Strength correlation with residual ferrite fraction in 9CrODS ferritic steel

The microstructure and tensile properties at 973 K in 9CrODS steels were investigated with respect to various fractions of the residual ferrite from zero to 47 vol.%. The formation of the residual ferrite was discussed from a balance between a chemical driving force for α to γ reverse transformation and the oxide particle pinning force, while 0.04 mass% carbon sample could contain an equilibrium δ-ferrite. With increasing volume fraction of the residual ferrite, strength at 973 K increases but ductility decreases, which is attributed to the fact that the residual ferrite is harder than the tempered martensite.