Job Knowledge 11
The use of the plasma arc as a gouging tool dates back to the 1960s when the process was developed for welding. Compared with the alternative oxyfuel and MMA gouging techniques, plasma arc has a needle-like jet which can produce a very precise groove, suitable for application on almost all ferrous and non- ferrous materials.
Plasma arc gouging is a variant of the plasma arc process. The arc is formed between a refractory (usually tungsten) electrode and the workpiece. Intense plasma is achieved by constricting the arc using a fine bore copper nozzle. By locating the electrode behind the nozzle, the plasma-forming gas can be separated from the general gas supply used to cool the torch/assist the plasma gas to blow away molten metal (dross) from the groove.
The temperature and force of the constricted plasma arc is determined by the current level and plasma gas flow rate. Thus, the plasma can be varied to produce a hot gas stream or a high power, deeply penetrating jet. This ability to control quite precisely the size and shape of a groove is very useful for removing unwanted defects from a workpiece surface.
Whilst gouging, normal precautions should be taken to protect the operator and other workers in the immediate area from the effects of intense arc light and hot metal spray. Unlike the oxyfuel and MMA processes, the plasma arc's high velocity jet will propel fume and hot metal dross some considerable distance from the operator. When using a deeply penetrating arc, noise protection is an essential requirement.
The power source for sustaining this gouging arc must have a high open circuit voltage, usually well in excess of 100V. The torch is connected to the negative polarity of the power source and the workpiece must be connected to the positive. The plasma torch is the same as the one used for cutting; it will be either gas or water cooled and have the facility for single and dual gas operation.
Electrodes are normally tungsten for argon and argon-based gases. However, when using air as the plasma gas, special purpose, for example hafnium tipped copper, electrodes must be used to withstand the more aggressive, oxidising arc.
Plasma and cooling gases
Plasma gas can be argon, helium, argon - H2 , nitrogen or air. Argon - 35% H2 is normally recommended as a general- purpose plasma gas for cutting most materials. Alternative plasma gases are argon and helium. Argon, a colder gas, will reduce metal removal rates. Helium, which generates a hot but less intense arc than argon - H2 , can produce a wider and shallower groove. Nitrogen and air are also used as plasma gases, especially for gouging C-Mn steels. Although gas costs will be substantially reduced, the groove surface profile will be inferior to that which can be achieved with argon - H2 gas. Air is not recommended for gouging aluminium as this requires an inert or reducing gas. Argon, nitrogen or air are all used as cooling gases. Use of argon will normally produce the best quality of gouge, but nitrogen or air will reduce operating costs.
Gouging is effected by moving the torch forward at a steady controlled rate. It is carried out in a progressive manner to remove metal over a distance of 200 to 250mm. The jet can then be repositioned, either to deepen or widen the groove, or to continue gouging for a further 200 to 250mm. Principal process parameters are current level, gas flow rate, and speed of gouging. These settings determine groove size and metal removal rate. In a typical gouging operation on C-Mn steel, metal is removed at about 100 kg/hr at a speed of 0.5 m/min, and groove size will be around 12mm wide and 5mm deep.
The torch stand-off and its angle to the surface of the workpiece have a major influence on speed of travel, groove profile and quality of surface. The torch is normally held at a distance of 20mm from the workpiece and inclined backwards to the direction of gouging at an angle of 40 to 45 degrees. Gouging will remove up to approximately 6mm depth of metal in a single pass.
The torch stand-off should not be reduced to less than 12mm, to avoid spatter build-up on the nozzle from the molten particles ejected from the groove. At standoff distances greater than 25mm, arc/gas forces are reduced and this lessens the depth of penetration of the jet. By reducing the torch angle to the workpiece surface, the plasma jet can be encouraged to 'skate' along the surface of the workpiece; this produces a shallower and wider groove. By increasing the angle of the torch the plasma jet is directed into the workpiece surface, resulting in a deeper and narrower groove.
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This Job Knowledge article was originally published in Connect, September 1995. It has been updated so the web page no longer reflects exactly the printed version.