Effect of Shot Peening on the Intergranular Corrosion Susceptibility of a Novel Super304H Austenitic Stainless Steel

The surface phase constituent of Super304H austenitic stainless steel, after shot peening and sensitization treatment at 600, 650, and 700 °C for 2 h, was characterized using x-ray diffraction method. The degree of sensitization (DOS) was investigated by means of double-loop electrochemical potentiokinetic reactivation (DL-EPR) test, and the morphology after DL-EPR test was observed by scanning electron microscopy (SEM). The results showed that nano-sized grains and strain-induced martensite together with compressive residual stress formed on the surface of Super304H steel after shot peening. Surface compressive residual stresses relaxed greatly after being sensitized at 600-700 °C for 2 h, and no systematic correlation was observed between the compressive residual stresses developed and the intergranular corrosion susceptibility (IGCS). Because of the occurrence of strain-induced martensite in the shot-peened specimens, their IGCS is much higher than that of the as-received specimen when being sensitized at 600-650 °C for 2 h. Besides, the DOS increased with the increasing of shot peening time and the content of strain-induced martensite. On the contrary, the IGCS of Super304H stainless steels subjected to shot peening was eliminated when being sensitized at 700 °C for 2 h because of the reverse transformation of strain-induced martensite and faster diffusion rate of Cr at higher temperature in ultrafine-grained austenite which had helped healing the chromium depletion zone in a very short time. In a word, shot peening promoted desensitization of Super304H steel in a time shorter than 2 h at higher temperature up to 700 °C.