REVERSION MECHANISM FROM DEFORMATION INDUCED MARTENSITE TO AUSTENITE IN METASTABLE AUSTENITIC STAINLESS-STEELS

Reversion mechanism from deformation induced martensite (alpha') to austenite (gamma) has been investigated in two metastable austenitic stainless steels, 15.6%Cr-9.8%Ni (the 16Cr-10Ni) and 17.6%Cr-8.8%Ni (the 18Cr-9Ni) steels, by means of magnetic analysis and transmission electron microscopy. Metastable-gamma almost completely transforms to lath-alpha' by 90% cold rolling, and the alpha' again reverts to gamma-during annealing at temperatures above 700 K. Deformation induced-alpha' in the 16Cr-10Ni steel undergoes a martensitic shear reversion during heating to 923 K annealing, while that in the 18Cr-9Ni steel does a diffusional nucleation-growth reversion on 923 K annealing. Grain refining processes are greatly influenced depending on the reversion mechanism, Martensitically reversed-gamma has a high density of dislocations immediately after the reversion and the gamma-grains are refined through recovery and recrystallization process just like that taking place in a deformed-gamma. On the other hand, diffusionally reversed-gamma is characterized by the nucleation of equiaxed gamma-grains within the alpha'-matrix and the gamma-grains gradually grow during annealing. The reversion mechanism significantly depends on the chemical compositions of steels and annealing temperature. An increase in the Ni/Cr ratio causes an increase in the Gibbs free energy change between fcc and bcc structure, leading to a fall-down of austenitizing temperature for the martensitic shear reversion, The critical driving force required for the complete martensitic shear reversion is about -500 J/mol. To obtain the critical driving force in the 18Cr-9Ni steel, it should be heated to a high temperature above 1 023 K. However, the diffusional reversion can easily occur because the martensitic shear reversion temperature is too high in the 18Cr-9Ni steel. The 16Cr-10Ni steel also undergoes the diffusional reversion when it was annealed at low temperatures below the martensitic shear reversion, 923 K.