Air Carbon Arc Gouging and Cutting Process

1. SCOPE
2. REFERENCES
3. SAFETY
4. DESCRIPTION
5. CAPABILITIES AND LIMITATIONS
6. PROCESS VARIABLES

6.1 Current
6.2 Arc Voltage, Arc Length
6.3 Travel Speed
6.4 Electrode Type, Diameter, and Characteristics
6.5 Polarity and Type of Current
6.6 Airflow and Direction
6.7 Electrode Stick Out
6.8 Electrode Angle
6.9 Weaving

7. ANTISPATTER COMPOUND
8. OPERATING MODES

8.1 Gouging
8.2 Piercing, Slitting, and through Cutting
8.3 Washing
8.4 Beveling

9. POWER SUPPLIES
10. TROUBLESHOOTING
11. ADVANTAGES
12. LIMITATIONS

FOLLOWING FIGURES TO BE SHOWN IN THIS ARTICLE.
1 Normal Setup for Air Carbon Arc Gouging and Cutting.
2 Air Carbon Arc Torch and Component.
3 Flat Position Air Carbon Arc Gouging.
4 Steep Angle Air Carbon Arc Gouging.
5 Shallow Angle Air Carbon Arc Gouging.
6 Vertical Position Air Carbon Arc Gouging.
7 Horizontal Position Air Carbon Arc Gouging.
8 Overhead Position Air Carbon Arc Gouging.
9 Air Carbon Arc Gouging with Flat Electrode.
10 Air Carbon Arc Beveling.
11 Air Carbon Arc Cutting.

TABLE I – Direct Current (dc) Reverse Polarity (rp) Electrodes.
TABLE II – Size of Welding Leads and Number of Cables Required for the Indicated Cable Work Lengths (Welding Power Supply to Workpiece).
TABLE III – Acceptable Power Supplies.

1. Scope

This article describes the air carbon arc gouging and cutting process, its capabilities and limitations, process variables, operating modes, equipment, and advantages and disadvantages.

4. Air Carbon Arc Gouging and Cutting Process

4.1 Air carbon arc cutting is a thermal cutting process where the removal or severing of metal is done by melting with the heat of an arc between an electrode and the workpiece and blowing the molten material away with an air stream. The electrode is a special carbon graphite material held by a special insulated electrode holder through which air is directed to the arc. Power is usually supplied by an arc welding power supply. Not only are the power requirements for a given diameter carbon electrode higher than that for a comparable diameter shielded metal arc welding electrode, but
the air carbon arc cutting electrodes are usually larger.

4.2 Although the air carbon arc cutting process is used primarily for gouging and in some cases smoothing away (washing off) excess metal, it can also be used for cutting metals, particularly those that are not readily cut by an oxy-fuel gas cutting process. For these metals, the air carbon arc cutting process is more suitable because it does not depend upon oxidation of iron for its operation, and chromium surface oxides do not impede its progress. Figure 1 shows a normal air carbon arc cutting setup. It should be noted, however, that if sufficient amounts of this type of material require cutting, plasma arc cutting is a more efficient process.

4.3 Figure 2 shows a typical air carbon arc cutting torch and the terms used for the various components.

4.4 The rotating head permits adjustment of the jets of compressed air and the electrode relative to the handle grip.

5. Air Carbon Arc Process Capabilities and Limitations

5.1 The air carbon arc process is suitable for gouging, smoothing, melting, and through cutting. It is also used for crack routing in preparation for weld repair, the removal of excess weld, and in some cases, the removing of backing straps.

Figure 1 – Normal Setup for Air Carbon Arc Gouging and Cutting

Figure 1 - Normal Setup for Air Carbon Arc Gouging and Cutting

Figure 2 – Air Carbon Arc Torch and Component

Figure 2 - Air Carbon Arc Torch and Component

5.2 The process is suitable for both manual or automatic operation. However, this standard only describes manual operation. The air carbon arc process provides rapid metal removal, shallow heat penetration, and reduced distortion. 

5.3 The process is used with plain carbon steels, low alloy steels, stainless steels, nickel alloys, and cast iron.

5.4 With materials such as high carbon steel and cast iron, hardening of the heated material often occurs. This hardened surface can be removed by grinding or machining. 

5.5 Stainless steels are readily air carbon arc cut. Atmospheric corrosion resistance is not affected as long as the thin surface layer of carbon enrichment is ground away.

5.6 High nickel alloys are more difficult to cut cleanly than carbon steels, low alloy steels, or stainless steels.

5.7 After cutting, all air carbon arced surfaces shall be ground or machined to clean metal. Depending upon the material and its tendency to crack, penetrant or magnetic particle inspection shall also follow. 

5.8 In the manual air carbon arc process the quality of the cut depends mostly upon the operator’s skill. The smoothness or quality of the cut surface is dependent upon the operator’s ability to maintain a steady travel speed. See Figure 3.

Figure 3 – Flat Position Air Carbon Arc Gouging

Figure 3 - Flat Position Air Carbon Arc Gouging

6. Process Variables used in Air Carbon Arc Cutting Process

6.1 Current

The weld current used is direct current (dc) from an ordinary welding power supply, either motor generator or rectified.

6.1.1 The current used depends upon the electrode size selected. A guide to required amperage is shown in Table I.

Table I – Direct Current (dc) Reverse Polarity (rp) Electrodes

6.1.2 The actual current used depends upon the individual job. In general, the maximum amperage works best.

6.2 Arc Voltage, Arc Length

6.2.1 Arc voltage and arc length are interdependent. An increase in arc length increases the arc voltage and conversely a decrease in length decreases the arc voltage.

6.2.2 With manual operation the arc voltage and arc length are controlled by the operator as in stick electrode welding.

6.2.3 It is advisable to maintain a short arc, but the electrode shall not touch the work except when striking the arc at the start. Use as constant an arc length as possible: do not draw the electrode back once the arc is struck. This technique is different from welding since in cutting, metal is being removed.

6.3 Travel Speed

6.3.1 Cutting depth is affected by cutting speed. With a constant electrode size and torch angle, a decrease in travel speed increases the depth of the cut, and an increase in speed reduces the cutting depth. 

6.3.2 When cutting manually, the steadiness of the arc speed controls the smoothness of the cut surface.
6.3.3 In general, the proper travel speed is determined by listening for a smooth hissing or uniform crackling sound from the arc.

6.4 Electrode Type, Diameter, and Characteristics (see Table I)

6.4.1 Electrodes are made of carbon graphite and are copper clad to maintain a constant diameter. This type of electrode cuts evenly, holds a steady arc, operates cooler, and in general is less subject to breakage than unclad electrodes.

6.4.2 During operation the copper coating burns back from the carbon electrode. Reduce the current if the burn back is more than 25.4 mm (1 in). This is an indicator for selecting the proper current for a given electrode diameter.

6.4.3 To remove excess surface metal, a flattened carbon graphite, copper clad electrode is sometimes used to good advantage. These electrodes are usually 4.0 by 9.5 mm or 4.8 by 9.5 mm. Use of these electrodes results in rapid metal removal, high efficiency, and a low temperature rise in the base metal.
6.4.4 Electrode diameter primarily determines width and depth of cut. Generally, the width of cut is 3.2 mm wider than the electrode diameter. The depth of cut is usually equal to the electrode diameter.

6.4.5 Most electrodes are for use with dc. Electrodes for ac, however, are available.

6.5 Polarity and Type of Current

Air carbon arc cutting is done with dc using reverse polarity. The torch lead is positive (+) and the ground lead is negative (-). If polarity is incorrect, the amount of burn back will be greater.  

6.6 Airflow and Direction

6.6.1 High speed jets of compressed air are used to blow away metal as it is melted by the carbon arc.

6.6.2 The use of proper air pressure 552-689 kPa (80-100 psig), flow rate 170-1416 l/min (6-50 cfm), and air-stream direction are required to ensure a clean slag free cut surface.

6.6.3 When cutting in any position the air stream shall be behind and underneath the electrode and directed towards the arc. The groove in the rotating head will normally assure the proper relation of the air stream and the arc stream.  

6.6.4 Positioning the arc stream and air stream is done by turning the rotating head so that the stream is pointed in the proper direction as indicated on Figure 2.  

6.6.5 The recommended size and number of welding leads for different cable work lengths and currents are given in Table II.
Table II – Size of Welding Leads and Number of Cables Required for the Indicated Cable Work Lengths (Welding Power Supply to Workpiece)

6.7 Electrode Stick Out

6.7.1 Electrode stick out is the distance from the air jet head of the torch to the arc end of the electrode.
6.7.2 A stick out of 152 mm (6 in) for manual gouging is best. Too long a stick out results in poor air jet action and consequently poor metal removal. Too short a stick out results in overheating of the torch and in poor metal removal due to premature cooling of the molten material by the air jet.

6.8 Electrode Angle

The steeper the electrode angle (see Figure 4) the more directly the arc strikes the surface, and the deeper the cut. A flat angle (see Figure 5) produces a shallow groove. Although changes in electrode angle can be used to vary the depth of cut, it is often better to use a 35 – 45 degree angle and change the depth of cut by changing the electrode diameter or travel speed, or both.

Figure 4 – Steep Angle Air Carbon Arc Gouging

Figure 4 - Steep Angle Air Carbon Arc Gouging

Figure 5 – Shallow Angle Air Carbon Arc Gouging

Figure 5 - Shallow Angle Air Carbon Arc Gouging

6.9 Weaving

For a given electrode size a wider groove may be obtained by weaving the electrode. Weaving, however, is advisable only with automatic cutting.

7. Anti spatter Compound

7.1 Compounds, which are free of lead, mercury and chlorides can be used to prevent the sticking of spatter during air carbon arc cutting.

7.2 Before putting anti spatter compound on the metal surface, clean the surface from grease and impurities. The compound may be used directly or it may be diluted with 1 part of water to 1 part of compound by volume. Allow the coating to dry before starting the operation. In any case, follow the manufacturer’s recommendations.

7.3 Compounds may be used on all surfaces where spatter from molten particles occurs. Extra shields made from stainless steel sheet metal or Transite boards may also be used. Take care to shield concrete floors below the work, to prevent spalling of concrete.

8. Air Carbon Arc Operating Modes

8.1 Gouging

8.1.1 Gouging is used to remove welding defects, to back-rout, and to remove tack welds.

8.1.2 It can be performed in all positions. In the vertical position gouging is best done in the downward direction where removal of material is helped by gravity (see Figure 6). Horizontal gouging (see Figure 7) can be done either to the right or to the left. When in the overhead position (see Figure 8), the electrode is set nearly parallel to the torch at a slight angle, to avoid the molten metal dripping onto the operator.

8.2 Piercing, Slitting, and through Cutting

8.2.1 With thin materials, hold the torch at a steep angle (approximately 90 degrees to the work) with the electrode tip piercing the section and cutting through completely.

8.2.2 With heavy materials, use a torch angle of 35-45 degrees and make a number of passes. With the final pass, use a 90-degree angle cutting through the material.

Figure 6 – Vertical Position Air Carbon Arc Gouging

Figure 6 - Vertical Position Air Carbon Arc Gouging

Figure 6 – Vertical Position Air Carbon Arc Gouging

Figure 7 – Horizontal Position Air Carbon Arc Gouging

Figure 7 - Horizontal Position Air Carbon Arc Gouging

Figure 7 – Horizontal Position Air Carbon Arc Gouging

Figure 8 – Overhead Position Air Carbon Arc Gouging

Figure 8 - Overhead Position Air Carbon Arc Gouging

Figure 8 – Overhead Position Air Carbon Arc Gouging

8.2.3 With deep cuts, widen the cut at the surface so as to prevent the electrode from arcing off the side walls and losing the cutting action.

8.3 Washing

Shallow passes are usually employed. Flat electrodes are often used (see Figure 9 and 6.4.3). Low base metal temperatures and high speeds can be obtained. It is also good for removal of clad surfaces. 

8.4 Beveling

To make a slant cut along an edge (see Figure 10), the electrode can be smoothly drawn almost parallel to the cut. The air blast is between the electrode and the work.  

Figure 9 – Air Carbon Arc Gouging with Flat Electrode

Figure 9 - Air Carbon Arc Gouging with Flat Electrode

Figure 9 – Air Carbon Arc Gouging with Flat Electrode

Figure 10 – Air Carbon Arc Beveling

Figure 10 - Air Carbon Arc Beveling

Figure 10 – Air Carbon Arc Beveling

9. Power Supplies for Air Carbon Arc Gouging

All standard welding power sources of adequate current capacity (see Table I) can be used for air carbon arc cutting. However, the open circuit voltage shall be sufficiently larger than the required arc voltage to allow for the voltage drop in the circuit. The arc voltage used in air carbon arc gouging and cutting, ranges from 35-55 volts. An open circuit voltage of at least 60 volts is required. The actual arc voltage in air carbon arc gouging and cutting is governed to a large extent by the size of the electrode. Table III shows acceptable power supplies.

Table III – Acceptable Power Supplies

10. Air Carbon Arc Gouging Troubleshooting

10.1 The air carbon arc cutting process is not complicated to use, but sometimes problems arise. Some common problems and their solutions are:

a. Large Carbon Deposit at the Beginning of the Groove. The cutting operator either neglected to turn the air jet on before striking the arc, or the torch was improperly positioned. The air is turned on before striking the arc and flows between the electrode and the work and behind the electrode relative to the direction of travel.  

b. An Unsteady Arc, Causing the Cutting Operator to Use a Slow Travel Speed Even on Shallow Grooves. The amperage is insufficient for the electrode diameter used. While the minimum amperage is sufficient, it does require a higher degree of skill. The middle of the range is more efficient, and the top of the range is even better; therefore, if the amperage is limited by the capability of the power supply, greater efficiency can be obtained by dropping down to the next smaller electrode diameter.

c. Erratic Groove, with the Arc Wandering from Side to Side and with the Electrode Heating Up Rapidly. The process being used is dc, straight polarity (electrode negative). DC electrodes shall be used with direct current reverse polarity on all metals with the exception of a few copper alloys. For these alloys, the added heat in the electrode produced by a high amperage for a given electrode diameter increases the cutting speed.

d. Intermittent Arc Action Resulting in an Irregular Groove Surface. The speed of air carbon arc gouging is much faster than shielded metal arc welding. The cutting operator shall assume a comfortable position to ensure his arm can move freely and his gloves do not drag on the work. If the operator fixes his position by resting his hand on the work (as most shielded metal arc welders do), friction between the gloved hand and the work will cause erratic forward motion.

e. Carbon Deposited at Various Groove Intervals During Gouging and at Various Spots on the Washed Surface When Pad Washing. The electrode has shorted out on the work. This condition is caused by using excessive travel speed for the amperage available and for the depth of the groove being made.

f. Irregular Groove Too Deep and Then Too Shallow. The cutting operator was not steady. The operator should relax and assume a comfortable position when gouging.

g. Slag Adheres To the Edge of the Groove. Slag ejection was inadequate. For adequate slag ejection, proper air pressure and volume shall be used. Air pressure between 552 and 689 kPa (80 and 100 psig) will not effectively eject all of the slag if the volume of air is insufficient. Adequate volume requires an air hose with a minimum inside diameter of 6.4 mm feeding the concentric cable assembly.

h. Groove Gets Progressively Deeper. The electrode travel speed is too slow.

11. Air Carbon Arc Advantages

11.1 Fast – It is five times as fast as chipping. It gouges a 9.5 mm groove at over 51 mm (2 ft) per minute.

11.2 Easily Controllable. It removes defects with precision. Defects are clearly visible in the groove and can be followed with ease. The depth of the cut is easily regulated and the welding slag does not deflect or hamper the cutting operation.

11.3 Compact. It is not much larger than a shielded metal arc welding electrode holder.

11.4 Versatile and Portable. It can be used almost anywhere it is possible to weld. It can be operated in spaces too restricted to accommodate a chipping hammer or an oxyacetylene gas cutting torch. It requires no difficult adjustments for use on different metals.

11.5 Cuts Cleanly. Resulting surfaces are clean and smooth. On carbon steel, welding can generally be done with a minimum of grinding or cleaning.
11.6 Equipment Cost. Minimal.

12. Air Carbon Arc Limitations

12.1 Under most conditions, other cutting processes are better for severing (see Figure 11) or cutting carbon steel plate and pipe.
12.2 A large volume of compressed air is required.
12.3 There are depths of cut limitations.

Figure 11 – Air Carbon Arc Cutting

Oval Gear and Helical

Figure 11 – Air Carbon Arc Cutting

 

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