Hydrogen Embrittlement of 8630M/625 Subsea Dissimilar Joints: Factors That Influence the Performance

Abstract

Following the failure of a small number of subsea dissimilar joints, there is significant interest in understanding the fracture mechanism(s) and in qualifying future items for avoidance of further failures. Subsea dissimilar joints typically comprise a butter weld deposit onto the joint face of an alloy steel forged hub/tee/elbow, postweld heat treated, and completed with a closure weld to a pipeline steel. The case examined in this paper, consists of a build up of alloy 625 onto an 8630M forging. Hydrogen embrittlement test methods are described and the results from the tests interpreted with respect to their implications for fabrication and service. The microstructure and chemistry across the dissimilar interface is found to be of fundamental importance, and this is illustrated by the relative performance of gas-tungsten arc (GTA), hot-wire GTA and friction welds. Guidance for fabrication, installation and service is given for avoidance of further hydrogen embrittlement failures.