Effects of heat treatments conditions and Mn contents on grain boundary segregation and mechanical properties in high manganese austenitic Fe-Mn alloys have been investigated. The Mn contents investigated were 25, 30, 35 and 40w/o, and heat treatments applied were varying austenitizing temperature and changing cooling rates. Charpy impact and tensile tests were performed at RT and -196℃.
In Fe-35Mn and Fe-40Mn alloys, it was found that the Charpy impact energy decreased with increasing austenitizing temperature due to enriched Mn segregation at the grain boundaries. The Charpy impact energy also increased with the decreasing cooling rate after the austenitization, which was attributed to the less segregation of manganese to the gain boundary. However, too slow cooling in the furnace was detrimental to the impact energy as a result of impurity segregation to the grain boundaries.
In Fe-30Mn alloys, the Charpy impact energy did not change with the cooling rate. On the other hand, the Charpy impact energy of Fe-25Mn aloy increased with the increasing cooling rate by supressing the formation of hcp ε-martensite.
Tensile elongations of Fe-35Mn and 40Mn alloys at -196℃ were highter than those at room temperature. The increased ductility with decreasing temperature was due to the strain-induced formation of deformation twin. The tensile elongation was analyzed by the proposed K-L equation. For the Fe-35Mn and Fe-40Mn alloys, it was found that the tensile strength and elongation at -196℃ in the as-rolled condition(without post rolling heat treatments)was higher than those of the heat treated normal to the tension direction were observed, resulting in the appearance of serrations in the stress-strain curve. The further propagation of the microcracks were inhibited by formation of the deformation twins.