Cavitation erosion resistance of a high temperature gas nitrided duplex stainless steel in substitute ocean water

The cavitation erosion (CE) resistance of a high temperature gas nitrided (HTGN) UNS S31803 duplex stainless steel (DSS), in substitute ocean water, was evaluated. The CE tests were performed in a vibratory cavitation testing equipment according to ASTM G32-92. For comparison purposes, solution treated samples of the same DSS, as well as of an austenitic UNS S30403 stainless steel (SS), were also studied. After high temperature gas nitriding (HTGN), the duplex stainless steel showed an austenitic surface layer, containing high nitrogen content in solid solution. Five sets of specimens with similar (0.8 wt%) nitrogen contents at the surface, but different grain diameters and increasing texture intensities, were investigated. Three sets of samples with similar grain size and texture but increasing nitrogen contents, between 0.65 and 1.15 wt%, were also studied. The CE mass loss rate of the nitrided samples decreased from 8 to 23 times as compared to the DSS solution treated samples. It was observed that the CE mass loss rate decreased with decreasing grain size, increasing the texture components sharpness or with increasing nitrogen content. The HTGN treatment allowed obtaining samples with CE resistance similar to the cobalt base Ireca and Stellite hard facing alloys. The results of CE tests were analyzed taking into account the major damaging mechanisms, which were examined by scanning electron microscopy observations of eroded surfaces.