Microstructural Contributions to Stress Corrosion Cracking in High Strength Steels

The effects of microstructural variables upon stress corrosion cracking resistance of 300M steel in 3.5% NaCl solution have been studied. Table 1 lists KIscc values for three heat treatment conditions. The martensite substructure was predominantly twinned plates for conventional heat treatment and dislocated laths for both high temperature and step heat treatments. A typical twinned region observed in the conventional heat treatment is shown in Fig. 1. Such twinned regions were less frequent in the other heat treatments. Both cementite and epsilon carbide were seen in all cases, as illustrated in Figs. 2 and 3 for conventional heat treatment. Epsilon carbide was usually observed within large grain boundary nucleated laths (autotempered martensite), with definite habit planes and growth directions. The formation of such laths have been discussed previously. Retained austenite, mostly in the form of interlath films, was observed in all cases with increased amounts present in coarser grained structures (Fig. 4).