Environment-Induced Cracking in Weld Joints in Subsea Oil and Gas Systems: Part II

Abstract

In subsea oil and gas systems, low-alloy steel (LAS) forgings need to be welded to leaner steels such as X and F-65. While the LAS needs to be post-weld heat treated (PWHT) to relieve stresses and temper the HAZ microstructure in order to avoid hydrogen cracking, the same heat cycle would, in general, result in the degradation of the properties of the leaner alloy. A buttering technique is, therefore, usually used so that the buttered LAS forging can be heat treated before the closure weld is carried out. In the case of clad components, nickel alloy filler materials such as Alloy 625 are commonly used for both buttering and closure welds. This is an issue for subsea structures protected from corrosion by cathodic polarisation (CP) using aluminium based anodes. Whilst CP has proven successful as a means of preventing corrosion of steel components within subsea structures, failures along the dissimilar metal interfaces have been observed. This is due to hydrogen evolution as a result of CP. To further our understanding on this issue, this paper focuses on the correlation between microstructures, obtained by changing material combinations and PWHT conditions, and the resistance to hydrogen assisted cracking. Slow strain rate single edge notched bend (SENB) tests were carried out on the interfaces between AISI alloy 8630 and Alloy 625 buttering, retrieved from subsea service and tested in 3.5% NaCl solution under an applied potential of −1100mVsce. Retrieved specimens were pre-charged with hydrogen and tested at 4°C and 80°C, approximately in-line with commissioning/shutdown and service temperatures, respectively. In addition to the retrieved specimens, a testing programme has been developed to explore the effect of PWHT time on the performance of 8630-Alloy 625 and F22-Alloy 625 interfaces. The microstructures most susceptible to hydrogen cracking in these systems have been assessed by examination of the SENB test specimens.