ASM Metals HandBook Vol. 14 - Forming and Forging

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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 CO

shielding. Aluminum cut quality is improved using argon-

hydrogen plasma and nitrogen as the secondary gas blanket. When high-duty cycles are used, a change from compressed

air to nitrogen/CO

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

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

Plate thickness, mm (in.)

Machine

rating,

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 V-

shaped 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 wi

th 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

Cause of imperfection

Type 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 ( to

in.), although some users achieve 0.8 mm ( in.). For thick work metal, width of kerf may exceed 9 mm ( 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 ( to in.) thickness can be economically

stack cut.

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

Kerf Narrow

Wide

Cut Both sides square

One side square

Speed Moderate

High

Heat-affected zone

Moderate

Narrow

Cutting ability:

Carbon steel

Yes

Stainless steel

Requires special process

Yes

Aluminum

Yes

Copper

Yes

Special alloys

Some

Yes

Nonmetallics

No Yes

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

Reference cited in this section

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 carbon-

graphite 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 high-

strength 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 motor-

generator, resistor, and resistor

grid

Direct current

All electrode sizes

Constant-voltage motor

generator, rectifier

Direct current

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 m

/min, or 3 to 33 ft

/min, for manual operations and 0.7 to 1.4

/min, or 25 to 50 ft

/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 f

rom 4 to 20 mm

( to in.)

• Dire

ct 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

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.

Its flexibility makes the laser an ideal tool for prototype or production work. Because laser cutting is a noncontact

process, no tool wear occurs. Laser systems can cut intricate parts with accuracies of ±0.025 mm (±0.001 in.) and with

surface finishes better than 1.3 μm (50 μin.) for some steels, and better than 0.50 μm (20 μin.) for some nonmetals.

Laser Cutting

Gregg P. Simpson, Peerless Laser Processors Division, Peerless Saw Company; Thomas J. Culkin, Lumonics Materials Processing

Corporation

Definition of a Laser

First, the word laser is an acronym for light amplification by stimulated emission of radiation. Second, there are

essentially three components that are necessary for lasing action. There must be an active media that can be excited, a

method of exciting the media, such as an electrical discharge between an anode and cathode, and a resonator.

The molecules of the active media must be excited in order to stimulate the emission of radiant energy or light. As the

molecules are charged to a higher energy state, they excite their electrons into a higher energy level. Because unstable

electrons seek their lowest energy state, they release this added energy as light particles, known as photons.

The resonator consists of two parallel mirrors that reflect light particles between them, thereby amplifying the stimulated

emission of light. Of the two mirrors in the resonator, the rear mirror is 100% reflective, while the front, or output, mirror

is typically only 50% reflective and therefore 50% transmissive. The percentage of light that is transmitted through the

front mirror is commonly known as the laser beam. It is this parallel, monochromatic, intense beam of light particles that

is used for material processing. The percentage of light remaining inside the resonator is necessary to maintain the

continuous stimulated emission of photons.

Laser Cutting

Gregg P. Simpson, Peerless Laser Processors Division, Peerless Saw Company; Thomas J. Culkin, Lumonics Materials Processing

Corporation

Laser Types

The carbon dioxide (CO

) laser and the neodymium-yttrium aluminum garnet (Nd-YAG) laser are by far the most

commonly used material-processing lasers. The CO

laser relies on a gas mixture of CO

, helium (He), and nitrogen (N)

as its active media. It usually uses an electrical discharge between an anode and cathode as the method of media

excitation, and the standard two-mirror resonator. The CO

laser produces a wavelength of 10.6 μin (420 μin.), which is

invisible to the human eye. Visible light falls between 0.4 and 0.7 μm (15 and 28 μin.). Although practical material-

processing lasers have powers ranging from 150 W to 8 kW, CO

lasers have been built with powers above 25 kW. Most

lasers used for cutting have powers that range from 150 W to 3 kW. Figure 1 shows a typical CO

gas laser design.