The TIG (tungsten inert gas) process is widely applied in the North Sea for pipeline welding of riser tie ins and other subsea connections and is steadily kept available for possible repair work. There are attmepts to add also Gas Metal Arc (GMA) welding with argon-mixtures to established hyperbaric underwater welding processes. The purpose of this work is to establish reliable information on possible gas pick-up from worst case sources at pressure and study the respective effects on weld at pressure, including crack avoiding procedures by preheating. At hyperbaric welding, the possibility of gas infiltration into the weld metal from the applied chamber gas or rust or water leakage in the shielding gas supply is given, The present workm is concernded with the weld metal pick-up of nitrogen, hydrogen and oxygen, and the possibility of hydrogen delayed cracking regarding to hyperbaric welding repair of offshore units up to 16 bar. The cracking tests were performed with a common material (ST 52-3) in the early days of the North Sea development projects. Pulsed GMA welding was used.The shielding gas was argon with different partial pressures of oxygen, nitrogen and water vapour. The chamber gas was argon. The pressure dependent arc characteristic, which changes the material transfer and the penetration from the conventional 1 bar welding conditions, demanded the investigation under such simulated conditions. The absorbed weld metal nitrogen content decreases with an increase in chamber pressure at a constant partial pressure of nitrogen in the shielding gas, mainly up to 4 bar, thereafter the weld metal nitrogen content remains nearly constant. The absorbed weld metal hydrogen content decreases with an increase in chamber pressure for a constant partial pressure of water vapour in the shielding gas up to 3,6 bar chamber pressure, thereafter the weld metal hydrogen content remains constant. The weld metal hydrogen content increases with an increase in partial pressure of hydrogen with a constant slope, which is independent of the chamber pressure. For each chamber pressure the linear increase is limited by the saturation of hydrogen in the weld metal.
Contribution to the effects of ambient pressure and arc atmosphere on weld metal chemistry and hydrogen assisted cracking in hyperbaric pulsed GMA welding of St 52-3
Beitrag zu den Wirkungen von Umgebungsdruck und Bogen-Athmosphäre zu Schweißmetall- Chemie und Wasserstoff-unterstütztem Rißbeginn beim pulsierenden GMA Schweißen von St 52-3 unter Überdruck
1996
113 Seiten, 98 Bilder, 9 Tabellen, 96 Quellen
Hochschulschrift
Englisch
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