Discovering the Benefits of Gouging with Plasma

Posted on: October 20, 2011

Originally featured on www.weldingdesign.com. ILMO Products Company is an award-winning Hypertherm distributor. Please contact us with any Hypertherm or plasma questions.

By Michelle Avila | Oct 15, 2011 11:01 PM

Plasma gouging is faster, easier, safer, and quieter than other metal removal methods.

They’ve been mining in Grant County, N.M., since before the Civil War, and huge copper mines like Chino and Tyrone still drive the local economy. It’s an economy that has been in high gear for several years thanks to rising copper prices, which have prompted mine operators to run around the clock: three shifts per day, seven days per week.

Non-stop activity is undoubtedly good for the economy, but it means there is no time to rest. Huge dump trucks and other heavy equipment are in constant demand, moving hundreds of thousands of tons of material a day. Maintenance crews are constantly busy, fixing one thing or another. Often their work involves gouging out old welds, a necessary first step before they can begin the actual work of repairing a truck bed or replacing a mold board liner on a track dozer. In the past the crew would reach for carbon-arc, but today these pros use plasma to make better repairs in less time.

The switch to plasma wasn’t exactly planned. It came after a supplier insisted on demonstrating the process. A senior supervisor explains his initial reaction, “I thought they were wasting my time because plasma can’t be used for gouging,” he recalled. “Well, we were wrong. You can gouge with plasma. We now use it in every application where we used to use carbon-arc.”

Many welders are realizing the benefits of gouging with plasma. It’s easy to understand why. Plasma gouging is faster, easier, safer, and quieter than other metal removal methods —benefits that save operations a considerable amount of money.

Another company sold on plasma gouging is Rockland Marine Corp., a business that fabricates, repairs, and services a range of different boats. Its first attempt at plasma gouging began more than 14 years ago with a Hypertherm MAX100 plasma cutter.

Rockland Marine purchased the system to cut metal because, as welder Francis Dennison explained, “Plasma can cut through rusted and painted metal and about 90% of our work is done outside in the elements.” To them, gouging was just an added benefit. Now it’s a necessity.

“Plasma is a lot faster than carbon-arc and grinding. The metal removal rate is faster. It takes less time, and the clean-up is easier because there is little to no mess,” Dennison continued.

His comments are echoed by Miller-St. Nazianz, a company that fabricates and manufactures spray-application agricultural equipment. Brian Schad, who heads Miller-St. Nazianz’ Special Projects Team, uses plasma to cut and gouge daily. “The plasma is cleaner, faster, and not nearly as loud,” he agreed. “We use fewer consumables too.”

The different gouging processes
There are four different gouging methods in common use: mechanical, oxyfuel, carbon-arc, and plasma. Mechanical grinding may involve hand-grinding, hand-milling, routing or chipping. While mechanical grinding eventually gets the job done, the process is time-consuming, loud, and dangerous: The crew working to build the 680-mile Ruby Pipeline in the Western U.S. have seen their share of injuries, especially eye injuries, a common occurrence in the pipeline industry, which reports at least one eye injury per day.

The next method, oxyfuel gouging, involves using fuel gas to heat steel to its ignition temperature. Once the metal is hot it is rapidly combusted, then blown away by a jet of oxygen gas. Oxyfuel is quieter and faster than mechanical gouging, but the process is hard to control, even if you have years of experience. Also, its use is limited to carbon steel.

In the carbon-arc process, an electric arc is generated between the tip of a carbon electrode and the metal workpiece. Then, a jet of air is directed around the tip to remove molten metal from the area, forming a groove. This process is more versatile than oxyfuel gouging as it works on a number of different metal types: mild steel, cast iron, nickel alloys, copper, and aluminum. It’s also easier to learn, and overall does a good job of removing metal.

The problem with carbon-arc gouging is that, like mechanical gouging, it is loud. In addition, it creates a lot of smoke and fumes, and like oxyfuel gouging carbon-arc is hard to control. It’s not uncommon to remove too much metal or to wind up with extra carbon deposits or inclusions on the base metal. Another problem: carbon-arc gouging is tedious and time consuming because the process requires operators to stop repeatedly to re-feed the rod. Rockland Marine’s Dennison explained, “You have to keep adjusting your rod. The rods are only about 12-in. long, so you adjust it once or twice and then it’s time to replace it.”

Another benefit, as with plasma cutting, is that the process works on most any type of metal, including mild steel, stainless steel, aluminum, copper, and more. Dennison said Rockland Marine can gouge more metals now. “If it can conduct electricity, we can gouge it. We couldn’t gouge stainless effectively until we got the plasma,” he said

The science behind plasma gouging
Plasma gouging works in a way similar to plasma cutting. When cutting with plasma, a plasma arc is formed between a negatively charged electrode inside the torch and a positively charged workpiece. Heat from the transferred arc melts the metal rapidly, and a high-velocity gas jet blows the molten material away.

The process is nearly identical for gouging, except that the operator doesn’t completely blow the molten material away. Instead only some of it is blown away. This is achieved by holding the torch at an angle and using special gouging consumables that produce a wider and softer arc. In addition, many plasma systems have a dedicated operation mode for gouging that enables the arc to stretch without going out. Some manufacturers even include gouging consumables in the start-up kit that comes with new machines.

The equipment needed for plasma gouging is nearly identical to what is needed for plasma cutting. For hand gouging, a plasma system consists of a power source, a gas supply (often just air), a torch, and lead. Special cut charts and instructions in the operator’s manual explain safety, parts, and operating parameters for gouging.

Many manufacturers supply accessories specifically designed for gouging, such as metal heat shields to surround and protect the operator’s hands and leather lead protectors. These generally do not come standard with a new system, but are worth the investment if you expect to do a lot of gouging.

A number of different plasma and shielding gases — including air, nitrogen, oxygen, an argon/hydrogen mix, and other combinations — can be used when gouging with plasma; however, it’s a good idea to check the operator manual first just to be sure the gas to be used is compatible with the particular plasma system. Some general rules when deciding on which gas to use:
• Air plasma provides the lowest cost of operation and acceptable quality on mild steel, stainless, and aluminum.
• An argon / hydrogen mix can produce a clean, bright, smooth gouge on aluminum and stainless.
• Nitrogen gas is good if you want to extend the life of your consumables.

Modern air plasma torches provide good cooling, high cutting capacity, long consumables life, and consistent cut / gouge quality — all attributes that improve gouging performance and allow for a more controlled gouging process. Another feature of modern air plasma torches is their design. In the past, all hand plasma torches were designed with a 75° or 90° angle. Today though, it is possible to purchase a straight torch, something the crew at Miller-St Nazianz has found especially useful for gouging.

“There is a big advantage to that torch. It blows sparks away so you can see where you are gouging. This is a plus because you don’t remove more metal than you need to,” said Schad, at Miller-St. Nazianz. “You can cut down to the vertical weld and see what you are doing, so you don’t take away too much material. The torch keeps the arc going constantly and doesn’t kick out.”

He added that the straight torch makes it easier to gouge in tight spaces or corners because the arc can hold a distance of up to two inches.

In addition to the straight torch, better plasma systems also come with a quick-disconnect feature that makes it easy to switch between torches. For example, you can have two torches—a standard 75° th 90° torch with cutting consumables, and a straight torch, ready with gouging consumables—and then quickly and easily switch between the two, depending on the job at hand. That feature is one many companies who gouge with plasma really appreciate. Schad said, “It takes about a matter of a minute to switch-over. It’s very convenient.”

In addition to different hand torches, it is also possible to move quickly to a mechanized torch as Canadian-based Hooper Engineered Vessels does. The company routinely uses a mechanized torch that they mount on a track burner to gouge plate metal. Plant superintendant Landon Klassen said he was able to figure out pretty quickly how to use the track cutter for gouging, and that his team hardly needed any training time. The company is now saving a significant amount of money on labor and saw blades, so much in fact that Klassen estimated they paid for their Powermax85 system the very first time they used it.

Gouging techniques
Operators use various techniques to achieve different gouge profiles and sizes, whether gouging by hand or using a mechanized method like a track cutter. The most common way involves positioning the torch at a 40° to 60° angle to the work surface while the pilot arc is formed and transferred to the plate. Once contact is made, the operator simply aims or feeds the arc into the area to be gouged, while moving the torch forward. Variations in the torch angle and speed, along with amperage levels, are used to control the depth of the gouge. A steeper angle and slower forward movement result in a deeper gouge, while a lower angle and faster forward movement result in a shallower gouge.

Today plasma arc gouging has wide industrial applications, from shipbuilding and maintenance, to heavy equipment manufacturing and repair, to manufacturing of truck bodies, tanks, and steel structures. Plasma gouging removes strong backs, lifting lugs, temporary brackets, tack welds, and rivets. It’s also suitable for demolition or salvage operations. In foundries it removes excess material from castings. It’s a good way to fix and maintain bridges, heavy equipment, pipelines, and more, and it’s useful at removing cracks and imperfections. And, it can help to prepare plate edges for welding by removing any extra metal that may be on the plate.

“With plasma, you just drop your hood and go,” Rockland Marine’s Dennison said. “If the quality of the steel is good, I can gouge all day on one electrode.”

Author Michelle Avila is the director of public relations at Hypertherm Inc., a designer and manufacturer of advanced plasma cutting systems for shipbuilding, manufacturing, and automotive repair.

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