Fusion Zone Microstructure Associated With Embrittlement of Subsea Dissimilar Joints

Abstract

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.

Butter-weld Interface Microstructural Analysis Employing NCS and LAMS Sections

“…However, at typical PWHT temperatures the carbon activity in the weld metal remains comparatively low. This is attributed to the diffusivity in the austenite being considerably smaller than in the ferrite and the solubility of the carbon is higher 22 .…”

“…The thickness of this zone varied and, in general, had a dimension of approximately 10μm. Dodge et al 22 found a planar zone with a thickness of 32 μm, applying a lower welding energy. A positive factor in reducing the thickness of zone Φ is the inhibition of formation of Cr 7 C 3 -type carbides in this zone, which may arise mainly after prolonged PWHT 22 .…”

“…Figure 3 (b) shows the predominance of ferrite and / or retained austenite microstructure in the BM, corroborating the effect of carbon diffusivity in the vicinity of the fusion line. Dodge et al 22 observed that high values of carbon activity are present in ASTM A182 F22 steel when compared to the addition metal, with the carbon likely to diffuse in the direction of its thermodynamic activity gradient. Alexandrov et al 19 observed that after PWHT carbon tends to migrate from BM to austenitic WM.…”

“…Considering the PWHT of 6 hours at 677ºC, represented in Figure 4 by the yellow point, there is a favoring in the formation of Fe 3 C, Cr 7 C 3 and Mo 2 C. Among these carbides mentioned, Fe 3 C and Cr 7 C 3 commonly found in the interface of dissimilar welded joints, are particularly sensitive to hydrogen embrittlement. These carbides may act as hydrogen trapping sites and may be present at the dissimilar interface of the ARBL-Inconel 625 23 , providing a low energy fracture path mainly along the zone M 22,30,31 .…”

“…The F22 / 625 joint has shown promise due to the lower carbon content in its composition. The occurrence of hydrogen embrittlement, among other factors, is due to the microstructure at the dissimilar interface, mainly due to the precipitation of carbides 21,22 . The microstructure of the interface is a function of the chemical composition of the materials, the welding process and the PWHT.…”