Effect of particle size on the microstructure evolution and microsegregation behavior of rapidly solidified high-carbon martensitic stainless steel powder

The size-dependent microstructural evolution and microsegregation behavior of gas-atomized high-carbon martensitic stainless steel M390 alloy powders were systematically investigated. The solidification microstructure of gas-atomized droplets consists of γ-Fe and metastable δ-Fe phases, along with a certain amount of primary M 7 C 3 and VC carbides. A substantial amount of metastable δ-Fe phase, along with carbides exhibiting a lower area fraction, is formed in small-sized powders (<50 μm) due to the high cooling rate and large undercooling. Elongated primary M 7 C 3 and needle-like VC carbides are discretely distributed in the interdendritic regions. As the powder size increases, the content of metastable δ-Fe sharply decreases, and microstructure is dominated by the γ-Fe phase. The continuity of primary M 7 C 3 carbides growth is enhanced, resulting in large clusters or brain-like structures consisting of primary M 7 C 3 carbides distributed in the interdendritic regions, while VC carbides gradually transform into a spherical morphology, with both their area fraction and size increasing significantly. The microsegregation coefficient ( k X ) and the area of the microsegregation regions between adjacent dendrites decrease with the decrease in powder size, and the cooling rate associated with 150 μm droplets is considered the critical cooling rate that effectively suppresses microsegregation. Thermally induced dendritic evolution and variations in local solidification time, driven by cooling rate and undercooling, are the direct factors of the size-dependent microstructural evolution and microsegregation behavior.