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 range of 950-1100 °C and the strain rate 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 aforementioned temperature range and strain rate ,the prevailing softening mechanism was the dynamic recrystallization process. Instability regions were observed at lower temperatures and higher strain rates, associated with the occurrence of necklace phenomenon in both grain sizes, as well as the semi-necklace structure in coarse-grained steel and in fine grain steel to the formation of geometrically necessary dislocation along the grain boundaries. Moreover, it was associated with the development of cell structure interior the grains. In addition, with an increase in temperature and a decrease in strain rate, both the power dissipation efficiency and the strain rate sensitivity increased, indicating an increase in the volume of the recrystallized materials in a constant strain and a decrease in the localization of the plastic flow.