A detailed analysis of martensite transformations in 18/8 (3b4) stainless steel,.utilizing transmission electron microscopy and diffraction in conjunction with X-ray and magnetization techniques, has established that the sequence of transformation is y ~ E ~ a. E is a thermodynamically stable hcp phase whose formation is greatly enhanced as a result of plastic deformation. Comparison with the E ~a transformation in pure Fe-Mn alloys lends further support to the above sequence and suggests that a transforma-.tion line between E and a in Fe-Cr:-Ni alloys can be expected. In the 304 stainless steel used in this investigation, formation of a was induced only by plastic deformation and subsequent to formation of E. Nucleation of. a occurs heterogeneously at intersections of E-bands or where E-bands abutt twin or grain boundari~s (which represent unilaterally compressed regions) .. From electron diffraction, the Nishiyama relationship between y and a phases appears to predominate at the start of the transformation, but then changes ·to that of Kurdjumov-Sachs. Based on these observations, a sequence of atom· movements from the hcp structure to the bee structure is proposed which has the basic geometric features of the martensitic transformation.

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SUMMARY INTRODUCTIONAt a recent symposium, Robinson l summarized the development of and the challenges faced by alloy steels in the automotive industry. Through the years since Henry Ford revolutionized car manufacture, constant changes in alloy steel compositions, to meet the demands of the times, were prevalent. At present, it appears that the technology of heat-treated alloy steels has reached its maturity, with work concentrated on both accurately predicting hardenability from chemical composition and balancing composition to minimize cost for the application desired . However, this is far from the reality, for specific effects of the various alloying elements, individually and in combination, on hardenability, materials properties, and service performance are far from being understood. l , 2 The current use of alloy steels depends to some extent on the mystique, developed with their development, that either individual or combinations of alloying elements were considered uniquely responsible for the steel properties.Despite this mystique, there persist changes in the selection of materials based on alloy costs and performance, i.e., response to varied and complex engineering and processing demands on the materials. The complexity of demands resulted in the establishment of an increased number of alloy grades to meet specific demands. In 1940, the SAE (Society of Automotive Engineers) listed 65 standard grades of lowalloy steels; today7 there are 86, not including the "H band" series, plus EX grades and numerous "modified" grades specified by individual manufacturers. In the course of the years, 96 former grades 7 were deleted, i.e, no longer of major commercial use. Such continuing activity suggests that the technology of alloy steels is still in a state of flux.Although wrought alloy steels face some competition in low-stress automotive components, they will continue to be the dominant materials for highly stressed components, such as power transmission gears and shafting, ball and roller bearings, and spring members. These components have high hardness (~50Rc ) and depend on the ability of the steel to be through hardened, case hardened by induction heating, or case carburized.In predicting hardenability from composition, one finds no readily available factor for vanadium,3 in spite of the fact that vanadium was used in the early alloy steel grades

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