Stress relief cracking in creep resisting low alloy ferritic steels.
Tait, Robert Andrew
University of Cambridge
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Tait, R. A. (1976). Stress relief cracking in creep resisting low alloy ferritic steels. (doctoral thesis).
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 good 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 failed by nucleation controlled intergranular cavitation in association with a particular stress/strain-rate/temperature regime where the deformation process is rate controlled by the diffusion of carbon. Further experiments described in the latter part of the thesis examine the effect of purity in promoting intergranular failure both at low and high temperatures. The results suggest that the presence of impurities like phosphorus, which are known to segregate to prior austenite grain boundaries, may act to allow easier nucleation and growth of cavities during failure by inter granular cavitation. The conclusions of the thesis outline the mechanism of stress relief cracking and make discussion in terms of current theories of intergranular failure at high temperatures. Recommendations are made concerning both the assessment of susceptibility of steels to this mode of failure, as well as the adoption of safer stress relief procedures.
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