Gas Tungsten Arc Welding Technique Research Articles

Decomposition of ferrite in commercial superduplex stainless steel weld metals; microstructural transformations above 700 °C

The microstructural stability at temperatures above 700 °C of weld metal of type 29Cr-8Ni-2Mo-0.39N and weld metal of type 25Cr-10Ni-4Mo-0.28N has been compared. Multipass welding was employed using the gas tungsten arc welding technique with a shielding gas of Ar+2 pct N2. The quantitative assessment of the intermetallic phase was performed using automatic image analysis in the light optical microscope (LOM). Detailed microanalysis was also performed using scanning and transmission electron microscopy. A computer program developed by the authors was used to calculate a continuous cooling-temperature (CCT) diagram on the basis of the experimentally determined time-temperature-transformation (TTT) diagram. Thermodynamic calculations for estimating phase stabilities and for interpreting experimental observations were performed. It was found that weld metal of type 29Cr-8Ni-2Mo-0.39N was microstructurally more stable than weld metal of type 25Cr-10Ni-4Mo-0.28N. A lower molybdenum concentration and a higher nitrogen concentration in the former alloy could explain the higher stability with respect to the intermetallic phase. The higher nitrogen concentration also provides a rationale for the higher stability against the formation of secondary austenite in weld metal of type 29Cr-8Ni-2Mo-0.39N. This effect, which is associated with a lower thermodynamic driving force for precipitation of secondary austenite during multipass welding, can be explained by nitrogen-enhanced primary austenite formation.

Influence of welding techniques on microstructure and pitting corrosion behaviour of 1441 grade Al–Li alloy gas tungsten arc welds

AbstractThe effects of changing the welding technique on the weld bead solidification structure and resistance to pitting corrosion of an aeronautical grade Al-Li based alloy have been studied. Russian alloy 1441, with a nominal composition Al-1.9Li-1.8Cu-1.0Mg-0.1Zr (wt-%) and in the form of sheet, was welded using constant current, pulsed current and arc oscillation gas tungsten arc welding techniques. Under the optimum conditions of welding, the solidification structure was coarse columnar in constant current welds, fine equiaxed in pulsed current welds and fine equiaxed intermixed with fine cellular grains in arc oscillation welds. The distribution of interdendritic (and intergranular) non-equilibrium solidification products became increasing discrete on changing from constant current to pulsed current and arc oscillation welding. Resitance to pitting corrosion was found to be greatest in welds having a discrete second phase distribution, i.e. those produced using the pulsed current and arc oscillatio...

Ｔｉ‐Ｎｉ形状記憶合金のＧＴＡ溶接に関する研究 １ Ｔｉ‐Ｎｉ形状記憶合金溶接部の形状回復及び引張特性に及ぼす酸素の影響

Effects of nitrogen on microstructure, shape recovery behavior and tensile property of Ti-Ni alloy weldments were investigated. A near-equiatomic Ti-Ni shape memory alloy was welded by a gas tungsten arc (GTA) welding technique under Ar-N2 welding atmosphere. The nitrogen absorbed into the weld metal increased with rising nitrogen partial pressure and welding current. Second phase particles were observed in the weld metal containing nitrogen. The second phase was identified as TiN by transmission electron microscopy. The shape recovery stress of the weldment was equivalent to that of the base material. As temperature of the weld metal decreased with increasing nitrogen content. Tensile strength of the alloy decreased by welding process, and the further reduction was observed for the weld metal absorbed nitrogen. Elongation also decreased by welding and rising nitrogen content.

Helium-induced weld cracking in austenitic and martensitic steels

Helium was uniformly implanted into type 316 stainless steel and Sandvik HT-9 (12Cr-1 MoVW) to levels of 0.18 to 256 and 0.3 to 1 a.p.p.m., respectively, using the “tritium trick” technique. Autogenous bead-on-plate, full penetration, welds were then produced under fully constrained conditions using the gas tungsten arc welding (GTAW) process. The control and hydrogen-charged plates of both alloys were sound and free of any weld defects. For the 316 stainless steel, catastrophic intergranular fracture occurred in the heat-affected zone (HAZ) of welds with helium levels ≥2.5 a.p.p.m. In addition to the HAZ cracking, brittle fracture along the centreline of the fusion zone was also observed for the welds containing greater than 100 a.p.p.m. He. For HT-9, intergranular cracking occurred in the HAZ along prior-austenite grain boundaries of welds containing 1 a.p.p.m. He. Electron microscopy observations showed that the cracking in the HAZ originated from the growth and coalescence of grain-boundary helium bubbles and that the fusion-zone cracking resulted from the growth of helium bubbles at dendrite boundaries. The bubble growth kinetics in the HAZ is dominated by stress-induced diffusion of vacancies into bubbles. Results of this study indicate that the use of conventional GTAW techniques to repair irradiation-degraded materials containing even small amounts of helium may be difficult.