INFLUÊNCIA DA PRESSÃO NA ABSORÇÃO DE OXIGÊNIO

Abstract

Few technological data are available concerning operational limits of shielding gases aiming the application of the GMAW process in hyperbaric welding. In order to contribute to this knowledge, this paper presents results correlating the pressure and the oxygen content in the shielding gas to the weld metal chemical composition. The welding of bead on plate and multipass test joints was performed in the hyperbaric simulator of the GKSS Forschungszentrum Geesthacht GmbH, in Germany. The absolute pressures of 5,1 MPa (51 bar); 7,1 MPa (71 bar); 9,1 MPa (91 bar) e 11,1 MPa (111 bar) were applied in combination with the following Helium and Argon based gas mixtures: He-0.1%O2, He-0.5%O2, Ar-0.3%O2, Ar-0.6%O2 Ar-1.0%O2. The welding consumable was a low alloy steel wire containing 1% Ni and 0.4% Mo. Considering the high partial pressures attained the oxygen contents in the weld metals were unexpectedly low. In addition, increasing the oxygen partial pressure, small variations on the contents of Mn, Si and C in the weld metal were observed. These results do not follow the corresponding tendency reported in the literature for welding under the room pressure. To explain such behaviour it is suggested that under high pressures the oxygen partial pressure does not play an important role anymore. The oxygen diffusion throughout the metallic vapour which emerges from the molten metal surfaces is claimed to be a new mechanism to be considered and probably it becomes the most important controlling mechanism in the oxygen absorption by the weld metal.