Effect of Stress on Variant Selection of Lath Martensite in Low-carbon Steel

The effect of stress on variant selection of lath martensite in a low-carbon steel (Fe-0.18%C-0.89%Mn-2.88%Ni-1.51%Cr-0.40%Mo) was investigated by EBSP analysis. The steel was continuously cooled from fully austenitic temperature to room temperature under uniaxial compressive stress applied during the martensitic transformation. It was demonstrated that, only in blocks larger than the average block size, some of the variants were preferentially selected under the applied stress while maintaining the Kurdjumov-Sachs (K-S) orientation relationship with the prior austenite. Otherwise, no clear variant selection was found. The external work done during the martensitic transformation evaluated from the transformation strain and the applied stress showed that the variants with greater external work were more likely to be selected. However, both the shift of martensite start temperature and the selected variants indicate that the transformation strain effective for the variant selection of lath martensite in the low-carbon steel was only the invariant line strain, unlike in nickel steels where the additional lattice invariant shear has been included in the literature.