Friction surfacing of precipitation hardening stainless steel coatings for cavitation erosion resistance

The cavitation erosion (CE) behavior of friction-surfaced (FSed) 17-4PH precipitation hardening stainless steel coatings without/with post-aging (H900) in 3.5 wt% sodium chloride solution at 25 °C was investigated. The FSed 17-4PH coatings were more CE resistant than the AISI 304 substrate by 1.35-fold owing to grain refinement, high dislocation density in martensite and increase in hardness. Despite crack initiation at the soft δ-ferrite, the surrounding refined grains of martensite (α’) retarded the growth of cracks during CE. Moreover, the initiation sites for CE including the sulfide inclusions and the incoherent copper precipitations in the AR 17-4PH, were fragmented and dissolved into the α’ matrix after FS. Thus, the synergistic corrosion effect was suppressed and beneficial to the CE resistance of the FSed coatings. The effect of FS processing parameters on the CE resistance of the FSed coatings was negligible. After H900, the CE resistance of the FSed coatings was further improved by 1.9-fold due to the effective hardness increment and the formation of reversed austenite.