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An Explanation of the Arc Air Gouging and Carbon Arc Techniques

 

Air Carbon Arc Gouging

The main difference between this gouging technique and the others is that a separate air jet is used to eject molten metal from the groove.

 
Process description
Air carbon arc gouging works as an electric arc is generated between the tip of a carbon electrode and the work piece. The metal becomes molten and high velocity air jet streams down the electrode to blow it away, thus leaving a clean groove. The process is simple to apply (using the same equipment as MMA welding like a DC welding power source), has a high metal removal rate, and gouge profile can be closely controlled. Disadvantages are that the air jet causes the molten metal to be ejected over quite a large distance and, because of high currents (up to 2000A) and high air pressures (80 to 100 psi), it can be very noisy.
Application
As air carbon arc gouging does not rely on oxidation it can be applied to a wide range of metals. DC (electrode positive) is normally preferred for steel and stainless steel but AC is more effective for cast iron, copper and nickel alloys. Typical applications include back gouging, removal of surface and internal defects, and removal of excess weld metal and preparation of bevel edges for welding.

Electrode
The electrode is a graphite (carbon) rod which has a copper coating to reduce electrode erosion. Electrode diameter is selected according to required depth and width of gouge. Cutting can be precisely controlled and molten metal/dross is kept to a minimum.
Power source
A DC power supply with electrode positive polarity is most suitable. AC power sources which are also constant current can be used but with special AC type electrodes. The power source must have a constant current output characteristic. If it does not, inadvertent touching of the electrode to the work piece will cause a high current surge sufficient to 'explode' the electrode tip. This will disrupt the operation and cause carbon pick-up.  As arc voltage can be quite high (up to 50V), open circuit voltage of the power source should be over 60V
Air supply
The gouging torch is normally operated with either a compressed air line or separate bottled gas supply. Air supply pressure will be up to 100psi from the airline but restricted to about 35psi from a bottled supply. Providing there is sufficient air flow to remove molten metal, there are no advantages in using higher pressure and flow rates.
Carbon pickup
Although carbon is picked up by the molten metal, the air stream will remove carbon-rich metal from the groove to leave only minimal contamination of the sidewalls. Poor gouging technique or insufficient air flow will result in carbon pick-up with the risk of metallurgical problems, e.g. high hardness and even cracking.
Operation
Gouging is commenced by striking the electrode tip on to the work piece surface to initiate the arc. Unlike manual metal arc (MMA) welding the electrode tip is not withdrawn to establish arc length. Molten metal directly under the electrode tip (arc) is immediately blown away by the air stream. For effective metal removal, it is important that the air stream is directed at the arc from behind the electrode and sweeps under the tip of the electrode. The width of groove is determined by the diameter of electrode, but depth is dictated by the angle of electrode to the work piece and rate of travel. Relatively high travel speeds are possible when a low electrode angle is used. This produces a shallow groove: a steep angle results in a deep groove and requires slower travel speed. Note, a steeply angled electrode may give rise to carbon contamination.
Oscillating the electrode in a circular or restricted weave motion during gouging can greatly increase gouging width. This is useful for removal of a weld or plate imperfection that is wider than the electrode itself. It is important, however, that weave width should not exceed four times the diameter of the electrode. The groove surface should be relatively free of oxidised metal and can be considered ready for welding without further preparation. Dressing by grinding the side-walls of the gouge should be carried out if a carbon rich layer has been formed. Also, dressing by grinding or another approved method will be necessary if working on crack-sensitive material such as high strength, low alloy steel.
 
 
Choosing a gouging method
Advantages and disadvantages of plasma, air carbon-arc gouging
Plasma gouging and air carbon-arc gouging have their advantages and disadvantages. The most distinct differences are in cost, fume production, and necessary post-cutting operations.

In recent years environmental and quality issues have taken center stage in the welding industry. This increased emphasis on safety and health often is accompanied by reconsideration of how certain tasks are performed, for example, cutting and gouging metal.
Gouging—a requirement for many years in several industries and applications—is one method to scrutinize. Especially in maintenance and repair, the ability to gouge or groove metal is critical and deserves proper consideration.
Two of the most common methods of gouging metal are plasma gouging and air carbon-arc gouging.
A Brief History
In maintenance and repair, operators must remove any of the following: welds or metal to replace a worn or defective part; worn hard-faced deposits so that hard-facing can be reapplied; defects in a weld so the part can be re-welded. Back-gouging welds to sound metal also may be necessary when both sides of a plate are to be welded. Gouging also is used in the foundry business to remove fins, risers, and defects from castings.
Common methods of gouging are mechanical techniques such as grinding, hand milling, routing, and chipping; oxy-fuel gouging, which can be used only on carbon steels; and air carbon-arc gouging.
The plasma cutting process was invented in 1954 at the Tonawanda Laboratory of the Linde Division of Union Carbide. A young scientist, Robert Gage, discovered that by forcing a gas tungsten arc through a small orifice in a process similar to focusing a beam of light through a lens, the temperature and intensity of the arc could be increased. By passing a fairly high gas flow through this focused arc, it could cut metal.
Plasma gouging is a variation of plasma cutting, in which the arc is "defocused" slightly by increasing the whole size in the constricting orifice. A cutting arc is directed downward through the metal to blow the molten metal down and out through the kerf, forcing the two pieces of metal to separate. In plasma gouging, the torch is inclined at an angle to the work piece, and the arc plows out a groove on the metal surface and blows the molten metal off to the side. A more intense cutting arc causes a groove too deep and narrow for most applications, so the defocused arc is used for gouging.
 
Fume, Noise Production
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 air 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 being gouged, exposure to particular toxins that are constituents of the base metal also can cause problems.
Plasma also uses an electric arc to melt the metal being gouged, but the plasma gas itself pushes the molten metal out of the groove. Because this is done less violently than in air carbon-arc gouging, less molten metal vaporizes, reducing the metallic vapor and reaction with the surrounding atmosphere. When air is used as the plasma gas, some reaction occurs, but the volume of air is lower than that found in 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.
However, aluminum applications are an exception to this. This metal's lightness and strong affinity for oxygen do create fumes. Also, the strong ultraviolet content of the radiation from the plasma arc increases the carbon monoxide, ozone, and nitrogen oxides generated. These amounts generally are below threshold limits.
Noise also can be a concern for cutting operators. Plasma gouging can reduce noise production. Typically, when measured at conditions that create a similar gouge size, plasma gouging is 5 to 10 decibels quieter than carbon-arc gouging.
Depending on the current level, the noise level of plasma gouging still may be high enough to require hearing protection for the operator, but it can eliminate the need for such protection for nearby workers. As always, actual measurements should be used as a guide.
Post-cutting Cleanup
In air carbon-arc gouging, the carbon rod is consumed; this releases carbon. Usually a layer of molten metal remains and re-solidifies in the gouge because it's not blown out completely by the air blast. When carbon dissolves, in this solidified layer, a brittle carbon-rich layer forms posing possible welding and cracking problems.  In stainless steel, this layer also can become a starting point for corrosion. Also, when air reacts with the layer of molten metal on the surface, an oxidized layer can form. This doesn't cause much difficulty in carbon steel, but it must be ground off of stainless steel and other corrosion-resistant alloys before welding. In aluminum, an oxide layer can form that requires cleanup.
Plasma gouging uses no carbon rod. The gas used in plasma gouging determines the condition of the final groove. With carbon steel, any oxidation left by using air as the plasma gas usually is of little consequence.
Gouging with air on carbon steel can leave a thin, dissolved, nitrogen-rich layer on the gouged surface. In certain welding situations, this can cause micro-porosity. Normally, this is a problem only in very high-quality welding situations and can be removed with light grinding on the surface or by using inert gas as the plasma gouging gas.
But with stainless steel and other corrosion-resistant alloys and aluminum, an inert gas should be used as the plasma gas. This inert gas shields the groove from the contaminating atmosphere and generally is free of oxidation and other contamination. In most cases, the groove can be re-welded with almost any welding process without additional cleanup.
Cost
The initial cost of air carbon-arc gouging is lower than that of plasma gouging. With air carbon-arc gouging, existing welding power supplies and air supply can be used; only a gouging torch must be added. Air carbon-arc gouging with compressed air also costs less than plasma gouging with an inert gas.
Maintenance costs for air carbon-arc gouging also can be less than in plasma gouging, in which the electrode and nozzle must be replaced periodically. An air carbon-arc gouging electrode also costs less than a plasma gouging electrode.
However, plasma gouging typically is four times faster than air carbon-arc gouging. Its rod is non-consumable, and the gouging electrode and nozzle can last longer than their plasma cutting counterparts. In addition, the heat source used for plasma gouging generally is more efficient than that for air carbon-arc gouging.
Depending on the application, plasma gouging can pay a company back indirectly, as secondary cleanup, particularly on stainless steel and aluminum, is typically less than in air carbon-arc gouging. This can result in labor and material savings. In addition, depending on a variety of factors—including duty cycle, location, local environmental laws, and the size of the facility—fume extraction and ventilation needs may be less with plasma gouging.
 
Arc Air Gouging
Arc Air Gouging Welders

The wide-ranging gouging needs of fabrication plants, shipyards, railroads and farms were top considerations as Arc-Air, ESAB and Gullco designed gouging torches, including angle-arc manual gouging torches, straight handle manual gouging torches and the tri-arc foundry gouging torch.

ArcAir gouging torches have been taken to the extreme level with many new design features and improvements. Extreme torches feature a new four-barrel air nozzle design that optimizes air flow to the arc to efficiently clean slag from the groove edge. Optimized air flow allows operation in areas where noise cannot exceed 115 decibels, yet achieve acceptable grooves at lower air pressure and flow. Improved electrical conduction decreases heat build-up in the cable and torch while superior outer cable cover materials improves cable life in harsh environments and resist breakdown from heat exposure.

The Arc-Air extreme family of gouging torches features four angle-arc torches, two straight handle torches and a single tri-arc torch with three head options. The four angle-arc torch models are designated for either light, medium, heavy duty or extra heavy-duty performance. Straight handle torches are specified for either medium or extra heavy-duty tasks. The tri-arc torch can be equipped with any one of three interchangeable torch heads, including those recommended for defect removal, pad-washing, cutting stainless steel and general purpose usages.

Extreme angle-arc gouging torches and extreme straight handle gouging torches speed up weld removal, back-gouging, edge preparation, defect repair and many other metal removal jobs. They are ideal for almost all metals with little or no deformation because the heat input is so low. Welders, if time and money savings are issues with a gouging task, angle-arc and straight handle torches can provide the solution.

Extreme tri-arc foundry gouging torch offers one body that accepts three types of heads so that a combination can be tailored to best fit a specific application, such as cutting stainless steel. Equipped with the proper head, the tri-arc torch speeds up pad-washing, removes defects in castings and generally cleans casting of fins, nails and other unnecessary metal. Tri-arc is ideal for users who prefer to eliminate the need for several different type torches, not wanting the hassles and confusion associated with stocking and supplying several torch types and related parts. This is the original triple purpose torch known worldwide as an ArcAir staple.

Gullco have developed a New Gullco ArcairMatic N6000 Gouging System follow this link for more details and photographs

 

 

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