Influence of cold-work on the cavitation erosion resistance and on the damage mechanisms in high-nitrogen austenitic stainless steels

Abstract

► Cold-rolling greatly improves the cavitation-erosion resistance of high temperature gas nitrided fully austenitic UNS 31803 stainless steel, when compared to the simply high temperature gas nitrided steel. ► Cold-rolled and recrystallized high temperature gas nitrided random textured fully austenitic UNS 31803 stainless steel specimens, showing a random texture are less cavitation-erosion resistant than the strongly (1 1 0) // surface textured as nitrided specimens. ► Cavitation-erosion damage in the cold-rolled specimens initiates mainly at coherent grain boundaries and slip lines initiation is rarely observed. ► Variations of the stacking fault-energy, grade of short range ordering and mechanical twinning are discussed as possible mechanisms of enhancing the cavitation erosion resistance of the cold-worked high temperature nitrided UNS 31803 stainless steel. UNS S31803 duplex stainless steel specimens, 1.5 mm in thick, were high temperature gas nitrided (HTGN) in order to obtain a 0.9 wt% N, through the thickness fully austenitic microstructure with increased cavitation-erosion (CE) resistance. The microstructure of the as received UNS S31803 duplex stainless steel consists of ferrite + austenite stringers (50% α + 50% γ). HTGN was performed at 1200 °C for 8 h, in a 0.1 MPa N 2 atmosphere, followed by direct quench in water. A fully austenitic microstructure, 330 HV 0.1 hard, containing 0.9 wt% N in solid solution, was obtained. Part of the specimens were 30% cold rolled and part of the specimens were annealed after cold-working, obtaining cold worked and recrystallized specimens. All the specimens, including the as received UNS 31803 duplex steel, were tested in a 20 kHz vibratory cavitation erosion testing equipment. The HTGN specimens showed lower CE wear rates and higher incubation times, when compared to the as received condition. The 30% cold worked HTGN specimens showed the greatest increase in incubation times and greatest decrease of cavitation wear rates. The recrystallized specimens showed lower CE resistance when compared to HTGN samples. The difference was attributed to development of a sharp {1 0 1}//surface texture during HTGN, which was weakened after recrystallization. The mechanisms responsible for the improvement of CE performance are discussed.