Hot working and recrystallization of as-cast 317L

Abstract

Stress-strain behavior and microstructure evolution during hot working of as-cast austenitic stainless steel alloy 317L is investigated by uniaxial compression of cylindrical specimens at a strain rate of 1 s−1 over the temperature range 1000 °C to 1150 °C and up to a strain of one. The measured flow curves show little strain hardening, attributed in part to the high stacking fault energy (SFE) of the alloy. Dynamic recrystallization is not observed. Static recrystallization is observed to nucleate within the austenite matrix in the dendrite cores at dislocation microbands and in austenite immediately adjacent to a vermicular microconstituent, composed primarily of sigma and austenite and, occasionally, some delta ferrite. The recrystallization kinetics of 317L are retarded compared to as-cast 316L steel. The relatively sluggish recrystallization behavior is attributed in part to the higher SFE of 317L, which favors recovery over recrystallization, and in part to gradients in chemical composition and SFE, not found in 316L, in the dendritic microstructure. Thus, in the austenite near the interphase boundary, with high SFE, recovery initially replaced recrystallization, in contrast to recrystallization in the austenite more distant from the boundary. The recrystallization kinetics of both as-cast 317L and 316L were relatively slow compared to wrought stainless steels of comparative grain size and SFE, presumably due to the crystallographic texture and associated relatively low flow stress in the former materials. A kinetic model for recrystallization in as-cast 317L is developed and utilized to simulate evolution of the first cycle of recrystallization during various thermal-mechanical treatment schedules typically employed during primary breakdown of as-cast material.