7 effects of shielding gas

Blend composition makes a difference

PRACTICAL WELDING TODAY® JANUARY/FEBRUARY 2007

January 9, 2007

By: ,

Depending on your application, various components of your shielding gas blend can help or hurt you. Find out what effects shielding gas has on your weld and what you can do to get the best results.

Effects of shielding gas

Although you probably know that shielding gas is essential in most welding procedure specifications, you may pay little attention when you're selecting it. A simple gas composition change can offer potential savings in seven key areas, particularly in gas metal arc welding (GMAW).

Shielding gas typically isn't considered to have much of an effect on the cost of a welding operation. Many welders don't fully understand the financial impact that shielding gas can have on the bottom-line cost of the welding operation.

Which shielding gas composition you choose for GMAW can affect your welding operation in seven key ways.

1. Filler Metal Deposition Rate and Efficiency

Shielding gas blends with high argon content generally result in high productivity. Placing the workpiece in the flat or horizontal welding position allows you to use spray transfer with these blends. Select shielding gas carefully: Wire feed rates and current levels are high.

Single-wire GMAW can exceed deposition rates of 15 pounds per hour at 100 percent duty cycle. Argon content should be 85 percent or more to accomplish spray transfer. In some cases, instead of using a conventional argon/carbon dioxide or argon/oxygen blend, using a helium-enhanced argon blend may increase weld metal deposition rates up to 15 percent.

Although helium costs more than argon, shielding gas typically comprises less than 5 percent of the total welding cost, making the addition of helium something to consider when choosing a shielding gas blend.

Electrode deposition efficiency is linked directly to the welding spatter level. High-argon blends typically produce the best results in spray transfer. Improved deposition efficiency also can be a function of choosing the right welding parameters. A nonoptimized system—in which any number of parameters, such as gas flow rate and voltage, aren't optimized—generally produces lower deposition efficiency and may contribute to increased postweld cleanup costs.

2. Spatter Control and Postweld Cleaning

Argon's low ionization potential results in improved arc stability which, in turn, helps eliminate spatter when you use conventional power supplies. Some recently developed power sources are designed to improve spatter with pure CO2 shielding gas. It's possible to reduce spatter even more and increase the operating window of these units by using argon-based shielding gases. You can increase the operating current and voltage by 10 percent or more while still maintaining spatter control.

If you use spray arc transfer, generally the argon level should be 85 percent or more. Pulsed spray transfer with a 95 percent argon blend typically will yield the lowest spatter levels when welding plain carbon steel. A three-part blend of argon, helium, and carbon dioxide can reduce weld spatter when joining galvanized steel or steel with some residual surface oil or scale.

In general, GMAW is a slag-free process, but slag islands still are common on the bead surface. Powder and paint won't adhere to these silicon deposits. Low shielding gas reactivity can help to reduce these surface residuals. If you're concerned about slag island buildup along the edge of the weld bead, work only with properly cleaned base material and use a blend containing at least 90 percent argon with no oxygen. Choosing filler metal carefully also will help limit island formations.

Postweld cleanup can increase cost, reduce your arc-on time, and decrease the welding operation's duty cycle. Because it reduces spatter, an optimal argon blend may help you reduce postweld grinding, which means you can dedicate more time to welding.

Color match typically is a concern when welding stainless steels. For the best color match, select a blend of argon or helium with low levels of oxygen or CO2 to minimize weld surface oxidation. Oxygen-free blends produce less weld metal discoloration. To further minimize discoloration, use a low operating current and a large gas nozzle with close nozzle-to-work distance to ensure good shielding gas coverage. When joining 300 series stainless steel, you can add a controlled amount of hydrogen—less than 5 percent—to lessen oxidation and therefore improve bead color and enhance productivity. If corrosion resistance is important, limit CO2 content to less than 5 percent.

3. Bead Profile and Overwelding

A CO2 shielded weld bead tends to have a convex shape, which contributes to overwelding; this increases welding cost. Argon-based blends offer good bead shape control, which can reduce overwelding. Because of the physical characteristics of a CO2 shielded arc and the weld puddle produced, CO2 can produce a convex bead shape. Argon blends tend to produce a flat bead face, which produces sufficient reinforcement but reduces overwelding.

Filler metal diameter also plays a critical role in optimizing bead shape. A large wire size can make it difficult to control the weld bead size. An oversized weld bead can increase welding costs by at least 50 percent. Choose your filler metal type and size based on the needs of your application.

4. Bead Penetration, Potential for Burn-through

When welding thin material—16- to 22-gauge—a gas blend's welding characteristics become important. One characteristic of pure CO2, for example, is that it results in increased weld pool energy when compared to an argon/ CO2 blend. By controlling the blend's CO2 content, you can control burn-through and increase welding productivity. Use argon/ CO2 blends in the 85 percent to 95 percent range to minimize burn-through.

Pure CO2 can help you achieve good weld penetration. The operating current, filler metal, and gas composition also affect the penetration profile. If you want deep penetration, use an argon blend with a high percentage of CO2—15 percent to 20 percent—or perhaps consider adding helium to broaden and deepen penetration. This also will contribute to better welding productivity.

5. Out-of-position Weldability

Shielding gases with greater reactivity, which use more CO2 or O2, will increase weld pool fluidity. For out-of-position work, this may force you to use slower wire feed rates, which will decrease productivity.

The type of metal transfer you choose also is critical when trying to improve out-of-position control. High-argon blends with low reactivity generally perform well.

6. Welding Fume Generation Rates

Many factors influence welding fume generation, including filler metal, base metal composition, operating parameters, and shielding gas.

High-argon blends are less reactive than pure CO2 and generally produce less welding fumes under similar operating conditions. But lower fume generation doesn't always equal lower exposure, so be sure to conduct measurements to ensure compliance with applicable permissible exposure limits.

7. Weld Metal Mechanical Properties

Since high-argon blends typically are less reactive than other blends, more alloying elements in the filler wire are transferred to the weld pool. This typically increases the weld strength. In all cases, consider your shielding gas when choosing a wire consumable so you know that the resulting weld meets the needs of your application.

The shielding gas you choose can affect many welding characteristics. Once you understand which properties are most important for your application, you can select the best blend for the job.



W.F. Garth Stapon

Marketing Manager, Metal Fabrication
Praxair Inc.
39 Old Ridgebury Road
Danbury, CT 06810
Phone: 800-225-8247

Kevin Lyttle

Development Scientist
Praxair Technology Center
175 E. Park Drive
Tonawanda, NY 14150
USA
Phone: 716-879-7290

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