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The effect of strain rate on stress-strain behavior of austenitic stainless steel 309 and 304L was investigated. Tensile tests were conducted at room temperature at strain rates ranging from 1.25 ϫ 10 Ϫ4 s Ϫ1 to 400 s Ϫ1 . The evolution of volume fraction martensite that formed during plastic deformation was measured with X-ray diffraction and characterized with light microscopy. Alloy 304L was found to transform readily with strain, with martensite nucleating on slip bands and at slip band intersections. Alloy 309 did not exhibit strain-induced transformation. Variations in ductility and strength with strain rate are explained in terms of the competition between hardening, from the martensitic transformation and a positive strain rate sensitivity, and softening due to deformational heating. Existing models used to predict the increase in volume fraction martensite with strain were examined and modified to fit the experimental data of this study as well as recent data for alloys 304 and 301LN obtained from the literature.

Deformation-induced phase transformation in atype 304 austenitic stainless steel has been studied in tension at room temperature and -50°C. The evolution of transformation products was monitored using X-ray diffraction (XRD) line profile analysis of diffraction peaks from as ingle XRD scan employing the direct comparison method. Crystallographic texture transitions due to deformation strain havebeen evaluated using (111) g pole figures. The tensile stress-strain data have been analyzed to explain the influence of underlying deformation-induced microstructural changes and associated texturechangesinthe steel. It is found that the initial stage of rapidlydecreasing strain hardeningrate in type 304 steel is primarily influenced by hcp e -martensite formation, and the seconds tage of increasing strain hardening rate is associated with an increasei nt he a # -martensite formation. The formation of e -martensite is associated with ag radual strengthening of the copper-type texture componentsupto15pct strain and decreasing with further strain at -50°C. Texture changes during low-temperature deformation not only change the mechanism of e -martensite formation but also influence the strain rate sensitivity of the present steel.

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Stress-strain behavior and microstructure evolution during hot working of as-cast austenitic stainless steel alloy 316L were investigated by uniaxial compression of cylindrical specimens at a strain rate of 1 s Ϫ1 over the temperature range 1000°C to 1150°C and up to a strain of one. The measured flow curves showed monotonic hardening, indicating that dynamic recrystallization was not important in microstructural evolution. Static recrystallization was observed to nucleate preferentially at the delta ferrite-austenite interphase boundaries. The recrystallization kinetics of the as-cast material was compared to a relatively fine-grained wrought 316L material and found to be somewhat slower. However, the difference between the two material conditions was not nearly as great as previously reported for as-cast and wrought 304L alloy. The difference in behaviors between 316L and 304L is attributed to the relatively large amount and vermicular morphology of the delta ferrite phase in the 316L, resulting in a relatively fine effective grain size, compared to the existing coarse columnar structure, and concomitant enhancement of recrystallization. Compared to wrought 316L, the recrystallization rate of the as-cast material was relatively sluggish, despite a relatively fine effective grain size. The difference is associated with the 100 orientations of the columnar grains with respect to the compression axis, producing a soft orientation and a reduced rate of accumulation of dislocation density in the substructure. Also, compared to wrought 316L, the recrystallization rate of the ascast material tends to decrease with time, the drop occurring concurrently with spheroidization and dissolution of the ferrite. It is suggested that (1) movement of the delta ferrite-austenite interphase boundary during spheroidization may poison incipient recrystallization and (2) dissolution of delta ferrite can locally enrich the austenite matrix in Mo and Cr, raising the local stacking fault energy and lowering grain boundary mobility to favor recovery over recrystallization in the vicinity of the ferrite-austenite boundary. A kinetic model for recrystallization was developed and used to simulate evolution of the first cycle of recrystallization during various thermal-mechanical treatment schedules typically employed during the primary breakdown of as-cast material.