The effect of heat treatment on the fracture toughness and subcritical crack growth characteristics of a 350-grade maraging steel

The heat treatment response of a 350-grade maraging steel, with the nominal composition 18.5 Ni, 12 Co, 4.6 Mo, 1.4 Ti, balance Fe, has been determined in billet and bar form. When aged at temperatures below 900°F, the material was very susceptible to subcritical crack growth, and premature brittle fracture occurred in unnotched tension specimens loaded at a slow strain rate in laboratory air. Fracture mechanics was used to interpret this behavior. The introduction of reverted austenite significantly decreased the strength level but had little effect on fracture toughness. The resistance to brittle fracture of this material is contrasted with that of high-strength steels currently used by the airplane industry.