Subsea Dissimilar Joints: Failure Mechanisms and Opportunities for Mitigation

Dissimilar joints are a common part of subsea oil and gas systems. Whilst the majority of these joints have given successful service, a small number have failed by a complex combination of fracture modes. Environmental testing was carried out on a dissimilar joint between a forging made from a low alloyed steel grade 8630M and buttered using alloy 625. Tests were monitored using a sensitive technique in order to capture the very first moment of crack initiation and to determine the minimum threshold stress intensity factor K_1H, below which there is no environmentally assisted cracking. Two distinct initiation mechanisms were identified; one dependent on hydrogen-charging in service (under cathodic protection) and one not. A detailed microstructural investigation, including chemistry, structure and strain, gave insight into the attributes of the ‘partially mixed zone’ (PMZ) immediately adjacent to the fusion boundary, on the weld side, in which one of the failure modes occurred. The chemical and microstructural characteristic of this PMZ are influenced by welding and post weld heat treatment (PWHT) parameters. Based on this investigation, the mitigation of cracking susceptibility by modifying the welding/PWHT procedure is discussed.