Hot working and recrystallization of As-Cast 316L
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7/8/03
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Hot Working and Recrystallization of As-Cast 316L MARTIN C. MATAYA, ERIC R. NILSSON, ELLIOT L. BROWN, and GEORGE KRAUSS 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 s1 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.
I. INTRODUCTION
RECRYSTALLIZATION of coarse as-cast ingot structure in stainless steels is a primary objective during the initial stages of ingot breakdown. Incipient recrystallization at grain boundaries, where solute and second phases collect during solidification and cooling of ingots, enhances hot workability by reducing the propensity for the phenomenon of slivering and for crack formation at columnar grain boundaries. Hot working of ingot material occurs either by radial forging or open die pressing to produce billet and bar
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