Heat Treatment-asm Metals Handbook Volume 4 6q3w71

against any alleged infringement of letters patent, copyright, or trademark, or as a defense against liability for such infringement. Comments, criticisms, and suggestions are invited, and should be forwarded to ASM International. Library of Congress Catag-in-Publication Data (for Print Volume) ASM Handbook (Revised vol. 4) Metals Handbook. Title proper has changed with v.4: ASM Handbook/Prepared under the direction of the ASM International Handbook Committee. Includes bibliographies and indexes. Contents: v. 4. Heat Treating. 1. Metals-Handbooks, manuals, etc. I. ASM International. Handbook Committee. II. Title: ASM Handbook. TA459.M43 1990 620.1'6 90-115 ISBN 0-87170-379-3 SAN 204-7586 Printed in the United States of America

Principles of Heat Treating of Steels Torsten Ericsson, Linköping Institute of Technology, Sweden

Introduction A STEEL is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steels can exhibit a wide variety of properties depending on composition as well as the phases and microconstituents present, which in turn depend on the heat treatment. In subsequent articles in this Section, various types of heat treatment are described in detail. In this article, an outline of the physical metallurgy associated with heat treating of steels will be given and some important "tools" will be introduced. Deliberately no microstructures are shown as a well-illustrated article exists in "Microstructures, Processing, and Properties of Steels," by G. Krauss in Properties and Selection: Irons, Steels, and High-Performance Alloys, Volume 1 of ASM Handbook. A companion article that emphasizes information systems for predicting microstructures and hardnesses of quenched steels follows (see the article"Quantitative Prediction of Transformation Hardening in Steels" in this Volume).

The Fe-C Phase Diagram The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig. 1). Because it is well explained in earlier volumes of ASM Handbook, formerly Metals Handbook (Ref 1, 2, 3), and in many elementary textbooks, it will be treated very briefly here. Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The FeC diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature. Table 1 provides a summary of important metallurgical phases and microconstituents. We distinguish at the low-carbon end ferrite (α-iron), which can at most dissolve 0.028 wt% C at 727 °C (1341 °F) and austenite (γ-iron), which can dissolve 2.11 wt% C at 1148 °C (2098 °F). At the carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures. Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite. In heat treating of steels, the liquid phase is

always avoided. Some important boundaries at single-phase fields have been given special names that facilitate the discussion. These include:   

A1, the so-called eutectoid temperature, which is the minimum temperature for austenite A3, the lower-temperature boundary of the austenite region at low carbon contents, that is, the γ/γ + α boundary Acm, the counterpart boundary for high carbon contents, that is, the γ/γ + Fe3C boundary

Sometimes the letters c, e, or r are included. Relevant definitions of associated with phase transformations of steels can be found in Table 2 as well as the Glossary of in this Volume and Ref 3. The carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content (0.77 wt% C). The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite lamellae that degenerates ("spheroidizes" or "coarsens") into cementite particles dispersed with a ferrite matrix after extended holding close to A1. Table 1 Important metallurgical phases and microconstituents Phase (microconstituent)

C

Ferrite (α-iron)

bc

δ-ferrite (δ-iron)

bc

Austenite (γ-iron)

fc

Cementite (Fe3C)

C

Graphite

H

Pearlite

Martensite

bc

Bainite

..

Table 2 Definitions of transformation temperatures in iron and steels See the Glossary of in this Volume for additional terminology. Transformation temperature. The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steels.

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