CHOOSING A GOUGING METHOD

Increasing emphasis on environmental issues and safety has led the welding industry to reconsider how certain tasks are performed. Gouging and cutting metal are primary operations that have come under scrutiny. Here is a comparison of carbon-arc vs. plasma gouging for various applications.

Welders must often remove welds or metal to replace worn or defective parts, repair defects in a weld, or remove worn hardfaced deposits so the hardfacing can be reapplied. Backgouging welds to sound metal can also be necessary when both sides of a plate are to be welded.

Plasma gouging is a variation of plasma cutting in which the arc is slightly “defocused” by increasing the hole size in the constricting orifice. The torch is inclined at an angle to the workpiece, so that the arc plows out a groove on the metal surface and blows the molten metal off to the side.

In carbon-arc gouging, an electric arc at the end of a consumable carbon rod melts the metal, and a continuous blast of compressed air violently blows the molten metal away. The constituents of the molten metal react strongly with air, and the force of the blast tends to vaporize much of the molten metal into fine droplets, creating a high level of fume consisting of metal vapor, carbon dust and metallic byproducts. Typically the fume level of an air carbon-arc gouging operation is higher than the allowed exposure level to welding fumes in a workplace. Depending on the material to be gouged, exposure to particular toxins may also be a problem.

Plasma gouging also uses an electric arc to melt the metal, but the plasma gas itself pushes the molten metal out of the groove. This operation is less violent than air carbon-arc gouging, so less molten metal vaporizes, reducing the level of metallic vapor and its reaction with the surrounding atmosphere. If air is used for the plasma gas, some reaction occurs but the volume of air is lower than what is used for air carbon-arc gouging. If inert gas is used, the molten metal in the gouge is protected from the surrounding atmosphere and has little chance to react with the air.

Plasma gouging is typically 5 decibels to 10 decibels quieter than air carbon-arc gouging. This still normally requires hearing protection for the operator but may eliminate the need for hearing protection for nearby workers. Actual measurements should always be taken to assure that proper safety procedures are implemented.

In addition to the safety issues, when evaluating a gouging method, clean up and cost should be considered. Typically, not all the molten metal is blown out and some may solidify in the gouge, creating a brittle, carbon-rich layer that can pose welding and cracking problems. In stainless steel, this also provides a starting point for corrosion. When air reacts with the molten metal on the surface, an oxidized layer can also form. Plasma gouging uses no carbon rod. The condition of the final groove is determined by the plasma gas used. With carbon steel, any oxidation from using air as the plasma gas is generally inconsequential.

As for cost, the initial cost of air carbon-arc gouging is less than plasma gouging and the process can use existing welding power supplies and air supply. The electrode and nozzle must be replaced periodically, and air carbon-arc gouging electrodes cost less than a plasma gouging electrode. Plasma gouging, however, is typically four times faster, its rod is non-consumable and the heat source used for plasma gouging is usually more efficient than its air carbon-arc gouging counterpart. Plasma gouging can also pay back a company indirectly, as secondary clean up, particularly on stainless steel and aluminum, is reduced, producing labor and material savings.

When evaluating the right gouging choice for your situation, be sure to consider all the factors, including initial costs, materials to be used, duty cycle, local environmental laws, the size of the facility and the cost for extraction and ventilation needs, in determining the best choice for your needs.