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Air Carbon Arc Gouging
The main difference between this
gouging technique and the others is that a separate air jet is used to eject
molten metal to form the groove.
Process description Air carbon arc gouging works as follows. An electric arc is generated
between the tip of a carbon electrode and the workpiece. The metal becomes
molten and a high velocity air jet streams down the electrode to blow it away,
thus leaving a clean groove. The process is simple to apply (using the same
equipment as MMA welding), has a high metal removal rate, and gouge profile can
be closely controlled. Disadvantages are that the air jet causes the molten
metal to be ejected over quite a large distance and, because of high currents
(up to 2000A) and high air pressures (80 to 100 psi), it can be very noisy.
ApplicationAs air carbon arc gouging does not rely on oxidation it can
be applied to a wide range of metals. DC (electrode positive) is normally
preferred for steel and stainless steel but AC is more effective for cast iron,
copper and nickel alloys. Typical applications include back gouging, removal of
surface and internal defects, removal of excess weld metal and preparation of
bevel edges for welding.
Electrode The electrode is a non-consumable graphite (carbon) rod which has a
copper coating to reduce electrode erosion. Electrode diameter is selected
according to required depth and width of gouge. Cutting can be precisely
controlled and molten metal/dross is kept to a minimum.
Power sourceA DC power supply with electrode positive polarity is most
suitable. AC power sources which are also constant current can be used but with
special AC type electrodes. The power source must have a constant current output
characteristic. If it does not, inadvertant touching of the electrode to the
workpiece will cause a high current surge sufficient to 'explode' the electrode
tip. This will disrupt the operation and cause carbon pick-up. As arc voltage
can be quite high (up to 50V), open circuit voltage of the power source should
be over 60V.
Air supplyThe gouging torch is normally operated with either a
compressed air line or seperate bottled gas supply. Air supply pressure will be
up to 100psi from the air line but restricted to about 35psi from a bottled
supply. Providing there is sufficient air flow to remove molten metal, there are
no advantages in using higher pressure and flow rates.
Carbon pickupAlthough carbon is picked up by the molten metal, the air
stream will remove carbon-rich metal from the groove to leave only minimal
contamination of the sidewalls. Poor gouging technique or insufficient air flow
will result in carbon pick-up with the risk of metallurgical problems, e.g high
hardness and even cracking.
Typical operating data for air carbon arc gouging:
| Electrode diameter (mm) |
Current A
Note: DC electrode |
Gouging dimensions |
Carbon electrode consumed (mm/min) |
Gouging speed (mm/min) |
| Depth (mm) |
Width (mm) |
| Manual |
6.4 |
275 |
6-7 |
9-10 |
120 |
609 |
| 8.0 |
350 |
7-8 |
10-11 |
114 |
711 |
| 9.5 |
425 |
9-10 |
12-13 |
100 |
660 |
| 13.0 |
550 |
12-13 |
18-19 |
76 |
508 |
| Automatic |
8.0 |
300-400 |
2-9 |
3-8 |
100 |
1650-840 |
| 9.5 |
500 |
3-12 |
3-10 |
142 |
1650-635 |
| 13.0 |
850 |
3-15 |
3-13 |
82 |
1830-610 |
| 16.0 |
1250 |
3-19 |
3-16 |
63 |
1830-710 |
OperationGouging is commenced by striking the electrode tip on to the
workpiece surface to initiate the arc. Unlike manual metal arc (MMA) welding the
electrode tip is not withdrawn to establish arc length. Molten metal directly
under the electrode tip (arc) is immediately blown away by the air stream. For
effective metal removal, it is important that the air stream is directed at the
arc from behind the electrode and sweeps under the tip of the electrode. The
width of groove is determined by the diameter of electrode, but depth is
dictated by the angle of electrode to the workpiece and rate of travel.
Relatively high travel speeds are possible when a low electrode angle is used.
This produces a shallow groove: a steep angle results in a deep groove and
requires slower travel speed. Note, a steeply angled electrode may give rise to
carbon contamination.
Oscillating the electrode in a circular or restricted weave motion during
gouging can greatly increase gouging width. This is useful for removal of a weld
or plate imperfection that is wider than the electrode itself. It is important,
however, that weave width should not exceed four times the diameter of the
electrode.The groove surface should be relatively free of oxidised metal and can
be considered ready for welding without further preparation. Dressing by
grinding the side-walls of the gouge should be carried out if a carbon rich
layer has been formed. Also, dressing by grinding or another approved method
will be necessary if working on crack-sensitive material such as high strength,
low alloy steel.
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