Austenite grain size control by insoluble carbide in martensitic stainless steels

Effect of insoluble carbide on the size of prior austenite (γ) grains was investigated in martensitic stainless steels with the chemical composition of 12%Cr-(0.12 to approximately 0.34)%C(in mass%). When fully tempered martensite (α′) is subjected to partial solution treatment; the solution treatment in (γ+M₂₃C₆carbide) two phase region, the growth of γ grains is effectively suppressed by insoluble carbide particles retained in the γ matrix owing to the grain boundary pinning effect. The mean diameter (D) of γ grains changes depending on the volume fraction (f) and diameter (d) of dispersed carbide particles and is given by the equation; D = β(d/f) (β = 1.0 to approximately 1.5). With increasing carbon content of steels, the growth of γ grains is suppressed up to higher temperature For instance, γ grain size of a 12%Cr-0.34%C steel can be kept as small as about 20 μm even at a high temperature around 1300 K. Smaller the size of dispersed carbide particles is, γ grain size is expected to become liner. However, the grain refining by partial solution treatment was limited to around 12 μm, because, in tempered martensitic steels, homogeneous dispersion of carbide particles is not obtained due to the fact that carbide precipitates preferentially on the boundaries of martensite laths and blocks.