REVERSION PROCESS OF DEFORMATION INDUCED MARTENSITE TO AUSTENITE IN METASTABLE AUSTENITIC STAINLESS STEELS.

The reversion process of martensite ( alpha prime ) to austenite ( gamma ) was investigated using 15. 62 percent Cr-9. 83 percent Ni (16-10) and 17. 62 percent Cr-8. 78 percent Ni (18-9) steels with metastable austenitic structure by means of transmission electron microscopy and magnetic analysis. The used steels transform to lath alpha prime from gamma by 90 percent cold rolling, and reverse to gamma again by the following annealing at temperature above about 900 K. The reversion processes are greatly different in the two steels. The difference is clear for 923 K isothermal annealing after 90 percent cold rolling. The results are: (1) for 16-10 steel, alpha prime reverses to gamma during heating up to 923 K by a martensitic shear transformation. The gamma structure contains high density of dislocations and traces of alpha prime block which was induced by cold rolling. During annealing at 923 K, the reversed gamma is refined to 0. 5 mu m in grain size by a recovery process. (2) for 18-9 steel, alpha prime reverses to gamma during annealing at 923 K by a diffusional nucleation-growth process. (3) The reversion process of alpha prime to gamma is greatly dependent on the composition of steels and annealing temperature.