Effect of activating fluxes on weld bead morphology of P91 steel bead-on-plate welds by flux assisted tungsten inert gas welding process

Gas tungsten arc welding is fundamental in those applications where it is important to control the weld bead shape and the metallurgical characteristics. This process is, however, of low productivity, particularly in the welding of large components. Maximum 2-3 mm thick plates of carbon steel, stainless steel can be welded with GTAW process in the Argon shielding under autogenous mode. The activated flux TIG (A-TIG) welding process, developed by the Paton welding Institute in the 1960s, is now considered as a feasible alternative to increase the process productivity. A-TIG welding uses a thin layer of an active flux that results in a great increase in weld penetration. This effect is generally, connected to the capture of electrons in the outer parts of the arc by elements of high electronegativity, which constrict the arc causing an effect similar to that used in plasma welding [1]. Grade 91 (modified 9Cr-1Mo or P91 steel) steels are structural material and are widely used for high temperature components of power plants, petrochemical and nuclear industry because of its superior mechanical properties such as yield, ultimate tensile, and creep rupture strengths matching or exceeding that of 9Cr-lMo, 214Cr-1Mo, HT9, EM12, and 304 stainless steel [2], but depth of weld penetration achievable in single pass with autogenous TIG welding is still lesser in Cr-Mo steel [3]. A novel variant of the autogenous TIG welding process (A-TIG) was applied on P91 steel in which oxide powders CaO, Fe2O3, TiO2, ZnO, MnO2 and CrO3 (elements of period IV in the periodic table of elements) were used to produce a bead on plate welds. Process parameters were optimized to achieve the desired depth of penetration at the lowest heat input possible. Variation in the depth of penetration and weld bead width as a function of current at fixed torch speed was determined employing A-TIG welding process. The purpose of the present work was to investigate the effect of oxide fluxes on weld morphology, arc voltage obtained with A-TIG welding, which applied to the welding of 6 mm thick modified 9Cr-lMo steel plates. The experimental results indicated that the increase in the penetration is significant with the use of Fe2O3, ZnO, MnO2 and CrO3. Full penetration weld secured with the use of these fluxes. (C) 2014 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.