In Situ Transformation of Cementite to M7C3 and Internal Defects of M7C3 in High Carbon-Chromium Steel by Tempering

Abstract

Transformation of cementite to M₇C₃ and internal defects of M₇C₃ in high carbon-chromium steel tempered after rapid quenching from the melt have been examined by transmission electron microscopy. The precipitates in the as-quenched state are mostly cementite. By tempering at about 600°C, nucleation of M₇C₃ takes place mostly at the cementite/ferrite interface and occasionally within the ferrite matrix. The nuclei of M₇C₃ on the cementite surface grows rapidly inward the cementite crystal. These results indicate that the transformation of cementite to M₇C₃ occurs mainly by the in situ mechanism. In grown particles of M₇C₃ a number of internal faults lie along crystal planes perpendicular to the basal plane and consequently in the electron diffraction patterns there exist long streaks around reflection spots in the directions perpendicular to {10.0} planes. Mutual relationships of crystal orientation among M₇C₃, cementite and ferrite matrix were decided.