Microstructure control for toughening a high carbon martensitic stainless steel

A High carbon martensitic stainless steel (Fe-12%Cr-0.7%C) was subjected to isothermal aging after full solution treatment, and then solution-treated again in the two phase region of austenite and M₂₃C₆ carbide; partial solution (PS) treatment. Microstructural development during these heat treatment was investigated, and mechanical properties for the PS treated steel were evaluated by comparison with a referencial steel without isothermal aging. During the isothermal aging at 923K, the supercooled austenite undergoes eutectoid transformation and decomposes to ferrite and M₂₃C₆ carbide, and the carbide particles are finely dispersed within the matrix. Insoluble carbide particles retained during the following PS treatment contribute to suppressing austenite grain growth effectively through the grain boundary pinning effect. Ductile-brittle transition in the PS treated steel is characterized by a lower transition temperature and a higher upper-shelf energy compared with the referencial steel. This is due to the grain refining of prior austenite as well as the homogeneous dispersion of carbide particles through the PS treatment with isothermal aging.