Mechanism and crystallography of ferrite precipitation fromcementite in an Fe-Cr-Calloy during austenitization

Abstract

Abstract The morphology of ferrite precipitates formed within cementite and the mechanism of their formation were examined by means of transmission electron microscopy in an Fe-2.6 wt% Cr-0.96 wt% C alloy after austenitizing in the range 800–950°C for various times. At the beginning of the austenitization process, the cementite particles dissolve rapidly by carbon diffusion in the austenite. The precipitation of ferrite occurs during the second stage of transformation when cementite dissolution is controlled by chromium diffusion in cementite or in the austenite matrix. The ferrite precipitates exhibit a lath-like shape elongated in the [010] cementite direction with the habit plane close to the (001) cementite. The cementite and ferrite were related to each other by two previously reported orientation relationships. In the case of the Pitsch-Petch orientation relationship, the habit plane of the ferrite lath is almost parallel to (152)f//(001)c and the ferrite-cementite interface is flat without any steps. The side facet of the lath is close to (2 15)f//(101)c, and the ferrite growth direction is close to [311]f//[010]c. In the case of the Bagaryatsky orientation relationship, the habit plane of the ferrite lath deviates several degrees from the broad face of (1 12)f//(001)c and structural ledges form at the ferrite-cementite interface, preserving coherency. The side facet of the lath is close to (121)f//(101)c and the ferrite growth direction is close to [111]c//[010]c. The results are discussed by assuming local equilibrium at the moving interfaces using the software THERMOCALC.