The effects of extrusion ratio and heat treatment on microstructures and tensile properties of a powder metallurgy Fe–22Mn steel

Powder metallurgy Fe–22Mn (wt.%) steel rods were fabricated by powder compact extrusion followed by heat treatment, and their microstructures and tensile properties were investigated. The microstructure of the as-extruded rods consisted of γ-austenite, ε-martensite, α'-martensite and a dispersion of ferrous manganese oxide (FMO) particles. By increasing the extrusion ratio from 9:1 to 16:1, the volume fractions of γ and ε increased significantly, while that of α' decreased significantly, which is due to more Mn atom diffused into the γ phase and improved its stability. This situation led to a significant decrease in yield strength without much change of the tensile strength and ductility. γ→ε and ε→α' transformations occurred during plastic deformation, resulting in a strengthening of the strain hardening capacity. The γ→ε transformation occurred only when the strain was <0.1, and beyond this strain, the volume fraction of γ decreased to a level that would not allow further phase transformation in γ. Annealing at 1200 °C for 1 h makes Fe atoms undergone segregation in FMOs, causing an obvious decrease of the hardness and Young’ s modulus of the FMO particles to a level lower than that of the matrix, which brings more severe damage to the ductility of the material.