Effect of hyperbaric gas composition on mechanical properties of the weld metal

Selection of chamber gas composition for a viable and time-consuming underwater repair operation is influenced by the final mechanical properties that each shielding environment can offer. Tensile and fracture toughness tests were performed on five bead-on-plate test samples that were welded under different chamber gas environments at 10bar. The chemical composition, microstructure and mechanical properties of the weld metal were investigated. It was found that the pure He chamber gas offers the best visual and mechanical properties required by standards despite its high costs. CO2 chamber gas was discovered to introduce abundant micro and nanopores in the matrix that can potentially degrade the mechanical properties. He–CO2 mixture however, was assessed to be detrimental due to both dominating bainite transformation and distributed pores in the weld metal. The properties of the weld metal using Ar and Ar–He mixtures were found to be slightly poorer than the properties in pure He case, yet better than the rest of the samples.