Effect of the Presence of the sigma Phase on the Chemical Composition and Structure of Carbide M sub 23 C sub 6 Segregating in Austenitic Steels of the 25/20 Type

摘要

Nucleation of sigma phase during annealing of austenitic steel of type 25/20 causes a drop in the carbide content in this steel and a change in its chemical composition and morphology. The chromium content in the carbide diminishes, and its atoms are replaced by iron atoms and partially by nickel atoms. Decomposition of the skeletal morphology of the carbide and a refinement of its particles ensue. A drop in the chromium content in carbide M sub 23 C sub 6 leads to a reduction in its stability and unmixing. Carbide M sub 7 C sub 3 leads to a reduction in its stability and unmixing. Carbide M sub 7 C sub 3 takes the place of carbide M sub 23 C sub 6 . The presence of the sigma phase and the inability of the carbide M sub 23 C sub 6 to achieve a chemical composition of equilibrium hinder the growth process and the adherance of particles during protracted steel annealing. The changes observed in the carbide suggests that the sigma phase nucleates at its particles. This assumption is supported by the carbide inclusion observed in the coagulated sigma phase.