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Gas Tungsten Arc Welding (GTAW/TIG) for Duplex Stainless Steels

Gas tungsten arc welding (GTAW), sometimes referred to as tungsten inert gas (TIG) welding, is especially useful for short runs of manual welding. It may be automated for simple geometries, but generally it is not economical as the primary procedure for large amounts of welding on large equipment. Because many fabrications will require some GTA welds even when another procedure is the primary welding method, it is generally appropriate to qualify GTAW procedures for repairs or local finishing.

Equipment

GTAW is best performed with a constant current power supply, with a high-frequency circuit to aid in starting the arc. GTA welding should be performed with direct current straight polarity (DCSP), electrode negative. Use of direct current reverse polarity (DCRP) will lead to electrode deterioration. The electrode should be a 2% thoriated tungsten electrode (AWS specification 5.12 Classification EWTh-2). Arc control is aided by grinding the electrode to a conical point with a vertex angle of 30 to 60 degrees, and with a small flat at the point. The ideal vertex angle for achieving penetration in automatic GTAW should be determined by a few tests in actual production.

Filler Metals

Most filler metals for duplex stainless steel welding are described as “matching”, but typically they are overalloyed in nickel with respect to the wrought products that they are said to match. Usually there is about 2-4% more nickel than in the wrought product. The nitrogen content is typically slightly lower in the filler metal than in the base metal. It is generally accepted that the more highly alloyed duplex stainless steel weld fillers are suitable for welding the lower alloyed duplex stainless steel products. The “matching” fillers have been reported to give acceptable results when joining duplex stainless steels to austenitic stainless steels or to carbon and alloy steels.

Shielding

It is essential in GTAW, as in all gas shielded welding processes, that the weld pool be protected from atmospheric oxidation and contamination. Most typically this protection is achieved with the inert gas, argon, a dry welding grade with purity of 99.95% argon or better. It is important that the gas handling system be clean, dry, and free from leaks, and that flow conditions be adjusted to provide adequate coverage, as well as to prevent turbulence and aspiration of air into the shielding gas. Gas flow should be initiated several seconds ahead of striking the arc, and it should be maintained for several seconds after the arc is extinguished, ideally long enough for the weld and HAZ to cool below the oxidation range of the stainless steel. For electrode coverage, suggested flow rates are 12-18 l/min (0.4-0.6 cfm) when using a normal gas diffuser screen (gas lens), and with half that rate required for a normal gas nozzle.

Backing gas (also pure argon) flow rates depend on the root volume, but should be sufficient to assure complete flushing of air and full protection of the weld as indicated by the absence of heat tint. Because argon is heavier than air, the feed should be from the bottom to the top of the enclosed volume, with purging by a minimum of seven times the volume.

Satisfactory welds have been obtained with pure argon, but further improvements are possible. The addition of up to 3% dry nitrogen will aid in retention of nitrogen in the weld metal, particularly of the more highly alloyed duplex stainless steels. While, the nitrogen addition has been found to increase electrode wear, the addition of helium partially offsets this effect.

Additions of oxygen and carbon dioxide to the shielding gas should be avoided because they will reduce the corrosion resistance of the weld. Hydrogen should not be used in the shielding or backing gas because of the possibility of hydrogen embrittlement or hydrogen cracking of the ferrite phase in duplex stainless steels. The gas handling system and the water cooling system, if the torch is so equipped, should be regularly inspected to ensure that the dry, clean nature of the gas is preserved.

Technique and Parameters

With duplex stainless steels, it is especially important to establish good consistent edge preparation, alignment, and root land or spacing. While austenitic stainless steels may accept some use of welding technique to overcome deficiencies in these areas, the duplex stainless steels risk extended time at temperature when these techniques are used. It is recommended that copper backing bars be avoided if possible, because the duplex stainless steels are sensitive to copper surface contamination.

Any arc strikes outside of the welding zone will create local points of autogenous welding with very high quench rates, resulting in locally high ferrite content and possible loss of corrosion resistance at those points. Arc strikes should be made in the weld joint itself to avoid this problem. Tacking welds should be made with full gas shielding. There should be no tack weld at the starting point of the root pass. Ideally, to avoid cracking of the root pass associated with tack welds, the root pass weld should be interrupted and the tack weld ground away, or the tack may be partially ground before the root pass. The width of the root gap should be carefully maintained to ensure consistent heat input and dilution in the root pass. The start and finish of the root pass should be ground before the start of filler passes.

The work piece should be allowed to cool below 150°C (300°F) between passes to provide for adequate cooling of the HAZ in subsequent passes. duplex stainless steels, for example, the superduplex filler for 2205 base metal welds, have been used successfully. Wire sizes of 1.6, 2.4, and 3.2 mm (1/16, 3/32, and 1/8 inch) diameter are commonly used. Filler wire should be clean and dry, and should be stored in a covered container until use. Best results are obtained when the welding is done in the flat position. The torch should be maintained as near as possible to vertical to minimize aspiration of air into the shielding gas.

There is substantial freedom in the selection of heat input to deal with a wide range of material thickness and joint design. The heat input is typically in the range of 0.5-2.5 kJ/mm(15 to 65 kJ/inch) as calculated by the following formula:

Heat input = (V x A x 60) / (S x 1000)

where V = voltage (volts)

A = current (amperes)

S = travel speed (in./min)

GTAW, when made with good shielding and appropriate management of time at temperature, provides a weld of good toughness and corrosion resistance, and is versatile in the range of situations in which it can be used. GTAW is often used to supplement and finish larger constructions assembled using other welding methods. It is important that the GTAW procedures be qualified to deal with the variety of situations in which it may be employed.

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