November 2007 Edition

WELDING

Metal-cored Wire Offers Speed and Savings

An expert from a welding consumables company shares his knowledge about the advantages offered by metal-cored welding wire

Metal-cored wire is a versatile and flexible welding wire for manufacturing requirements. It can be used to weld a variety of material thicknesses with equal success and offers potential increased deposition rates and travel speeds to produce aesthetic bead appearances and little spatter compared to solid- and flux-cored wires.

Metal-cored wire costs about 33 percent more than solid wire, but it has the potential to increase travel speeds and deposition rates by 25 percent or more. Given that labor comprises about 85 percent of a welding operation's costs, the time savings can outweigh the additional expenditures as shown in the table.

In order for metal-cored wire to be worth the investment, users need to choose the proper wire and adjust welding technique for optimum performance.

Metal-cored wire is often a good choice for applications with material thicknesses ranging from 3/16" through 4", welded in flat and horizontal positions.

Problems with metal-cored welding operation, or where it has not increased welding productivity compared to solid wire, are often the result of using incorrect welding parameters, wire model or technique.

A ropy bead with poor penetration and a convex profile can be solved by adjusting wire feed speed, voltage, travel speed, and possibly even the gas mixture. Similarly, burn through or other problems caused by excessive heat and force can be fixed by adjusting the same variables.

The Welding Sweet Spot

Finding the metal-cored sweet spot – where the welds look and penetrate perfectly and the travel speeds are high – can be a bit tricky. Yet, if there is a cardinal rule in optimizing the performance of metal-cored welding, it is experimentation.

What works for one company or welder might not work for the next. Getting the best results from metal-cored wire require fine tuning variables to achieve the right combination of travel speed, deposition, and weld quality. Filler metal manufacturing representatives or welding distributors can help with fine-tuning these variables.

One of the most critical factors in metal-cored welding is selecting the proper wire. With the many varieties of wire under the same AWS classification – each with very different characteristics – choosing the right one may require a little investigation.

Available in diameters ranging from 0.035" to 3/32", metal-cored wire is available for carbon steel, low-alloy steel, and stainless steel applications.

What it Is

Metal-cored wire is a hollow metal tube filled with a blend of metal, mineral, and chemical powders. The majority of the core is iron, and additional elements are added to produce specific results based on the materials and applications.

Welding operations that involve painted equipment, such as bulldozers, cranes, fork lifts, or tractors, may want to select a metal-cored wire that produces little spatter and minimizes the formation of silicon islands, or brings the islands to the center of the weld bead, where they are easily removed.

Since, as stated above, an average of 85 percent of a welding operation's expenses is labor, metal-cored wire can contribute to a company's bottom line by reducing the amount of time spent on cleaning weldments and other activities that do not contribute to the throughput cycle.

Likewise, operations that weld material with mill scale and other surface contaminants should look to a wire with sufficient deoxidizers – specifically manganese and silicon – to bring those contaminants to the surface of the weld so that they don't remain trapped in the bead.

Choosing a wire with a strong "driving" arc provides better penetration through mill scale and other contaminants.

Getting the Gas Right

Shielding gas plays a role in the success of metal-cored welding. The wrong shielding gas mix can lead to insufficient penetration, burn through, excessive spatter, silicon deposits, or a number of other imperfections.

Thicker materials with surface contaminants may require an arc with more force and drive to break through those contaminants. In such cases, argon/CO2 mixtures provides a broader penetration profile and the needed arc drive.

Higher CO2 levels create more spatter, so use as low of a percentage of CO2 as necessary. The arc stabilizing and spatter reducing benefits of argon aren't significant until it is at least 75 percent of the mixture.

Like solid- and gas-shielded flux-cored wire, metal-cored wire can be used with 100 percent CO2 or an argon/CO2 mixture, with the richer argon mixtures being the most common. Metal-cored wire can also be used with a small amount of oxygen as part of an argon/oxygen mixture.

The reactivity of the oxygen with the arc intensifies its heat and accelerates the spray transfer process without producing additional drive. The result is that an argon/oxygen mixture produces a softer arc with high deposition rates, which allows metal-cored wire to weld very thin materials at high travel speeds and without the burn through that might otherwise be caused by an argon/CO2 mixture. For mild steel applications, a 95 percent argon/five percent oxygen mixture is commonly recommended. Stainless steel applications usually use less oxygen, such as a 98 percent argon/two percent oxygen mix.

Joint configuration dictates a number of factors when using the metal-cored process, including wire selection, gas mixture, and welding parameters. For example, deep v-grooves require a hard-driving arc. Such an arc is possible through proper wire selection, 75/25 argon/CO2 shielding gas, and proper voltage settings.

Speeding Along

Maximizing metal-cored welding performance depends on the application. Although prized for its ability to achieve high deposition rates and high travel speeds with a spray transfer process on thick material, metal-cored wire can perform very well on thin material as well.

When welding thin material, typically 11 gauge and thinner, a spray transfer process may introduce too much heat and penetrate too deeply into the base material, causing burn through or warping. In these instances, a short circuit, globular transfer, or pulse process often provides successful results – usually at the expense of travel speed, however.

Applications that require bridging gaps or welding in a vertical or overhead position benefit from the lower heat input and calmer weld pool created by a pulse process or standard short circuit transfer with a metal-cored wire. Other variables, such as gas mixture, can be adjusted to achieve good results when welding thin material with metal-cored wire.

Because of the heat it creates, metal-cored welding usually requires the contact tip to be recessed from 1/8" to 1/4" inside the nozzle. Keeping the contact tip recessed keeps it cooler, which increases consumable life and prevents degrading of the electrical conductivity.

A recessed tip also produces a longer wire stick-out, which increases the voltage passing through the wire and facilitates the spray transfer process.

In some instances, robotic welding applications will use an extended contact tip as a way of accessing confined joints.

The Need for Feed

Maintaining proper wire feed speeds, particularly in relation to travel speeds, is another important factor in optimizing metal-cored performance. Proper wire feed speeds depend on the material type, its thickness, electrical parameters, and other factors. Like solid- and flux-cored wire, running a travel speed that is too fast for the wire feed speed can result in a ropy, convex bead with poor penetration and tie-in to the base material.

Conversely, running wire feed speed too fast can cause an unstable arc with excessive spatter and a turbulent weld pool. The resultant weld bead will not spread out properly and may result in poor penetration and undercut, a condition where the weld metal is not fully bonded with the base metal. Consider using a semi-spray transfer to prevent excessive voltage and undercut during high-speed welding.

Of course, metal-cored wire will not be perfect for every application. If not used to its full potential, many of the characteristic benefits of metal-cored wire – travel speed, deposition, and bead appearance – can be lost. Welded to its full potential, metal-cored wire can improve weld cycle times, bead appearance, and overall productivity. It can reduce a welding operation's costs. Getting to that point will require consideration of a number of factors as well as some trial-and-error to find what works best. Hobart Brothers Co.

Amos Davis is the business development manager at Hobart Brothers Co.

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