Effects of tempering temperature on microstructure and tensile properties of Fe-12Mn steel

Effects of tempering temperature on microstructure and tensile properties of Fe-12% Mn steel have been investigated in relation to phase transformation of blocky α′-martensite and ε-martensite during tempering process. In Fe-12% Mn steel, transformations of ε→γ and α′→γ occurred during the heating process, while a two-stage transformation of γ→ε→α′ was observed during an air cooling process. This steel contained mainly blocky type α′-martensite with very small amount of ε-martensite and γ phases. A large number of dislocations were observed both within and boundaries of blocky α′-martensite, which showed the BCC twin relationship with (112) twin planes. The ε-martensite was formed along the boundaries of blocky α′-martensite having the size of about 50 nm. During the tempering at 500 °C, transformation of ε→γ occurred first during the heating and then γ phase transformed to ε-martensite during the subsequent air cooling process. The shape of blocky α′-martensite changed to irregular and coarse ε-martensite having the size of 0.5-1.0 μm. Tempering at 600 °C transformed both the ε and α′ phases to γ phase during the heating process, while transformations of γ to ε phase occurred during the subsequent air cooling process. Tempering at 600 °C formed coarse plate type ε-martensite with a very small amount of γ phase and much reduced dislocation density in α′-martensite. The present Fe-12Mn steel exhibited a continuous yielding behavior with very low yield stress due to high mobile dislocation density in α′-martensite. By tempering at 500 °C, the blocky boundaries of α′-martensite appeared to decompose to reduce mobile dislocation density, which in turn raises effectively the elastic limit at an early stage of tensile test and yield strength. On the other hand, tempering at 600 °C was observed to reduce the volume fraction of α′-martensite by 30% and also to form coarse plate type ε-martensite to reduce both the yield strength and ultimate tensile strength.