CORROSION-FATIGUE RESISTANCE OF POWDER-METALLURGY AND FORGED DUPLEX STAINLESS-STEELS

The high cycle corrosion fatigue resistance of powder metallurgically (PM) fabricated and hot isostatically pressed (hipped) duplex stainles s steels (DSSs) was investigated and compared with that of a conventio nal forged DSS. Tests were performed in rotating bending fatigue in a chloride and sulphate containing aqueous solution at room temperature. The hipped PM DSSs studied had small grain size and homogeneous micro structure and isotropic properties and thus, their high cycle corrosio n fatigue resistance was generally markedly higher than that of the co rresponding forged DSS tested in the longitudinal direction. Fatigue c racks were observed to grow preferentially in the ferrite phase and a retarding effect of the austenite phase on corrosion fatigue crack gro wth was observed both in the hipped PM and wrought DSSs. Slow cooling after solution annealing improved the corrosion fatigue resistance, ev en though it induced slight second phase precipitation. Localised corr osion attack was observed at inclusion/matrix interfaces, when oxide i nclusions were cerium modified (forged DSS). When oxide inclusions wer e either unmodified or calcium modified (hipped PM DSSs), no localised corrosion attack was observed.