Effects of Mn and Al contents on cryogenic-temperature tensile and Charpy impact properties in four austenitic high-Mn steels

Effects of Mn (19 and 22 wt.%) and Al (0 and 2 wt.%) contents on tensile and Charpy impact properties in four austenitic high-Mn steels were investigated at room and cryogenic temperatures. The cryogenic-temperature tensile test results indicated that the yield strength was higher in the Al-added steels than non-Al-added steels, which could be explained by a stress-induced martensitic transformation in the non-Al-added steels. The reduction in ductility was largest in the 19Mn steel, where the transformations to ε- and α′-martensites occurred and their fraction was highest. Charpy impact energies of the 19Mn and 22Mn steels rapidly dropped with decreasing temperature, whereas those of the 19Mn2Al and 22Mn2Al steels slowly decreased. According to the EBSD analysis data of the cryogenic-temperature Charpy impact specimen, the transformation to ε- and α′-martensites readily occurred in the 19Mn and 22Mn steels, which resulted in the large reduction in impact energy. In the 19Mn2Al steel composed of highly stable austenite, the time needed for sufficient deformation to trigger the martensitic transformation was very short under the impact testing condition. In the Al-added steels, any martensites were not found, while many deformation twins were formed, thereby leading to high Charpy impact energy.