Plasma Arc Cutting

cutting oxygen. When the flux strikes the refractory oxides that are formed when the cutting oxygen is turned on, it reacts with them to form a slag of lower melting temperature compounds. This slag is driven out, enabling oxidation of the metal to proceed. Chemical fluxing methods are used to cut stainless steel. The operator should have an approved respirator for protection from toxic fumes generated by the process. Thermal Cutting Revised by Ed Craig, AGA Gas, Inc.

Plasma Arc Cutting Plasma arc cutting employs an extremely high-temperature, high-velocity, constricted arc between an electrode contained within the torch and the piece to be cut. The arc is concentrated by a nozzle onto a small area of the workpiece. The metal is continuously melted by the intense heat of the arc and then removed by the jetlike gas stream issuing from the torch nozzle. Because plasma arc cutting does not depend on a chemical reaction between the gas and the work metal, because the process relies on heat generated from an arc between the torch electrode and the workpiece, and because it generates very high temperatures (28,000 °C, or 50,000 °F, compared to 3000 °C, or 5500 °F, for oxyfuel), the transferred arc cutting mode can be used on almost any material that conducts electricity, including those that are resistant to oxyfuel gas cutting. Using the nontransferred arc method, nonmetallic objects such as rubber, plastic, styrofoam, and wood can be cut with a good quality surface to within 0.50 to 0.75 mm (0.020 to 0.030 in.) tolerances. The past decade has seen a great increase in use of plasma arc cutting, because of its high cutting speed (Fig. 11). The process increases the productivity of cutting machines over oxyfuel gas cutting without increasing space or machinery requirements.

Fig. 11 Typical cutting speeds for plasma arc cutting of carbon steel or stainless using 6.8 m3/h (240 ft3/h) of air at 345 kPa (50 psi) from a single source. This information represents realistic expectations using recommended practices and well maintained systems. Other factors such as parts wear, air quality, line voltage fluctuations, and operator experience may also affect system performance.

Operating Principles and Parameters (Ref 1) The basic plasma arc cutting torch is similar in design to that of a plasma arc welding torch. For welding, a plasma gas jet of low velocity is used to melt base and filler metals together in the joint (see the article "Plasma Arc Welding" in Welding, Brazing, and Soldering, Volume 6 of the ASM Handbook). For the cutting of metals, increased gas flows create

a high-velocity plasma gas jet that is used to melt the metal and blow it away to form a kerf. The basic design and terminology for a plasma arc cutting torch are shown in Fig. 12.

Fig. 12 Components of a plasma arc cutting torch.

All plasma arc torches constrict the arc by passing it through an orifice as it travels away from the electrode and toward the workpiece. As the orifice gas passes through the arc, it is heated rapidly to high temperature, expands, and accelerates as it passes through the constricting orifice. The intensity and velocity of the arc plasma gas are determined by such variables as the type of orifice gas and its entrance pressure, constricting orifice shape and diameter, and the plasma energy density on the work. The basic plasma arc cutting circuitry is shown in Fig. 13. 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.

Fig. 13 Plasma arc cutting circuit. The process operates on direct current electrode negative (straight polarity). The arc is initiated by a pilot arc between the electrode and torch nozzle. Pilot arc is initiated by the highfrequency generator, which is connected to the electrode nozzle.

Plasma arc cutting was originally developed for severing nonferrous metals using inert gases. Modifications of the process and equipment to allow the use of oxygen or compressed air in the orifice gas permitted the cutting of carbon and alloy steel with improved cutting speeds and a cut quality similar to that obtained with oxyfuel cutting. Because the plasma constricting nozzle is exposed to the high plasma flame temperatures (estimated at 10,000 to 14,000 °C, or 18,000 to 25,000 °F), the nozzle is sometimes made of water-cooled copper. In addition, the torch should be designed to produce a boundary layer of gas between the plasma and the nozzle. Several process variations are used to improve the plasma arc cutting quality for particular applications. They are generally applicable to materials in the 3 to 38 mm ( to 1 in.) thickness range, depending on the current rating of the plasma machine. Auxiliary shielding in the form of gas or water is used to improve cutting quality. Dual-flow plasma cutting provides a secondary gas blanket around the arc plasma, as shown in Fig. 14. The usual orifice gas is nitrogen or compressed air. The shielding gas is selected for the material to be cut. It may be compressed air for mild steel, CO2 for stainless steels, and an argon-hydrogen mixture for aluminum.

Fig. 14 Dual-flow plasma arc cutting.

Water Shield Plasma Cutting. This technique is similar to dual-flow plasma cutting. Water is used in place of the

auxiliary shielding gas. Water Injection Plasma Cutting. This modification of the plasma arc cutting process uses a symmetrical impinging

water jet near the constricting nozzle orifice to further constrict the plasma flame. The arrangement is shown in Fig. 15. The waterjet also shields the plasma from turbulent mixing with the surrounding atmosphere.

Fig. 15 Water injection plasma arc cutting arrangement.

Selection of Gas. Any gas or gas mixture that does not degrade the properties of the tungsten electrode or the

workpiece can serve as a plasma gas. The gas mixture varies according to the plasma equipment design criteria. The most commonly used gas is compressed air; all common metals, such as carbon and alloy steels, stainless steels, and aluminum, can be cut with compressed air. As the metal thickness increases (over 25 mm, or 1 in., with steels and stainless steels), benefits are derived from the use of nitrogen plasma with CO2 shielding. Aluminum cut quality is improved using argonhydrogen plasma and nitrogen as the secondary gas blanket. When high-duty cycles are used, a change from compressed air to nitrogen/CO2 prolongs the consumable life. Machine Ratings. In selecting a plasma cutting unit, the thickness of plate to be cut and the required cutting speed

should be considered. Table 4 shows thickness capacity and midrange cutting speeds of four plasma units. These ratings are an average of speeds quoted by two manufacturers of plasma arc cutting equipment. Table 4 Cutting speed of plasma arc cutting machines for stainless steel Machine rating, A

Cutting speed, m/min (in./min)

Plate thickness, mm (in.)

1.5 (

)

3(

)

6(

)

9(

)

13 (

25 (1.0)

50 (2.0)

75 (3.0)

)

30

0.75-1.5 (30-60)

0.5-0.75 (20-30)

0.13-0.25 (5-10)

...

...

...

...

...

50

1.5-3.0 (60-120)

1.3-2.5 (50-100)

0.6-1.3 (25-50)

0.13-0.25 (5-10)

0.025-0.13 (1-5)

...

...

...

100

...

1.5-2.8 (60-110)

0.75-1.5 (30-60)

0.5-1.0 (20-40)

0.4-0.5 (15-20)

0.13-0.25 (5-10)

...

...

400

...

3-4 (120-150)

4-4.3 (150-170)

3-3.5 (120-140)

2.5-3 (100-120)

1.0-1.5 (40-60)

0.25-0.5 (10-20)

0.08-0.2 (3-8)

The thickness capacity of a cutting unit should first be examined to determine the cutting speed it can achieve for a given application. Next the speeds quoted for the next-larger unit should be studied to see whether the greater speed justifies its higher cost. For example, a 30 A unit cuts 6 mm (

in.) stainless steel plate at 125 to 250 mm/min (5 to 10 in./min); the

50 A unit cuts 6 mm ( in.) plate at 635 to 1270 mm/min (25 to 50 in./min), a significant increase. If the average required cutting thickness exceeds 75% of the maximum thickness capacity of a unit, the next larger size should be considered. Pierce capacity is usually half the cutting thickness capacity, an important consideration in selection of plasma equipment. To cut plate thicker than 75 mm (3 in.), connecting 400 A or 500 A units in parallel extends thickness capacity. Technique. With a machine-operated plasma arc torch, standoff distance from the work metal is about 5 to 20 mm (

to in.). In manual operation, the current and rate of gas flow are set, and the arc is struck by pressing a button on the torch, which is guided manually over the work. At the end of the cut, the arc is automatically extinguished, and the control opens the contactor and closes the gas valves. The operator can extinguish the arc at any time by moving the torch away from the work metal. Quality of Cut. Most plasma cutting torches impose a swirl on the orifice gas flow pattern by injecting gas through

tangential holes or slots (Fig. 14 and 15). As a result of the swirl of the plasma gas, walls of plasma arc cuts have a Vshaped included angle of 2 to 4° on one of the cut edges. When a straight edge is required on the cut part, the operator

must operate the torch carefully so that the bevel is on the scrap side of the cut. When the operator is facing the direction of torch travel, if the gas swirls clockwise, the bevel will be on the left side of the cut. In many cases, a small bevel is acceptable; it may even be used as a weld preparation. The relationship of torch travel direction to the part with clockwise swirl of the orifice as is illustrated in Fig. 16.

Fig. 16 Relationship of torch travel direction to the part with clockwise swirl of the orifice gas. With the clockwise swirling plasma gas, the bevel side of the cut is on the left when the operator is looking in the direction of the torch travel. To achieve straight cuts on the inner diameter and the outer diameter of the ring, torch directions must reverse to keep the right side of the cut on the part edge.

Quality of cut includes surface smoothness, kerf width, degree of parallelism of the cut faces, dross adhesion on the bottom of the cut, and sharpness of top and bottom faces. Table 5 provides data on the causes of imperfections in plasma arc cutting of low-carbon steel, stainless steel, and aluminum. Table 5 Causes of imperfections in plasma arc cuts Type of imperfection

Cause of imperfection

Low-carbon steel

Stainless steel

Aluminum

Top edge rounding

Excessive speed, excessive standoff

Excessive speed, excessive standoff

Seldom occurs

Top edge dross

Excessive standoff, dross easily removed

Excessive standoff, excessive hydrogen

Excessive standoff, dross easily removed

Top side roughness

Seldom occurs

Excessive hydrogen or standoff, insufficient speed

Insufficient hydrogen

Side bevel--positive

Excessive speed, excessive standoff

Excessive speed, excessive standoff

Excessive speed, insufficient hydrogen

Side bevel--negative

Seldom occurs

Seldom occurs

Excessive hydrogen

Top side undercut

Excessive hydrogen

Excessive hydrogen

Insufficient speed, insufficient hydrogen

Bottom side undercut

Seldom occurs

Slight effect at near-optimum conditions

Seldom occurs

Concave surface

Seldom occurs

Excessive hydrogen

Excessive hydrogen, insufficient speed

Convex surface

Excessive speed

Insufficient hydrogen, excessive speed

Seldom occurs

Bottom edge rounding

Excessive speed

Seldom occurs

Seldom occurs

Bottom dross

Excessive hydrogen or speed, insufficient standoff

Insufficient speed, excessive hydrogen

Excessive speed

Bottom side roughness

Insufficient standoff

Seldom occurs

Insufficient hydrogen

Width of kerf is 1

to 2 times the kerf of conventional oxyfuel gas cutting. The range is usually 5 to 10 mm (

in.), although some users achieve 0.8 mm (

in.). For thick work metal, width of kerf may exceed 9 mm (

to

in.).

Heat-Affected Zone. The high speeds possible with plasma arc cutting result in relatively low heat input to the

workpiece. Heat-affected zones are therefore narrow. The HAZ on stainless steel plate 25 mm (1 in.) thick cut at 1270 mm/min (50 in./min) is 0.08 to 0.13 mm (0.003 to 0.005 in.). Sensitization is usually avoided. Bevel cutting for weld preparation is an important application of plasma arc cutting. The intense heat of the process

makes it suitable for all types of beveling at a higher efficiency than oxyfuel gas cutting. Applications Plasma arc cutting can be used to cut any metal. Most applications are for carbon steel, aluminum, and stainless steel. It can be used for stack cutting, plate beveling, shape cutting, and piercing. In stack cutting, the plates should be clamped together as closely as possible. However, plasma arc cutting can usually tolerate wider gaps between carbon steel plates than can oxyfuel gas cutting. When high plasma arc cutting speeds are used, there is less distortion of the top plate. Several plates of 1.5 to 6 mm ( stack cut.

to

in.) thickness can be economically

For shape cutting, plasma arc cutting torches are used on shape cutting machines similar to those used for oxyfuel gas cutting (Fig. 6). Generally, plasma arc shape cutting machines can operate at higher travel speeds than is possible with oxyfuel gas cutting machines. Because of the fumes and heat produced by the cutting action, water tables are sometimes used with plasma arc shape cutting machines. The water just touches the bottom of the plate, where it traps the fumes, slag, and dross as they emerge from the bottom of the kerf. It also helps reduce noise. Plasma arc cutting of carbon steel plate can be done faster than with oxyfuel gas cutting processes in thicknesses below 75 mm (3 in.) if the appropriate equipment is used. For thicknesses under 25 mm (1 in.), plasma arc cutting speed can be up to five to eight times greater than that for oxyfuel gas cutting (Fig. 17). For thicknesses over 38 mm (1

in.), the

choice of plasma arc or oxyfuel gas cutting depends on other factors such as equipment costs, load factor, and applications for cutting thinner plates and nonferrous metals. Characteristics of plasma arc cutting and oxyfuel gas cutting are compared in Table 6. Table 6 Comparison of OFC and PAC processes Oxyfuel

Plasma arc

Flame temperature

3040 °C (5500 °F)

28,000 °C (50,000 °F)

Action

Oxidation, melting, expulsion

Melting, expulsion

Preheat

Yes

No

Kerf

Narrow

Wide

Cut

Both sides square

One side square

Speed

Moderate

High

Heat-affected zone

Moderate

Narrow

Carbon steel

Yes

Yes

Stainless steel

Requires special process

Yes

Aluminum

No

Yes

Copper

No

Yes

Special alloys

Some

Yes

Nonmetallics

No

Yes

Cutting ability:

Fig. 17 Comparison of oxyfuel gas cutting and plasma arc cutting of plain carbon steel.

Reference cited in this section

1. W.H. Kearns, Ed., Welding Handbook, Vol 2, Welding Processes--Arc and Gas Welding and Cutting, Brazing, and Soldering, 7th ed., American Welding Society, 1978, p 499-507 Thermal Cutting Revised by Ed Craig, AGA Gas, Inc.

Air Carbon Arc Cutting and Gouging Air carbon arc cutting and gouging severs or removes metal by melting it with the heat of an arc struck between a carbongraphite electrode and the base metal. A stream of compressed air blows the molten metal from the kerf or groove. Its most common uses are for weld joint preparation; removal of defective welds; removal of welds and attachments when dismantling tanks and steel structures; and removal of gates, risers, and defects from castings. The process cuts almost any metal, because it does not depend on oxidation to keep the cut going. A holder clamps the carbon-graphite electrode in position parallel to an air stream, which issues from orifices in the electrode holder to strike the molten metal immediately behind the arc. The electrode holder contains an air flow control valve, an air hose, and a cable. The cable connects to the welding machine; the air hose connects to a source of compressed air. Cutting action in the air carbon arc process is illustrated in Fig. 18.

Fig. 18 (a) Air carbon arc cutting action. (b) Manual air carbon arc cutting.

The low heat input of air carbon arc gouging makes this process ideal for joint preparation and for weld removal on highstrength steels. Base-metal temperatures rise very little, about 80 °C (150 °F) in most applications. Rough cutting is done manually. Accurate work calls for electrode holders mounted on motor-driven carriages. Pipe Fabrication. Fabricators of structural steel, pressure vessels, tanks, and pipe use hand torches, semiautomatic

torches, and fully automatic torches. A typical pipe fabrication plant uses two automatic air carbon arc torches. One, mounted on a large traveling manipulator, works with several sets of turning rolls and in tandem with submerged arc welding units. Longitudinal and circumferential seams are square butted, welded on the inside, backgouged to sound weld metal, then welded on the outside. The second torch, mounted on a pedestal, backgouges circumferential seams at another station. Power Supply. Constant-voltage direct current with a flat to slightly rising voltage characteristic is best for most air

carbon arc cutting applications. Direct current is preferred; copper alloys, however, cut better with alternating current. Table 7 provides data on power sources for air carbon arc cutting and gouging. Table 7 Power sources for AAC and gouging Equipment

Polarity

Use

Variable-voltage motorgenerator, resistor, and resistor grid

Direct current

All electrode sizes

Constant-voltage generator, rectifier

Direct current

motor

Electrodes >

in. in diameter

Transformer

Alternating current

Alternating current electrodes only

Rectifier

Alternating current, direct current

Direct current from three-phase transformer only; single-phase source not recommended. Use alternating current with alternating current electrodes only.

Air Supply. Compressed air from a shop line or a compressor at 550 to 700 kPa (80 to 100 psi) should be used; pressure

as low as 275 kPa (40 psi) is suitable for light work. Deep grooves in thick metal require pressures up to 860 kPa (125 psi). Air hoses should have a minimum inside diameter of 6 mm ( in.) with no constrictions. Air pressure is not critical in air carbon arc cutting; the process requires a sufficient volume of air to ensure a clean, slag-free surface. The amount of air required depends on the type of work (0.08 to 0.9 m3/min, or 3 to 33 ft3/min, for manual operations and 0.7 to 1.4 m3/min, or 25 to 50 ft3/min, for mechanized operations). Air carbon arc cutting electrodes are made from mixtures of carbon and graphite. The three basic types of air

carbon arc cutting electrodes are: •

Direct current copper-coated electrodes, which are used most frequently because of long life, stable arc characteristics, and groove uniformity. These electrodes are produced in diameters from 4 to 20 mm

•

( to in.) Direct current uncoated electrodes, which have limited use. These electrodes, although generally

•

restricted to diameters of less than 9 mm ( in.), are available with diameters from 3 to 25 mm ( to 1 in.) Alternating current copper-coated electrodes, which have additions of rare-earth metals to provide arc stabilization with alternating current. These electrodes are produced in 5, 6, 9, and 13 mm ( and

,

,

,

in.) diameters

Cross sections vary; round electrode rods are most common. Electrodes also come in flat, half-round, and special shapes to produce specially designed groove shapes. Technique. The angle of the electrode, speed of cut, and amount of current determine depth and contour of the cut or

groove. The electrode is held at an angle, and an arc is struck between the end of the electrode and the work metal. The electrode is then pushed forward. Data on groove depth, electrode size, current, and travel speed for air carbon arc gouging is available from various equipment manufacturers. For through-cutting, the electrode is placed at a steeper angle, almost vertically inclined. Plate thicknesses greater than 13 mm (

in.) may require multiple passes.

Grooves as deep as 25 mm (1 in.) can be made in a single pass. A steep angle, approaching that used for through-cutting, and rapid advance produce a deep, narrow groove; a flatter angle and slower advance produce a wide, shallow groove. Electrode diameter directly influences groove width. Operators should use a wash or weave action to remove excess metal such as risers and pad stubs, or in surfacing. Smoothness of the gouged or cut surface depends on the stability of electrode positioning, as well as on the steadiness of the electrode as it advances during the cutting operation. Mechanized gouging, with the electrode and holder traveling in a carriage on a track, produces smoother surfaces five times faster than does manual work. Absorption of Carbon. Reverse polarity air carbon arc cutting removes metal faster than does straight polarity.

However, the current carries carbon from the electrode to the base metal, increasing its carbon content. To minimize hardenability, the air stream must be adjusted to ensure removal of all molten metal. Thermal Cutting Revised by Ed Craig, AGA Gas, Inc.

Exo-Process

A relatively new electric arc cutting process, called the Exo-Process, has been developed. Similar to flux-cored processes, it uses a consumable tubular electrode and a specially designed gun that feeds high-speed compressed air to the arc (Fig. 19). The air flow functions to push molten metal from the gouge cavity, to constrict the arc for more precise control, and to cool the electrode. The system can be adapted to conventional gas metal arc welding equipment (it requires a direct current constant-voltage power source--150 A minimum--and a conventional wire feeder).

Fig. 19 The Exo-Process for gouging.

The velocity of the air flow at the arc is the key factor for straight cutting. A 1.5 mm (

in.) wire size can cut up to 6

mm ( in.) thick carbon steel. Speed and edge cut quality on most commercial metals and alloys is good, particularly for sheet metal thicknesses. Gouge quality on carbon steels is also good. The process would be well suited for automated equipment, in that high travel speeds may be attained. An obvious benefit of the process is that it can be mounted on a gas metal arc dual-wire feed system to provide the operator with a multifunctional welding and cutting unit. Thermal Cutting Revised by Ed Craig, AGA Gas, Inc.

Oxygen Arc Cutting Oxygen arc cutting uses a flux-covered tubular steel electrode. The covering insulates the electrode from arcing between it and the sides of the cut. The arc raises the work material to combustion temperature; the oxygen stream burns the material away. Oxidation, or combustion, liberates additional heat to support continuing combustion of sidewall material as the cut progresses. The electric arc supplies the preheat necessary to obtain and maintain ignition at the point where the oxygen jet strikes the surface of the work. The process finds greatest use in underwater cutting. When cutting oxidation-resistant metals, melting action occurs. The covering on the electrode acts as a flux; it functions in a manner similar to that of powdered flux or powdered metal injected into the gas flame in the flux-injection method of oxyfuel gas cutting of stainless steel.

Equipment. Oxygen arc cutting uses direct or alternating current, although direct current electrode negative (DCEN) is

preferred. The electrode and the electrode holder convey the electric current and oxygen to the arc. Electrode holders must be fully insulated; underwater cutting requires a flashback arrester, and the electrode must have a watertight plastic coating. Components of an oxygen arc electrode are shown in Fig. 20.

Fig. 20 Components of an oxygen arc electrode.

Thermal Cutting Revised by Ed Craig, AGA Gas, Inc.

Reference 1. W.H. Kearns, Ed., Welding Handbook, Vol 2, Welding Processes--Arc and Gas Welding and Cutting, Brazing, and Soldering, 7th ed., American Welding Society, 1978, p 499-507 Laser Cutting Gregg P. Simpson, Peerless Laser Processors Division, Peerless Saw Company; Thomas J. Culkin, Lumonics Materials Processing Corporation

Introduction INDUSTRIAL LASERS are being used in numerous material processing applications. They can weld microswitches and auto transmission gears, scribe and machine ceramic substrates, and drill jet engine turbine blades and baby bottle nipples. They are also used in heat treating, cladding, ablating, and marking. However, cutting represents their largest single application. The versatility of the laser in cutting operations is responsible for its widespread use. The same laser can be used to cut men's suits, newspapers, circuit boards, motorcycle fenders, circular saw blades, stainless steel auto exhaust tubing and 13 mm (0.5 in.) thick alloy steel for aircraft disc brakes.