Showing papers on "Shielding gas published in 2004"

Nickel based superalloy welding practices for industrial gas turbine applications

Abstract: The continued drive for increased efficiency, performance and reduced costs for industrial gas turbine engines demands extended use of high strength-high temperature capability materials, such as nickel based superalloys. To satisfy the requirements of the component design and manufacturing engineers, these materials must be capable of being welded in a satisfactory manner. The present paper describes the characteristic defects found as a result of welding the more difficult, highly alloyed materials and reviews a number of welding processes used in the manufacture and repair of nickel alloy components. These include gas tungsten arc (GTA) and electron beam (EB) welding, laser powder deposition and friction welding. Many of the more dilute nickel based alloys are readily weldable using conventional GTA processes; however, high strength, precipitation hardened materials are prone to heat affected zone and strain age cracking defect formation. A number of factors are found to affect the propensity f...

Principles of welding : processes, physics, chemistry, and metallurgy

Abstract: THE PROCESS AND PROCESSES OF WELDING. Introduction to the Process of Welding. Classifying Welding Processes. Fusion Welding Processes. Nonfusion Welding Processes. THE PHYSICS OF WELDING. Energy for Welding. The Flow of Heat in Welds. Thermally Induced Distortion and Residual Stresses During Welding. The Physics of Welding Energy or Power Sources. Molten Metal Transfer in Consumable Electrode Arc Welding. Weld Pool Convection, Oscillation, and Evaporation. THE CHEMISTRY OF WELDING. Molten Metal and Weld Pool Reactions. Weld Chemical Heterogeneity. THE METALLURGY OF WELDING. Weld Fusion Zone Solidification. Eutectic, Peritectic, and Postsolidification Fusion Zone Transformations. The Partially Melted Zone. The Weld Heat-Affected Zone. Weldability and Weld Testing. Closing Thoughts. Appendices. Index.

Marangoni convection and weld shape variations in Ar-O2 and Ar-CO2 shielded GTA welding

Abstract: Increasing the oxygen or the carbon dioxide concentration in the argon-based shielding gas leads to an increase in the weld metal oxygen content when the oxygen or carbon dioxide concentration is to be lower than 0.6 vol.% in the shielding gas. However, when the O2 or CO2 concentration is higher than 0.6 vol.% in the Ar-based shielding gas, the weld metal oxygen is maintained around 200 ppm–250 ppm. An inward Marangoni convection mode in the weld pool occurs when the weld metal oxygen content is more than 100 ppm. When it is lower than 100 ppm, the Marangoni convection would change to the outward direction and the weld shape varies from a deep narrow to a shallow wide shape. The effective ranges of O2 and CO2 concentrations for deep penetration are same. A heavy layer of oxides is formed when the O2 or CO2 concentration in the shielding gas is more than 0.6 vol.%. Based on the thermodynamic calculation of the equilibrium reactions of Fe, Si, Cr and Mn with oxygen in liquid iron for the oxide products, FeO, SiO2 ,C r 2O3 and MnO and the experimental oxygen content in the weld metal, Cr2O3 and SiO2 oxides are possibly formed at the periphery area of the liquid pool surface under the arc column during the welding process. One model is proposed to illustrate the role of the oxide layer on the Marangoni convection on the pool surface at elevated temperature. The heavy oxide layer inhibited the fluid flow induced by the Marangoni convection and also became a barrier for the oxygen absorption into the molten weld pool. © 2004 Elsevier B.V. All rights reserved.

Feasibility study of acoustic signals for on-line monitoring in short circuit gas metal arc welding

Abstract: The acoustic waves produced by the gas metal arc welding (GMAW) contain information about the behaviour of the arc column, the molten pool and droplet transfer. In this study measurements of acoustic waves generated during GMAW process were performed. Acoustic waves were measured in the surrounding air and in the parts being welded by employing a microphone and PZT sensor. To evaluate influences on sound generation extensive experiments were performed with two different unalloyed carbon steels: DIN RSt13 with 0.1% C and DIN Ck45 with 0.46% C, using two types of shielding gas: CO 2 and gas mixture by its brand name Crystal (90% Ar, 10% CO 2 ) and welding on a slope to vary the distance between welding nozzle and welding part. Acoustic signals were processed to obtain time domain and frequency domain descriptors. Some relationships between descriptors and the weld process characteristics were investigated. Results indicate that the arc sound exhibits distinct characteristics for each welding situation and that the main source of acoustic waves in short circuit metal transfer mode is arc reignition. From acoustic signals one can easily assess process stability and detect welding conditions resulting in weld defects.

Electrode evaporation and its effects on thermal arc behavior

Abstract: In circuit breakers, arc heaters and arc welding apparatus, metal vapor resulting from electrode erosion is inevitably injected into the arc plasma. The arc then burns in a mixture of the working gas (SF/sub 6/ in the case of circuit breakers) and electrode vapor, whose properties are substantially different from those of pure SF/sub 6/. The present work is a computational investigation into the effects of electrode vapor on the behavior of a supersonic nozzle arc under dc conditions. The arc and electrodes are treated as a coupled system. The arc plasma is assumed to be in local thermodynamic equilibrium (LTE). The thin non-LTE layer between the electrode and the arc is not included in the computation but its effects are taken into account in the energy balance at the electrode surface. A conservation equation for the electrode vapor is solved together with the governing equations for mass, momentum, and energy of the gas mixture. Comparisons were made between the results with and without electrode vaporization. Results show that vaporization of the electrode material (copper in the present investigation) has a cooling effect at the arc center and broadens the arc column. The arc voltage is, however, not sensitive to the presence of electrode vapor for the gas discharge conditions investigated.

Time-dependent spectroscopy of plasma plume under laser welding conditions

Abstract: Momentary emission spectra of iron and argon lines were measured in a plasma plume induced during welding with a continuous wave CO2 laser. Time-dependent spectra were registered using a fast gate, lens coupled microchannel plate image intensifier placed between a spectrograph and a 1254 silicon intensified target detector connected to an optical multichannel analyser. The results, together with the analysis of the colour images from a fast camera, show that in the case when argon is the shielding gas, two plasmas exist: the argon plasma and the iron plasma. It has been found that during strong bursts the plasma plume over the keyhole consists mainly of metal vapour, not being diluted by the shielding gas. No apparent mixing of the metal vapour and the shielding gas has been observed. The space-averaged electron densities determined from the Stark broadening of the 7503.87, 7514.65 A Ar I lines amounts to (0.75–1.05) × 1023 m−3 depending on the distance from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermodynamic equilibrium these electron densities correspond to the temperatures of 12–13 kK. At the peaks of strong vapour bursts the space-averaged electron densities determined from the Stark broadening of the 5383.37 A Fe I line are (0.6–1) × 1023 m−3. Numerical simulations showed that the maximum densities in the plasma centre are considerably higher and amount to ~1.8 × 1023 m−3 and ~2.45 × 1023 m−3 in the case of the argon and metal plasma, respectively. Consequently the absorption of the laser beam in the plasma plume amounts to ~5% of the beam power in the case of argon and 10% in the case of metal plasma.

Stress and distortion mitigation technique for welding titanium alloy thin sheet

Abstract: A stress and distortion mitigation technique for Gas Tungsten Arc Welding (GTAW) of titanium alloy Ti–6Al–4V thin sheet is presented. The proposed welding technique incorporates a trailing heat sink (an intense cooling source) with respect to the welding torch, and it is also called the Dynamically Controlled Low Stress No-Distortion (DC-LSND) technique. The development of this mitigation technique is based on both detailed welding process simulation using the advanced finite element technique and systematic laboratory experiments. The finite element method is used to investigate the detailed thermomechanical behaviour of the weld during conventional GTAW and DC-LSND GTAW. With detailed computational modelling, it is found that by the introduction of a heat sink at some distance behind the welding arc, a saddle shaped temperature field is formed as a result of the cooling effects of the heat sink; the lowest temperature exists in the zone where the heat sink is applied. High tensile action on the ...

Process for welding

Abstract: A welding process for non-stainless steel workpieces (12) using GTAW equipment (20) and hydrogen containing shield gas on a first root pass is improved by using a shield gas delivery system comprising hoses or conduits (30) made of elastomeric material having a moisture permeability coefficient of less that 275, preferably less than 100, and using a tungsten electrode (22) composition comprising at least tungsten and lanthanum oxide, and preferably tungsten, lanthanum oxide, yttrium oxide and zirconium oxide Preventing moisture permeation through the elastomeric hoses (30) delivering hydrogen containing shield gas eliminates expulsion of fused weld metal during second pass filler welding over the root pass weld Electrode life is enhanced using the tungsten compounds

Laser processing head

Abstract: A nozzle of a laser processing head for laser cutting and laser welding includes a laser beam outlet for directing a laser beam towards a processing location of a workpiece to be processed, a first gas supply channel for supplying a cutting gas towards the processing location of a workpiece to be processed when the laser processing head is used for laser cutting, and a second gas supply channel for supplying a welding gas towards the processing location of a workpiece to be processed; when the laser processing head is used for laser welding.

Marangoni convection in weld pool in CO2-Ar-shielded gas thermal arc welding

Abstract: Small CO2 additions of 0.092 to 10 vol pct to the Ar shielding gas dramatically change the weld shape and penetration from a shallow flat-bottomed shape, to a deep cylindrical shape, to a shallow concave-bottomed shape, and back to the shallow flat-bottomed shape again with increasing CO2 additions in gas thermal arc (GTA) welding of a SUS304 plate. Oxygen from the decomposition of CO2 transfers and becomes an active solute element in the weld pool and reverses the Marangoni convection mode. An inward Marangoni convection in the weld pool occurs when the oxygen content in the weld pool is over 80 ppm. Lower than 80 ppm, flow will change to the outward direction. An oxide layer forms on the weld pool in the welding process. The heavy oxide layer on the liquid-pool surface will inhibit the inward fluid flow under it and also affects the oxygen transfer to the liquid pool. A model is proposed to illustrate the interaction between the CO2 gas and the molten pool in the welding process.

Diffusible Hydrogen Content in Rutile Flux-Cored Arc Welds as a Function of the Welding Parameters

Abstract: The objective of the current work was to establish the effect of flux-cored arc welding (FCAW) parameters, such as welding current, contact-tip to workpiece distance (CTWD) and shielding gas type, on diffusible hydrogen content for single run, horizontal position, bead-on-plate welds using seamed and seamless rutile consumable wires. The work included an investigation of arc characteristics under typical welding conditions, using high-speed digital imaging and laser backlighting, in order to provide information on metal transfer and arc length. The work has shown that under most conditions investigated, the hydrogen levels for the seamed rutile wire were above the 10 ml/100 g specified by the consumable’s classification (H10). In contrast, the seamless wire met the requirements of the H5 classification for all welding conditions investigated. In general, lower diffusible hydrogen levels were achieved when using CO2 shielding gas, although the effect is less significant with the H5 seamless rutile wire. It is further concluded that the time that the wire resides in the resistive heating zone, tRHZ, and the arc length are important factors in determining diffusible hydrogen content.

A Comprehensive Model on the Transport Phenomena During Gas Metal Arc Welding Process

Abstract: A comprehensive mathematical model and the associated numerical technique have been developed to simulate the coupled, interactive transport phenomena between the electrode (droplets), the arc plasma, and the workpiece (weld pool) during a stationary axisymmetric gas metal arc welding process The simulation involves arc plasma generation, electrode melting, droplet formation, detachment, transfer, and impingement onto the workpiece, and weld pool dynamics During transfer from the tip of the electrode to the workpiece, the droplet subjects to gravity, electromagnetic force, surface tension, and arc plasma drag force Transient temperature and velocity distributions of the arc plasma, shapes of the electrode, droplet, and weld pool, and heat transfer and fluid flow in the weld pool are all calculated in a single, unified model The predicted solidified weld bead shape compares favourably with the experimental result

Different kinds of metal welding method

Abstract: PROBLEM TO BE SOLVED: To provide a different metals-welding method, in which an oxide film on a weld interface is removed so as to enable firm welding while suppressing generation of intermetallic compounds in a welding step. SOLUTION: When a steel material 1 and an aluminum alloy material 2 which are different kinds of metal are welded, the steel material 1 with a galvanized layer 1z deposited thereon as a third metal different from the above metals is overlapped between both metal materials 1, 2, so as to weld while generating a eutectic molten metal 3 of Al and Zn in a welding interface. COPYRIGHT: (C)2006,JPO&NCIPI

Method and apparatus for composite YAG laser/arc welding

Abstract: A combined YAG laser/arc welding apparatus is disclosed which performs a welding operation with a large depth of fusion and a deep fusion at a high efficiency. The welding apparatus includes an arc electrode which targets a point on a welded member where a laser beam from a YAG laser is focused. While an arc power supply is connected between the arc electrode and the welded member, the welded member is irradiated by the YAG laser to produce plasmoid gasses and metal vapors or a plume which is effective to start an arc from the arc electrode. A combined YAG laser/arc welding operation takes place while keeping the plume within and without a keyhole. Welded surface is covered by a hood, and a shielded gas passed through a commutation metal network surrounds the welded focus to prevent an oxidation and keeps the plume maintained. A filler wire is fed to a molten pool.

An investigation on the effects of gases in GTA welding of a wrought AZ80 magnesium alloy

Abstract: Magnesium alloy components are frequently gas tungsten arc welded despite magnesium's high thermal diffusivity. Gases such as argon, helium, -and nitrogen - enriched or not with hydrogen - have been investigated to determine if melting, and in particular weld penetration, can be increased. Images of the arcs, voltage readings, dimensions, defects, and microstructure of weld fusion zones have been examined. Due to a greater first ionization potential, helium increased the constant-current voltage and created more melting than argon. With diatomic gases such as nitrogen and hydrogen, voltage and weld dimensions were even further increased. However, hydrogen caused porosity, and nitrogen interacted with magnesium by leaving a nitride deposit at weld surfaces. While consequences of alloying with nitrogen were probably not disadvantageous, hydrogen pores were of greater concern. Both welding parameters and hydrogen concentration in the arc atmosphere were important in controlling porosity. The two-dimensional heat-flow conditions of fully penetrating welds were capable of eliminating porosity and could make welding with hydrogen additions a possibility to consider.

Mechanism governing nitrogen absorption by steel weld metal during laser welding

Abstract: The existence of monatomic nitrogen in the plasma just over the keyhole during CO2 laser welding was confirmed by the monochromatic image of a specific spectrum line emitted by monatomic nitrogen. The smaller reaction area of the molten pool with monatomic nitrogen is considered to lead to less nitrogen absorption during CO2 laser welding than that during arc welding. The effect of the penetration mode shows that the nitrogen absorption during CO2 laser welding mainly occurs on the upper surface of the molten pool. The nitrogen content in a reduced-pressure nitrogen atmosphere during CO2 laser welding is in good agreement with that obtained during yttrium aluminum garnet (YAG) laser welding within the range of low nitrogen (partial) pressures. This result supports the supposition that the different behaviors of nitrogen absorption between CO2 laser welding and YAG laser welding can be reasonably attributed to the lesser amount of monatomic nitrogen during YAG laser welding.

Proposal for a modified fowler-milne method to determine the temperature profile in TIG welding at low currents

Abstract: It is possible to assess the thermal efficiency of welding shielding gases by means of the arc temperature field analysis. Since this approach opens a remarkable study field to assess different shielding gases, giving support for dealing with advantages and disadvantages of commercial gas mixtures, there is a research line under development, which goal is to find techniques to measure arc temperatures. This work describes a proposed method containing different procedures to quantify plasma jet temperature profiles from experimental data. This method was applied on data taken from TIG welding arc, at low current (40 A). This low current was selected to contrast to and amplify the current literature focus, i.e., high currents. The experiment was conducted using emission spectroscopy, picking punctual luminescence from the plasma through an optic system. The TIG arc was stroked on a water-cooled copper plate and shielded by pure argon. The temperature field was determined through the modified Fowler-Milne method. The introduced modification aimed to overcome the limitation that this method has for low radiation intensity presents in low current arcs: the Fowler-Milne method has an intrinsic threshold of 10,000 - 25,000 K. For a 40-A arc, the lower 10,000-K limiting isotherm is reached close to the cathode, restricting the analysis field, especially for the anode region. The proposed modification suggests a linear distribution of the particle density instead of a Maxwellian one, at temperatures below 12,500 K. The experimental temperature field was compared to a previous publication that deals with numerical simulation and the results were found in good agreement, what indicates the supposition of a linear distribution it is not far from the reality.

Welding of titanium alloys with activating flux

Abstract: In the present paper, the effects of an activating flux on Ti–6Al–4V alloy welding were investigated. Tungsten inert gas welding was used to weld 8.0 mm thickness Ti–6Al–4V alloy plates. Results show that applying the activating flux on the Ti–6Al–4V alloy surface leads to an increase in weld penetration depth, whereas the corresponding weld bead width is reduced. It was also found that various welding conditions, particularly flux thickness, influence the effectiveness of the activating flux. Furthermore, a data acquisition system was used to monitor the current and voltage signals during welding. Results from monitoring of the welding current and voltage signals reveal that there is a clear correlation between the signals and the weld penetration when the welding arc is steady. Analysis of the acquired signals can be used to identify inconsistencies in weld penetration. In summary, to take advantage of the use of activating flux in Ti alloy welding, it is important that a uniform flux layer is p...

Girth Welding Development For X120 Linepipe

Abstract: This paper discusses the development of a girth welding technique for X120 linepipe. The solid-wire, pulsed gas metal arc welding (PGMAW) process with Ar-based shielding gas was selected for its process control, and low oxygen and hydrogen potentials. The weld metal microstructure was designed using fundamental metallurgical principles and verified through the evaluation of about 20 experimental wires and several commercial-scale heats of wire. Weld metal chemistry and shielding gas composition were optimized. The developed microstructure is termed acicular ferrite interspersed in martensite (AFIM). Transmission electron microscopy was the primary tool used for microstructural analysis and weld metal inclusion design.

Method for laser welding steel sheets and composite material

Abstract: A method for laser welding steel sheets in which a gas mixture comprising at least one of argon, helium and nitrogen, and 20 to 50% by volume of a carbon dioxide gas, or a gas mixture comprising at least one of argon, helium and nitrogen, 10 to 40% by volume of a carbon dioxide gas and 1 to 5% by volume of an oxygen gas is used as a shield gas.

Metal-core gas metal arc welding of ferrous steels with noble gas shielding

Abstract: The gas-metal arc welding of metal-core wile electrodes in the pure Ar shielding gas for carbon steel, low alloy steel, and ferritic stainless steel is described. Such shielding gas provides several benefits not realized ??the gas-metal arc welding process with solid wires. When compared to standard argon/oxygen containing gas mixtures normally used for metal cored wires, these benefits include reduced silicate islands on the weld surface for improved weld appearance, reduced welding fume, and lower weld spatter, all of which provide easier clean-up after the welding operation. Benefits also include reduced arc penetration desirable for welding on thinner materials or handling poor joint fit-up. Lower voltage requirement further makes it possible to weld on thinner materials. Lower oxygen content in the weld deposits provide better toughness and easier welding in all-positions.

Weld shape comparison with iron oxide flux and Ar–O2 shielding gas in gas tungsten arc welding

Abstract: The influence of iron oxide flux and O2–Ar mixed shielding gas on weld shape and penetration in gas tungsten arc welding is investigated by bead-on-plate welding on SUS 304 stainless with low oxygen and low sulphur contents. The oxygen content in the weld metal is measured using a HORIBA EMGA-520 oxygen/nitrogen analyzer. The results show that both the iron oxide flux and the O2–Ar mixed shielding gas can significantly modify the weld shape from shallow wide to deep narrow. A large weld depth/width ratio around of 0.5 is obtained when the oxygen content in the shielding gas is in the range of 3000–6000 vol. ppm. Oxygen over a certain critical value, i.e. 70 wt. ppm, in the weld pool alters the temperature coefficient of the surface tension on the pool surface, and hence changes the Marangoni convection. A thick oxide layer on the weld pool surface is generated when the oxygen content in the shielding gas is over 6000 vol. ppm, which becomes a barrier for the oxygen conveyance to the liquid pool an...

The effect of welding parameters on levels of diffusible hydrogen in weld metal deposited using gas shielded rutile flux cored wires

Abstract: Hydrogen assisted cold cracking (HACC) represents a significant threat to the integrity of welded steel constructions. HACC occurs at near ambient or lower temperatures and may be delayed for several hours, or even days, after welding is completed. Significant advances in developing thermomechanically controlled processed steels, with reduced carbon and alloy content, resulted in steels that are more resistant to HACC in the heat affected zone (HAZ). This has enabled the use of lower preheat temperatures during fabrication of welded structures. However, the improvement in the resistance of the HAZ to HACC has shifted the problem of cold cracking to the weld metal. The objective of this current work was to establish the effect of flux cored arc welding (FCAW) welding parameters, such as welding current, contact-tip to workpiece distance (CTWD) and shielding gas type, on diffusible hydrogen (HD) content for single run, bead-onplate welds using low strength seamed and seamless gas shielded rutile wires of E71 T-1 classification. The work has shown that under most conditions investigated, the weld metal HD levels for the seamed rutile wire were above the 10 ml/100g specified by the consumable’s classification (H10). The measured range of diffusible hydrogen for the H10 wire was 8.3 to 17.0 ml/100g, with the highest hydrogen content being obtained at the lowest welding current of 280 A, shortest CTWD of 15 mm and deposited using 75Ar-25CO2 shielding gas. In contrast, the seamless wire met requirements of the H5 classification (HD ≤5 ml/100g) for all welding conditions investigated, with a range of HD levels of 0.9 to 3.5 ml/100g. v In general, lower HD levels were achieved when using CO2 shielding gas, although the effect is less significant with the H5 seamless rutile wire. The work included an investigation of arc characteristics under typical welding conditions, using high speed digital imaging and laser backlighting, in order to provide information on metal transfer and arc length. Several tests were carried out using the H10 consumable. It was established that the amount of heat generated by resistive heating of the wire prior to melting can exert a strong influence on the weld metal HD content and is more pronounced in welds deposited using 75Ar-25CO2. The measured arc length was reduced significantly when welding under CO2 shielding gas. Despite suggestions in the literature there was no evidence of a change in metal transfer mode to spray transfer on increasing the welding current from 280 to 320 A, transfer mode was globular for all conditions used. Following the weld metal diffusible hydrogen testing and welding arc imaging work, the weld metal susceptibility to cold cracking was assessed using the gapped bead-on-plate (G-BOP) test at different preheat temperatures. For this part of the work, identical welding parameters those used for the diffusible hydrogen testing were selected. It was found that the H5 wire weld deposits did not reveal any cracking at ambient temperature, whereas all the welds deposited using the H10 wire exhibited cold cracking with no preheat. Weld metal deposited using 75Ar-25CO2 shielding gas resulted in higher susceptibility to cold cracking than with CO2, which correlated to lower HD levels the CO2 tests. Besides the higher hydrogen content, it was also found that higher weld metal hardness corresponded to the greater crack susceptibility in the welds deposited using 75Ar-25CO2 shielding gas. These factors are considered to contribute to higher susceptibility to transverse cold cracking compared with CO2 shielding gas. vi Although the overall results indicate that the weld metal susceptibility to cold cracking corresponds to the relevant levels of HD, this relationship was found to be ambiguous in welds deposited at the shortest CTWD of 15 mm, using CO2 shielding gas at all welding currents investigated. While the amount of diffusible hydrogen was marginally increased from 11.7 to 12.8 ml/100g, resulting from the welding current increase from 280 to 320 A, the amount of cold cracking at room temperature was significantly decreased from 89 to 25 %RTC. This is explained by a significant difference in the cross section of the weld beads, suggesting a need to more closely evaluate the G-BOP testing, particularly examining the effects of weld bead profiles on the weld susceptibility to HACC. Preheat was found to decrease the amount of cold cracking in the H10 welds and it was concluded that preheat significantly reduces the main contributor to decrease the HD in the weld metal. Although the cracking susceptibility of welds using 75Ar-25CO2 shielding gas decreased more slowly with an increasing preheat temperature, compare to those deposited using CO2, no cracking was observed at 120 °C in welds under both shielding gases. This indicates that the same welding consumable (H10) deposited using different shielding gases can result in a different response to preheat temperature. Based on the results of this work, a number of changes are proposed to hydrogen testing standards AS 3752-1996 and ISO 3690-2000, particularly with respect to the effects of CTWD and shielding gases on levels of diffusible hydrogen in weld metal deposited using gas shielded rutile flux-cored wire.

An introduction to physical phenomena in arc welding processes

Abstract: As is well known, arc discharges have been applied to various processes such as welding, cutting, spray coating, melting and refining. Unlike electrodeless discharge methods such as high frequency discharge of inductive coupling type (eg. RF discharge), arc discharge is a polarized discharge in which the arc is generated between the positive and negative electrodes.1 Accordingly, when the arc discharge is applied to welding processes, the material becomes one of the electrodes. In TIG welding, the material, that is, the molten pool, generally acts as the anode to the tungsten cathode. As shown in Fig. 1, TIG welding processes are based on the close energy balance between the ‘electrode–arc plasma–molten pool’. On the other hand, for the formation of the molten pool, energy transfer from the arc is also important, but energy transfer in the molten pool after that is extremely important, too. In TIG welding of steels, in which energy transfer by convection current becomes dominant rather than therm...