Two-Pass Friction Stir Welding of Cladded API X65

Abstract

Cladded materials are used by the oil and gas industry due to the combination of good mechanical properties and low cost of high-strength low-alloy steel (HSLA) API 5L X65 with the excellent corrosion resistance of the Inconel 625. This combination of materials are specially used on rigid risers, where aggressive substances such as organic acids, carbon dioxide and H2S are present inside the pipeline. Conventional arc welding processes bring typically several issues, such as long welding times, melting of the involved materials, Fe dilution inside the weld metal (generally Inconel 625) and brittle phase precipitation. Friction Stir Welding (FSW) joints can produce better mechanical properties and lower tensile residual stresses when compared with conventional arc welding processes, having at the same time greater reproducibility and control of process parameters. Thus, a two-pass FSW process on 7mm cladded API 5L X65 HSLA steel plates were analyzed, the first pass of the process consisted in welding a 4mm API 5L X65 steel layer while the second pass consists of joining the 3mm Inconel 625 layer. Detailed hardness and microstructural analysis were performed. Correlation with the underlying physical mechanisms are discussed.