon Content 268
6.2.2 Hardness 269
6.2.3 Structure 269
6.3 Retained Austenite Measurement Methods 270
6.3.1 Calculation Methods 270
6.3.2 Calculation Methods Using X-Ray Diffraction 270
6.4 Incomplete Hardening Due to Intergranular Oxidation 273
6.5 Durability Evaluation of Carburized Materials 274
6.5.1 Hardness274
6.5.2 Retained Austenite 274

10. 6.5.3 Relationship Between Retained Austenite, Hardness and Amount of Carbon Content
274
6.5.4 Tempering Temperature 276
6.5.5 X-Ray Half Width 276
6.6 Retained Stress Measurements 277
6.7 Abnormal Structure 277
6.8 Maintenance and Safety 277
6.8.1 Generators 277
6.8.2 Carburizing Furnaces 277
6.8.3 Oil Tank 278
APPENDIX
(Damaged Gear Analysis Techniques)
1. Investigation Target 279
2. Practical Viewpoint 279
3. Example of Investigation 281
4. Discussion 284