Plasma Arc Gasses(4) Gas selection. (a) Cutting gas selection depends on the material being cut and the cut surface quality requirements. Most nonferrous metals are cut by using nitrogen, nitrogen-hydrogen mixtures, or argon-hydrogen mixtures. Titanium and zirconium are cut with pure argon because of their susceptibility to embrittlement by reactive gases. Nitrogen is used with the water injection method of PAC. Some systems use nitrogen for the plasma forming gas with oxygen injected into the plasma downstream of the electrode. This arrangement prolongs the life of the electrode by not exposing it to oxygen. (c) For some nonferrous cutting with the dual flow system, nitrogen is used for the plasma gas with carbon dioxide (C02) for shielding. For better quality cuts, argon-hydrogen plasma gas and nitrogen shielding are used. (b) Carbon steels are cut by using compressed air (80 percent N2, 20 percent 02) or nitrogen for plasma gas. c. Principles of Operation. (1) The basic plasma arc cutting circuitry is shown in figure 10-72.
The process operates on direct current, straight polarity (dcsp), electrode negative, with a constricted transferred arc.
In the transferred arc mode, an arc is struck between the electrode in the torch and the workpiece. The arc is initiated by a pilot arc between the electrode and the constricting nozzle. The nozzle is connected to ground (positive) through a current limiting resistor and a pilot arc relay contact. The pilot arc is initiated by a high frequency generator connected to the electrode and nozzle. The welding power supply then maintains this low current arc inside the torch. Ionized orifice gas from the pilot arc is blown through the constricting nozzle orifice. This forms a low resistance path to ignite the main arc between the electrode and the workpiece. When the main arc ignites, the pilot arc relay may be opened automatically to avoid unnecessary heating of the constricting nozzle.
(2) Because the plasma constricting nozzle is exposed to the high plasma flare temperatures (estimated at 18,032 to 25,232ºF (10,000 to 14,000ºC)), the nozzle must be made of water-cooled copper. In addition, the torch should be de-signed to produce a boundary layer of gas between the plasma and the nozzle. (3) Several process variations are used to improve the PAC quality for particular applications. They are generally applicable to materials in the 1/8 to 1-1/2 in. (3 to 38 mm) thickness range. Auxiliary shielding, in the form of gas or water, is used to improve cutting quality.
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