Simplifying Shielding Gas Selection

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Home \ Consumables \ Articles \ Simplifying shielding gas selection

Simplifying shielding gas selectionHow different gases and gas blends affect your welding application

By Kevin A. Lyttle , W.F. Garth Stapon

January 11, 2005

Have you ever asked yourself any of the following questions:

1. Why is welding affected by the shielding gases used?

2. Why are so many blends available for carbon steel?

3. Why can I use some gases for some materials and not

others?

Answering these questions will help you take the mystery out

of gas selection and make a choice that meets the needs of

your job.

You can choose from a variety of gas blends for gas metal arcwelding (GMAW), gas tungsten arc welding (GTAW), or flux

cored arc welding (FCAW). For each of these processes, the

shielding gas performs multiple tasks. It not only protects the

molten weld pool from exposure to the atmosphere, but it also

can promote a stable arc, determine the type of metal transfer

obtained, affect the welding travel speed, and impact the

quality of the finished weld deposit. Proper shielding gas

selection is critical to the ultimate success of the joining

operation.

You can take several approaches to selecting a gas, but to

make the best selection, you should know what your finished weld requirements are.

Ask yourself:

1. What kind of material must be joined?

2. How important is weld appearance?

3. Is spatter a concern?4. Is productivity improvement a prime interest?

5. Is deep penetration required, or should penetration be minimized to reduce burn-through in the joint?

6. Is it important to reduce welding fumes?

Keep these questions in mind as you address how to choose the best shielding gas for your application.

Which Process Is Best for This Application?

When deciding which welding process works best in your application, consider base material type, base material

thickness, and welding position.

As a rule of thumb, use GTAW for the most consistent joining results for materials thinner than 0.040 in. For thicker

materials, GMAW usually is a more economical process. FCAW should be considered in some instances to

enhance production rates, especially on scaled or rusty material or when welding out of position.

Additional considerations include joint penetration, welding position, and weld quality. It's more economical to weld

in the flat, horizontal, or slightly downhand positions because higher deposition rates are achieved. With FCAW or

pulsed and conventional spray GMAW, productivity can increase. Optimum weld quality can be achieved with

GTAW, but it requires greater welder skill and deposits less weld metal at slower speeds than GMAW or FCAW.

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Note: The multiple gas recommendations for the same process and material type and

thickness indicate that the selection will be based on the specific needs of the application

being considered.

One Gas, Two Gases, or Three Gases?

Three pure gases form the foundation for shielding arc welding processes: argon (Ar), helium (He), and carbon

dioxide (CO2). In many cases, other gases such as oxygen (O2), nitrogen (N2), and hydrogen (H2) may be added to

modify arc characteristics, the molten weld pool, or the weld. They also can affect the metal transfer and the overall

performance obtained in GMAW and FCAW. Tailoring the blend composition helps meet job requirements.

The next question to ask is which gases do what with GMAW, GTAW, and FCAW.

Argon. Argon is heavier than air, has low thermal conductivity, and can ionize easily in a welding arc. This means

that argon blankets the weld zone (so lower flow rates are needed), providing a relatively narrow arc characteristic

with good electrical conductivity (which means easy arc-starting). It can be used alone in GTAW and for GMAW of

aluminum and other nonferrous materials. Argon is a major component of shielding gas when spraylike, high-

productivity welding is desired in GMAW or FCAW for joining steel and stainless steel.

Helium. Helium is considerably lighter than air, which means higher flow rates are required than for argon or carbon

dioxide. It offers good thermal conductivity but lower electrical conductivity when compared to argon (arc-starting

requires higher voltage). Helium generally is combined with other gases to optimize performance characteristics.

Helium-enhanced mixtures can join all material types using GMAW, GTAW, or FCAW.

Carbon Dioxide. Carbon dioxide dissociates at arc temperatures and recombines as it contacts the cooler base

material, t ransferring arc energy t o the weld puddle. Additions of carbon dioxide provide wider and deeper bead

penetration. The oxidizing atmosphere produced within the arc zone results in more slag on the surface of the

solidified GMAW bead. Carbon dioxide can be used alone or as a major component (typically 5 to 25 percent)

mixed with argon for GMAW and FCAW.

Oxygen. Oxygen can improve arc performance in GMAW by enhancing arc stability while reducing the surface

tension of the weld puddle. This causes the puddle to be more fluid and have better wetting characteristics. Since

oxygen reacts with components of the welding wire or electrode, it contributes to the slag deposit found on the weld

bead surface. It's typically used as a 2 to 5 percent addition to an argon-based blend.

Nitrogen and Hydrogen. Nitrogen and hydrogen typically are used only for welding 300 series (austenitic) or duplex

stainless steels. Nitrogen can increase weld penetration and arc stability. Hydrogen can improve weld puddle fluidity

and surface cleanness. Their use generally is confined to stainless steel applications, because nitrogen can cause

porosity in carbon steel, while hydrogen can increase cracking potential in some of the same materials.

How Are Gases Blended to M ee t Application Needs?

To understand how different gas blends work in different applications, first it's important to understand the differencebetween two- and three-part blends.

Two-part Blends. Traditional two-part blends for GMAW of carbon steel are mixtures of argon with controlled

amounts of oxygen or carbon dioxide. Argon/oxygen blends were a s tandard choice for conventional or pulsed-spray

transfer, but in many applications, they have been replaced by argon/carbon dioxide mixes.

When argon/carbon dioxide blends are used in place of argon/oxygen mixtures, bead appearance improves with less

surface oxide and better bead shape and wetting control. Simultaneously, it can provide a more easily controlled,

broader, less fingerlike penetration profile. As the carbon dioxide level increases, penetration deepens, which can

cause burn-through on thin materials. Spatter and fume levels increase as the carbon dioxide content of the blend

increases. Converting from oxygen-enhanced blends to argon/carbon dioxide mixt ures generally produces more

consistent, higher-quality results. Travel speed increases of 15 to 20 percent can be achieved by selecting argon

with 5 to 15 percent carbon dioxide in applications in which argon with 1 to 5 percent oxygen has been used

previously.

For FCAW of carbon or stainless steel, argon with 25 percent carbon dioxide can be used to improve out-of-position

performance and reduce weld spatter. In special circumstances, lower carbon dioxide additions can be used with

specially formulated cored wires to reduce welding fumes. To prevent weld quality problems, the flux-cored wire




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Author

must be used following the manufacturer's shielding gas recommendation.

Argon with 25 to 50 percent helium can be used to shield both GMAW and GTAW of aluminum and some other

nonferrous materials. Helium increases the heat input to the base material to enhance penetration and improve weld

puddle fluidity. Hydrogen (less than 10 percent) also can be added to argon for GTAW of austenitic stainless steel

to increase weld puddle fluidity and improve travel speeds by 10 to 25 percent while providing good weld bead

appearance.

Three-part Blends. To further enhance bead appearance, improve ease of operation when some base material

contamination is present, and increase flexibility and overall welding productivity, three-part shielding gas blends

may be a good choice.

Three-part blends can work well with short-circuiting, spray transfer, and pulsed-spray transfer. Benefits include

improved arc stability for reduced spatter and better weld bead wetting characteristics.

Argon with carbon dioxide and oxygen can provide versatility to join different types and thicknesses of carbon steel.Argon/helium/carbon dioxide mixes (25 to 35 percent helium, 1 to 10 percent carbon dioxide) can increase travel

speed. Argon/helium blends with controlled carbon dioxide content (1 to 2 percent) are suitable for joining stainless

steels when weld metal carbon control is important. For best results with GMAW, pulsed-spray transfer often is

recommended.

FCAW also is suitable for joining stainless steels, especially when welding out of position. Using cored wires

typically results in cost savings via higher weld metal deposition rates. They are suitable for joining material thicker

than 14 in. or when pulsed transfer isn't an option.

When joining 300 series alloys, optimum bead color and shape can be obtained by using argon/carbon

dioxide/hydrogen blends because of the reducing atmosphere created by the presence of the hydrogen. This mixture

minimizes ox ide formation on the bead surface and enhances the fluidity of the weld puddle. Hydrogen-enhanced

mixtures are not recommended for joining plain carbon steel.

The industry standard for short-circuiting GMAW of stainless steel has been a helium-based (85 to 90 percent) mix

with small additions of argon (5 to 10 percent) and carbon dioxide (2 t o 5 percent). This blend provides good bead

shape and color match, but it's not as versatile as some other mixtures. An argon/carbon dioxide/nitrogen blend can

be used in its place while also permitting use in high-productivity spray and pulsed-spray transfer. When combined

with high-silicon filler metals, weld puddle fluidity and wetting action can improve. Mixtures containing nitrogen arenot recommended when joining stainless to carbon steel.

What Is the Optimal Gas Flow?

Shielding gas flow rates are determined by the process, welding position, and operating parameters. With GTAW,

flow rates typically are from 10 to 20 cubic feet per hour (CFH). For GTAW, using a torch with a gas lens will help

ensure a laminar flow, which contributes not only to better weld quality, but also to 10 percent or lower gas

consumption rates.

With GMAW and FCAW, recommended flow rates vary widely30 to 45 CFHdepending on the welding position,

operating current, and shielding gas composition. For flat-position welding, helium-enhanced mixtures will require

slightly higher flow rates than argon-based blends. Gas flow rates may be reduced if the nozzle-to-work distance is

kept as close as possible. In many instances, production site surveys determine that shielding gas flow rates

typically are set in excess of 50 CFH. This can contribute to poor weld quality as atmospheric gases are drawn into

the arc zone because of excessive gas turbulence. Optimized flow enhances quality and reduces shielding gas

usage.

Shielding Gas and Cost-effective Joining

Shielding gas selection is critical to achieving cost-effective joining of carbon steel, stainless steel, and aluminum.

You can select one gas, such as argon for aluminum welding, to provide suitable arc stability, minimum spatter, and

good bead shape.

Two-part blends, such as argon/carbon dioxide mixtures, can perform well in most carbon steel and stainless steel

welding applications in which you use conventional or pulsed-spray t ransfer.

Lower carbon dioxide content will reduce the amount of welding fumes generated. Where you require enhanced bead

appearance, low spatter, and less postweld cleanup, three-part gas blends can provide good results while still

improving welding productivity.

Base metal thickness, welding position, operator skill level, and production demands require careful consideration

when selecting and optimizing the joining process. With an understanding of shielding gases, you can optimize your

welding process to reduce cost and improve quality.

Kevin Lyttle is manager of welding research and development for Praxair Inc., and Garth Stapon is marketing

manager, metal fabrication, eastern region, for Praxair Distribution Inc., 39 Old Ridgebury Road, Danbury, CT

06810, 800-772-9247, fax 800-772-9985, [email protected], [email protected],

www.praxair.com.

Related Company Showrooms:Praxair Inc.

Additional Information

Kevin A. LyttleManager, Welding Research and Deve lopmentPraxair Inc.

39 Old Ridgebury Road

Danbury, CT 06810

USA




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Phone: 800-PRAXAIR

Contact via email

W.F. Garth StaponMarketing Manager, Metal FabricationPraxair Inc.

39 Old Ridgebury Road

Danbury, CT 06810

Phone: 800-225-8247

Contact via email

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Simplifying Shielding Gas Selection