Grain Size Effect on the Hot Deformation Processing Map of AISI 304 Austenitic Stainless Steel

In this study, the hot deformation processing map of AISI 304 austenitic stainless steel in two initial grain sizes of 15 and 40 μm was investigated. For this purpose, cylindrical samples were used in the hot compression test at the temperature ranges of 950-1100 °C and the strain rates of 0.005-0.5% s-1. At first, the relationship between the peak stress and Zener-Hollomon parameter was obtained and their microstructures were studied, then the strain rate sensitivity and the processing map were determined at the strains of 0.5 and 0.7. It was found that in the range of temperature and strain rate studied here, the prevailing softening mechanism is the dynamic recrystallization process. Instability regions were observed at lower temperatures and higher strain rates, associated with the occurrence of necklace phenomena in both grain sizes especially the emphasized semi-necklace structure in the coarse-grained steel, and moreover to the formation of geometrically necessary dislocation along the grain boundaries as well as the development of cell structure interior the grains in the fine grain steel. Results also showed that with increasing temperature and decreasing strain rate, the power dissipation efficiency and the strain rate sensitivity are both increased, indicating an increase in the volume of recrystallized material in a constant strain and a decrease in the susceptibility to the localization of the plastic flow.