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

The effect of stress on the variant selection in 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 using electron backscatter diffraction pattern (EBSP) analysis. The steel was continuously cooled from a fully austenitic temperature to room temperature under uniaxial compressive stress applied during the martensitic transformation. It was demonstrated that certain variants maintaining the Kurdjumov-Sachs (K-S) orientation relationship with the prior austenite were preferentially selected under the applied stress only in blocks larger than the average block size. Otherwise, no clear variant selection was found. The applied stress and the external work done during the martensitic transformation, which was evaluated from the transformation strain, showed that the variants with greater external work values were more likely to be selected. However, both the shift in the martensite start temperature and the selected variants indicate that only the invariant line transformation strain was effective for variant selection in lath martensite in the low-carbon steel, unlike in nickel steels where the lattice-invariant shear has been additionally included in the literature.