Effect of Surface Nanocrystallization on the Sensitization and Desensitization Behaviors of the Novel Super304H Stainless Steel

Abstract

The newly developed Super304H austenitic stainless (SS) steel is being widely used in ultra supercritical fossil fired boilers because of its superior high temperature strength and oxidation resistance, but its high carbon content has increased greatly its intergranular corrosion susceptibility (IGCS). In the present paper, we have made an attempt to understand the sensitization and desensitization behaviors of the shot-peened Super304H SS during aging around service temperature. The shot peening on the Super304H SS resulted in its surface nanocrystallization accompanied with strain-induced αʹ-martensite. The strain-induced αʹ-martensite had good thermal stability at 600℃ and was blamed mainly for the significant increase of the degree of sensitization (DOS) of shot-peened Super304H SS when aged at 600℃ for different time. However, the DOS decreased greatly when aged at 650℃ for 24 h or 700℃ for 2 h, indicating the occurrence of a faster self-healing process of chromium depletion zones due to long range Cr diffusion enhancement by surface nanocrystallization and also the reverse transformation of αʹ-martensite to austenite at the same time. Figure 1