The effect of trace impurities on the stress-corrosion cracking susceptibility and fracture toughness of 18Ni maraging steel

Abstract

A series of 18Ni maraging steels of overall commercial purity but containing also deliberate impurity additions of S, P, C, Cr and Si + Mn has been studied. The fracture toughness and stress-corrosion resistance (determined using plane-strain fatigue-precracked specimens tested in 35x NaCl solution) of these steels have been compared to the fracture toughness and stress—corrosion resistance of a commercial-purity 18Ni maraging steel with no deliberate impurity additions, and to a similar steel prepared from special high-purity melting stock. The most important conclusions reached are that: (I) Ultra-high purity steels do not have significantly improved stress—corrosion resistance but show useful increases in fracture toughness when the C content is less than 0005 per cent; (II) Simultaneous additions of Mn + Si result in extremely low fracture toughness values; (III) High C contents (greater than 003 per cent) result in marginally improved stress—corrosion resistance; (IV) High Cr contents result in rather poor stress—corrosion properties. The fracture toughness results have been correlated with the electron transmission microstructure of the steels and the results of a fractographic analysis of the fracture surfaces.