Gouging is the term to define a type of corrosion which occurs on a metallic surface in which a hole, groove or indentation is created. In welding, gouging has been a requirement for many years in several industries and applications – and is one method to scrutinise. Particularly in maintenance and repair, the ability to gouge or groove metal is critical and deserves full consideration.
Two of the most common methods of gouging metal are plasma gouging and air carbon-arc gouging.
Air Carbon-arc Gouging
The process of air carbon-arc gouging is generated between the tip of a carbon electrode and the workpiece. The metal becomes molten and a high velocity air streams down the electrode to blow it away and thus leaves a clean groove. It is a simply process to apply, has a high metal removal rate and gouge profile can be closely controlled. However, there are disadvantages: 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.
This type of gouging 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, removal of excess weld metal and preparation of bevel edges for welding.
The main advantage of manual metal arc (MMA) gouging is that the same power source can be used for welding, gouging or cutting, simply by changing the type of electrode.
The arc is formed between the tip of the electrode and the workpiece in conventional MMA welding. MMA gouging differs due to requiring special purpose electrodes with thick flux coatings in order to generate a strong arc force and gas stream. Unlike MMA welding where a stable weld pool must be maintained, this process can force the molten metal away from the arc zone to leave a clean-cut surface.
The MMA gouging process is characterised by the large amount of gas which is generated to eject the molten metal. However, because the arc/gas stream is not as powerful as a gas or a separate air jet, the surface of the gouge is not as smooth as an oxyfuel gouge or air carbon arc gouge.
This type of gouging is used for localised gouging operations such as the removal of defects. It is also used where it is more convenient to switch from a welding electrode to a gouging electrode, rather than using specialised equipment.
If this process is correctly applied, MMA gouging can produce relatively clean gouged surfaces. For general applications, welding can be carried out without the need to dress by grinding. However, when gouging stainless steel, a thin layer of higher carbon content material will be produced – this should be removed by grinding.