Investigation On Activating Flux In Tungsten Inert Gas Welding Of T91 Steel

High quality weld can be obtained by conventional tungsten inert gas welding (TIG), for which process automatization can be carried out easily. So the TIG welding has been utilized widely in all aspects of commercial application. However, the defect of shallow penetration strongly limits the development of TIG welding, resulting in low productivity. Consequently, activating TIG welding (A-TIG) technology is developed to overcome the deficiency of conventional TIG welding. First, activating flux powder is mixed with acetone to form a thin layer and applied on the surface of work-piece to be weld. Then, the welding process is conducted using the same welding process as in the conventional TIG welding. Results indicate that weld with greatly increased penetration is obtained by A-TIG welding.T91 heat-resistant steel is widely used in piping system of power station. During welding of T91 pipes with wall thickness of 5mm, multi-layer and multi-weld method is applied, and groove preparation is needed before welding. In this study, a kind of TIG activating flux of T91 is developed to increase the depth of weld penetration. Using this kind of flux, weld penetration of 5mm can be obtained with on single TIG welding without groove preparing, and good mechanical properties of weld joint are obtained.In this study, effects of several kinds of single-component fluxes on weld penetration and surface condition of weld were studied. Based on the experiments of single-component fluxes, basic ingredients of T91 flux were determined, and best contents of these ingredients were chosen by orthogonal test method. In the study, effects of welding parameters such as welding current, welding speed, distance between tip of tungsten electrode and work-piece, and coating thickness of activating flux on weld penetration, weld width, and surface condition of weld were also researched. Effect of T91 activating flux on micro-structure of weld was investigated. And mechanical properties of weld joint were tested by tensile test and bend test. During welding process, high-speed camera system and oscillograph were used for capturing instantaneous arc shape and arc voltage respectively to research the effect of activating flux on arc shape and arc voltage of TIG welding and the mechanism of activating flux on weld penetration.By orthogonal test method, the activating flux of T91 steel are composed by SiO₂, MnO₂, Cr₂O₃, TiO₂ and NaCl with contents of 45%, 20%, 8%, 15% and 12%, respectively. Using this kind of flux, weld penetration of T91 steel is increased obviously. And T91 pipe of 5mm thickness can be fully penetrated by A-TIG welding at proper welding parameters without groove preparing. At the same time, the activating flux has no bad effect on micro-structure of weld. The experimental results show that the hardness of weld is higher a little than base metal. The fracture points of tensile samples are all existed on base metal in tensile test, no crack appears on weld tense surface of face bending samples and back bending samples, which indicate that fully penetrated weld with good mechanical properties is obtained by TIG welding with this activating flux.The experimental results indicate that, obvious arc deflection towards the rear of arc moving direction is observed by high-speed camera system when using SiO₂ as activating flux. The deflection phenomenon becomes heavier with decreasing of welding currents, and intermittent weld is obtained at low welding current. Obvious increased arc voltage is also found when using SiO₂. Results also indicate that no obvious changes occur on arc shape when using MnO₂, TiO₂, CaF₂ and etc, but a little lower arc voltages are observed. Results show that when using TF-5 activating flux, increased weld penetration and weld pool area are caused by the synthesis reason of arc shrinkage and alteration of surface tension temperature gradient of liquid metal in weld pool.