Choosing the right shielding gas and supply system for GMAW

WWW.THEFABRICATOR.COM JANUARY 2002

July 26, 2001

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Selecting the right shielding gasses for your welding operation can mean the difference between mediocre production rates and peak efficiency.

Welder in Pipe

Selecting a gas metal arc welding (GMAW) shielding gas and supply system demands a cost and capability analysis to determine the most economical, efficient solutions for your operation. The cost of the welding gas selected may be a small percentage of the overall process cost, but the correct gas can decrease labor and overhead or produce a faster welding speed.

Costs you should assess include labor and overhead, shielding gas, consumables(wire and/or electrodes), and gas supply.

Labor and Overhead

Proper selection of shielding gas can reduce your operation's labor, overhead, and bottom-line costs by increasing welding speed and duty cycle while decreasing cleanup time.

Increasing Welding Speed. Maximizing welding speed depends largely on the combination of heat transfer properties, oxidizing potential, and metal transfer characteristicsof the selected gas. High-thermal-conductivity gases produce the hottest puddles with the greatest fluidity. High-oxidizing gases reduce weld bead surface tension more effectively and provide better weld metal coalescence. Inert gas blends that allow spray transfer provide the highest level of deposition efficiency and, generally, higher travel speeds.

For example, in the same time period, an argon blend in the spray arc mode produces a weld that is about 17 percent to 20 percent longer than a weld made with carbon dioxide in the globular mode (on 0.025-inch and larger welds, all other factors being the same). The most common mistake in selecting the shielding gas is choosing the least expensive one; other factors affect the total cost of the welding process and depend on the quality of the shielding gas used.

In addition to shielding gas and weld size, other factors that affect welding speed include current, voltage, weld position, joint fit-up, and the mode of travel (mechanized or manual). Maximum possible welding speeds for any shielding gas are more attainable in mechanized applications with clean plate and good fit-up. In a manual application, operator skill often is the limiting factor. A welder generally can track a weld seam consistently and deposit an acceptable weld is generally at about 35 to 40 IPM. Automatic and/or robotic applications produce higher welding speeds; however, the maximum speed depends on the system employed.

In regard to manual welding of thinner material, all shielding gases can accommodate higher welding speeds than operators can produce. In a mechanized operation, carbon dioxide can produce a higher-speed weld on thin-gauge material when burn-through is not a problem. However, when fit-up or burn-through problems are evident, argon blends with a lower carbon dioxide percentage can achieve more consistent results. Even lowering the carbon dioxide level from 25 percent to 8 percent sometimes can dramatically improve linear travel speed. In most shops, where different materials and thicknesses may be welded, various blends of gases may be required.

Shielding Gas Effect on Duty Cycle and Cleanup

Operator duty cycle is the amount of time an operator actually is welding, as opposed to time spent on associated functions such as setup, cleanup, or other nonwelding activities.

When ferrous metal is welded using carbon dioxide as the shielding gas, spatter accumulates in the gas nozzle, disturbing gas flows, or collects on the end of the contact tube, causing wire feed problems. The use of argon-based gas blends greatly reduces weld spatter, in many cases eliminating spatter entirely. As a result, operators can weld for longer periods before stopping to clean spatter from nozzles and contact tips. Any factor that reduces weld spatter can improve the deposition efficiency of the welding process.

Fumes and gases also are created by the chemical reaction between hot metal and active gases such as oxygen or carbon dioxide. Using argon mixtures that produce fewer fumes and gases can lower pollution control and cleanup costs.

Using the proper shielding gas helps to ensure that deposition efficiency is as high as possible for the process used. For example, the deposition efficiency of a GMAW operation using pure carbon dioxide typically is 89 percent. When the shielding gas is changed to a mixture of argon with 25 percent carbon dioxide, the deposition efficiency can increase to 97 percent.

Shielding Gas Cost. Shielding gas cost should be estimated at no more than about 5 percent of total costs. Even though argon or argon mixtures cost substantially more than carbon dioxide, when the increases in operator duty cycle and deposition efficiency are considered, the total cost per foot of weld is less with an argon mixture than with carbon dioxide. Gas costs vary across geographic regions, but, on average, argon or argon mixes cost five times more than straight carbon dioxide.

Consumables Costs. When determining consumables costs, you should consider wire and shielding gas prices together because of the effect that shielding gas has on deposition efficiency. Remember, the lowest-priced wire and gas also may yield the lowest deposition efficiency. Although the initial price may be higher, the recommended wire and gas combination can increase deposition efficiency and result in lower overall cost.

Highly oxidizing shielding gases, such as pure carbon dioxide, require welding wires with additional deoxidizers to counter the loss of alloy in the welding arc. If carbon dioxide shielding is changed to argon with 20 percent carbon dioxide, a less expensive welding wire can be substituted because of the good alloy retention of the argon shielding gas. A shop can save up to 15 percent in wire costs with this change while maintaining mechanical properties, making operation easier, and increasing weld speeds.

Gas Supply System

Shielding gas is available in high-pressure cylinders, liquid cylinders, or bulk liquid supply. The choice depends on the gas blends you use, your consumption patterns, location of your equipment, and presence of a piping distribution system. Product costs and the rental cost of storage systems—amortized over monthly product consumption-should be considered. When a variety of gas blends are used, a large store of cylinders may have to be inventoried.

One method of obtaining a gas blend is to use an onsite gas blender. However, the cost of storage and blending equipment must be compared to the cost of premixed cylinder products to determine which form of supply is more economical.

Managing gas flow at use points requires a dedicated and constant effort to prevent waste. Recommendations about sizing a system correctly, the proper use of regulation and mixing equipment, and flowmeters to ensure economical gas consumption are available from your gas supplier.

After you have selected a shielding gas, consider the various gas supply systems available.

Cylinder Storage Systems. When gas volume requirements are small, individual cylinders of argon, helium, carbon dioxide, oxygen, or blends of these gases can be used directly in the welding operation. If the consumption rate increases, two or more cylinders can be manifolded together in banks to provide greater supply and to reduce the amount of cylinder handling. A manifold usually has two independent sets of controls to permit alternate or simultaneous operation of the two cylinder banks. Empty cylinders can be replaced with full ones after shutting down one or both cylinder banks.

If you use premixed blended gases even in small volumes, your cost can be higher than expected, because blended gas normally is priced at the more expensive component level. A 75/25 argon/carbon dioxide blend costs the same as 100 percent argon, negating any savings gained for the lower-cost carbon dioxide.

Gas Blending. Shielding gases often are used in blends to optimize the arc welding process. Blenders can provide mixes that meet tight tolerances, ensuring users that they are receiving the correct blend consistently. You can purchase and blend pure gases to meet changing production requirements by changing mixer settings. When using a blender system, you can select a variety of blends and no additional cylinders will be required; plus, the lower cost of carbon dioxide can be realized. Even in small amounts (5,000 cubic feet/month), the cost of the equipment can be offset by the lower cost of the gas. In larger applications, cryogenic cylinders or on site bulk systems can add to the cost savings.

Bulk Storage Systems. If you consume more than 50,000 cu. Ft. of gas per month consider bulk cryogenic storage systems because they offer such significant space and cost savings. Bulk storage tanks are fairly common, with standard capacities ranging from 1,500 to 11,000 gallons.

Inert Gas Distribution Systems. When individual cylinders are used to supply gas, a pressure regulator is attached to the cylinder to reduce the exiting gas pressure. A flowmeter, connected to the welding equipment by tubing with appropriate fittings, is used to maintain preset gas flow rates.

When large volumes of gas are required or where there are multiple weldstations, it may be preferable to install a distribution piping system. A system of this type contains main and secondary distribution lines. Note that gas supply, a blender, a main distribution line, and shutoff valves are required.

A typical secondary line has a station valve that is connected to the welding machines through a pressure regulator and flowmeter (often a combined unit). This is connected to the welding machines by an inert gas hose or tubing with appropriate fittings and a solenoid-operated shutoff valve.

It is recommended that the design, materials, fabrication, inspection, and tests meet the standards set in the Compressed Gas Association's pamphlet Industrial Practices for Gaseous Transmission and Distribution Piping Systems.



Witt Gas Controls LP

David Bell

President
Witt Gas Controls LP
380 Winkler Drive
Suite 200
Alpharetta, GA 30004
Phone: 770-664-4447
He also is a member of the Practical Welding Today® Editorial Review Committee.

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