The Crystal Structure of As-quenched Fe–C Martensite

Abstract

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.

It is argued that martensite is always tetragonal at the moment when it forms from austenite but it can subsequently decompose into cubic by auto-tempering during the quench. Kinetics of decomposition have been measured using higher carbon steels and these are extrapolated to the situation of auto-tempering. A model is developed whereby the creation of tetragonal martensite according to the Koistinen-Marburger relationship is modified by its rate of decomposition. The final structures are predicted to depend on both the martensite start temperature (Ms) and the cooling rate.