Process of reversion and mechanical properties in metastable austenitic stainless steels containing molybdenum

Effect of molybdenum on the behavior of reversion of deformation induced martensite to austenite and the mechanical properties of specimens containing reversed austenite were investigated within (15.5 approximately 13)% Cr-11%Ni-(0 approximately 2.5)%Mo steels by means of transmission electron microscopy, magnetic analysis and tensile testing. Reversion treatment of martensite to austenite was performed at a higher heating rate using a salt bath and at lower heating rate using an electric furnace. The results obtained are as follows. (1) The steels used almost transform to lath martensite (α′) from austenite (γ) by 90% cold rolling at 290 K. Effect of molybdenum on γ stability against deformation is almost same as that of chromium. (2) The temperature range for reversion rises with increasing molybdenum content. (3) In case of 923 K annealing at a higher heating rate, a process of reversion is changed from a martensitic type to a diffusional one with increasing molybdenum content. A 13%Cr-11%Ni-2.5%Mo steel which reverts to γ by a diffusional process has an ultra fine austenitic structure with a mean grain size of 0.2 μm giving a high 0.2% proof stress of 0.8 GPa, tensile strength of 1.0 GPa, and elongation of 30%. (4) In case of 923 K annealing at a lower heating rate, α′ reverts to γ by a diffusional process at lower heating temperatures. Therefore, molybdnum content of 0.5% is enough to obtain a high strength steel.