TWI Industrial Member Report Summary 939/2009
V C M Beaugrand and L S Smith
Background
Dissimilar joints are a common part of subsea oil and gas systems. Typically, forged and machined hubs or tees made from low alloy steel 8630M, 4130, or F22, are buttered with a nickel alloy, such as Alloy 625, and then joined to X65 or X70 pipeline micro-alloyed steels. The main advantage of buttering is that it enables the forgings to be postweld heat treated prior to making the completion weld in the field, as the HAZ of the completion weld lies wholly in the buttered layers. As for any fusion welded dissimilar joint, a narrow, partially mixed zone (PMZ) is observed at the dissimilar interface, where the composition is graded between that of the parent forging and the bulk chemistry of the first layer buttering runs. In order to prevent corrosion of piping systems subsea, cathodic protection (CP) is applied. Whilst this does inhibit corrosion it can result in a certain amount of hydrogen charging. Whilst the vast majority of subsea joints have given successful service, a small number has failed with significant consequence, warranting the present study.
Hydrogen induced cracking at dissimilar interfaces has been observed in vessels for high temperature hydrogen service, specifically at the interface between austenitic claddings and low alloy Cr-Mo steels. Here, rather than hydrogen charging through CP, thermal hydrogen charging is experienced and cracking is a risk upon cooling during shutdowns. Here, Zone M cracking was defined as occurring at the interface itself, Type II cracking followed the first grain boundary in the fusion zone, close to the interface in the PMZ, and Type III cracking was in the steel HAZ (Asami et al, 1981).
Whilst there are similarities in terms of the interface between the high temperature hydrogen service claddings and the butter layers of the subsea dissimilar joints, there are some obvious differences in parent metals and consumables employed, fabrication and, of course, hydrogen charging conditions. This report describes work conducted to better understand the microstructure and chemistry of a typical subsea dissimilar metal joint, and its relationship with the failure mechanism in hydrogen charging conditions. The primary aim is to understand the failure mechanism(s) that may operate in dissimilar joints in subsea service. In addition, a small number of single layer butter deposits were examined to determine the influence, if any, of changing the butter weld parameters.
Objectives
- Comprehensively characterise the microstructure and chemistry of a steel forging to nickel alloy butter weld deposit dissimilar metal interface, and to identify those microstructural features most susceptible to hydrogenembrittlement.
- Determine the crack initiation/growth mechanisms under CP and to establish the relationship between microstructure and cracking mechanism.