Decomposition of martensite and formation of carbide phases during tempering of chromium steels

Abstract

1. The presence of weakly tetragonal α solid solution (decomposed martensite) in naturally aged samples of chromium steels is due not to selftempering of martensite during quenching but to processes occurring during warming of the samples from the temperature of liquid nitrogen to room temperature and rest at room temperature. 2. Aging of martensite at temperatures below 120° is not accompanied by precipitation of carbide phase. At these temperatures the x-ray diffraction patterns show diffuse scattering effects, indicating sections with an elevated carbon concentration in the matrix. With increasing aging temperatures the carbon atoms in the enriched sections are redistributed, their concentration increasing, and then (at 120°) these sections are rebuilt into carbide phase with hexagonal symmetry. 3. An increase of the chromium content of the steel weakens the intensity of diffuse effects from the enriched sections. Reflections of ɛ carbide are correspond'ngly weakened, indicating a smaller amount of it, and it disappears completely in the steel with 7% Cr. With increasing chromium concentrations the carbide phase with the orthorhombic lattice of cementite is formed at a lower temperature.