Microstructural stability of thermal-mechanically pretreated type 316 austenitic stainless steel

Studies of the microstructural stability of Type 316 austenitic stainless steel were performed for a wide range of thermal-mechanical pretreatments in the limited aging temperature range of 550° to 760°C. The pretreatments were selected in order to investigate the effects of varying solution treatment temperature, amount of cold reduction by rolling, initial grain size, and initial precipitate distribution. Large variations in both phase stability and recrystallization behavior can be effected by appropriate pretreatments. Cold work accelerates precipitation of M₂₃C₆ carbide and the intermetallic compounds (Laves, χ, and σ phases). Both the amount and kinetics of σ phase formation are especially enhanced by recrystallization occurring in the aging temperature range. It is suggested that this occurs due to ready σ nucleation at slowly moving (recrystallizing) grain boundaries together with enhanced growth rates due to diffusion along the boundary. Fine grain size enhances phase instability by providing additional nucleating sites and decreased diffusion paths for precipitate forming elements, but in the grain size range studied (ASTM No. 3.5 to No. 13) the effect is not as significant as the effect of cold work, particularly when recrystallization occurs during the aging treatment. Fine grain size and pretreatments which precipitate the carbides prior to the final cold working step enhance recrystallization kinetics relative to solution treated and cold-worked materials. This is apparently due to stabilization of the cold-worked substructure in the solution treated samples by precipitation of carbide and Laves phases on the dislocations and stacking faults.