| TIP TIG welding is 100 to 500% faster than TIG with superior quality than traditional TIG - Pulsed MIG - FCAW |
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Strict welding regulations are applied to welding cryogenic applications. The weld metal properties should contain low nitrogen, low ferrite, low carbon and high nickel. Filler metals such as Nickel Chrome Molybdenum, Nickel Chrome Iron or high alloy austenitic electrodes. The Nickel alloy consumables have a coefficient of thermal expansion that is close to the 9% nickel this reduces the risk of thermal fatigue in applications subject to thermal cycling. Typically the mechanical properties of nine percent nickel will be higher than those of the weld consumables utilized. This requires special consideration to weld qualification tests. Note that with the 30X in centrast to the 30XL (low carbon grades). The higher the carbon content the lower the impact toughness.Shop built stainless steel cryo vessels in the USA are built to ASME Boiler Pressure Vessel Code Section V111. Field erected vessels may use the API 620 Q. Austenitic stainless accounts for the majority of metals used for cryo applications. The rest of the applications use 5 to 9% nickel or aluminum. Where high strength is required nine nickel may be chosen instead of an austenitic steel. Its important to remember that nine percent nickel is an alloy that can rust.
Weld Note: Due to hardening potential and the formation of refractory oxides consideration is required for Precipitation Hardenable nickel steels. Use TIP TIG to get the best weld results. With the MIG welding process, short circuit, spray and pulsed transfer modes are available. The primary weld differences between carbon steel MIG welds and nickel alloy welds will be; [1] The nickel welds will be much more sluggish, weld fusion is always a primary concern. [2] The nickel welds are very sensitive to oxidation that can lead to extensive weld porosity. With MIG be concerned about the quality of the gas mixes available at your local gas distributor. Many cylinders used for argon may have previously been used for argon CO2 and argon Oxy. The remains of these reactive gases may be present in your cylinders. If you order 99 argon 1 CO2 have a certificate of gas composition presented at the time the gas is delivered. Ensure you adequate pre-flow, post-flow., [3] The magentic influence on the arc is much more noticeable with nickel alloys, again this is a good reason to use Ed gas mix as the CO2 provides improved electron transfer and improved arc stability. [4] The crack sensitivity is much greater with nickel alloy so use low to moderate weld parameters. [5] The cleanliness in the weld areas is super critical when welding nickel alloys. Welding and post weld heating should only be carried out on nickel alloys that are clean and free of contaminates. Grinding and shot blasting are effective. With grinding use wheels that are dedicated only to the nickel welds. Wire brushing will typically not fully remove the surface oxides. If brushes or power brushes are utilized ensure they are made of stainless steels. [6] Nickel alloys are sensitive to embattlement from phosphorous, and sulfur and these elements are found in many of the materials used in metal forming. Plasma or laser cutting oxides which will have higher melting temperature than the base metal should be removed from nickel alloy plate edges that will be part of the welds. The higher temp cutting oxides can act as a barrier against the sluggish nickel welds impeading weld fusion potential. The oxides from the cutting surfaces can also create internal weld porosity and cause a reduction in the nickel mechanical properties. In contrast with carbon steels in which the oxides and inclusion typically rise at a fast pace to the weld surface, with the sluggish composition of nickel welds, the contaminates on the plate are more likely to become trapped in the weld. The sluggish nature of the nickel welds can also cause extensive lack of weld fusion especially on MIG welded parts > 4 mm. Lack of weld penetration can cause a point for stress concentration. When welding tube or pipe or butt welds with full penetration treat the weld like a stainless weld and ensure the backside of the root has an argon purge. [7] Weld heat typically does not have a negative impact on the nickel alloys. A small amount of grain growth and annealing will occur in the welds HAZ. Short circuit will obviously have a much less affected HAZ than a spray transfer weld and with a specific wire diameter, pulsed will provide less weld heat than spray. [8] When you do a tensile test on a nickel welded sample, please keep in mind that the annealed part of the HAZ will be the first location to elongate. The plastic elongation will cause strain hardening which "actually increases the yield strength". The bottom line is the work hardening influence on the elongation is influenced by the size of the HAZ, and its important to remember that transverse tensile elongation or the noted transverse yield strength attained can be misleading. [9] With multi-pass welds be aware of the weld heat input build up, especially when welding those 
oxidation sensitive, precipitation hardenable alloys which can leave an oxide
surface on the weld that can impead multi-pass weld fusion potential. All Nickel
welds subject to excess weld heat will be influenced by atmospheric contamination
creating a severe oxide on the weld's surface. For mult-pass welds use interpass
temperature controls (typically 300 to 350F) to minimize both the heat influence
on the weld HAZ and oxidation potential.[10] Pre-heat is typically not necessary for nickel alloys if the metals are at or above indoor shop temperature. If the metals have been stored outside or moisture is suspect to reduce the weld porosity potential, pre-heat the metals between 70 and 100F. [11] Post heat treatment is usually not required for the common nickel alloys after welding to attain the desired corrosion resistance. However with nickel chrome 600 alloy, stress relief is required for fused-caustic service applications and also for alloy 400 applications as used in hydrofluoric acid service. Also note the nickel molybdenum and nickel silicon alloys HAZ can lower the corrosion resistance therefore these alloys may require a postweld solution-annealing treatment to restore the corrosion resistance of the HAZ. Pulsed MIG, flux cored traditional TIG, none of these processes will provide the weld quality attained from the TIP TIG process, extensive info on theis process here.
Filler Metal Selection. As corrosion potential is the primary concern in the selection of nickel alloys the filler metal should have similar chemistry composition to the base metal to be welded. The 600 series nickel chrome and nickel- iron - chrome alloys can end up with that austenitic problem caused by carbide precipitation (CP) in the HAZ, see the stainless section. Its reported that the CP does in most cases not result in accelerated corrosion attacks. Like stainless weld consumables, additions of columbian or titanium are added to specific filler metals such as the popular Inconel 625 are used to help stabilize the welds and minimize the CP influence. With MIG welding remember you will get greater current density (less sluggish welds) from smaller wire diameters. Welding under 6 mm thickness, I would recommend an 0.035 (1 mm) nickel MIG wire. Welding thicker than 6 mm, consider an 0.045 (1.2 mm) wire. However if you want the best weld quality forget MIG and consider TIP TIG.  ED'S
MIG GAS MIXES FOR NICKEL ALLOYS:
You can use straight argon for the Nickel MIG welds, however when using MIG spray
transfer consider argon with 1% CO2, for applications 3 to 6 mm. For spray applications
over 6 mm, to attain more weld energy try a three part mix containing argon -
40% helium - 1 % CO2. Use gas flow rates in the range of 40 to 60 cuft/hr. For
those of you that are considering pulsed rather than spray, remember nickel welds
are sluggish going from a pulsed peak to a low background weld current does not
improve a sluggish weld in contrast to traditional spray. When TIP TIG welding
use the same filler metals as MIG, no special gas mixes are required simply use
straight argon. []
When welding the 300 series of stainless to carbon steels the austenitic 309 filler
metal and sometimes 310 are utilized. The 310 25% Cr - 20% Ni, can cause the austenitic
welds to fail due to microfissuring which resulted in cracks in applications subject
to thermal stresses. The weld failures were often a result of the differences
of the coefficient of thermal expansion (CTE). The 309, 23% Cr - 13% Ni filler
metal when used on stainless to carbon steels results in a weld with ferrite reducing
the potential for mico-fissuring, however keep in mind depending on the application
chemistry, thickness, weld process and parameters used, the dissimilar weld joints
are still dilution sensitive. The 309 filler when used on stainless to steel welds
still have large CTE differences therefore one should be concerned when the welds
or parts are subject to temperatures over 600F in which high stresses or thermal
fatigue effects the ferritic / austenitic weld interface. [] Where the 309 and 310 have problems the weld solutions are frequently found with the 600 series Ni Alloy filler metals.  Ed developed this boiler, Inconel 625, pulsed MIG water wall clad procedure, patented by Aquilex (Welding Services Atlanta) in the USA 2007 and in Europe 2009: Check the MIG clad section for more data. []
The 600 series as many of you know are often called Inconel. These high Ni-alloy
filler metals typically contain up to 72 nickel 15 % Chrome and 8% Fe. These filler
metals have a much lower CTE than the 300 series austenitic alloys.When welding
the lower CTE results in less weld thermal stresses. The Inconel alloys are also
less sensitive to weld microfissuring or weld dilution concerns from dissimilar
metals. [] When parts are in service at temperatures> 700 F, welds that contain high nickel to chrome ratios can be sensitive to sulfur corrosion. This risk is reduced with filler metals that have higher chrome / moly. Alloys 625 / 671. The 671 is AWS (ERNiCr-4 rod) [] The 625filler, EniCrMo-3 rod , MIG and flux cored wire should be restricted to applications <1000F as weld embrittlement can occur. For inconel welds the best weld process is TIP TIG. [] For a story on how not to use Inco 625 for cladding boiler water wall tubes click.
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If
you want all position, defect free alloy welds at superior quality than conventional
TIG, Pulsed MIG or the flux cored process and you would like to produce all position
weld deposition rates equal to pulsed MIG and flux cored, consider the TIP TIG
process. A five minute TIP TIG demo will show any weld professional that when
welding / cladding in any weld position, thin or thick metals, any alloy , the
TIP TIG process is the world's most cost effective process for producing defect
free welds.
The
Fossil and Nuclear industry will never attain the construction weld quality or
productivity (10 to 40 times faster than manual TIG) that the ATT manual and automated
weld process can deliver. Oil Platforms - Ship Yards - Naval Vessels and Submarines
- The Space and Aircraft Industries - Cryogenic Vessels - Petro Chemical - Refining
- Waste to Energy - Industrial Processing - Pulp and Paper - Military Equipment
- Medical Equipment - Food and Beverage, none of the North American industries
have in their weld shops a weld process that can deliver the weld quality / productivity
attainable from the easy to use, semiautomatic ATT process. 
ED'S
MIG GAS FOR NICKEL ALLOYS:
Site Established 2001. Ed Craig Weld Reality. E-Mail ecraig@weldreality.com. Phone Eastern Time USA 828 658 3574.
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