The aging and tempering of iron-nickel-carbon martensites
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INTRODUCTION
BECAUSE of the wide range of mechanical properties obtainable through the hardening and tempering of steel, the structures of iron-carbon martensites and their decomposition products have been of lasting interest to metallurgists. Crystallographic and kinetic theories have been developed to account for the formation of ferrous martensite in its various forms, and numerous studies have identified several stages of decomposition during the tempering of martensite. Since there are excellent review papers available dealing with these topics, only a brief description of certain pertinent aspects will be given here. 1-4 Two major morphological types of ferrous martensite have been observed. As shown in Figure 1, alloys with high Ms temperatures tend to form martensites consisting of packets of lath-shaped units containing highly dislocated regions. Alloys with lower Ms temperatures tend to form plate-like morphologies containing fine internal twins. The latter martensites are usually associated with significant amounts of retained austenite, while the lath martensites typically retain relatively little austenite. Some alloys transform to martensites with hybrid or mixed morphologies, such as irregular plates which are only partially twinned. These intermediate cases suggest that the transition from the lath morphology (where the lattice-invariant shear involved in the transformation occurs by slip) to the plate morphology (where twinning is the lattice-invariant shear mode) is not abrupt. It has been proposed that a change in the relative ease of slip vs twinning is responsible for this transition in martensite morphology)'6
A. M. SHERMAN is Principal Research Scientist Associate, Ford Motor Company, Dearborn, M148121. G. T. ELDIS is Research Manager, Climax Molybdenum Company, Ann Arbor, MI 48106. MORRIS COHEN is Institute Professor Emeritus, Massachusetts Institute of Technology, Cambridge, MA 02139. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.
METALLURGICALTRANSACTIONS A
Since ferrous martensites form at such a fast rate and because the solubility of interstitial elements, such as carbon, is much greater in austenite than in body-centered iron, the transformation traps interstitials in metastable positions in the martensite. The changes which occur during the aging or tempering of martensite consist largely of the movements of carbon atoms from their as-quenched sites to lowerenergy locations. Martensite which has not been subjected to any intentional aging treatment is often termed "freshly quenched." However, there is now substantial evidence that even at subambient temperatures, and also during the quenching of alloys with high Ms temperatures, carbon movements and/or other changes take place. Speich 7 has calculated that carbon atoms in high Ms martensites can di
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