Development of Ultra-Fine-Grained Structure in AISI 321 Austenitic Stainless Steel

Ultra-fine-grained (UFG) structure was developed in AISI 321 austenitic stainless steel (ASS) using cryogenic rolling followed by annealing treatments at 923 K, 973 K, 1023 K, and 1073 K (650 degrees C, 700 degrees C, 750 degrees C, and 800 degrees C) for different lengths of time. The alpha'-martensite to gamma-austenite reversion behavior and the associated texture development were analyzed in the cryo-rolled specimens after annealing. The activation energy, Q, required for the reversion of alpha'-martensite to gamma-austenite in the steel was estimated to be 80 kJ mol(-1). TiC precipitates and unreversed triple junction alpha'-martensite played major roles in the development of UFG structure through the Zener pinning of grain boundaries. The optimum annealing temperature and time for the development of UFG structure in the cryo-rolled AISI 321 steel are (a) 923 K (650 degrees C) for approximately 28800 seconds and (b) 1023 K (750 degrees C) for 600 seconds, with average grain sizes of 0.22 and 0.31 mu m, respectively. Annealing at 1023 K (750 degrees C) is considered a better alternative since the volume fraction of precipitated carbides in specimens annealed at 1023 K (750 degrees C) are less than those annealed at 923 K (650 degrees C). More so, the energy consumption during prolonged annealing time to achieve an UFG structure at 923 K (650 degrees C) is higher due to low phase reversion rate. The hardness of the UFG specimens is 195 pct greater than that of the as-received steel. The higher volume fraction of TiC precipitates in the UFG structure may be an additional source of hardening. Micro and macrotexture analysis indicated {110} < uvw > as the major texture component of the austenite grains in the UFG structure. Its intensity is stronger in the specimen annealed at low temperatures. (C) The Minerals, Metals & Materials Society and ASM International 2017