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I. INTRODUCTION

AUSTENITIC stainless steels represent a large family of alloys and end uses. These steels enjoy a wide range of applications because of good corrosion resistance, good high- and low-temperature strength and ductility, and excellent weldability. The high work-hardening rate and ductility of austenitic stainless steels in the annealed state allows the application of severe forming operations. Some grades are metastable and are prone to transformation from the initial face-centeredcubic austenite () to body-centered-cubic martensite () upon plastic deformation, commonly referred to as straininduced martensite. Stability is mainly dependent on composition and temperature. Other factors that can affect the extent of transformation are plastic strain, strain rate, stress state, and grain size. The transformation results in increased strength, because martensite is stronger than austenite, and can, at the same time, give increased ductility if it occurs rapidly just prior to severe strain localization. Transformation from austenite to martensite can also occur by two other means, spontaneous transformation and stressassisted nucleation. Spontaneous transformation occurs when the material is cooled below the martensite start temperature (Ms) as occurs in conventional steels. Slightly above the Ms, stress-assisted martensite can form in response to an applied elastic stress. This type of martensitic transformation occurs until the austenite yields at a temperature designated by Mss. Above Mss, where the material yields by slip, new highly potent transformation sites are generated and straininduced transformation on these new sites is possible at stresses lower than observed for the stress-assisted transformation. As temperature increases, austenite stability increases to limit the transformation. The temperature above which strain-induced martensite is not produced by plastic deformation is referred to as Md.[1] JOSHUA A. LICHTENFELD is with Firth Rixon-Schlosser Forge Co., Cucamonga, CA 91730. CHESTER J. VAN TYNE is with the Department of Metallurgical and Materials Engineering, Colorado School of Mines Golden, CO 80401. MARTIN C. MATAYA is with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] Manuscript submitted June 17, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

The susceptibility of an alloy to strain-induced transformation depends on austenite stability, which can be roughly assessed by its martensite start temperature (Ms). Eichelman and Hull[2] studied the effect of composition on Ms and found that Cr and Ni have a moderate effect while C and N have a stronger effect on the Ms, which decreases with solute addition. The direct effect of composition on the formation of strain-induced martensite in alloy 304 was first investigated by Angel,[3] who correlated elemental compositions to the temperature at which half of the austenite is transformed by the application of 0.3 true strain in tension, denoted by MD30.