The Crystal Structure of As-quenched Fe–C Martensite

Confusion exists in the literature as to whether the crystal structures are cubic or tetragonal in lath martensites of Fe–C alloys and low-alloy steels. Steels with a range of carbon contents have been quenched and examined by synchrotron x-ray diffraction. The presence of dislocations and residual local strains complicates the analysis since peak splitting of tetragonal lines is obscured by the broadening. Asymmetry of the 200,020/002 lines has been examined and synthesised using model peak functions. A new approach has been to study the 222 peaks which are unique (not split) for both cubic and tetragonal crystals. For low carbon steels (<~0.2%C) the structures are fully or almost completely cubic. Above about 0.7%C the martensite has tetragonal symmetry. Intermediate, medium carbon, steels consist of mixtures of cubic and tetragonal structures.