Slow, environmentally assisted crack growth has been observed during the fracture-toughness testing of a Fe-20Co-15Ni-5%Mo maraging alloy in the ‘underaged’ condition. This phenomenon has been shown to be associated with fracture along a soft, corrosion-sensitive path at the prior austenite grain boundaries. The susceptibility of the alloy is influenced by both its oxygen content and the humidity of the environment. As ageing proceeds the effect diminishes; toughness first recovers, as microvoid coalescence develops towards peak hardness, and then falls during over ageing when a cleavage mode becomes established.
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