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Library of Congress Cataloging-in-Publication Data (for Print Volume) ASM Handbook !evised vol. "# Metals Handbook. $itle proper has changed with v."% ASM Handbook&'repared under the direction of the ASM International Handbook Committee. Includes bibliographies and inde(es. Contents% v. ". Heat $reating. ). Metals*Handbooks, manuals, etc. I. ASM International. Han dbook Committee. II. $itle% ASM ASM Handbook. $A"+.M"- ) /0.)1/ *))+ IS23 *45)5*-5*SA3 0"*5+4/ 'rinted in the 6nited States S tates of America
Principles of Heat of Heat Treating of Steels of Steels $orsten 7ricsson, 8ink9ping Institute of $echnology, Sweden
Introduction A S$778 is usually defined as an alloy of iron and carbon with the carbon content between a few hundreds of a percent up to about 0 wt:. ;ther alloying elements can amount in total to about + wt: in low*alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steels can e(hibit 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 subseeliberately no microstructures are shown as a well*illustrated article e(ists in =Microstructures, 'rocessing, and 'roperties of Steels,= by ?. @rauss in Properties and Selection: Irons, Steels, and High-Performance and High-Performance Alloys, olume Alloys, olume ) of ASM Handbook ASM Handbook . A companion article that emphasiBes information systems for predicting microstructures and hardnesses of
The Fe-C Phase Diagram $he basis for the understanding of the heat treatment of steels is the De*C phase diagram Dig. )#. 2ecause it is well e(plained in earlier volumes of ASM Handbook , formerly Metals Handbook !ef ), 0, -#, and in many elementary te(tbooks, it will be treated very briefly here. Digure ) actually shows two diagramsE the stable iron*graphite diagram dashed lines# and the metastable De*De -C diagram. $he 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. $he De* C diagram shows which phases are to be e(pected at e
always avoided. Some important boundaries at single*phase fields have been given special names that facilitate the discussion. $hese include% • •
•
A), the so*called eutectoid temperature, which is the minimum temperature for austenite A-, the lower*temperature boundary of the austenite region at low carbon contents, that is, the & K G boundary Acm, the counterpart boundary for high carbon contents, that is, the & K De -C boundary
Sometimes the letters c, e, or r are included. !elevant definitions of terms associated with phase transformations of steels can be found in $able 0 as well as the ?lossary of $erms in this olume and !ef -. $he carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content .55 wt: C#. $he ferrite*cementite phase mi(ture 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 =spheroidiBes= or =coarsens=# into cementite particles dispersed with a ferrite matri( after e(tended holding close to A). Table 1 Important metallurgical phases and microconstituents Phase (microconstituent)
Derrite G*iron#
b
J*ferrite J*iron#
b
Austenite *iron#
f
Cementite De-C#
C
?raphite
'earlite
Martensite
b
2ainite
.
Table Definitions of transformation temperatures in iron and steels See the ?lossary of $erms in this olume for additional terminology. Transformation temperature! $he temperature at which a change in phase occurs. $he term is sometimes used to denote the limiting temperature of a transformation range. $he following symbols are used for iron and steels.
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