Plasma Arc Cutting

Plasma arc cutting has always been seen as an alternative to the oxy-fuel process. However, the important difference between the two processes is that while the oxygen-fuel process oxidises the metal and the heat from the exothermic reaction melts the metal, the plasma process operates by using the heat from the arc to melt the metal. The ability to melt the metal without oxidation is essential when cutting metals, such as stainless steel, which form high temperature oxides. The plasma process was therefore first introduced for cutting stainless steel and aluminium alloys. The first plasma torches gave poor quality cuts and the process itself suffered from excessive noise and fume, especially when cutting thicker material. Over the last thirty years, the plasma cutting process has been highly refined and is now capable of producing high quality cuts, at increased speeds, in a wide range of material thicknesses.

Process variants

The basic plasma torch (Fig 1a) consists of a central tungsten electrode for forming the

arc but, unlike in the conventional TIG welding process, the arc is constricted by a fine

bore copper nozzle. This has the effect of increasing the temperature and velocity of

the plasma emanating from the nozzle. The temperature of the plasma is in excess of

20,000K and its velocity can approach the speed of sound [1]. The process variants

(Figs 1b to 1g) have been principally designed to improve the quality of the cut and the

cutting performance, or to reduce operating costs. The most important process

variants are described below.

Dual gas: The process operates basically in the same manner as the conventional system but a secondary gas shield is introduced around the nozzle (Fig 1b). The cutting gas is normally argon, argon/H2 or nitrogen and the secondary gas is selected according to the metal being cut. For example, when cutting mild steel, air or oxygen can be used to increase the cutting speed.

Water injection: Water can be injected radially into the plasma arc (Fig 1c) to induce a greater degree of constriction. The temperature of the plasma is considerably increased (30,000°C) which facilitates higher cutting speeds and, because of the greater constriction of the arc, much improved cut quality. The presence of an annular film of water around the plasma will protect the nozzle bore, reducing nozzle erosion.

Water shroud: The plasma arc can also be operated either with a water shroud (Fig 1d) or even with the workpiece submerged some 50 to 75 mm below the surface of the water. The water will act as a barrier in reducing fume and noise levels. In a specific example of noise levels at high current levels in excess of 115 dB, this can be reduced to about 96 dB with a water shroud and 52 to 85 dB when cutting underwater [2].

Air plasma: The inert or unreactive plasma forming gas (argon or nitrogen) can be replaced with air but this requires a special electrode of hafnium or zirconium mounted in a copper holder (Fig 1e). The use of compressed air instead of the more expensive cylinder gas, makes this variant of the plasma arc process highly competitive with the oxy-fuel process. A variant of the air plasma process is the monogas torch in which air is used for both the plasma and the cooling gas.

It is generally considered that air plasma is more widely applied in general engineering and light fabrication industries, eg, in cutting sheet steel within the thickness range 1 to 20 mm [3]. The more popular materials are C-Mn and stainless steels but the process has also been used for cutting SG (spheroidal graphite) iron and non-ferrous materials [4]. For thin section material of a few millimetres, the process is much faster than oxy-

fuel, but at thicknesses approaching 30 to 40 mm, air plasma becomes relatively slow [5].

The obvious cost advantages of using air in preference to expensive gases (for the plasma and oxy-fuel processes) must be considered when other operating costs have also been taken into account. For example, the air must be fed at a relatively high pressure (typically 150 1/min at 5 bar) and clean.

This will require an adequate size compressor for a line feed supply with suitable filters for dust particles and oil. Additionally, special purpose electrodes will be required and the operating life of the electrodes and nozzles can be severely shortened if there are frequent stop/starts [6].

Low current air plasma torches, typically less than 40A, are particularly attractive for cutting thin sheet material, in that compressed air is used for both the plasma forming gas and cooling the torch. Moreover, the torch head can be held in contact with the surface of the metal being cut (Fig 1f), without the risk of damaging the nozzle by forming a secondary or series arc between the electrode/nozzle and the workpiece. As nitrogen and oxygen suppress the formation of a series arc, compared to argon, contact cutting can be practised with the air plasma system [6]. The process is becoming more widely used for manual cutting of thin sheet components in both C-Mn and stainless steel, where contact cutting greatly deskills the cutting operation.

High tolerance plasma: In an attempt to improve cut quality and to compete with the superior cut quality of laser systems, a number of plasma systems are available commercially which operate with a highly constricted plasma arc; the systems under the generic name high-tolerance plasma arc cutting (HTPAC) are manufactured by Hypertherm, Koike Aronson, Abicor Binzel and Komatsu-Cybernation [7]. The common features of the torches (Fig 1g) are that the oxygen plasma jet is forced to swirl as it enters the plasma orifice and a secondary flow of gas is injected down stream of the plasma nozzle. The Komatsu-Cybernation torch has a separate magnetic field surrounding the arc which stabilises it by maintaining the rotation induced by the swirling gas

It is claimed that the cut quality lies between a conventional plasma arc cut and laser beam cutting, but the cutting speed is significantly lower than conventional plasma arc cutting and approximately 60 to 80% the speed of laser cutting [2]. Cutting

speeds can be several m/min for materials with thicknesses up to 6 mm.

However, for high quality cuts, ie, minimum kerf width, the optimum speed is more typically 1.5 m/min for 1 mm, reducing to about 500 mm/min for 6 mm (C-Mn) sheet material. An example of a high definition system, which is mounted on torch manipulation equipment with an accuracy of ±0.1 mm is shown in Fig 2.

Applications

The plasma process can be used for cutting a wider range of materials than the oxy-fuel process which is largely restricted to C-Mn steels. It is, therefore, not surprising that the plasma arc process is being used increasingly for cutting materials such as stainless steel (Fig 3), aluminium and coated steels. Even in cutting C-Mn steel, the plasma process

can have advantages over the oxy-fuel process such as a higher cutting speed and a narrow heat affected zone.

For example, there is a substantial speed advantage for cutting thin section materials of less than 30 mm but this disappears very quickly as the plate thickness increases.

The gas in the plasma process has a significant effect on performance and it is generally considered that the best quality is achieved with oxygen for mild steel, and nitrogen or argon/ hydrogen for aluminium and stainless steel [8]. However, from an evaluation of gases for cutting C-Mn steel, aluminium and stainless steel, the maximum cutting speeds were obtained with air and argon/35% H2 compared to nitrogen and argon/15% H2 [3]. In a comparison with nitrogen, air plasma was faster for C-Mn steel and aluminium, but slightly slower for stainless steel.

The overall conclusion was that in terms of cut quality and cutting speeds, air plasma would normally be recommended for cutting C-Mn steels in preference to gas plasma, while gas plasma with argon/15% H2 is preferred for cutting stainless steel and aluminium.

Current industrial practice for mixed gas cutting equipment is to use argon 25/35% H2 for cutting stainless steel and aluminium, which is a compromise between quality and cutting speed.

Heavy duty cutting systems can be used for cutting stainless steel up to 130 mm and aluminium up to 150 mm. Argon/H2 mixtures are recommended, but for materials below 75 mm in thickness, nitrogen, air or argon/H2 can be used [8].

The advantages and disadvantages of plasma cutting are summarised below :

Advantages

Can be used with a wide range of materials, including stainless steel and aluminium

High quality cut edges can be achieved, e.g. the HTPAC process

Narrow HAZ formed

Low gas consumable (air) costs

Ideal for thin sheet material

Low fume (underwater) process

Disadvantages

Limited to 50mm (air plasma) thick plate

High noise especially when cutting thick sections in air

High fume generation when cutting in air

Protection required from the arc glare

High consumable costs (electrodes and nozzles)

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