1991 Volume 31 Issue 12 Pages 1431-1437
Reversion mechanism from deformation induced martensite (α') to austenite (γ) has been investigated in two metastable austenitic stainless steels, 15.6%Cr-9.8%Ni (the 16Cr-10Ni) and 17.6%Cr08.8%Ni (the 18Cr-9Ni) steels, by means of magnetic analysis and transmission electron microscopy. Metastable γ almost completely transforms to lath α' by 90% cold rolling, and the α' again reverts to γ during annealing at temperatures above 700 K. Deformation induced α' 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 γ has a high density of dislocations immediately after the reversion and the γ grains are refined through recovery and recrystallization process just like that taking place in a deformed γ. On the other hand, diffusionally reversed γ is characterized by the nucleation of equiaxed γ grains within the α' matrix and the γ 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.