Isolation of carbon and grain boundary carbide effects on the creep and intergranular stress corrosion cracking behavior of Ni-16Cr-9Fe-xC alloys in 360 °C primary water

The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity Ni-16Cr-9Fe-xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT) tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models.