quenching theory and technology -...

Quenching Theoryand Technology

Second Edition

2010 by Taylor and Francis Group, LLC

Edited byBozidar LiscicHans M. Tensi

Lauralice C. F. CanaleGeorge E. Totten

Quenching Theoryand Technology

Second Edition

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Library of Congress Cataloging-in-Publication Data

Quenching theory and technology / Boidar Licic [et al.]. -- 2nd ed.p. cm.

Rev. ed. of: Theory and technology of quenching. 1993.Includes bibliographical references and index.ISBN 978-0-8493-9279-5 (hardcover : alk. paper)1. Metals--Quenching. I. Licic, B. (Boidar) II. International Federation for Heat Treatment and

Surface Engineering. III. Theory and technology of quenching. IV. Title.

TN672.Q46 2010671.36--dc22 2009042419

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This book is dedicated to all our colleagues who have supported both the development and

the writing of many portions of Theory and Technology of Quenching Technology,

2nd editionespecially the IFHTSE Quenching and Cooling Committee.

Special appreciation to our families for their sacrifi ce and continuing

support of our dedication to this technology, often at their expense.


vii

Contents

Foreword .........................................................................................................................................ixPreface to the Second Edition .......................................................................................................xiPreface to the First Edition ......................................................................................................... xiiiContributors ...................................................................................................................................xv

1. Hardening of Steels................................................................................................................1Lauralice C.F. Canale, and George E. Totten

2. Quenching of Aluminum Alloys ......................................................................................43Ralph T. Shuey and Murat Tiryakioglu

3. Quenching of Titanium Alloys .........................................................................................85Lemmy Meekisho, Xin Yao, and George E. Totten

4. Mechanical Properties of Ferrous and Nonferrous Alloys after Quenching .................................................................................................................. 105Heinz-Joachim Spies

5. Thermo- and Fluid-Dynamic Principles of Heat Transfer during Cooling ........... 129F. Mayinger

6. Heat Transfer during Cooling of Heated Metals with Vaporizable Liquids ......... 159R. Jeschar, E. Specht, and Chr. Khler

7. Wetting Kinematics ............................................................................................................ 179Hans M. Tensi

8. Wetting Kinetics and Quench Severity of Selected Vegetable Oils for Heat Treatment .............................................................................................................. 205K. Narayan Prabhu

9. Residual Stresses after Quenching .................................................................................229Volker Schulze, Otmar Vhringer, and E. Macherauch

10. Effect of Workpiece Surface Properties on Cooling Behavior .................................. 289F. Moreaux, G. Beck, and Pierre Archambault

11. Determination of Quenching Power of Various Fluids ............................................. 315Hans M. Tensi and Boidar Licic

12. Cooling Media and Their Properties .............................................................................. 359Wackaw Luty


viii Contents

13. Gas Quenching....................................................................................................................445Gabriela Belinato, Lauralice C.F. Canale, and George E. Totten

14. Techniques of Quenching .................................................................................................485Howard E. Boyer, Pierre Archambault, and F. Moreaux

15. Intensive Steel Quenching Methods ..............................................................................509Nikolai I. Kobasko

16. Prediction of Hardness Profi le in Workpiece Based on Characteristic Cooling Parameters and Material Behavior during Cooling .................................................... 569Hans M. Tensi and Boidar Licic

17. Simulation of Quenching .................................................................................................605Caner Simsir and C. Hakan Gr

Appendix A: Temperature Conversion Table ...................................................................... 669

Appendix B: Common Conversion Constants ..................................................................... 673

Appendix C: Equivalent Hardness Scale Conversion Table ............................................. 675

Appendix D: Water Quenching Data: 7075-T73 Aluminum Bar Probes ....................................................................683

Appendix E: Type 1 Polymer Quenchant Data: 7075-T73 Aluminum Bar Probes ..................................................................... 687

Appendix F: Type I Polymer Quench Data: 7075-T73 Aluminum Sheet Probes .................................................................. 689

Appendix G: Type I Polymer Quench Data: 2024-T851 Aluminum Sheet Probes .............................................................. 691

Index ............................................................................................................................................. 693


ix

Foreword

The fi rst edition of this book was the most tangible and outstanding result of an immense amount of detailed and long-term collaborative work directly undertaken, or inspired, by the International Federation for Heat Treatment and Surface Engineering (IFHTSE) Technical Committee (TC) Scientifi c and technological aspects of quenching. The TC was launched in May 1978, at the 7th Assembly of the Governing Council of the IFHTSE in Barcelona, Spain, following proposals that had been circulated in January; Boidar Licic was immediately elected as the chairman. With input originally from representatives of organizations in Germany, France, the Netherlands, Poland, Sweden, Switzerland, the United States, the USSR, and Yugoslavia, he was able to present his fi rst activities report in Detroit the following year, detailing working meetings held in the meantime.

Negotiations with publishers for the fi rst edition, published by Springer Verlag in 1992, were announced at the 16th Assembly of the IFHTSE held in Senlis, France, in 1987. The appointment of the three original editors was confi rmed at the 17th Assembly of the IFHTSE in Chicago in 1988.

Originally, from 1978, the TC considered

Laboratory methods for testing the quenching capacity of quenching oils Laboratory methods for testing the quenching capacity of polymer solutions Workshop methods for testing the quenching intensity of different quenchants in different conditions

Later it addressed the following:

Computerized spray and fog quenching Self-quenching Pressurized and streaming gas cooling in vacuum furnaces Programming and automatic control Waterair spray Fluidized beds Salt bath

Therefore, important and signifi cant as the fi rst edition of this book was, it is essential to remember that it was by no means the only outcome of the committees efforts. Many other examples may be cited:

Collaboration with the ASM Heat Treat Society Quenching Committee agreed to in Lisbon in 1989Industrial collaboration, for example, with Union Carbide, TU Munich, and the University of ZagrebISO 9950 Industrial quenching oilsDetermination of cooling characteristics: Nickel-alloy probe test method (published in 1995)


x Foreword

Numerous papers, initially in the English language, Hrterei Mitteilungen in German and Promyshlennaya teplotekhnika in RussianInput of papers and whole sessions to many seminars and conferencesand indirectly the launch of the series Quenching and control of distortion and Distortion engineeringand much of the inspiration and subject matter for the Modelling and simulation series

The IFHTSE has good reason to be grateful for, and very proud of, the dedicated work of many individuals committed to global activity. Among these many, Boidar Licic clearly stands out for his expert dedication to the subject, his talent for friendly but persuasive leadership, and his stamina and energy in steering this productive multinational collabo-ration for almost 30 years until, very recently, when IFHTSEs quenching studies were sep-arated into gas and liquid aspects. He ensured the productive cooperation with Hans M. Tensi and W. Luty as his coeditors, which resulted in the fi rst edition, representing Croatia, Germany, Polandand now four countries: Croatia, Germany, Brazil, and the United States in the second edition. The whole exercise is an excellent example of the continuing IFHTSE global network for knowledge transfer. In the context of quenching, the IFHTSE also gratefully acknowledges the services of Hartmut Beitz, then of Houghton Hildesheim, appointed as the secretary of its quenching committee in support of Licic and his collabo-rators as the necessary organizational and record-keeping work expanded.

Robert WoodIFHTSE


xi

Preface to the Second Edition

This book is a signifi cant revision of the fi rst edition, Theory and Technology of Quenching, Second Edition, which was originally edited by Profs. Boidar Licic, Hans M. Tensi, and Wacaw Luty and published nearly 20 years ago. Over this time, Prof. Luty has passed away. However, quenching processes continue to be among the most important in the heat treatment of metals and various important developments that have been made in this fi eld. Since there is no other book of this kind addressing this critically important topic, a decision was made by the International Federation for Heat Treatment and Surface Engineering (IFHTSE) to pursue the development and publication of a second edition.

In addition to revising and updating the original content of the fi rst edition, the objective of the second edition was to add a number of chapters addressing important technological developments and also the quench processing of aluminum and steel, although the pri-mary focus continues to be on the quenching of steel. These additional chapters include quenching of aluminum alloys, quenching of titanium alloys, wetting kinetics and quench severity of selected vegetable oils, gas quenching, and intensive quenching and simulation of quenching. Therefore, this new and revised book makes a substantial contribution to the general fi eld of the thermal processing of metals.

Two of the original editors from the fi rst edition, Profs. Bozidar Licic and Hans M. Tensi, with two new editors, Profs. Lauralice C.F. Canale and George E. Totten, have served as editors for this edition. We are deeply indebted to the contributing authors for their vital assistance in completing this project. We would also like to express our appreciation to members of the IFHTSE Quenching and Cooling Committee, formerly chaired by Prof. Boidar Licic, for their assistance and suggestions in developing this book. Our special appreciation is extended to Robert Wood, Secretariat of IFHTSE, for his constant encour-agement and enthusiasm during the development of both the fi rst and second editions. Most importantly, the encouragement of our families is particularly appreciated.

Boidar LicicHans M. Tensi

Lauralice C.F. CanaleGeorge E. Totten


xiii

Preface to the First Edition

Heat treatment of metallic alloys constitutes an important step within the production pro-cess. The heat treatment process itself is considered as a cycle of heating the workpieces to a predetermined temperature, keeping them at this temperature for the time period required, and cooling them to room temperature in an appropriate way.

The process of heating and keeping workpieces at the required temperature is nowa-days well mastered and mostly automatized. The process of cooling or quenching which determines actually the resulting properties, is handicapped with many physical and technical uncertainties. Good results can already be obtained predominantly by using empirically based practice. But increased demands on the properties of the products as well as demands ones safety and environment conditions of the quenching media require efforts to investigate the details of the quenching process and to transfer the results of the research to practical application.

Advances in the knowledge about quenching processes have been achieved by mod-ern applied thermodynamics especially by the heat and mass transfer researchers; fur-ther the application of computer technology was helpful to new approaches in quenching processes.

Special emphases has been given to:

The theory of heat transfer and heat exchange intensifi cation during quenching Wetting kinematics Residual stresses after quenching Determination of the quenching intensity Prediction of microstructural transformation and hardness distribution after quenching, the latter with some limitations.

The idea to write this book originated with the Technical Committee: Scientifi c and Technological Aspects of Quenching of the International Federation for Heat Treatment and Surface Engineering (IFHT).

While the development of quenching media is pushed on by the chemical industry, the development of quenching techniques lies with heat treatment equipment manufacturers. The above named Committee deals primarily with standardization of methods for testing the quenching intensity (cooling power) of different quenchants in laboratory and in prac-tical conditions, as well as with the upgrading of the theoretical explanation of different quenching phenomena.

As a consequence of the multidisciplinary approach of the very complicated process of quenching, 17 authors from 6 different countries, have contributed to this book. Only in this way we have been able to deal with this specifi c matter from many different aspects.


xv

Contributors

Pierre ArchambaultEcole des Mines de NancyInstitut Jean LamourNancy, France

G. BeckEcole des Mines de NancyInstitut Jean LamourNancy, France

Gabriela BelinatoDepartamento de Engenharia

de MateriaisAeronautica e AutomobilisticaEscola de Engenharia de Sao CarlosUniversidade de Sao PauloSao Paulo, Brasil

Howard E. Boyer

Formerly at Consulting Service Materials,Manufacturing Processes

Chagrin Falls, Ohio

Lauralice C.F. CanaleDepartamento de Engenharia

de MateriaisAeronautica e AutomobilisticaEscola de Engenharia de Sao CarlosUniversidade de Sao PauloSao Paulo, Brasil

C. Hakan GrMetallurgical and Materials Engineering

DepartmentMiddle East Technical UniversityAnkara, Turkey

H.P. Hougardy (now retired)Formerly at Max-Planck Institute fr

Eisenforschung GmbHDusseldorf, Germany

R. JescharInstitt fr EnergieverfahrenstechnikTechnische Universitt ClausthalClausthal-Zellerfeld, Germany

Nikolai I. KobaskoIQ Technologies, Inc.Akron, Ohio

Chr. KhlerInstitt fr EnergieverfahrenstechnikTechnische Universitt ClausthalClausthal-Zellerfeld, Germany

Boidar LicicDepartment of Material ScienceFaculty of Mechanical

EngineeringUniversity of ZagrebZagreb, Croatia

Wackaw Luty

Instytut Mechaniki PrecyzyjnejWarsaw, Poland

E. MacherauchInstitute fur Werkstoffkunde IUniversity of KarlsruheKarlsruhe, Germany

F. Mayinger (now retired)Lehrustuh fur ThermodynamikTechnische Universitat MunchenMunich, Germany

Lemmy MeekishoDepartment of Mechanical and Materials

EngineeringPortland State UniversityPortland, Oregon

Deceased


xvi Contributors

F. MoreauxEcole des Mines de NancyInstitut Jean LamourNancy, France

K. Narayan PrabhuDepartment of Metallurgical & Materials

EngineeringNational Institute of Technology,

KarnatakaMangalore, India

Volker SchulzeInstitut fr Werkstoffkunde I and Institut

fr ProduktionstechnikUniversitt Karlsruhe (TH)Karlsruhe, Germany

Ralph T. ShueyAlcoa Technical CenterAlcoa Center, Pennsylvania

Caner SimsirStiftung Institut fr WerkstofftechnikBremen, Germany

E. SpechtInstitt fr EnergieverfahrenstechnikTechnische Universitt ClausthalClausthal-Zellerfeld, Germany

Heinz-Joachim SpiesInstitute for Materials EngineeringTechnische Universitt Bergakademie

FreibergFreiberg, Germany

Hans M. TensiInstitute for Materials and Processing

SciencesTechnical University of MunichMunich, Germany

Murat TiryakiogluRobert Morris UniversityMoon Township, Pennsylvania

George E. TottenDepartment of Mechanical and Materials


Otmar VhringerInstitut fr Werkstoffkunde IUniversitt Karlsruhe (TH)Karlsruhe, Germany

Xin YaoDepartment of Mechanical and Materials



1

1Hardening of Steels*

Lauralice C.F. Canale, and George E. Totten

CONTENTS

1.1 Introduction ............................................................................................................................11.1.1 Steel Classifi cation .....................................................................................................2

1.2 Construction Iron Alloys ......................................................................................................21.3 Alloying Elements .................................................................................................................6

1.3.1 Austenite-Forming Elements ...................................................................................71.3.2 Ferrite-Forming Elements .........................................................................................71.3.3 Carbide-Forming Elements ......................................................................................7

1.3.3.1 Carbide Stabilizers ......................................................................................71.3.4 Multi-Alloyed Steels ..................................................................................................81.3.5 Effect of Alloying Elements on Eutectoid Concentration ....................................9

1.4 Kinetics of Transformation ...................................................................................................91.4.1 Principles .....................................................................................................................91.4.2 Microstructures of Steels ........................................................................................ 10

1.4.2.1 Types of Microstructure ........................................................................... 101.4.2.2 Ferrite and Pearlite .................................................................................... 111.4.2.3 Martensite .................................................................................................. 121.4.2.4 Bainite ......................................................................................................... 14

1.5 TTT Diagrams ...................................................................................................................... 151.5.1 Isothermal Diagrams ............................................................................................... 151.5.2 Continuum Cooling Transformation Diagrams .................................................. 16

1.6 Hardenability. ....................................................................................................................... 201.6.1 Grossmann Hardenability ......................................................................................231.6.2 Jominy Curves ..........................................................................................................25

1.7 Tempering ............................................................................................................................. 321.7.1 Tempering Reactions ...............................................................................................35

References ....................................................................................................................................... 39

1.1 Introduction

The properties of steels can be infl uenced over a wide range by changing the thermo-dynamic properties (e.g., the composition) by alloying elements or by suppression of the equilibrium states during cooling.

* This chapter was edited and revised from the fi rst edition. Originally, the author was Hans P. Hougardy and the chapter was titled Transformation of steels during cooling.


2 Quenching Theory and Technology

For steels with an austeniteferrite transformation, by varying the cooling rate from extremely slow to extremely fast, the yield strength can be changed from 200 (microstruc-ture of ferrite and carbide) to 2500 MPa (martensitic microstructure). Therefore, to obtain suffi cient predictability and reproducibility of the service performance of steel compo-nents, correct selection of the cooling rate during heat treatment is important. A survey on these correlations has been published [1].

1.1.1 Steel Classification

Before continuing the discussion on steel transformations, steel classifi cations will be briefl y summarized. Steels are commonly classifi ed as [2]

1. Plain carbon steelsThese steels are solid solutions of iron and small amounts of manganese, phosphorous, sulfur, and silicon. These steels may be further classifi ed as low-carbon (mild) steels, which typically contain

Hardening of Steels 3

In this diagram, the region of greatest interest (up to 2% Carbon) is that of austenite and the two-phase ferrite plus iron carbide region below it. Carbon content of more than 2% forms cast iron that will not be considered in this chapter. So, the FeC phase diagram could be simplifi ed and it is represented in Figure 1.2. Therefore, the simplifi ed Fe-C diagram show in Figure 1.2 is typically used.

For reference, solid solutions that are formed in steels are given names. For example, all of the solid solutions that are formed with gamma () iron as a solvent are called austenite and all the solid solutions that are formed with alpha () iron as the solvent are called ferrite [6].

TABLE 1.1

Common AISI Steel Designations

Series Designation Types and Classes

10XX Nonresulfurized carbon steel grades (plain carbon steel)11XX Resulfurized carbon steel grades (free-cutting carbon steel)13XX Manganese 1.75%20XX Nickel steels23XX Nickel 3.5%25XX Nickel 5.0%30XX Nickelchromium steelsa

31XX Nickel (1.25%), chromium (0.65% or 0.80%)33XX Nickel (3.5%), chromium (1.55%)40XX Molybdenum (0.25%)41XX Chromium (0.5%0.95%), molybdenum (0.12% or 0.20%)43XX Nickel (1.8%), chromium (0.5% or 0.8%), molybdenum (0.25%)a

46XX Nickel (1.55% or 1.80%), molybdenum (0.20% or 0.25%)47XX Nickel (1.05%), chromium (0.45%), molybdenum (0.25%)a

48XX Nickel (3.50%), molybdenum (0.25%)50XX Chromium (0.28% or 0.40%)51XX Chromium (0.8%, 0.9%, 0.95%, 1.0%, or 1.05%)5XXX Carbon (1.0%), chromium (0.5%, 1.0%, 1.45%)60XX Chromium vanadium steels61XX Chromium (0.8% or 0.95%), vanadium (0.1% or 0.15% min)70XX Heat-resisting casting alloys80XX Nickelchromemolybdenum steels86XX Nickel (0.55%), chromium (0.5% or 0.65%), molybdenum (0.20%)87XX Nickel (0.55%), chromium (0.50%), molybdenum (0.25%)90XX Siliconmanganese steels92XX Manganese (0.85%), silicon (2.00%)93XX Nickel (3.25%), chromium (1.20%), molybdenum (0.12%)94XX Manganese (1.00%), nickel (0.45%), chromium (0.4%), molybdenum (0.12%)97XX Nickel (0.55%), chromium (0.17%), molybdenum (0.2%)98XX Nickel (1.00%), chromium (0.8%), molybdenum (0.25%)a

Source: Adapted from Capudean, B., Carbon content, steel classifi cations and alloy steels, thefabricator.com, August 28, 2003, Internet: http://www.thefabricator.com/Metallurgy/Metallurgy_Article.cfm?ID = 685

a Stainless steels always contain high chromium content, often considerable amounts of nickel, and some-times contain molybdenum and other elements. Stainless steels are identifi ed by a three-digit number beginning with 2, 3, 4, or 5.



Austenite is a microstructural phase characterized by a face-centered cubic (FCC) iron crystal structure. It is the desired solid solution microstructure produced prior to the hardening. Ferrite is a nearly carbon-free solid solution of one or more elements in a body-centered cubic (BCC) arrangement in which alpha iron is the solvent. Fully fer-ritic steels are only obtained when the carbon content is very low [79]. These names are strictly names of structures and imply nothing about the composition or properties.

Cementite is another phase, which can be observed in Figure 1.2. It is a brittle compound of iron and carbon, which is known as iron carbide with the approximate chemical formula Fe3C, and is characterized by an orthorhombic crystal structure.

When it occurs as a phase in steel, the chemical composition will be affected by the pres-ence of manganese (Mn) and other carbide-forming elements. Pearlite is a microstructure characterized by a lamellar aggregate of ferrite and cementite. It is formed from austenite, and lamellae may be coarse or thin depending on the cooling rate and temperature range where they are formed (isothermal heat treatment) [79]. It is also possible to get ferrite and spherical cementite, but in this case it is not named pearlite.

Looking at Figure 1.2, there is an area labeled austenite. This area covers broad ranges of composition and temperature, confi rming the fact that austenite is distinctly the name of the structure and it is not related to one particular composition. Even alloy steels form austenite at elevated temperatures in the same way as formed for plain steels. The range of temperature between 723C and the GE

line, or the EF

line, is called the critical tem-

perature range, where two phases coexist, either austenite and ferrite (between 723C and the GE

line), or austenite and cementite (between 723C and the EF

line).

Roomtemperature Steel

Pearlite andferrite

Pearlite andcementite

Cast iron% C

723

0 0.4 0.8 1.3 3.0 4.3 5.56.67

1130

Austenitein liquid

Solidus

ACM +Fe3C

+Fe3C

Primaryaustenitebegins to solidify

Austentiteledeburite

andcementite

Cementiteand

ledeburite

Solidus

Austeniteto pearlite

Cementite,pearlite and transformedledeburite

+ L

L L +Fe3C

= Austenite = Ferrite

CM beginsto solidiffy

EAustenite solid solutionof carbon in gamma iron

1535

Liquidus

Liquidus

Solidus

Tempe

rature

(C)

Fe3C

CM = Cementite

FIGURE 1.1The ironcarbon (FeC) phase diagram representing the metastable equilibrium. This is a simplifi ed diagram. Delta iron transformation is not shown in this diagram. (Adapted from Colpaert, H., Micrografi a. Captulo 3. In Metalografi a dos Produtos Siderrgicos Comuns, 3rd ed., Ed. da Universidade de So Paulo, Edgard Blcher, So Paulo, 1974, pp. 121198.)



To become familiar with the ironcarbon diagram, transformation of a plain carbon steel will be described. Taking a piece of steel (0.8% C) and heating it until a temperature T2 will completely austenitize its microstructure, which means that transformation only occurs via the austenite structure. At a slow cooling rate, austenite will be transformed into coarse pearlite at 723C. This transformation is also called eutectoid transformation. Because of this, steel composition with 0.8% C is called the eutectoid composition.

Steel compositions having less than 0.8% C are called hypoeutectoids, and those with more than 0.8% C are named hypereutectoids.

For a hypoeutectoid composition like 0.4% C, a piece of steel will be austenitic at a tem-perature T1, as shown in Figure 1.2. At a slow cooling rate, austenitic transformation will begin when the temperature reaches the GE

line. At this point, ferrite is being formed from

FIGURE 1.2Simplifi ed FeC phase diagram related to the steel transformations. (Adapted from Colpaert, H., Micrografi a. Captulo 3. In Metalografi a dos Produtos Siderrgicos Comuns, 3rd ed., Ed. da Universidade de So Paulo, Edgard Blcher, So Paulo, 1974, pp. 121198.)

0

723

910

Ferrite

Ferrite

Austenite

PerlitePerlite Perlite

Cementite

Cementite

Austenite

1535

Liquid+Austenite

Austenite (Gamma)

G

E

T1

T3

T2

F

Tempe

rature

(C)

Liquidus

Solidus

0.4

Hypoeutectoic HypereutectoidEutectoid

0.8Carbon (%)

1.2 2.0



austenite. Between the GE

line and 723C, more and more ferrite is being formed while the remaining austenite is becoming richer in carbon, reaching 0.8% at 723C, also known as the eutectoid temperature. Just below this temperature, the remaining austenite transforms in coarse pearlite. At room temperature, the microstructure of hypoeutectoid steel will be a compound of ferrite and pearlite. The relative phase percentage between ferrite and pearlite are dependent on the steel carbon content and presence of element alloys in the steel.

In the case of hypereutectoid steels, for example 1.2% C, austenitization is completed at a temperature T3 (Figure 1.2). Under slow cooling, transformation will begin at the EF

line.

In this case, austenite begins the transformation to cementite (around the grain boundary) and the remaining austenite is losing carbon proportionally to that transformation. At 723C, the remaining austenite contains 0.8% C, and below the eutectoid temperature transforms into pearlite. At room temperature, hypereutectoid steels are formed by cementite as a net-work around pearlite. The relative phase percentage between cementite and pearlite will vary according to the carbon percentage and the presence of the alloying elements in the steel.

1.3 Alloying Elements

Alloying elements are added to steel to promote hardenability, strength, toughness, and machinability and are summarized in Table 1.2. Often the rule of thumb that is cited is chromium makes steel harder, and nickel and manganese make it tougher. However, the rule that chromium makes steel harder only applies to a stainless steel with 2% carbon and 12% chromium. For other steels, the effect of chromium may be more modest. Similarly, the rule that nickel and manganese make steel tougher best applies to Hadfi eld steel (13% manganese), and the effect on toughness is widely variable for other steel compositions.

TABLE 1.2

Alloying Element Effect on Steel Properties

Hardenability Strength Toughness Machinability

Chromiuma Carbona Nickela Sulfura

Boron Cobalt Calcium LeadCarbon Copper Cerium ManganeseManganese Chromium Chromium PhosphorousMolybdenum Manganese Magnesium SeleniumPhosphorous Molybdenum Molybdenum TelluriumTitanium Niobium Niobium

Nickel TantalumPhosphorous TelluriumSilicon VanadiumTantalum ZirconiumTungstenVanadium

Source: Adapted from Kopeliovich, D., Effect of alloying elements on steel properties, Internet: http://www.substech.com/dokuwiki/doku.php?id = effect_of_ alloying_elements_ on_steel_properties

a Indicates the element in the series exhibiting the greatest effect.



Therefore, what is important to understand is the ability of an alloying element to enhance the formation of a specifi c phase or to stabilize it.

Alloying elements may be classifi ed as austenite-forming, ferrite-forming, carbide-forming, or nitride-forming elements.

1.3.1 Austenite-Forming Elements

The most important austenite-forming elements are carbon, nickel, and manganese. If suf-fi cient quantities are used, steels that are austenitic even at room temperature may be produced. Austenite-forming elements perform by increasing the temperature at which austenite exists by raising the A4 temperature (the temperature of formation of austenite from the liquid phase) and decreasing the A3 temperature [10,11]. Typically, they possess the same crystal structure as the FCC austenite. In the case of Hadfi eld steel discussed above, the presence of carbon and manganese stabilize austenite formation.

1.3.2 Ferrite-Forming Elements

Ferrite-forming elements stabilize ferrite by decreasing the temperature range in which austenite exists by lowering the A4 temperature and increasing the A3 temperature. They function by reducing the solubility of carbon in austenite, which increases the concentra-tion of carbides in the steel. Ferrite-forming elements include chromium (Cr), tungsten (W), molybdenum (Mo), vanadium (V), aluminum (Al), and silicon (Si). Ferrite-forming elements have the same crystal structure as that of the BCC ferrite [10].

1.3.3 Carbide-Forming Elements

The presence of hard carbides increase steel hardness, strength, and wear resistance. Carbide-forming elements include (with increasing affi nity for carbon): chromium, tungsten, molybde-num, vanadium, titanium, niobium, tantalum, and zirconium. Some ferrite-forming elements may also promote carbide formation. Non-iron carbides that are formed include Cr7C3, W2C, VC, Mo2C as well as complex carbides such as Fe4W2C. High-speed and hot-work tool steels normally contain three types of carbides: M6C, M23C6, and MC. The letter M represents col-lectively all the metal atoms [11]. All carbide-forming elements are also nitride formers.

Alloying elements such as nickel, silicon, cobalt, aluminum, copper, and nitrogen do not form carbides. Therefore, they can only be present in steel as a solid solution with iron.

The ironcementite phase diagram can be changed remarkably by the addition of alloy-ing elements. Figure 1.3 shows that in alloyed steel, instead of cementite, a carbide (Fe, X)3C is the stable phase (X represents alloying elements). Instead of (Fe, X)3C, only M3C (M for metal) is typically written. At room temperature, an M3C carbide with only 0.7 wt.% Cr is a stable phase while Fe3C is metastable. With increasing alloy content, carbides such as M7C3, M23C6, MC, or M2C may precipitate.

1.3.3.1 Carbide Stabilizers

Carbide stability depends on the overall elemental composition of the steel and the partition coeffi cient governing the amount of the carbides present in the cementite and the matrix. The partition coeffi cient (K), the ratio of percent by weight of a given elemental carbide in cementite/matrix, for various alloying elements are provided in Table 1.3. These data show why chromium is most often the element of choice for use as a carbide stabilizer. Also, it is interesting that while manganese is a relatively poor carbide former, it is a relatively strong carbide stabilizer.



1.3.4 Multi-Alloyed Steels

Most steels contain at least three components. However, the interpretation of multicomponent phase diagrams is of rela-tively limited interest with respect to practical heat treat-ment since they represent equilibrium conditions only. One approach to examining the effect of alloying elements on phase transformation product formation during heat treatment is to apply the use of modifi ed Schaeffl er diagrams shown in Figure 1.4 [1214]. Although these diagrams are most commonly used in welding and for stainless steels, they are helpful in assess-ing the effects of multiple alloying elements on transforma-tion structures obtained after rapid or normal cooling in heat treatment also. However, it is important to note that Schaeffl er diagrams are not equilibrium phase diagrams.

In the modifi ed Schaeffl er diagram shown in Figure 1.4, the austenite formers are represented on the y-axis and the ferrite formers are represented on the x-axis. Although in the origi-nal Schaeffl er diagram, only Ni (austenite former) and Cr (fer-rite formers) were shown [12]; the modifi ed Schaeffl er diagram includes other elements by converting them to Ni equivalents and Cr equivalents by the following calculations [13]:

Ni equivalent ( ) Ni Co 30 ( C) 25 ( N) 0.5 ( Mn) 0.3 ( Cu)% = % + % + % + % + % + %

% = % + % + % + %

+ % + % + % + %

Cr equivalent ( ) Cr 2 ( Si) 1.5 ( Mo) 5 ( V)

5.5 ( Al) 1.75 ( Nb) 1.5 ( Ti) 0.75 ( W)

TABLE 1.3

Partition Coeffi cients (K) for Various Alloying Elements

Element K Element K

Al 0 W 2Cu 0 Mo 8P 0 Mn 11.4Si 0 Cr 28Co 0.2 Ti >28Ni 0.3 Nb >28

Ta >28

Source: Adapted from Anon, Infl uence of alloying ele-ments on steel micro-structure, Knowledge article from www.Keyto-Steel.com, 1949, August 16, 2008, Internet: http://steel.keytometals.com/Articles/Art50.htm

1100

Accm

Ac1e

Ac1b

Ac3

+M3C

+ +M3C

+M3C

+

1000

900

800

700

600

5000 0.2 0.4 0.6 0.8

Carbon content (wt.%)

Tempe

rature

(C)

1.0 1.2 1.4

FIGURE 1.3Section through a three-component equilibrium system FeMC with a low content of M schematically illus-trating the Ac-temperatures. (From Eisenhttenleute, V.J., Steel: A Handbook for Material Research and Engineering, Vol. 1, Springer-Verlag, Berlin, 1991, Verlag Stahlesien mbH, Dsseldorf, 1992.)



Figure 1.4 shows that increasing the chromium equivalent concentration stabilizes the formation of -ferrite. Increasing the nickel equivalent concentration stabilizes the formation of -ferrite and extends the fi eld of austenite formation.

1.3.5 Effect of Alloying Elements on Eutectoid Concentration

The addition of alloying elements to steel will vary the position of the A1, A3, and Acm boundaries and the eutectoid composition in the FeFe3C diagram as follows [10]:

1. All important alloying elements decrease the eutectoid carbon content. 2. Austenite-stabilizing elements manganese and nickel decrease A1. 3. Ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten

increase A1.

1.4 Kinetics of Transformation

1.4.1 Principles

When austenite is cooled, it becomes metastable and may undergo a diffusionless transfor-mation to a new phasemartensite. Diffusionless transformation is a thermally reversible process that does not occur by long-range diffusion of atoms. Instead, the atoms maintain

FIGURE 1.4Illustration of a modifi ed Schaeffl er diagram. (Adapted from Totten, G., Steel Heat Treatment: Metallurgy and Technologies, Taylor & Francis Group LLC, Boca Raton, FL, 2006.)

F+M

0

2

4

6

8

10

12

14

16

1820

22

24

26

28

2 4 6 8 10 12 14 16 18 20Cr equivalent (%)

Ni equ

ivalen

t (%)

22 24 26 28 30 32 34 36 38 40

B+M

Martensite

A+F

A+M

FNO

FN5

FN10

0%F FN

20

FN40

FN80

FN100 100%F

Ferrite

Austenite

M+F

M+A+ F



their relative position, and displacement of atoms in the crystal lattice occurs at less than interatomic distances with a distortion of the lattice. Since austenite is metastable, it may exist at temperatures below the transformation temperature and if given suffi cient time will transform isothermally to martensite.

If high cooling rates are used, the description of the transformation process must also include time instead of phase diagrams. Transformations and precipitations as described in Section 1.2 are controlled by the diffusion of iron, carbon, and alloying elements and are therefore time-dependent. Cooling the solid solution of austenite with increased cooling rate leads to an incomplete diffusion process. Therefore, transformation and precipitation temperatures are lowered, supersaturated solid solutions are formed, or diffusionless austenite occurs. Metastable phases, not present in the equilibrium state, may also occur [1]. During fast cooling, as opposed to the equilibrium state, small grains, small precipitations, and phases with high interfacial energy such as pearlite or bainite are formed. During industrial production of steel components, nonequilibrium states are produced by increased cooling rates.

1.4.2 Microstructures of Steels

1.4.2.1 Types of Microstructure

During very slow cooling (about 100 K per week) in unalloyed steels, austenite transforms to ferrite and spheroidized cementite, a microstructure that can also be produced by prolonged tempering below Ac1 (see Figure 1.5). This microstructure is close to the equi-librium. During industrial heat treatment process where cooling rates greater than 100 K per week are used, three groups of microstructures can be differentiated as shown in Figure 1.6:

1. Pearlite microstructures whose growth is primarily diffusion-controlled 2. Bainite microstructures whose growth is partly diffusion-controlled 3. Martensite microstructures whose growth is primarily diffusionless

The temperature range of the formation of these three groups depends on the chemical composition of the steel and the cooling rate. An excellent summary of microstructures in steel is provided in references [1518].

FIGURE 1.5Ferrite (white) and cementite (gray or black) in a SAE 52100 alloy steel for bearings (Fe1.05% C0.35% Mn1.45% Cr) etched with 4% picral. Original at 1000. (Courtesy of G.F. Vander Voort.)



In the subsequent discussion, isothermal transformation of low-alloyed steel after aus-tenitization is assumed. In high-alloy steels, carbides other than M3C or nonmetallic and intermetallic phases eutectoids other than pearlite may be formed but these transformations will not be discussed here.

1.4.2.2 Ferrite and Pearlite

As mentioned earlier, in unalloyed steel with eutectoid composition (carbon concentra-tion with the lowest temperature of the austenite range, Figure 1.2, corresponding to the concentration of point E), austenite transforms between about 700C and 600C by a simul-taneous precipitation of ferrite and cementite in a metastable, lamellar-like arrangement called pearlite (Figures 1.7 and 1.8) [6,19].

The interlamellar distance of pearlite decreases with decreasing transformation tem-perature. In most construction steels, pearlite has such a small interlamellar distance that ferrite and cementite plates cannot be resolved with optical light microscopy (Figure 1.9). An area with parallel cementite lamella or lamella grown from one central point is called a colony.

In hypoeutectoid steels, in general transformation to proeutectoide ferrite starts at the austenite grain boundaries. The thickening rate of this grain-boundary ferrite decreases with temperature. Therefore, the volume fraction of ferrite transformed at a temperature

20 m

FIGURE 1.7Pearlite: ferrite (white) and cementite (gray or black) in an arrangement of parallel plates. Etched 2% Nital. (Courtesy of Dr. R. Muoz.)

FIGURE 1.6Temperature range of the formation of microstructures in unalloyed steels. (From Hougardy, H., Umwandlung and Gefge unlegierter Sthle, eine Einfhrung, Verlag Stahleisen mbH, Dsseldorf, 1990.)0

100

200

300

400

Tempe

rature

(C) 500

600

700

Ferrite+pearlite Pearlite range

Bainite Bainite range

Martensite Martensite range



below Ac1 increases with decreasing grain size of austenite. At high volume fractions of ferrite, nucleation at austenite grain boundaries is not visible after complete transforma-tion (Figure 1.9). Steels with low carbon concentration may form ferrite in Widmannsttten-arrangement, see Figure 1.10. After precipitation of ferrite, with increasing transformation time, the retained austenite transforms into pearlite (Figure 1.9).

1.4.2.3 Martensite

If, by very high cooling rates, the solid solution of austenite is maintained down to tempera-tures of about 300C, a diffusionless transformation to martensite occurs [1]. In steels with carbon concentrations greater than 0.6 wt.%, this microstructure exhibits a mainly plate-like substructure (Figure 1.11) and in low-carbon steels a lath-like (Figure 1.12) substructure. In medium-carbon steels, both of these substructures of martensite arise. In specimens with

FIGURE 1.8Pearlite after a deep etching 4 min in 5% Nital. The micrograph from a scanning electron microscope shows the gray cementite plates. Ferrite between cementite plates is dissolved by etching.

5 m

FIGURE 1.9Proeutectoid ferrite (white) and pearlite (gray). Most of the pearlite colonies have a lamellar distance below resolution. 4140 alloy steel (Fe0.4% C0.9% Mn0.2% Si1% Cr0.2% Mo) austenitized at 843C (1550F), isother-mally transformed at 677C (1250F) for 45 min, and water quenched, to completely transform the austenite to ferrite and pearlite. Aqueous 10% sodium metabisulfi te etch (darkens pearlite uniformly and brings up ferrite grain boundaries). (Courtesy of G.F. Vander Voort.)

10 m



200 m

FIGURE 1.10Ferrite in Widmannsttten arrangement and pearlite. Steel with 0.45 wt.% C. Heat treatment: 1000C 15 min continuous cooling from 800C to 500C in 100 s.

5

FIGURE 1.11Microstructure of improperly carburized SAE 9310 alloy steel (Fe0.1% C0.55% Mn3.25% Ni1.2% Cr0.12% Mo) tint etched with Berahas reagent (100 mL water10 g Na2S2O33 g K2S2O5). Carburizing treatment: 954C (1750F)11 h, air cool. Note the complete coverage of the prior-austenite grain boundaries with proeutectoid cementite. Plate martensite was colored blue and brown, retained austenite is cream colored. Some bainite was observed. (Courtesy of G.F. Vander Voort.)

(a)

50 m 2 m

(b)

FIGURE 1.12(a) Lath martensite in a low-carbon steel attained by rapid cooling in a weld. (b) Micrograph from transmission electron microscope of the same structure exhibiting parallel and long lath martensite plates with high disloca-tion density. (Adapted from Totten, G., Steel Heat Treatment: Metallurgy and Technologies, Taylor & Francis Group LLC, Boca Raton, FL, 2006.)



100% martensite, it is diffi cult to make the microstructure visible by etching and to differen-tiate between plates and laths (Figure 1.13). The formation of martensite starts at a tempera-ture Ms. At a temperature T, less than Ms austenite transforms to a defi ned volume fraction of martensite, independent of time. If the Ms temperature is above about 250C or after temper-ing of martensite, carbides precipitate.

1.4.2.4 Bainite

In a temperature range between martensite and pearlite (see Figure 1.4), austenite transforms to bainite by a mechanism which is partly diffusion-controlled and partly diffusionless [1,20]. In upper bainite (Figure 1.14), transformation starts with formation of bainitic ferrite, which possesses a microstructure similar to that of lath martensite. The austenite that is retained between these ferrite lathes is enriched with carbon. During further transformation from that carbon-enriched austenite, cementite or other carbides precipitate. The fi nal microstructure is ferrite surrounded by carbides. At transformation temperatures at the lower end of the bainite range (Figure 1.6), ferrites and carbides are very fi ne and are diffi cult to resolve by optical light microscopy (Figure 1.14). Areas with parallel lathes of bainite are called packets.

In steels with carbon contents above 0.3 wt.%, austenite transforms to lower bainite. In this case, austenite transformation is partly diffusionless to a highly supersaturated ferrite, which is similar to plate martensite. Within this ferrite, with subsequent transformation, carbides precipitate. Differentiation between upper and lower bainite is nearly impossible by optical light microscopy. Therefore, bainite should be characterized as fi ne (Figure 1.15) or coarse (Figure 1.14) [21], corresponding to the mean distance of bainitic carbides, a parameter that can be correlated to mechanical properties [1].

FIGURE 1.13Martensite. Carbon steel with 0.45% C. Etched 2% Nital. (Courtesy Dr. R. Muoz.) 20 m

FIGURE 1.14Upper bainite aggregate of ferrite (white) and carbide (black). Steel with 0.17 wt.% C. Heat treatment: 1300C continuous cooling from 800C to 500C in 15 min.

100 m



1.5 TTT Diagrams

As discussed previously, microstructures like ferrite, coarse pearlite, and cementite are predictable in the FeC diagrams, and are associated with very slow cooling rates simulating equilibrium conditions.

However, martensite, bainite, and fi ne pearlite are nonequilibrium microstructures. The formation of these products and the proportions of each are dependent on the austeniti-zation conditions (which infl uence the austenite grain size and also the alloy elements content in solution), the time and temperature cooling history of the particular alloy, and composition of the alloy. The transformation products formed are typically illustrated with the use of transformation diagrams that show the temperaturetime dependence of the microstructure formation process for the alloy being studied. Two of the most com-monly used TTT (timetemperaturetransformation) diagrams or IT (isothermal trans-formation) and CCT (continuous cooling transformation) diagrams. These diagrams are affected by chemical composition and austenite grain size, which are factors that affect the rate of nucleation and rate of growth of pearlite, bainite, primary ferrite, and primary iron carbide. These diagrams always presuppose that the austenite has been formed by heating to a proper temperature as indicated by the ironcarbon diagram.

1.5.1 Isothermal Diagrams

IT diagrams are developed by heating small samples of steel to the temperature where austenite transformation structure is completely formed, that is, austenitizing tempera-ture, then rapidly cooling to a temperature (intermediate between the austenitizing and the Ms temperature), then holding for a fi xed period of time, and immediately cooling to 25C. Different hold times are used, and in each sample transformation products are determined. This is done repeatedly for different temperatures and then the tempera-ture dependence of the process is examined from the observations made at different temperatures until an IT diagram is constructed. Another technique for diagram con-struction is dilatometry where the length of the sample is recorded as a function of time at the transformation temperature. When the phase transformation occurs, the pattern of contraction or dilation is changed. A typical IT diagram for AISI 4130 steel is shown in Figure 1.16 [22].

Since austenite is only stable at elevated temperatures and with faster cooling martensite is formed, this diagram shows Ms line, corresponding to the temperature where martensite begins to transform from the austenite (if the cooling is fast enough to maintain austenite

FIGURE 1.15Fine bainite in a steel with 0.5% C, 1 wt.% Cr, and 0.25 wt.% Mo. Heat treatment: 850C 20 min/360C 1700 s/brine.

10 m



by this temperature). Temperature M50 means 50% of the austenite was transformed in martensite and M90 means the transformation was 90%.

The Ms temperatures of many steels have been determined experimentally and have been approximated using several empirical formulas as follows [14,15]:

sM ( F) 1000650x C 70x Mn 35x Ni 70x Cr 50x Mo. = % % % % %

sM ( C) 539 432x C 30.4x Mn 17.7x Ni 12.1x Cr 7.5x Mo. = % % % % %

All elemental concentrations are expressed in weight percent and assumes all of the carbides are dissolved in the austenite.

Lines As and Af are the critical temperature range, where austenitizing is partial. A, F, and C indicate austenite, ferrite, and cementite respectively.

IT diagrams can only read along the isotherms. This procedure is to be contrasted to that where the heat treatment involves the temperature changing with time.

1.5.2 Continuum Cooling Transformation Diagrams

CCT curves correlate the temperatures for each phase transformation, the amount of transformation product obtained for a given cooling rate with time, and the cooling rate necessary to obtain martensite. These correlations are obtained from CCT diagrams by using different cooling rate curves [23].

The critical cooling rate is the time required to avoid formation of pearlite for the par-ticular steel being quenched. As a general rule, a quenchant must produce a cooling rate equivalent to, or faster than, that rate indicated by the nose of the pearlite transformation curve to maximize the formation of martensite.

00.5 1 2 5 10 102 103 104 105 106

100

300

I-T diagram

A

Stable austenite

Austenite+

Ferrite

Austenite+Ferrite+CarbideFerrite+Carbide

M*50M*90

M*s

Time (s)

*Estimated temperature

1 min 1 h 1 day

50%

Tempe

rature

(C)

Hardn

ess H

RC

600

800

12182424283744

56

Unstableaustenite

FIGURE 1.16IT diagram for AISI 4130 steel. (Adapted from Tarney, E., Heat treatment of tool steels, Tooling & Production, May 2000, pp. 102104.)



If the temperaturetime cooling curves for the quenchant and the CCT curves of the steel are plotted on the same scale, then they may be superimposed to select the steel grade that will provide the desired microstructure and hardness for a given cooling condition [24]. This assumption is limited to round bars up to 100 mm diameter quenched in oil and round bars up to 150 mm quenched in water.

CCT diagrams may be constructed in various forms. Steel may also be continuously cooled at different specifi ed rates using a dilatometer and the proportion of transformation prod-ucts formed after cooling to various temperatures intermediate between the austenitizing temperature and the Ms temperature are used to construct a CCT diagram.

Figure 1.17 is a CCT diagram for an unalloyed carbon steel (AISI 1040), which provides curves for the beginning and ending of the different phase transformations [2527].

An alternative form of a CCT diagram is shown by Figure 1.18 [24,28]. This curve was not generated using a dilatometer but instead cooling curves were measured at different distances from the end of a Jominy test bar. The corresponding Jominy curve is shown along with a diagram for a particular quenchant and agitation condition that permits the prediction of cross-sectional hardness for a round bar [24,29].

There are a number of heat treatment processes where only the use of a CCT diagram is appropriate. These include continuous slow cooling processes such as normalizing annealing by cooling in air, direct quenching to obtain a fully martensitic structure, and continuous cooling processes resulting in mixed microstructures.

200

200

400

600

800

50% Ferrite505050

50F

Ac1

P50

5050

5030

10

8

486

85

90

70

2

2

1 10

HardnessDph

Rockwell

3

B

634

16,300

M

7,300 4,100 2,300 1,200 550 250 50 10 2.5 F/min

C 57 C 38 C 28C 28C 21 B 95 B 95 B 92 B 88 B 86374 287 284 242 215 215 199 178 170

102Cooling time (s)

SAE 1040 SteelComposition: 0.39% C0.72% Mn0.23% Si0.010% P0.0018% S, Grain size: 78

102 102 102

400

600

800

1,000

Tempe

rature

(F)

Tempe

rature

(C)

1,200

1,400

1,600AISI 1040

10% Barnite

50% Pearlite

0.39 C0.72 Mn0.23 Si0.018 S0.010 PAc1 = 1,342 FAc1 = 1,446 FGrain size, ASTM No. 78F-ferriteP-pearliteB-bainiteM-martensite

FIGURE 1.17CCT diagram for an unalloyed steel (AISI 1040). (From Totten, G., Steel Heat Treatment: Metallurgy and Technologies, Taylor & Francis Group LLC, Boca Raton, FL, 2006. With permission.)



A number of points should be noted:

The CCT diagram is only valid for the steel composition for which it was determined. It is NOT correct to assume that the area of intersection of a cooling curve with the transformation product is equivalent to the amount of product that is formed.Scheil has shown that transformation begins later in time for a continuous cooling process than for an isothermal process [24]. This is consistent with IT and CCT curve comparison.

FIGURE 1.18Experimentally determined CCT diagram (solid lines) for a DIN 42CrMo4 steel IT diagram is also shown. (From ASM. Properties and selectionIrons, steels and high-performance alloys, in ASM Handbook, Vol. 1, 10th ed., ASM, Materials Park, OH, 1990. With permission.)

0

60

50

30

A B C D

200 10 20 30 40

Distance from quenched end (mm)

Hardn

ess (HRC

)

50 60 70 80 90 100

40

100

200

300

400

Tempe

rature

(C)

500A B

Bainite formation

Austenitizing temperature=860C

Ferrite formationAc3 (0.25C/min)

Pearlite formation

= Hardness, HRC,after cooling toroom temperature

C D

600

700

800

1Time (s)

End-quench test

101

CCT diagram

Martensitestart

Ms

IT diagramCooling curves

102

53 52 52 37 33 31 28 27 20 17 16 12

10

103 104 105



Since increasing the austenitizing temperature will shift the curves to longer transfor-mation times, it is necessary to use CCT diagrams generated at the desired austenitizing temperature.

Steel chemical composition has a strong infl uence on IC and CCT diagrams. Austenitizing temperature and grain size will also modify the IC and CCT diagrams. Alloy element and its amount present in the steel will determine the ability to obtain martensite during fast cooling from the austenitizing temperature; it is called harden-ability. Figures 1.19 through 1.21 show CCT diagrams for three different steels. Figures 1.19 and 1.20 are CCT curves for SAE 1020 steel and SAE 1080 steel, respectively.

0

100

200

300100C/s

20C/s

10C/s

2C/s

400

500

Tempe

rature

(C) 600

700

800

900

Fs

F+P

F+MF+B+M F+

P+B+M

F+P+B

Ps

BsBs

Ms

Pf

Bf

1 10 102Time (s)

F = FerriteP = PearliteB = BainiteM = Martensite

103 104

FIGURE 1.19CCT curve for SAE 1020 steel. (Adapted from Askeland, D.R., The Science and Engineering of Materials, 4th ed., PWS Publishing Company, Boston, MA, 1989.)

0

100

200

300

400

500

Tempe

rature

(C)

600

700

800

0.1 1 10210Time (s)

103 104 105 106


Ms

PsPf

Mf

Martensite Pearlite +martensite

Finepearlite

Coarsepearlite

140C/s

40C/s

5C/s

FIGURE 1.20CCT curve (solid lines) and IT curve (dashed lines) both for SAE 1080 steel. (Adapted from Askeland, D.R., The Science and Engineering of Materials, 4th ed., PWS Publishing Company, Boston, MA, 1989.)



Figure 1.21 is a CCT curve for SAE 4340 steel. In this last type of steel, even for relatively low cooling rate it is possible to obtain martensite as microstructure. Among these steels, SAE 4340 has the highest hardenability [30].

1.6 Hardenability

Hardenability is the ability of the FeC alloy to be hardened by forming martensite. Hardenability is not hardness. It is a qualitative measure of the rate at which hardness decreases with distance from the surface because of decreased martensite content [7].

High hardenability means the ability of the alloy to produce a high martensite con-tent throughout the volume of specimen. The ability to achieve a certain hardness level is associated with the highest attainable hardness, which depends on the carbon content of the steel and more specifi cally on the amount of carbon dissolved in the austenite after austenitizing.

However, with increasing carbon concentration, martensitic transformation from aus-tenite becomes more diffi cult resulting in a greater tendency for retained austenite and correspondingly lower strength. Hardenability also refers to the hardness distribution within a cross-section from the surface to the core under specifi ed quenching conditions. It depends on the carbon content that is interstitially dissolved in austenite and the amount of alloying elements substitutionally dissolved in the austenite during austenitization. Also, increasing carbon content infl uences the Mf temperature relative to Ms during rapid cooling as shown in Figure 1.22 [31]. In this fi gure, it is evident that for steels with carbon content above 0.6%, the transformation of austenite to martensite will be incomplete if the cooling process is stopped at 0C or higher.

900

800

700

600

500

Tempe

rature

(C)

400

300

200

100

01 10 102 103

Time (s)104 105 106


Ms

B+M F+B+M

F+P

F+P+B+M

Bs

Fs

PsPf

Mf

M

8C/s

0.3C/s

0.02C/s

0.006C/s

FIGURE 1.21CCT curve for SAE 4340 steel. (Adapted from Askeland, D.R., The Science and Engineering of Materials, 4th ed., PWS Publishing Company, Boston, MA, 1989.)



The depth of hardening depends on the following factors:

Size and shape of the cross-section Hardenability of the material Quenching conditions

The cross-section shape exhibits a signifi cant infl uence on heat extraction during quenching and, therefore, on the hardening depth. Heat extraction is dependent on the surface area exposed to the quenchant.

The effect of steel composition on hardenability may be calculated in terms of the ideal critical diameter or DI, which is defi ned as the largest bar diameter that can be quenched to produce 50% martensite at the center after quenching in an ideal quench, that is, under infi nite quenching severity. The ideal quench is one that reduces surface tempera-ture of an austenitized steel to the bath temperature instantaneously. Under these condi-tions, the cooling rate at the center of the bar depends only on the thermal diffusivity of the steel [24].

The ideal critical diameter may be calculated from

= I I Base Mn Si Cr Mo V Cu Ni (carbon concentration and grain size) nD D f f f f f f f f

where fn is a multiplicative factor for the particular substitutionally dissolved alloying element. The base DI Base value and one set of alloying factors are provided in Table 1.4.

The ASTM grain size number (G), referred to in Table 1.4, is a grain size designation bearing a relationship to average intercept distance at 100 diameters magnifi cation according to the following equation:

= 210.00 2 logG L

where L = the average intercept distance at 100 diameters magnifi cation. The smaller the ASTM grain size, the larger the diameter of the grains.

Grain size also has infl uence in hardenability. Figure 1.23 shows this relationship.

00 0.2 0.4 0.6 0.8

Carbon weight (%)

Mf

Ms

1.0 1.2 1.4

100

200

300

Tempe

rature

(C) 400

500

600

FIGURE 1.22Infl uence of the carbon content in steels on the temperature of the start of martensite formation (Ms) and the end of martensite formation (Mf). (From Totten, G., Steel Heat Treatment: Metallurgy and Technologies, Taylor & Francis Group LLC, Boca Raton, FL, 2006. With permission.)



FIGURE 1.23Infl uence of grain size (ASTM number) in the hard-enability (DI). (Adapted from Thelning, K.-E., Hardenability, in Steel and Its Heat Treatment, 2nd ed., Chap. 4, Butterworths, London, U.K., 1984.)

0.160 0.2 0.4

Carbon (%)0.6 0.8

0.20

0.24

0.28

Ideal d

iameter

DI (in

.)

0.32

0.36

G =4

G =5

G =6

G =7

G =8

TABLE 1.4

Hardenability Factors for Carbon Content, Grain Size, and Selected Alloying Elements in Steel

Carbon Content (%)

Carbon Grain Size No. Alloying Element

6 7 8 Mn Si Ni Cr Mn

0.05 0.0814 0.0750 0.0697 1.167 1.035 1.018 1.1080 1.150.10 0.1153 0.1065 0.0995 1.333 1.070 1.036 1.2160 1.300.15 0.1413 0.1315 0.1212 1.500 1.105 1.055 1.3240 1.450.20 0.1623 0.1509 0.1400 1.667 1.140 1.073 1.4320 1.600.25 0.1820 0.1678 0.1560 1.833 1.175 1.091 1.54 1.750.30 0.1991 0.1849 0.1700 2.000 1.210 1.109 1.6480 1.900.35 0.2154 0.2000 0.1842 2.167 1.245 1.128 1.7560 2.050.40 0.2300 0.2130 0.1976 2.333 1.280 1.146 1.8640 2.200.45 0.2440 0.2259 0.2090 2.500 1.315 1.164 1.9720 2.350.50 0.2580 0.2380 0.2200 2.667 1.350 1.182 2.0800 2.500.55 0.273 0.251 0.231 2.833 1.385 1.201 2.1880 2.650.60 0.284 0.262 0.241 3.000 1.420 1.219 2.2960 2.800.65 0.295 0.273 0.251 3.167 1.455 1.237 2.4040 2.950.70 0.306 0.283 0.260 3.333 1.490 1.255 2.5120 3.100.75 0.316 0.293 0.270 3.500 1.525 1.273 2.62 3.250.80 0.326 0.303 0.278 3.667 1.560 1.291 2.7280 3.400.85 0.336 0.312 0.287 3.833 1.595 1.309 2.8360 3.550.90 0.346 0.321 0.296 4.000 1.630 1.321 2.9440 3.700.95 4.167 1.665 1.345 3.0520 1.00 4.333 1.700 1.364 3.1600



The effect of quenching conditions on the depth of hardening are not only dependent on the quenchant being used and its physical and chemical properties but also on the process parameters such as bath temperature and agitation.

There are numerous methods to estimate steel hardenability and the two most common are Grossmann hardenability and Jominy curve determination.

1.6.1 Grossmann Hardenability

Grossmanns method of measuring hardenability uses a number of cylindrical steel bars of different diameters hardened in a given quenching medium [32]. After sectioning each bar at mid-length and examining it metallographically, the bar that has 50% martensite at its center is selected, and the diameter of this bar is designated as the critical diameter Dcrit. Other bars with diameters smaller than Dcrit will have more martensite and correspond-ingly higher hardness values and bars with diameters larger than Dcrit will attain 50% martensite only up to a certain depth as shown in Figure 1.24 [7]. The Dcrit value is valid only for the quenching medium and conditions used to determine this value.

To determine the hardenability of a steel independent of the quenching medium, Grossmann introduced the term ideal critical diameter, DI, which is the diameter of a given steel bar that would produce 50% martensite at the center when quenched in a bath of quenching intensity H = . Here H = indicates a hypothetical quenching intensity that reduces the temperature of heated steel to the bath temperature in zero time.

To identify a quenching medium and its condition, Grossmann introduced the quenching intensity (severity) factor H. Table 1.5 provides a summary of Grossmann H-factors for different quench media and different quenching conditions [3]. Although this data has been published in numerous reference texts for many years, it is of relatively limited value. One of the most obvious reasons is that quenchant agitation is not adequately defi ned and is often unknown, yet it exhibits an enormous effect on quench severity.

There is a correlation between Dcrit and DI as shown in Figure 1.25. Once quench severity is known, this graph permits to convert Dcrit to DI and vice versa [33].

60

40

HRCcrit=50% MDcrit

Hardn

ess (HRC

)

20

0

80 60 50 40

FIGURE 1.24Determination of critical diameter Dcrit according to Grossmann. (From Spur, G. (Ed.), Handbuch der Fertigungstechnik, Band 4/2, Warmebehandeln, Carl Hanser, Munich, 1987, p. 1012. With permission.)



The Grossmann value H is based on the Biot (Bi) number, which interrelates the inter-facial heat transfer coeffi cient (), thermal conductivity (), and the radius (R) of the round bar being hardened:

= = /Bi R H D

/(2 )H =

FIGURE 1.25Chart correlating Dcrit, DI, and quench severity (H). (Adapted from Grossmann, M.A. and Asimow, M., The Iron Age, 36, 25, April 25, 1940.)

40DI (mm)

H factor

Dcrit (

mm)

60 70

0.01

0.10

0.20

0.40

10.0

5.0 2.0 1.00

0.80

5000

10

10

20

30

40

50

20 30

TABLE 1.5

Effect of Agitation on Quench Severity as Indicated by Grossmann Quench Severity Factors (H-Factors)

Agitation

Grossmann H-Factor

Oil Water Caustic Soda or Brine

None 0.250.3 0.91.0 2Mild 0.300.35 1.01.1 22.2Moderate 0.350.4 1.21.3 Good 0.40.5 1.41.5 Strong 0.50.8 1.62.0 Violent 0.81.1 4 5

Source: Lyman, T. and Boyer, H.E., Metals Handbook, Vol. 2: Heat Treating, Cleaning and Finishing, 8th ed., ASM, Materials Park, OH, 1964. With permission.



Since the Biot number is dimensionless, this expression means that the Grossmann value, H, is inversely proportional to the bar diameter. This method of numerically analyzing the quenching process presumes that heat transfer is a steady-state, linear (Newtonian) cooling process. However, this is seldom the case and almost never the case in vaporizable quenchants such as oil, water, and aqueous polymers. Therefore, a signifi cant error exists in the basic assumption of the method.

Another diffi culty is the determination of the H-value for a cross-section size other than one experimentally measured. In fact, H-values depend on cross-section size. Values of H do not account for specifi c quenching characteristics such as composi-tion, oil viscosity, or temperature of the quenching bath. Tables of H-values do not specify the agitation rate of the quenchant either uniformly or precisely (see Table 1.5). Therefore, although H-values are commonly used, more current and improved pro-cedures, such as those discussed in this book, ought to be used when possible. For example, cooling curve analyses and the various methods of cooling curve interpre-tation that have been reported [3,32] are all signifi cant improvements over the use of Grossmann Hardenability factors.

1.6.2 Jominy Curves

The Jominy bar end-quench test is the most familiar and commonly used procedure for measuring steel hardenability. This test has been standardized and is described in ASTM A 255, SAE J406, DIN 50191, and ISO 642. For this test, a 100 mm (4 in.) long by 25 mm (1 in.) diameter round bar is austenitized to the proper temperature, dropped into a fi xture, and one end rapidly quenched with 24C (75F) water from a 13 mm (0.5 in.) orifi ce under speci-fi ed conditions [34]. The austenitizing temperature is selected according to the specifi c steel alloy being studied; however, most steels are heated in the range of 870C900C (1600F1650F). Cooling velocity of the test bar decreases with increasing distance from the quenched end. After quenching, parallel fl ats are ground on opposite sides of the bar and hardness measurements made at 1/16 in. (1.6 mm) intervals along the bar as illustrated in Figure 1.26 [32].

Hardness as a function of distance from the quenched end is measured and plotted and, together with measurement of the relative areas of the martensite, bainite, and pearlite that is formed, it is possible to compare the hardenability of different steels using Jominy curves. As the slope of the Jominy curve increases, the ability to harden the steel (hard-enability) decreases. Conversely, decreasing slopes (or increasing fl atness) of the Jominy curve indicates increasing hardenability (ease of hardening).

Figure 1.27 illustrates that steel hardenability is dependent on the steel chemistry, unal-loyed steels exhibit poor hardenability, and that Jominy curves provide an excellent indi-cator of relative steel hardenability [30].

Quenchant selection for a particular steel is dependent on the hardenability of the steel being hardened. For through-hardened steels, this is relatively straightforward and pro-cedures such as Jominy hardenability characterization can usually be readily applied. However, hardenability determination of carburized steels is considerably more complex since the hardenability of the case is substantially different from the hardenability of the core. Identifying the optimal quenching parameters to achieve the desired hardness gra-dient of a carburized steel is signifi cantly more complicated than a typical through-hard-ening process since it is important to consider a wider range of quenching performance variables including: core hardness, surface hardness, depth of hardening (typically to a hardness of 550 HV), and the hardness gradient through the case.



As Jatczak [35] reported earlier, there are two methods that may be used to assess steel hardenability from the Jominy end-quench test. One method is to correlate Jominy end-quench hardness data with equivalent hardnesses of various quenched cross-section sizes. This correlation will produce the so-called Jominy equivalent hardness (Jeh).

Alternatively, it is possible to correlate the cooling rate at different positions (J-values) along the Jominy end-quench bar with cooling rates in the center of different cross-section

FIGURE 1.27Jominy curve comparison of the hardenability of dif-ferent steels, alloyed and unalloyed. (From Askeland, D.R., The Science and Engineering of Materials, 4th ed., PWS Publishing Company, Boston, MA, 1989. With permission.)

0

20

30

40

Rockwell C

hardn

ess 50

60

70

10 20 30

4340

864093101080

4320

1050

Jominy distance (1/16th of an inch)

3.0 in.2.01.0

00

10

20

30

40

Hardn

ess (HRC

)

50

60

270 70Cooling rates

Distance from quenched end, in.

18 5.6 K/s

4891/16 8/16

4/16 16/16

124 32.3 10F/s

25 50 75 mmDistance from quenched end

FIGURE 1.26Measuring hardness on the Jominy test specimen and plotting hardenability curves. (From Krauss, G., Steels: Heat Treatment and Processing Principles, ASM International, Materials Park, OH, 1990. With permission.)



sizes of the steel alloy used to determine the Jominy curve. Experimentally, the proce-dure described by Jaczak [35] and Davis [36] is performed by determining the Jominy end-quench curve (hardness versus distance from the quenched end). Then test bar of the steel alloy of interest is normalized, austenitized at the specifi ed temperature for the alloy of interest, quenched, and the hardness from the quenched end (J = 0) is determined in the manner specifi ed by ASTM A255.

For comparison, a test bar of the same heat is then normalized, austenitized, and then cooled by immersion into the quenchant of interest. After quenching, the bar is sectioned and the hardness versus position from the surface to the core is determined. The mea-sured hardness values from the cross-sectional hardness survey after immersion into the quenching media of interest are compared with the location (J-value) on the Jominy bar of the same heat of steel that will produce the same hardness.

The Jominy distance obtained in this way is used to obtain the cooling rate (at 700C) at the position on the Jominy end-quench bar that will produce the same hardness as obtained at the position of interest in the bar that is immersion quenched. To obtain this cooling rate, a Jominy bar is instrumented with thermocouples at different positions from the quenched end and the cooling rate at 700C is obtained at different J-values (distances). A table or reference curve is then constructed of cooling rate at 700C versus J-value. Reference data used for the work reported here was previously published by Luty [37]. Similar data has been published by Krauss [38]. Note: by defi nition, Jominy end-quench curves are obtained using water at a temperature of 5C30C (40F85F) as the end-quenching medium. The bar that is immersion quenched is cooled into the quenching medium of interest. The assumption is that the distance on the end-quenched bar (water) that produces the same hardness as obtained with the immersion quench must have experienced the same cooling rate to achieve the same hardness. The cooling rate at 700C obtained from Jominy bar data to produce the Jominy equivalent hardness is known as the Jominy equivalent cooling rate (V700) or Jominy equivalent conditions (Jec). The cooling rate at 700C (V700) was selected since it represents the approximate pearlite transformation region for many steels.

There are practical applications of the Jominy curves helping steel selection for a specifi c application. A very detailed procedure can be found in Silva and Mei [39]. A brief description will be offered as following.

In order to get the steel selection, there are a sequence of steps that may be obeyed:

1. Mechanical resistance in a specifi c position (related to the diameter) of the compo-nent (to be manufactured) must be determined. This step is obtained from project requirements.

2. Determine the hardness correlated with the required mechanical properties. For this, the following empirical equations may be used:

r(MPa) 3.55 HB(HB>175) =

r(MPa) 3.88 HB(HB>175) =

wherer is fracture strengthHB is Brinell hardness

Correlation between fracture strength and yield strength is found in Figure 1.28.



3. Verify the necessary martensite percentage in order to obtain the required hard-ness, which is a function of the load type (dynamic [fatigue], static). Since mar-tensite hardness depends on the carbon content, it is important to note that the lower the carbon content, better the toughness property. Figure 1.29 shows a relationship between hardness, martensite percentage, and carbon content.

4. The hardness decrease during tempering must be considered. In Figure 1.30, it is possible to estimate the minimum as-quenched hardness necessary to achieve the hardness required after tempering.

5. A compatible quenchant must be selected considering geometric complexity of the com-ponent (potential risks of crack and distortion). Table 1.5 provides such information.

400 500 600Yield strength (MPa)

700 800 900 10002000.5

0.6

0.7

Yield streng

th/fr

acture

streng

th

0.8

0.9

1.0

300

FIGURE 1.28Elastic relationship (yield strength/fracture strength) as a function of yield strength. Valid only for steels. (Adapted from Silva, A.L.V.C. and Mei, P.R., Aos e Ligas especiais, Edgard Blcher, So Paulo, Brazil, 2006.)

FIGURE 1.29As-quenched hardness as a function of the carbon con-tent and martensite percentage in the microstructure. (From ASM, Properties and selectionIrons, steels and high performance alloys, in ASM Handbook, 10th ed., ASM, Materials Park, OH, 1990. With permission.)

0.50.4

Martensite

Carbon (%)

50%

80%

90%

95%99.9%

0.30.210

20

30

40

50

Hardn

ess (HRC

)

60

70

0.6



6. Using heat transfer correlation, it is possible to determine which position of the Jominy test corresponds to the same cooling rate of the specifi c position (with respect to the diameter) of the component. This step may be obtained using numer-ical solutions or using Lamont curves, as presented in Figure 1.31a through e. Lamont curves correlate bar diameter Jominy distance quenchant severity (H factor) hardened depth (this last one is given as a radius fraction of the bar). This last one is related to specifi c position (considering to the diameter), as mentioned before.

For example, if the required as-quenched hardness is necessary to obtain the center of the bar, Figure 1.31a must be used. With the bar diameter and the selected quenchant (H factor), it is possible to obtain Jominy distance.

30201030

40

50

Hardn

ess a

s quenc

hed (H

RC)

Hardness required after tempering (HRC)

60

70

40 50 60 70

FIGURE 1.30As-quenched minimum hardness recommended as a function of the required hardness after tempering. (Adapted from Silva, A.L.V.C. and Mei, P.R., Aos e Ligas especiais, Edgard Blcher, So Paulo, Brazil, 2006.)

0

r

1/4

1.0

2.0

3.0

(a)

4.0

0.2

0.3

0.50.71.01.52.05.0

H

Bar d

iameter

(in.)

5.0

6.0

7.0

1/2 3/4 1Distance from quenched end of specimen (in.)

21 1 1

r = 0.0

R

R

FIGURE 1.31(a) Lamont curves for center of quenched bars.

(continued)



0

rR

Rr

= 0.3

0.2

0.35

0.50.71.01.52.05.0

H

0

(b)

1.0

2.0

3.0

4.0

Bar d

iameter

(in.) 5.0

6.0

7.0

1/2Distance from quenched end of specimen (in.)

21 1

0(c)

0.2

0.35

0.5

0.71.01.52.05.0H

0

1.0

2.0

3.0

4.0

Bar d

iameter

(in.) 5.0

6.0

7.0

1/2Distance from quenched end of specimen (in.)

21 1

Rr

Rr = 0.5

FIGURE 1.31 (continued)(b) Lamont curves for 30% radius of quenched bars. (c) Lamont curves for 50% radius of quenched bars.



Rr = 0.7

0

Rr

1.0

2.0

3.0

4.0Bar d

iameter

(in.)

5.0

6.0

7.0

8.0

9.0

10.0

1/2Distance from quenched end of specimen

21(d)

1

0.2

0.35

0.5

0.71.0

1.52.0

5.0 H

Rr = 0.9

0.2

0.35

0.50.7

1.01.5

2.0 H

0

(e)

1.0

2.0

3.0

4.0Bar d

iameter

(in.)

5.0

6.0

7.0

8.0

9.0

10.0

1/2Distance from quenched end of specimen

21 1

Rr

FIGURE 1.31 (continued)(d) Lamont curves for 70% radius of quenched bars. (e) Lamont curves for 90% radius of quenched bars. (Adapted from Totten, G., Steel Heat Treatment: Metallurgy and Technologies, Taylor & Francis Group LLC, Boca Raton, FL, 2006.)



7. Verify which steels can achieve the hardness specifi ed, using value obtained from step 6, required hardness and Jominy curves from different steels. Some of these curves were presented in Figure 1.27.

8. Obtaining possible steel candidates; fi nal choice must be made considering avail-ability, price, etc.

1.7 Tempering

Tempering is a term historically associated with the heat treatment of martensite in steels, changing microstructure and mechanical properties

quenching theory and technology -...

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