Within subsea oil and gas systems, nickel alloy filler metals are commonly used in the joining of ferritic steels with different mechanical properties. An example of this is the joining of low alloy steel (LAS) forged manifold hubs (e.g 8630 or F22) to low hardenability pipelines steels, such as X65). The joint is part of a two stage welding process that simplifies offshore installation. Initially, a weld deposit is made on the forging using a multi-pass ‘buttering’ technique, providing intermediate layers of a suitable material, such as Alloy 625. A postweld heat treatment (PWHT) is applied to the buttered forging, onshore, to temper the hard heat affected zone (HAZ). After machining a bevel into the buttering layer, a closure weld, applied offshore, is employed to join the pipeline to the forging. As the buttering layer and linepipe are not critically hardened by the closure weld, no PWHT is required.
To prevent corrosion, subsea systems of this kind, are subjected to cathodic protection, via aluminium based anodes. Whilst successful in protecting the ferritic parts within the manifold structure, a number of high profile failures has been attributed to the evolution and ingress of hydrogen, and its diffusion to, the fusion zone of the dissimilar weld.
To investigate the susceptibility of fusion zone microstructures to hydrogen embrittlement, three dissimilar weld samples were fabricated: 8630-Alloy 625, F22-Alloy 625 and F22-309LSi. The latter is not a combination normally used to complete joins of this type. Each specimen was the subject of thermodynamic and kinetic modelling studies using Thermo-Calc™ and Dictra™, with microscopic examination by scanning electron microscopy (SEM) combined with energy-dispersive X-ray (EDX) and electron backscattered diffraction (EBSD). Nano-scale features were also investigated by transmission electron microscopy (TEM). By comparing the fusion zones of the three different joints in both as-welded and heat-treated conditions, the metallurgical aspects relating to hydrogen embrittlement are revealed. The results indicate that precipitates, which form in a zone of carbon diffusion during PWHT, are particularly harmful in the presence of hydrogen. The formation of this hydrogen susceptible region is partly due to the initial solidification structure morphology, the formation of which is also discussed.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.