Welding stainless steel requires the selection of shielding gas to maintain its metallurgical composition and associated physical and mechanical properties.Common shielding gas elements for stainless steel include argon, helium, oxygen, carbon dioxide, nitrogen, and hydrogen (see Figure 1).These gases are combined in different ratios to suit the needs of different delivery modes, wire types, base alloys, desired bead profile and travel speed.
Due to the poor thermal conductivity of stainless steel and the relatively “cold” nature of short-circuit transfer gas metal arc welding (GMAW), the process requires a “tri-mix” gas consisting of 85% to 90% helium (He), up to 10 % Argon (Ar) and 2% to 5% Carbon Dioxide (CO2).A common triblend mixture contains 90% He, 7-1/2% Ar, and 2-1/2% CO2.The high ionization potential of helium promotes arcing after a short circuit; coupled with its high thermal conductivity, the use of He increases the fluidity of the molten pool.The Ar component of Trimix provides general shielding of the weld puddle, while CO2 acts as a reactive component to stabilize the arc (see Figure 2 for how different shielding gases affect the weld bead profile).
Some ternary mixtures may use oxygen as a stabilizer, while others use a He/CO2/N2 mixture to achieve the same effect.Some gas distributors have proprietary gas blends that provide the promised benefits.Dealers also recommend these blends for other transmission modes with the same effect.
The biggest mistake manufacturers make is trying to short-circuit GMAW stainless steel with the same gas mixture (75 Ar/25 CO2) as mild steel, usually because they don’t want to manage an extra cylinder.This mixture contains too much carbon.In fact, any shielding gas used for solid wire should contain a maximum of 5% carbon dioxide.Using larger amounts results in a metallurgy that is no longer considered an L-grade alloy (L-grade has a carbon content below 0.03%).Excessive carbon in the shielding gas can form chromium carbides, which reduce corrosion resistance and mechanical properties.Soot can also appear on the weld surface.
As a side note, when selecting metals for shorting GMAW for the 300 series base alloys (308, 309, 316, 347), manufacturers should select the LSi grade.LSi fillers have a low carbon content (0.02%) and are therefore especially recommended when there is a risk of intergranular corrosion.Higher silicon content improves weld properties, such as wetting, to help flatten the crown of the weld and promote fusion at the toe.
Manufacturers should exercise caution when using short-circuit transfer processes.Incomplete fusion can result due to arc extinguishing, making the process sub-par for critical applications.In high volume situations, if the material can support its heat input (≥ 1/16 inch is approximately the thinnest material welded using the pulse spray mode), a pulse spray transfer will be a better choice.Where material thickness and weld location support it, spray transfer GMAW is preferred as it provides a more consistent fusion.
These high heat transfer modes do not require He shielding gas.For spray transfer welding of 300 series alloys, a common choice is 98% Ar and 2% reactive elements such as CO2 or O2.Some gas mixtures may also contain small amounts of N2.N2 has a higher ionization potential and thermal conductivity, which promotes wetting and allows for faster travel or improved permeability; it also reduces distortion.
For pulsed spray transfer GMAW, 100% Ar may be an acceptable choice.Because the pulsed current stabilizes the arc, the gas does not always require active elements.
The molten pool is slower for ferritic stainless steels and duplex stainless steels (50/50 ratio of ferrite to austenite).For these alloys, a gas mixture such as ~70% Ar/~30% He/2% CO2 will promote better wetting and increase the travel speed (see Figure 3).Similar mixtures can be used to weld nickel alloys, but will cause nickel oxides to form on the weld surface (e.g., adding 2% CO2 or O2 is enough to increase the oxide content, so manufacturers should avoid them or be prepared to spend a lot of time on them). Abrasive because these oxides are so hard that a wire brush usually won’t remove them).
Manufacturers use flux-cored stainless steel wires for out-of-situ welding because the slag system in these wires provides a “shelf” that supports the weld pool as it solidifies.Because the flux composition mitigates the effects of CO2, flux-cored stainless steel wire is designed for use with 75% Ar/25% CO2 and/or 100% CO2 gas mixtures.While flux-cored wire may cost more per pound, it is worth noting that higher all-position welding speeds and deposition rates may reduce overall welding costs.In addition, the flux-cored wire uses a conventional constant voltage DC output, making the basic welding system less costly and less complex than pulsed GMAW systems.
For 300 and 400 series alloys, 100% Ar remains the standard choice for gas tungsten arc welding (GTAW).During GTAW of some nickel alloys, especially with mechanized processes, small amounts of hydrogen (up to 5%) may be added to increase travel speed (note that unlike carbon steels, nickel alloys are not prone to hydrogen cracking).
For welding superduplex and superduplex stainless steels, 98% Ar/2% N2 and 98% Ar/3% N2 are good choices, respectively.Helium can also be added to improve wettability by about 30%.When welding super duplex or super duplex stainless steels, the goal is to produce a joint with a balanced microstructure of approximately 50% ferrite and 50% austenite.Because the formation of the microstructure depends on the cooling rate, and because the TIG weld pool cools quickly, excess ferrite remains when 100% Ar is used.When a gas mixture containing N2 is used, the N2 stirs into the molten pool and promotes austenite formation.
Stainless steel needs to protect both sides of the joint to produce a finished weld with maximum corrosion resistance.Failure to protect the backside can result in “saccharification,” or extensive oxidation that can lead to solder failure.
Tight butt fittings with consistently excellent fit or tight containment at the rear of the fitting may not require support gas.Here, the main issue is to prevent excessive discoloration of the heat-affected zone due to oxide build-up, which then requires mechanical removal.Technically, if the backside temperature exceeds 500 degrees Fahrenheit, a shielding gas is required.However, a more conservative approach is to use 300 degrees Fahrenheit as the threshold.Ideally, the backing should be below 30 PPM O2.The exception is if the back of the weld will be gouged, ground and welded to achieve a full penetration weld.
The two supporting gases of choice are N2 (cheapest) and Ar (more expensive).For small assemblies or when Ar sources are readily available, it may be more convenient to use this gas and not worth the N2 savings.Up to 5% hydrogen can be added to reduce oxidation.A variety of commercial options are available, but homemade supports and purification dams are common.
The addition of 10.5% or more of chromium is what gives stainless steel its stainless properties.Maintaining these properties requires good technique in selecting the correct welding shielding gas and protecting the backside of the joint.Stainless steel is expensive, and there are good reasons to use it.There is no point in trying to cut corners when it comes to shielding gas or choosing filler metals for this.Therefore, it always makes sense to work with a knowledgeable gas distributor and filler metal specialist when choosing a gas and filler metal for welding stainless steel.
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