Early investigations of the phenomenon of Stress Relief Cracking in low alloy steels suggested that the observed failure along prior austenite grain boundaries was in large part promoted by the relative strengthening of the grains with respect to the grain boundaries, during the stress relieving heat treatment. In this respect, it was felt that certain Cr-Mo-V steels containing vanadium carbide should be particularly susceptible to this mode of failure, since V4 C3 is well known to confer very

more »

... ood creep strengthening in these low alloy steels. As a result of this work, it has been established that the above view is both inaccurate and misleading. Most of the experiments were performed on two samples of commercially produced 1/2Cr 1/2Mo 1/2V steel. One of these samples had proved to be susceptible to stress relief cracking during fabrication. Detailed observations of the austenitic grain growth and secondary hardening response of the two samples are described. The results facilitated the design of a series of high temperature hardness and tensile tests in which the separate effects of grain strengthening and grain size in promoting high temperature intergranular failure could be identified. In particular, it is observed that although both steels exhibit very similar strength characteristics, the effect of grain size in promoting low ductility in association with intergranular failure is more marked in one steel than in the other. The steel in which a marked tendency towards stress relief cracking had been observed is seen to show low ductilities at high temperatures even in fine grain size specimens ; suggesting that the state of the boundaries, independent of the strengthening effect, is an important factor in promoting high temperature intergranular failure. By using an anisothermal stress relaxation testing procedure, it was possible to identify precisely the conditions of stress,temperature and microstructure under which stress relief cracking occurs. In particular, it was observed that the susceptible sample [...]