Martensite Formation in Conventional and Isothermal Tension of 304 Austenitic Stainless Steel Measured by X-ray Diffract
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INTRODUCTION
AUSTENITIC 304 stainless steel transforms to martensite under tensile loading at and below room temperature by a strain-induced mechanism[1–3]: the plastic deformation creates additional nucleation sites for the progressive transformation of retained austenite to martensite. This transformation substantially increases the macroscopic work-hardening rate because the external stress must supply the extra work required to deform the austenite further and cause the transformation to occur. In addition, the associated transformation strain can increase the uniform elongation and the ductility,[4–6] making this phenomenon interesting for applications, e.g., metal shaping. Hecker et al.[7] used a ferritescope to measure the volume transformed to martensite in thin sheets of 304 stainless steel tested between 193 K and 323 K (80 C to 50 C) at moderate strain-rates of 103/s and high strain-rates of 103/s in uniaxial and biaxial tension. They observed a 313 K (40 C) temperature rise in the high strain-rate specimens, but the low rate specimens stayed within 1 K (1 C) of ambient. They fitted their results to the Olson-Cohen model[8–12] and attributed the enhanced ductility to the c fi a¢ martensite transformation. They also found that a¢ martensite saturates at about 85 pct volume fraction. Huang et al.[13] performed isothermal tests on 304 stainless between 193 K and 433 K (80 C to 160 C) and evaluated the contributions of strain-induced transformation and dislocation substructure formation on the ductility, strain-harden-
NEWELL H. MOSER, Undergraduate Research Assistant, TODD S. GROSS, Professor, and YANNIS P. KORKOLIS, Assistant Professor, are with the Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824. Contact e-mail: [email protected] Manuscript submitted February 1, 2014. Article published online July 2, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
ing rate, and strain-rate sensitivity. They found that at lower temperatures, the transformation dominates the material behavior, while at higher temperatures the deformation is governed by slip. Shin et al.[14] used a ferritescope to measure transformation to martensite in well annealed 304 stainless as a function of strain and at temperatures between 77 K and 313 K (196 C and 40 C). They estimated Md, the temperature above which neither stress nor plastic strain can cause austenite to transform to martensite, to be 322 K (49 C). In our previous work, we have shown that the sample temperature is considerably above the ambient temperature for testing in air, due to deformation-induced heating.[15] De et al.[16] used X-ray diffraction (XRD) to determine the transformation to martensite as a function of strain at 223 K (50 C) in stirred ethanol which can be assumed to have rapid enough heat transfer so that the bath and sample temperature are the same. They proposed that martensite transformation begins with e-martensite (hexagonally closed packed), which is then transformed a¢ martensite (body centered tetrag
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