A metallurgical investigation of hottap and riser repair MIG welds produced under deepwater hyperbaric conditions of 160bar

摘要

A metallurgical evaluation of welds produced at 160bar pressure with ten different welding consumables was produced.The aims of the research were to prove the robustness of the welding process (capability of welding in various positions: downhand, vertical-up, vertical-down, overhead), and to produce improvements in the mechanical properties of the hyperbaric gas metal arc welds, in particular the weld-metal toughness. The welds were produced in a habitat made of pure argon using structural steel as base material. The metallurgical evaluation included the study of the hardness properties, Charpy impact toughness, tensile behaviour, and weld-metal microstructural characteristics, amongst others. In general, it was found that the use of three commercial wires designed for one-atmosphere applications reported poor impact toughness as a consequence of low oxygen content (< 100 parts per million). On the other hand, welds produced with specially-designed consumables with an oxygen level between 200-350ppm oxygen showed remarkably-better Charpy performance. In both cases, the nucleation of non- metallic inclusion was linked with the oxygen content. The acicular ferrite was seen to have a direct influence on the weld-metal notch-impact toughness. However, the nucleation of acicular ferrite was strongly influenced by the oxygen level present in the welds that were produced.