Table 1: Consumables for Welding Stainless

Welding Process Designations			Consumables
			Filler Metal Alloy	Specifications		Classifications
AWS	Common	Form		AWS	ASME	AWS	UNS
GTAW	TIG	Bare Welding Rods and Wire	625 276 22	A5.14 A5.14 A5.14	SFA5.14 SFA5.14 SFA5.14	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	N06625 N10276 N06022
GMAW	MIG	Bare Welding Rods and Wire	625 276 22	A5.14 A5.14 A5.14	SFA5.14 SFA5.14 SFA5.14	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	N06625 N10276 N06022
SMW	Stick or Covered Electrodes	Coating Electrodes	112 276 22	A5.11 A5.11 A5.11	SFA5.11 SFA5.11 SFA5.11	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	W86112 W80276 W86022

• Use alloy weld rings as the filler metal for orbital welding in the field. For other welds, use weld rings or wire. The filler alloy must have higher molybdenum content than the AL-6XN alloy to compensate for alloy dilution on cooling. Typically a 9% Molybdenum alloy (Alloy 625) is used. If Alloy 625 is not available Alloy C276, Alloy C-22® or Alloy 622 (15% Mo) may be substituted. (Table 1)
  
• Use inert gas for both the welding and shielding gas. Either helium or argon may be used, although argon is more commonly used. It is acceptable to add 3-5% nitrogen to both the torch and shielding gas. This added nitrogen compensates for nitrogen lost from the alloy during welding.
  
• Minimize the heat tint on the tubing and weld, no darker than a light straw color. A silver weld and heat-affected zone are the best. Any darker weld heat tints must be removed before placing in service. Dark blue heat and black tints are the most susceptible to corrosion. Remove these tints using aluminum oxide grit followed by acid cleaning/ passivation. A poorly cleaned surface may be just as susceptible to attack as the original heat tint.
  
• Do not preheat the weld unless the material is below 50º F (10º C). When temperature of the metal is below the dew point, allow it to warm above the condensation temperature to prevent moisture condensate on the surface. Remember: moisture causes heat tint.
  
• Start the weld within the area to be welded. If that is impossible, grind the ignition point after welding to remove it completely.
  

Why "Over Alloy" AL-6XN Stainless Weld Areas?
Because of two words: Intergranular Corrosion. Although the AL-6XN alloy is classified as a single-phase alloy, when it is melted, as in welding, it will solidify as a three-phase alloy: austenite, chi phase and delta ferrite. Chi phase, a chromium-iron-molybdenum compound, depletes the grain boundary in molybdenum and chromium, which reduces corrosion resistance, and delta ferrite has poor corrosion resistance. Alloying by using weld insert rings, the alloy balance, and therefore corrosion resistance, of the weld is equal to or better than the base alloy.

Orbital welding equipment consists of a solid-state DC power supply, associated cables, and an enclosed weld head. The weld head contains an internal rotor that holds the tungsten electrode. This allows the electrode to rotate around the work and to make the weld. The 115V VAC portable power supply controls the entire weld sequence starting with the inert-gas pre-purge, the arc strike, rotation delay, rotational speed (RPM), and multiple timed levels of welding current with pulsation. This is followed by a downslope that gradually terminates the current, and a postpurge to prevent oxidation of the heated material. These weld parameters are dialed into the power supply from a weld schedule sheet and are determined from test welds made on matching samples. Fusion welding, using automatic orbital TIG welding equipment, is practical for tubing or small diameter pipe in sizes from 1/8 inch (3mm) OD tubing to 6" schedule 10 pipe with wall thickness up to 0.154 inch (4 mm) wall. AL-6XN alloy is easily welded using similar weld parameters as Type 316L stainless steel, including travel speed (RPM) and weld current. Weld appearance is excellent, with a smooth, shiny, flat weld bead on both the OD and ID. When using weld ring inserts, simply place the weld ring between the two sections to be welded and fusion weld as usual. The weld current must be increased slightly to compensate for the increased thickness of material contributed by the insert ring. These welds also have a pleasing appearance, with a slight crown on the OD and some inner-bead reinforcement.

Autogenous (Without Filler) Welding For AL-6XN Stainless Steel
Autogenous welding may be used with the following precautions:

• Use 3 to 5 volume percent nitrogen in the shielding gas and a post-weld anneal above 2150°F (1180°C) followed by rapid cooling and pickling if a protective annealing atmosphere is not used.


• The duration of anneal, at least five minutes at temperature, must be sufficient to re-homogenize the weld segregation and to dissolve any chi phase.


• The G48-B crevice test may be use to assess the quality of autogenously welded and annealed AL-6XN alloy. In many applications, a post-weld anneal and pickle may not be possible, as in large vessel fabrication or field welding of piping systems. In these cases, the exposure conditions must be carefully reviewed to determine if autogenous welds are satisfactory. Autogenous AL-6XN welds are more resistant to corrosion than similar welds in Types 316L, 317L and 904L. Such autogenous AL-6XN welds have a corrosion resistance approximately the same as that of Alloy 904L base metal and superior to that of Types 316L and 317L base metal.