Decomposition of martensite by discontinuous-like precipitation reaction in an Fe–17Cr–0.5C alloy

Abstract

The mechanism of martensite decomposition and the kinetics of carbide precipitation have been studied in an Fe–17 wt% Cr–0.55 wt% C alloy. The morphology of carbide precipitates formed within the decomposed regions and the crystallography of their formation were examined by means of transmission electron microscopy after tempering at 735°C for various times. The martensite decomposition starts within less than 10 s, but it is not completed even after 10 min. The reaction initiates with the nucleation of fine cementite particles preferentially at the prior austenite grain boundaries and occasionally at the martensite lath boundaries. Cementite particles are related to the ferritic matrix with the Bagaryatsky orientation relationship. The decomposition of martensite proceeds heterogeneously by the migration of a reaction front. Various carbide morphologies were observed in the region close to the reaction front: rod-like, spherical or lamellae. The kinetics of martensite decomposition changes from carbon diffusion controlled to chromium diffusion controlled. After long time tempering, the alloy carbides, M 23C 6 and M 7C 3, precipitate at the reaction front. The M 23C 6 carbides are related with respect to the ferrite by the Kurdjumov–Sachs orientation relationship. Two specific orientation relationships were found between the M 7C 3 carbide and the ferrite, which are related to each other by a rotation of 30° about their common axis of [0001] h //[110] α . One of them has previously been reported. The specific features of discontinuous-like precipitation in martensite are discussed and are attributed to the presence of carbon and chromium atoms, which have different mobilities. The driving forces for diffusion of carbon and chromium were qualitatively determined with the software and database ThermoCalc by assuming local equilibrium at the moving interfaces.