I I 1 The Effect of Trace Imparitieo on the Stress -

A series of 18 Ni (300 grade) maraging steels of overall commercial purity but containing also deliberate impurity additions of sulphur, phosphorus, carbon, chromium and silicon + manganese has been studied. The fracture toughness and stress-corrosion resistance (determined using planestrain fatigue-precracked specimens tested in 3. 5% sodium chloride solution) of these steels have been compared to the fracture toughness and stresscorrosion resistance of a commercial purity 18 Ni (300 grade) 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 carbon content is less than 0. 005%; (ii) Simultaneous additions of Mn + Si result in extremely low frr.cture toughness values; (iii) High carbon contents (greater than 0. 03%) result in marginally improved stress-corrosion resistance; (iv) High Cr contents result in rather poor stress-corrosion properties. These results have been correlated with the electron transmission microstructure of the steels and the results of a fractographic analysis of the fracture surfaces.