Abstract
Dissimilar metal welds are necessary in high-pressure subsea systems and in cases where forged components must be welded to pipelines. F22 (2.25Cr-1Mo) steel is often used in such forged steel components and, since this steel cannot enter service without undergoing post-weld heat treatment (PWHT), the components are usually prepared for field welds through the application of a buttering layer. Furthermore, a weld overlay is deposited for the purpose of mitigating corrosion. This combination of multiple welding tasks and dissimilar materials leads to the possibility of developing substantial residual stresses. This study aims to provide insights to the evolution of residual stresses at each stage of the welding operation. The assessment has been undertaken on laboratory-scale weld mock-ups using the contour method for residual stress measurement, and incremental centre hole drilling. It was found that both buttering and cladding introduce near-yield levels of tensile residual stresses, but that these stresses are successfully relieved upon PWHT.
Highlights
Introduction and backgroundDissimilar metal welds (DMWs) are used in the construction of many components in the oil and gas industry, including in highpressure subsea systems
The cladding operation took place after buttering and, the tensile stresses across large regions of the buttering layer are in the range between 150 and 400 MPa, they are higher at the interface between the buttering layer and the cladding overlay
The evolution of welding residual stresses during the buttering, cladding and post-weld heat treatment (PWHT) processes in dissimilar metal welding was investigated in this study
Summary
Dissimilar metal welds (DMWs) are used in the construction of many components in the oil and gas industry, including in highpressure subsea systems. F22 steel forgings are widely used in the manufacturing of pressure retaining valves that need to be welded to steel pipelines (typically API X65 steel). As F22 components are sensitive to hydrogen cracking, post-weld heat treatment (PWHT) is essential in order to relieve the residual stresses and to temper the brittle microstructures that form in the heat affected zone (HAZ) of welds. PWHT becomes impractical after components have been joined to pipework and it would in general, result in degradation of the material properties of the steel pipeline. The residual stresses that are introduced by welding can affect the structural integrity of safety-critical components [1,2] A buttering technique, involving the deposition of several layers of low-alloy steel, is usually used so that the buttered F22 steel can be heat treated before the closure weld (i.e. to adjoining pipework) is carried out.
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