Room Temperature Creep Behaviour of Pipeline Steels and Its Influence on Stress Corrosion Cracking

Room temperature creep is often of significant importance in structural materials. Its occurrence, for example, may be an important factor contributing to crack growth during stress corrosion cracking. For pipeline steels used in gas transmission, room temperature creep deformation near the crack tip may result in a time dependent crack growth. In this research, room temperature creep of two pipeline steels with different grades was studied under various loading conditions including pure static, pure cyclic and a combination of static and cyclic loading. The creep deformation under a stress higher than the yield strength may represent the deformation behaviour at the crack tip. Due to cyclic hardening, all the steels crept at a stress higher than the yield strength exhibit cyclic creep retardation, which is less pronounced at lower stress-ratio (minimum stress/maximum stress). Pre-cyclic loading has significant effect on subsequent static creep. In general, pre-cyclic loading causes a burst of creep deformation under subsequent static loading, which may result in significantly larger cumulative creep strain than that of pure static creep depending on the initial loading strain and the number of cycles in pre-cyclic loading. The burst in creep deformation requires an incubation period that increases with number of prior load cycles. The burst strain is dependent on the number of cycles of prior cyclic loading in a more complicated manner. The implication of the creep behaviour observed in these tests is also discussed in terms of stress corrosion cracking in the pipeline steels.