Air carbon arc gouging - a process review

TWI Bulletin, November 1987

Colin Eileens, AWeldI, Senior Welding Inspector, was a Senior Welding Engineer in the Advisory Section of the Production Systems Department and now provides a mobile welding service.

David Abson, BA, MMet, PhD, MIM, CEng, FWeldI, is a Principal Research Metallurgist in the Materials Department.

One of the fastest and most effective metal removal processes developed, air carbon arc gouging, is examined here and the range of equipment and consumables available outlined. Operating techniques are described and the authors discuss the metallurgical consequences of using the technique on ferritic steels.

Air carbon arc gouging was first developed in the 1940s. ^[1] It has since become widely accepted as an efficient metal working tool, particularly in foundries and in welding and allied industries. The process has earned its popularity by its ability to remove metal faster than manual grinding, chipping and flame gouging, and it readily lends itself to many applications, for example cleaning castings, cutting edge preparations, back gouging root runs and defect removal. Metal removal rates of up to 70 kg/hr can be achieved, using 19mm diameter electrodes. ^[2]

Air carbon arc gouging uses an electric arc maintained between a consumable carbon-graphite electrode and the material surface to be cut. The arc produces a high temperature which melts a portion of the surface of the underlying material, while a high velocity air jet is directed at the arc, blowing away the molten metal.

Equipment

Air carbon arc equipment is commercially available from several manufacturers. The basic requirements for conventional air carbon arc gouging are:

- a cutting torch or electrode holder;

- cutting electrodes;

- a power source, and

- an air supply.

Figure 1 shows a typical set-up for using the air carbon arc cutting or gouging process.

Cutting torch or electrode holder

Electrode holders are available in three forms, which may be categorised by their mode of application: manual; semi-automatic and automatic.

Electrode holders for manual applications are similar in design to the conventional heavy duty holders used for MMA welding, but with the addition of air passages and, usually, a swivel head which grips the electrode. The swivelhead allows the electrode angle to be adjusted to suit the application. Integral air holes are normally incorporated in the swivel head to ensure that, whatever electrode angle is held, the air flow remains parallel to the electrode,providing an efficient jet to remove the molten metal out of the arc region and clear of the area, thereby preventing solidification in the gouged region.

Early torches were generally bulky and uncomfortable to operate, but use of modern high strength and heat resistant materials has brought about significant progress, particularly in hand-held torches, with the development of morecomfortable and easier maintenance features compared with those first available. An example of a modern electrode holder design is shown in Fig.2. ^[3]

Semi-automatic electrode holders for gouging are designed for mounting on a tractor unit or carriage. The operator manually feeds individual electrodes, as required, to produce the gouge, while the electrode holder is moved along automatically. For fully automatic holders, the carbon electrodes (which are joined for continuous use) are controlled and fed mechanically, either by a feed motor activated with a spring loaded device or by a voltage controlled automatic feed. ^[2] In marked contrast to the limited control of groove shape and depth provided by manual gouging, semi-automatic and fully automatic systems are capable of producing accurate and consistent grooves, with depth tolerances of the order of ± 0.6mm. ^[2] This is a major advantage for exacting applications such as gouging to remove backing strips, back gouging of root runs and cutting of edge preparations. When used in this last application, the automatic air carbon arc process produces substantially less distortion than that which occurs with an oxy-fuel process. ^[4] The automatic holders are also mounted on a tractor unit or carriage and, like the semi-automatic variant, may be fixed while the workpiece is moved past the stationary electrode holder. A typical mounted electrode holder is shown in Fig.3. ^[5]

A promising development is the use of a continuous wire, [6,7] which is fed by a conventional wire feeder. A DC power supply is used, for which is stipulated a characteristic of 0.15-0.20 V/A. Air may be fed through a nozzle which is independent of the gun, when the preferred angles of inclination are ≤ 30° for the air supply and ~60° for the wire; [6] alternatively, the air may be fed through nozzles on a modified gun. [7] Metal removal rates of up to 38.5 kg/hr are claimed, [6] operating at currents of 500-520A with a 3mm diameter solid wire and an arc voltage of 70-80V. The system described by Cullison [7] uses a tubular wire, either 1.14 or 1.6mm diameter, with current ranges of 50-300A and 80-600A, respectively, and gives metal removal rates of up to 27 kg/hr. Clearly, these systems lend themselves to fully automated operation and, where gouging or cutting must be carried out routinely, even to robot controlled operation.

Electrodes

For conventional air arc gouging, there are essentially three types of carbon electrode, varying in coating and current supply requirements.

Coated electrodes for DC positive operation are the most widely used because of their comparatively long life, stable arc characteristics and inherent ability to maintain groove uniformity. The electrodes are manufactured from a mixture of graphite and amorphous carbon with a suitable binder. Baking this mixture produces dense, homogeneous graphite electrodes of low electrical resistance, which are usually coated with a controlled thickness of high purity copper or aluminium. The coating improves electrical conductivity and reduces erosion of the electrode in the jet of air. [2,8] This helps to maintain the electrode diameter and therefore gives a more uniform groove width. [8] In addition to the conventional round section electrodes, there are available rectangular coated electrodes which are used for gouging flat bottomed grooves or for removing surface layers such as cladding. Half-round coated electrodes are also available, and these serve the dual purpose of being usable either as flat or round electrodes.

Plain electrodes for DC usage are manufactured like the coated electrodes, but without the copper or aluminium coating. When used for gouging, this particular type of electrode erodes more than the coated type, and in practice they are therefore predominantly employed for soldering and heating. [8] Diameters are generally small, usually under 10mm.

For AC operation, carbon electrodes coated with copper or aluminium are also made from a mixture of amorphous carbon and graphite with a suitable binder, and rare earth elements are incorporated to provide arc stability. [8] The electrodes are marketed in diameters from 4.8 - 12.7mm.

Typical operating currents for coated electrodes are listed in Table 1; [2] DC electrodes are usually used near the upper end of the current range recommended for each diameter, while AC electrodes re-generally used at currents which are about in the middle of the range recommended for DC operation. [9] Table 2 [2] shows typical metal removal capabilities for the various diameters available.

Table 1 Recommended arc current ranges for common sizes of DC positive and AC copper-coated electrodes (after Ref. [2] )

Table 2 Air carbon arc metal removal capabilities on steel, per millimetre length of electrode consumed (after Ref. [2] )

Power sources

Generally, any standard welding power source of sufficient capacity may be used for the conventional air carbon arc gouging process, with one proviso, namely, it must provide a minimum open circuit voltage of 60V to ensure good results, particularly with the larger diameter electrodes. [10] For manual operation, DC constant current or constant potential power sources, motor generators, rectifiers and resistor grids may all be used. Where large diameter electrodes are used, for example in foundries, rectifiers are generally preferred because, as long as they have the same open circuit voltage, they can be connected in parallel to increase power output.

For automatic gouging, a DC constant current drooping type power source is a necessity, because the automatic voltage control unit monitors arc voltage and adjusts arc length to maintain a preset voltage. [11] Power sources suitable for conventional air carbon arc gouging, and restrictions on their use, are listed in Table 3. [12]

Table 3 Power sources suitable for air carbon arc gouging (after Ref. [12] )

Air supply

During air carbon arc gouging, it is essential that the air flow passes between the electrode and the workpiece to blow the molten metal away. Ordinary compressed air is normally used. Goldberg [13] and Toropov [14] recommended a minimum pressure of 5bar (5x10 5 Pa), while others have recommended a minimum pressure of 5.3bar (5.3x10 5 Pa) [9] and 5.5bar (5.5x10 5 Pa). [11] Recommended maximum values are 6bar (6x10 5 Pa), [14] 6.6bar (6.6x10 5 Pa) [9] and 6.9bar (6.9x10 5 Pa). [11,15] Milyutin et al [16] reported that increasing the air pressure reduced the efficiency of the process. Table 4 [6,7,10] gives the typical air requirements; including air flow rates, for conventional air carbon arc gouging, and for the continuous wire processes. Air pressures as low as 2.8x10 5 Pa have been employed for conventional air carbon arc gouging with small diameter electrodes; however, the use of such low pressures is not recommended, [15] as carburised metal may not be removed completely from the groove. Correct air line diameters are of paramount importance; for conventional torches, a minimum inside diameter of 10mm is recommended, whereas for semi-automatic torches a larger air line diameter of 13mm is preferred.

Table 4 Typical air consumption for air carbon arc gouging (after Ref. [6,7,10] )

Concerning the continuous wire processes, the air consumption figures of 142-566 litre/min [7] and <833 litre/min [6] are broadly in line with those for conventional air carbon arc gouging. However, the range of operating pressures is well below that for conventional gouging; so far, no metallurgical studies appear to have been carried out to confirm that these pressures are adequate to produce a clean gouge.

Process operation

Most metals can be gouged or cut by the air carbon arc process, but the different thermal and slag characteristics of various materials require careful selection of the equipment and consumable. Table 5 lists the recommended power supply characteristics and electrode types for conventional air carbon arc gouging of different materials. [2]

Table 5 Recommended electrode type and current type for air carbon arc gouging of different materials (after Ref. [2] )

The gouge quality which is achievable, particularly for manual applications, is critically dependent upon the skill of the operator. However, irrespective of the mode of application, a number of process parameters have to be considered to avoid an unacceptable surface finish. Electrode extension or 'stickout', which is the distance between the torch jaws and the electrode tip, should be a maximum of 150mm for ferrous materials and 100mm for aluminium. [9,10] This is to ensure the effective removal of the molten metal from the gouge by the air stream. The electrode may be consumed to within 25-38mm from the torch jaws before it has to be repositioned, although such short electrode extensions give increased operating noise. [17]

The air stream should be turned on before striking the arc. The electrode then has to make contact with the workpiece to strike the arc. It is not necessary to retract the electrode to establish the arc length, as the molten metal directly beneath the electrode tip is immediately blown away. Once the arc has been established, touching the base metal with the electrode should be avoided, as carbon could be deposited on the material surface. Operation in the optimum mode with the correct arc length and electrode motion creates a smooth hissing sound.

The travel speed, the depth of cut and the angle at which the electrode is held in relation to the workpiece surface are all inter-related. Relatively high travel speeds are possible when a low electrode angle is held: this produces a shallow groove, whereas a steep electrode angle results in a deep gouge and requires a lower travel speed. However, using a steep angle can give rise to carbon contamination, as discussed later.

When air carbon arc gouging, it is necessary to ensure that the electrode is moved in a smooth, steady and consistent manner. Irregular, interrupted or shaky movement creates rough surfaces which may be contaminated by slag and carbon deposits.

Use of preheat

The surface layer adjacent to a groove produced by air carbon arc gouging is subjected to a thermal cycle which is sufficiently rapid to form martensite in ferritic steels, and which leaves a narrow heat affected zone (HAZ). Preheating may be expected to give a reduction in HAZ hardness only when the cooling rate is reduced to below the critical value for the formation of martensite; this is most likely to occur in thin section C-Mn steels. Christensen [18] found no reduction in hardness for his 25mm thickness C-Mn steels when a 93°C preheat was used, and the hardness even increased by a few percent for a 204°C preheat. Middleton [19] also found a hardness increase as a consequence of using a 300°C preheat when gouging steels containing up to 0.45% carbon. Preheating is normally specified for gouging where it would have been used for welding. [20] However, to assess this at least for C-Mn structural steels, hypothetical values of arc energy and hydrogen level, by reference to BS 5135, [21] must be assumed. Failure to preheat causes a risk of cracking in the high hardness HAZ. Such cracking has been avoided in low alloy steel castings [22] by using a preheat 50°C below that specified for the welding of steel castings according to BS 4570. [23] Ridal [20] recommends that the preheated material should also be allowed to cool slowly; this would not reduce HAZ hardness, but a post-heat does permit any hydrogen in the high hardness HAZ to diffuse away, and this may well be the main benefit derived from preheating. However, the topic does not appear to have been the subject of a systematic study.

Operating techniques

Air carbon arc gouging may be used for a wide range of metal removal applications, which may require slightly different techniques. [1] These techniques may be summarised as follows:

i) Gouging
Gouging is underside of weld roots or defective areas, normally used for excavating the underside of weld roots or defective areas, or for removing temporary attachments. Once the arc has been established and the desired electrode angle assumed (normally pointing in the direction of travel), the electrode is moved along the workpiece surface to be gouged. The width of the groove is determined by the electrode diameter, whereas the angle at which the electrode is held and the travel speed dictate the groove depth. A slightly wider groove may be produced by oscillating the electrode end in a circular, or weave motion. [15] Typical data [12] for automatic air arc gouging are shown in Table 6.

ii) Severing
When air carbon arc gouging is used for cutting ( e.g. for dismantling steelwork, foundry work, etc) the technique is similar to that adopted for gouging, except that the electrode is held at a much steeper angle [9] to produce a deeper groove. On thinner sections the metal is usually pierced by the electrode tip, and by moving the arc up and down through the hole in a sawing motion the metal is severed. [15] For thicker sections the depth of the groove is increased by subsequent gouging, usually from both sides if access permits, until the metal is severed.

iii) Washing
Washing is frequently adopted for removing metal from large areas ( i.e. the removal of surfacing layers and of riser pads on castings). Because removal of only a relatively thin layer of metal is required, the electrode is held at a shallow angle and weaved from side to side as it is moved forward to remove metal to the desired depth.

iv) Bevelling
As the name implies, bevelling is normally used for plate edge preparation. For thin plates, the air stream should be behind the electrode, which points in the direction of travel. However, for thicker material, the electrode is inclined in a plane which is perpendicular to the line of travel and at an angle which determines the angle of the bevel.

Table 6 Typical data for automatic air carbon arc gouging (after Ref. [12] )

Water carbon arc gouging

The same basic principles of metal removal by melting and physically blowing away are used for underwater gouging applications. [2] Water carbon arc gouging equipment is now commercially available; it is usually supplied as a complete underwater cutting system, and affords the same advantages that the air carbon arc process provides on land. Safety precautions must be observed meticulously, to prevent the operator receiving an electric shock.

Electrode holders are fully electrically insulated, to reduce the danger of shock to the user. Electrodes are similar to the conventional coated type, with the addition of a plastic coating, which prevents water impregnation and provides additional protection for the user against electrical shock. The end of the electrode which is held in the electrode holder remains bare. A DC positive power supply is used. The arc characteristics are basically the same as the air carbon arc; the arc produces sufficient heat to melt the material surface immediately beneath the electrode tip. A high pressure water jet is used as a substitute for the air stream, and this is directed below the electrode. A variant of the process, which is normally used for cutting, uses a DC negative power supply and a jet of oxygen which passes down the centre of the hollow electrode. Underwater, these processes can be used for the same type of work for which the air carbon is used above water ( e.g. weld preparation, defect removal and cutting) and on the same wide range of metals.

Safety

Generally, the safety precautions which are normally used for MMA welding, with respect to protective clothing, ventilation and fume extraction, etc, are adequate for air carbon arc gouging applications. Care must be taken to ensure adequate fume extraction when gouging metals containing significant quantities of Cr, Zn, Cu or Ni. [8] Wearing of air-fed screens or half-face masks supplied by compressed air while operating air carbon arc gouging equipment has been recommended. [24] Also, because of the high noise level inherent in the process, typically in the range 90-115dB, [10] some form of ear protection is needed. A modified electrode which is quieter in operation has been devised, and measures to reduce noise (including restricting the air pressure and using a maximum arc voltage of 40V and a minimum electrode extension of 100mm) have been discussed. [17] In view of the fire hazard from molten metal ejected by the air stream, any combustible material should be at least 11m from the workpiece.

Metallurgical consequences

Formation of local high hardness regions

The rapid cooling rate to which the surface layer adjacent to a gouged groove is normally subjected results in formation of martensite in ferritic materials, and thus the HAZ is a region of high hardness. Several authors [9,13,18,19] have reported the HAZ width as ≤~1mm in C-Mn steels; for example, Goldberg [13] reports the formation of a layer of martensite 0.2mm wide, and beyond that a partially transformed layer 0.75mm wide. In a low alloy steel, Ridal [20] has reported the HAZ width as 1.0-5.9mm, but typically 1.0-1.3mm. It appears likely that a width substantially greater than 1mm includes also the width of carburised metal which has resolidified in the groove - a phenomenon which can occur when the process is carried out - incorrectly, as discussed below.

Carbon pick-up

When air carbon arc gouging is carried out correctly, no carburised metal resolidifies in the groove, and minimal carburisation of the walls of the groove occurs. The small extent of carburisation is a result of carburised metal being blown away.

This has been demonstrated by Christensen, [18] who normalised gouged samples of C-Mn steel and observed that a uniform ferrite plus pearlite microstructure persisted right up to the edge of the groove, and by Goldberg, [13] who examined autoradiographs from steel containing radioactive carbon [14] a which had been gouged.

In a weld bead deposited in a gouged groove in a C-Mn steel, Christensen [18] reported an increase in deposit carbon content of 0.04% compared with a control weld in a machined groove. This observation is in conflict with his metallographic study mentioned above, and it suggests that some variation occurred in his gouging conditions. Both Hard, [25] and Brook and Moore [26] reported regions at groove surfaces with a carbon content of ~0.68%, and Boekholt [27] determined a carbon content of ~1% for particles removed by air carbon arc gouging. Ostrovskaya and Novikova [28] reported that the carbon content of the resolidified carburised metal remaining in the groove in their studies ranged from 0.7-3.1% and was a mixture of ledeburite (cementite/austenite eutectic), with a hardness >600HV, and high carbon martensite, with a hardness in the range 520-540HV. The presence of this carburised metal indicates that their gouging was carried out incorrectly. A further consequence of this was that the steel adjacent to the groove was carburised to a depth of 0.2-0.6mm. Goldberg [13] also demonstrated the formation of ledeburite, implying a carbon content greater than 4%, as its depth was up to 4mm, it was not necessarily all remelted during subsequent welding. Goldberg reported that such deposits are generally porous, so that it is normally possible to see, during grinding, when they have been removed completely.

Grinding of the faces of air carbon arc gouged grooves is commonly carried out in case a carbon-enriched layer has been formed. The British Standard BS 5500 [29] requires that dressing by machining or grinding be carried out to remove severe notches, slag and scale arising from thermal cutting. For ferritic steel, a depth of 1.5mm must be removed, unless the manufacturer can demonstrate, to the satisfaction of the Inspecting Authority, that the material has not been affected by the cutting process. Several authors have reported an increased carbon content in weld beads deposited in air carbon arc gouged grooves. These include increases in carbon content of 0.04% [18] and 0.01-0.03%, [30] and variations in carbon content from 0.3-0.4% [13] and from 0.055-0.19%, with one region containing 0.65% carbon. [31] By increasing the angle of inclination of the gouging electrode, Toropov [14] was able to increase the carbon content to ~1.3%, while Ostrovskaya and Novikova [28] found ledeburite (>4% carbon) in some of their weld beads. Clearly, such increased levels of carbon in production welds are undesirable, as they give an increased risk of solidification cracking [28] and of weld metal hydrogen-induced cracking, [32] and a decrease in toughness. [33] The extent of the increase in carbon content depends on the size of a weld bead, the amount of carburised metal incorporated into it, and its carbon content. Small weld beads are particularly at risk, not only because of their greater surface to volume ratio, but also because the shallow penetration may incorporate primarily the underlying carburised layer into the weld.

Electrode angle

It is important not only to use the correct values for the current, electrode extension and air pressure, but also to select the right angle between the gouging electrode and the workpiece. It is well established that, for gouging, this angle should be shallow. [31] Doshchechkina et al, [34] who studied air arc cutting, recommended an angle of ~40°, while Yeo [9] recommended a similar angle, 45°, for cutting and a smaller angle, 30°, for gouging. Goldberg [13] recommended an angle of 45° for gouging. Toropov [14] claimed that the angle should be 25-30°. He used angles of 25, 45 and 60° in turn, with an air pressure of 5-6bar (5x10 5 -6x10 5 Pa). Each groove was then filled with four beads, deposited one on top of the other with MMA electrodes, and the carbon content of each bead was determined. The findings, which are summarised in Fig.4, suggest that carburisation of the walls of the groove, and consequently an increase in weld metal carbon content, occurs for angles of inclination greater than 25°. This angle of inclination is similar to that stipulated by Gubenko et al [6] for the air supply used with their continuous wire gouging equipment. For many applications, surfaces produced by air carbon arc gouging which has been carried out correctly do not need to be ground. For example, Yeo [9] has suggested that in foundries subsequent cleaning by shot blasting is adequate, while for many situations in which the gouge surfaces will be completely melted or re-austenitised by welding, wire brushing of the gouged region to remove loose dross will be sufficient.

Recommendations

To avoid the adverse metallurgical consequences resulting from carburisation of the surface of air carbon arc gouged grooves:

1. The angle between the gouging electrode and the workpiece must be kept low, i.e. ~25°.
2. The air pressure must be kept high i.e. in the range 5-6.5bar (5x10 ⁵ -6.5x10 ⁵ Pa). However, for slightly quieter operation, and increased efficiency of metal removal, a pressure near the lower limit may be preferred.
3. The air supply must be turned on before the arc is struck. A failsafe device which only allows current to flow when the air is flowing is a useful safeguard.
4. Where these conditions can be maintained reliably, e.g. in semi-automatic or fully automatic air carbon arc gouging, or in closely supervised manual air carbon arc gouging, subsequent grinding to remove carburised metal should be unnecessary.
5. Where these conditions cannot be maintained reliably, e.g. where access is poor or where air carbon arc gouging is used to remove short defective regions or to produce deep grooves, subsequent grinding is required to remove not only any carbon enriched deposits which are porous but also a minimum of at least a further 0.5mm of parent material which is likely to be carburised.
6. Where small weld beads are to be deposited over air carbon arc gouged surfaces, the need for grinding should be especially considered.

Merits and limitations

An AWS publication ^[15] discusses the merits and limitations of the process, essentially as follows.

Merits

1. Improved productivity; metal removal may be up to five times faster than chipping. ^[1]
2. The depth of cut is easily regulated.
3. Initial equipment capital outlay is quite low. Gas cylinders or regulators are only necessary for on-site applications.
4. It is economical to operate. No oxygen or fuel gas is required, although large quantities of compressed air are used, and the cost of this should not be overlooked.
5. It is simple to operate for either gouging or cutting applications. Welders require only a short training time to become proficient.
6. It is versatile. The electrode holder is not much larger than a conventional MMA electrode holder, so it can be used in areas too restricted to accommodate other metal removal methods, and normally in those locations where it is possible to weld. It also has all-positional capabilities, as the electrode holder contains an air control valve and swivel electrode jaws which accommodate changing the electrode angle to suit almost any application, while maintaining correct alignment of the air stream.
7. It is capable of producing clean cuts, with smooth surfaces. Welding or brazing may be done without further grinding or cleaning, subject to the metallurgical factors discussed above.
8. Once they have been located, planar defects can be removed readily as they show up in the bottom of the gauge and the operator can 'follow' them until they disappear.
9. Equipment is available for on land or underwater gouging applications.

Limitations

1. Other cutting processes, for example oxy-fuel gas cutting, may be better for severing thick materials, although oxy-fuel gas cutting is usable on a narrower range of materials.
2. A compressed air supply providing large volumes of air is normally required.
3. There may be some risk of carbon pick-up, which may give increased HAZ hardness in some materials, i.e. C-Mn steels, air hardenable alloys and cast iron; this may influence the properties of subsequent welds adversely.
4. The depth of cut on any one pass is limited to the effective electrode penetration. Control of groove shape and depth is limited, unless automated equipment is used.
5. The process is usually accompanied by considerable noise, fume and discharge of molten metal and sparks.

Concluding remarks

Air carbon arc gouging is widely used for rapid removal of defective or surplus metal. In view of the high rates of metal removal and the flexibility of the process, it is likely to continue to find wide application. Misgivings about possible adverse consequences of the process can be allayed by correct selection of operating parameters when, for some applications, subsequent grinding may well be unnecessary.

Acknowledgements

The authors gratefully acknowledge the assistance given by the Institute's Library and Information Service in the compilation of this article, and helpful discussions with other colleagues, particularly T G Gooch, P H M Hart, O K Gorton and J Haugh.

References