Thermal stability of a nanostructured layer on the surface of 316L stainless steel

To obtain a nanocrystalline surface layer, 316L stainless steel was treated by fast multiple rotation rolling (FMRR). The microstructure, after FMRR treatment and annealing treatment, was characterized by transmission electron microscopy and x-ray diffraction. Equiaxed nanocrystalline with the average grain size about 12 nm is obtained on the surface layer of FMRR sample. The investigation of thermal stability of the nanocrystalline layer indicates that the grains are still nanocrystalline and the average grain size is about 60 nm for annealing at 500 degrees C. In addition, the amount of alpha-martensite increases markedly as the annealing temperature increases from 300 to 500 degrees C. However, it begins to reduce at 600 degrees C due to the reversion transformation from martensite to austenite. After annealing at 400 degrees C, the microhardness of the annealed FMRR sample reaches a maximum value of about 660 HV, and it is four times higher than that of the original sample.