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

Abstract

Effects of tempering temperature on microstructure and tensile properties of Fe-12% Mn steel have been investigated in relation to phase transformation of blocky alpha'-martensite and epsilon-martensite during tempering process. In Fe-12% Mn steel, transformations of epsilon ->gamma and alpha'->gamma occurred during the heating process, while a two-stage transformation of gamma ->epsilon ->alpha' was observed during an air cooling process. This steel contained mainly blocky type alpha'-martensite with very small amount of epsilon-martensite and gamma phases. A large number of dislocations were observed both within and boundaries of blocky alpha'-martensite, which showed the BCC twin relationship with (112) twin planes. The epsilon-martensite was formed along the boundaries of blocky alpha'-martensite having the size of about 50 nm. During the tempering at 500 degrees C, transformation of epsilon ->gamma occurred first during the heating and then gamma phase transformed to epsilon-martensite during the subsequent air cooling process. The shape of blocky alpha'-martensite changed to irregular and coarse epsilon-martensite having the size of 0.5-1.0 mu m. Tempering at 600 degrees C transformed both the epsilon and alpha' phases to gamma phase during the heating process, while transformations of gamma to epsilon phase occurred during the subsequent air cooling process. Tempering at 600 degrees C formed coarse plate type epsilon-martensite with a very small amount of gamma phase and much reduced dislocation density in alpha'-martensite. The present Fe-12Mn steel exhibited a continuous yielding behavior with very low yield stress due to high mobile dislocation density in alpha'-martensite. By tempering at 500 degrees C, the blocky boundaries of alpha'-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 degrees C was observed to reduce the volume fraction of alpha'-martensite by 30% and also to form coarse plate type epsilon-martensite to reduce both the yield strength and ultimate tensile strength. (C) 2015 Elsevier B.V. All rights reserved.