2.4 Air Carbon Arc Cutting and Gouging (CAC-A): Electrodes, Current, Air Supply, Technique & Defects
Key Takeaways
- CAC-A removes metal by melting it with a carbon arc and blowing the pool clear with compressed air at a minimum of 80 psi and 25 to 33 cfm at the torch.
- Electrode current ranges on DCEP run 90–150 A for 5/32 in., 300–400 A for 1/4 in., and 450–600 A for 3/8 in. carbon electrodes.
- Electrode stickout must stay between 2 in. and 7 in., and the air stream must sit between the electrode and the work, behind the arc.
- Gouging uses a push angle of about 35 degrees from the work surface; severing uses a push angle of 10 to 20 degrees from vertical.
- The finished groove runs roughly 1/8 in. wider than the electrode diameter, so electrode diameter is what sets groove width.
Air Carbon Arc Cutting and Gouging (CAC-A): Electrodes, Parameters, Technique & Defects
Core Trade Concept: Section 2.2 covered why a boilermaker gouges — back-gouging a root, excavating a defect, and the mandatory grind-back of the recast layer afterwards. This section covers how the process is set up and run: which electrode, how many amps, how much air, what stickout, what travel angle, and how to read a bad gouge and correct it. Air Carbon Arc Cutting (CAC-A) melts metal with an arc between a carbon-graphite electrode and the work, then blows the molten pool clear with a high-velocity compressed air jet. It removes metal; it does not cut by oxidation the way an oxyfuel torch does, which is exactly why it works on stainless steel, cast iron, copper alloys and aluminum as well as carbon steel.
1. Equipment String and Power Source Selection
DC WELDING POWER SOURCE (DCEP)
CC, CV, or AC/DC transformer COMPRESSED AIR SUPPLY
100% duty cycle rating 80 psi min. at the torch
| 25-33 cfm, air dryer fitted
| |
+---------> GOUGING TORCH <---------+
(rated for the
electrode size)
|
CARBON ELECTRODE
stickout 2 in. min / 7 in. max
|
WORK ---- work lead: >= 1 sq in. of
contact per 1000 amps
- Power source. A DC constant current, DC constant voltage, or AC/DC transformer machine, rated 100% duty cycle. On a CC machine the amperage setting is critical and must sit inside the electrode's published range. On a CV machine the electrode draws the current it needs, which makes CV more forgiving for gouging.
- Torch capacity. Gouging torches are rated by maximum amperage (commonly 450 A, 600 A, 1000 A and 1250 A classes). Running a 600 A electrode through a 450 A torch cooks the torch head and the air valve.
- Work lead. CAC-A pulls enormous current. Provide at least one square inch of clamp contact area per 1000 amperes. Grounding through tack welds, or extending reach with a steel flat bar, adds resistance — copper conducts roughly seven times better than steel, so the bar would have to be seven times larger to be equivalent. Hot cables mean undersized cable or a bad ground.
- Air supply. Minimum 80 psi at the torch while gouging, with 25 to 33 cfm of flow. Fit an air dryer; water vapor in the line produces erratic slag removal. Never substitute a cylinder of oxygen for compressed air — oxygen plus a carbon electrode plus molten steel is an uncontrolled burn.
2. Electrode Types, Sizes and Current Ranges
Three electrode families exist, and picking the wrong one wastes carbon:
| Electrode | Construction | Where it is used |
|---|---|---|
| DC copper-coated | Graphite core with a copper cladding | The default. Longest life, highest current capacity |
| DC plain (uncoated) | Bare graphite, generally under 3/8 in. | Consumes noticeably faster; light work only |
| AC coated | Rare-earth additions for arc stabilization | Only where an AC machine is the available power source |
Suggested current ranges
| Electrode dia. | DC electrode, DCEP | AC electrode on AC | AC electrode on DCEN |
|---|---|---|---|
| 5/32 in. (4 mm) | 90–150 A | — | — |
| 3/16 in. (5 mm) | 200–250 A | 200–250 A | 150–180 A |
| 1/4 in. (6 mm) | 300–400 A | 300–400 A | 200–250 A |
| 5/16 in. (8 mm) | 350–450 A | — | — |
| 3/8 in. (10 mm) | 450–600 A | 350–450 A | 300–400 A |
| 1/2 in. (12 mm) | 800–1000 A | — | — |
| 5/8 in. (16 mm) | 1000–1250 A | — | — |
Electrode storage. Carbon electrodes absorb moisture. Damp electrodes should be baked for approximately 10 hours at 300°F before use; wet carbons spit, pop and leave carbon deposits in the groove.
3. Setup Geometry: Stickout, Air Stream and Copper Peel-Back
Four setup dimensions decide whether the gouge is smooth or ragged:
- Stickout: 7 in. maximum, 2 in. minimum. Longer than 7 in. and the electrode wanders and overheats; shorter than 2 in. and the arc heat damages the torch head and jaws.
- Air stream position. The air orifices must sit between the electrode and the work, behind the arc in the direction of travel, so the jet lifts the molten pool out of the groove ahead of the solidification front. Air directed over the top of the electrode blows the puddle back into the groove.
- Air first, arc second. Open the air, then strike the arc by lightly touching the electrode to the work. Close the arc, then close the air. Striking dry leaves a slug of frozen metal in the groove that must be chipped out.
- Copper peel-back of 3/4 in. to 2 in. As the graphite consumes, the copper cladding peels back from the tip. Peel-back longer than about 2 in. means the electrode is running too hot for its diameter or the stickout is too long.
4. Technique: Gouging Versus Cutting
Gouging (removing a groove)
- Push (forehand) travel, torch angled about 35° from the work surface.
- Hold the torch with both hands for a steady, straight groove. The operator position that looks down the groove line watches the defect line disappear as the arc passes over it; the position looking across the groove cannot verify removal.
- Correct travel speed produces a smooth hissing sound. Popping and spitting means too slow or too little air; an unstable, wandering arc means amperage below the electrode's range.
- Groove width runs about 1/8 in. wider than the electrode diameter, so electrode selection sets groove width. Groove depth per pass is controlled by torch angle and travel speed, not by pressing harder.
- Finish by grinding the gouged surface to bright, shiny metal — this is the same recast-layer removal requirement developed in Section 2.2.
Cutting (severing)
- Same process, steeper angle: a push angle of 10° to 20° from vertical.
- Maximum thickness severed in a single pass is about 1.5 times the electrode diameter. A 3/8 in. electrode therefore severs roughly 9/16 in. plate in one pass.
- Keep at least 1/16 in. of arc stickout below the bottom of the plate so the cut fully penetrates, and favour the side the air stream is on.
- On heavier plate, work the arc up and down through the section with a sawing motion rather than trying to sever in a single straight pull.
Preheat
Apply preheat for gouging whenever preheat is required for welding that material and thickness. On repairs it is common practice to run roughly 50°F above the required welding preheat, and preheat must extend through the full thickness, not just the surface the torch is playing on.
5. Reading a Bad Gouge
| Symptom | Root cause | Correction |
|---|---|---|
| Carbon deposits (hard, glassy black film in the groove) | Travel too slow, amperage too low for the electrode, or arc extinguished with air off | Raise amperage into the published range, increase travel speed, grind the deposit out completely before welding |
| Irregular, scalloped groove bottom | Unsteady torch, single-handed operation, excessive stickout | Two-hand grip, reduce stickout, steady the torch on a rest |
| Slag and metal re-freezing in the groove | Air pressure below 80 psi, wet air, or air orifices misaligned | Verify 80 psi/25–33 cfm at the torch, drain and dry the line, re-index the electrode in the jaws |
| Copper smears on the groove wall | Copper cladding melting from over-current or excessive peel-back | Drop to the correct current for the diameter; grind the smear out — copper in a weld causes hot cracking |
| Excessive groove width | Electrode diameter too large for the excavation | Select a smaller electrode; groove width tracks electrode diameter plus about 1/8 in. |
Why this matters on code work: every one of these defects, if welded over, becomes a rejectable indication on the radiograph. Section 2.2's grind-back rule is not a suggestion — it is the last line of defence against carbon pickup and copper contamination in the finished weld.
6. Safety: Arc, Noise, Sparks and Fume
- Eye protection. AWS Z49.1 Table 1 lists air carbon arc cutting as minimum shade 10 / suggested shade 12 for light work below 500 A, and minimum shade 11 / suggested shade 14 for heavy work from 500 to 1000 A. Boilermaker gouging on heavy shell plate routinely runs in the heavy band.
- Noise. CAC-A is one of the loudest processes in the plant; double hearing protection (plugs plus muffs) is normal in a boiler setting where reverberation adds several dB.
- Sparks and molten metal. The air jet throws molten metal a substantial distance — spark travel in excess of 35 ft is documented for arc cutting processes. Fire watch, blankets and clearing of combustibles must extend well beyond the normal 35 ft hot-work radius, and anything below the work must be covered.
- Fume. Metal fume plus carbon and copper vapor in the confined volume of a boiler drum or a duct requires local exhaust ventilation, not just general dilution. Gouging stainless releases hexavalent chromium.
- Full leathers. Not a jacket over a T-shirt: sleeves, bib and spats, because the ejected metal travels sideways and downward, not just forward.
7. Realistic Trade Scenario: Excavating a Rejected Header Seam
A radiographic film on a 2 in. thick SA-106 Grade B header girth seam shows a 6 in. run of incomplete side-wall fusion at mid-wall on the far side.
- Preheat. The WPS calls for 175°F preheat for this thickness. The crew applies preheat and, per repair practice, holds roughly 225°F for gouging so the excavation does not chill-crack.
- Electrode. A 1/4 in. copper-coated DC electrode is chosen: it gouges a groove roughly 3/8 in. wide, narrow enough to excavate only the defect area, and runs at 300–400 A DCEP on the crew's CC machine.
- Air. The line is checked at the torch with the trigger open: 85 psi, and the air dryer bowl is drained.
- Gouging. Push angle about 35°, both hands on the torch, operator positioned to sight down the groove and watch the defect line disappear. Stickout is re-gripped before it exceeds 7 in.
- Verification. The excavation is ground to bright metal, then magnetic particle tested to confirm the defect is fully removed before re-welding — grinding alone can smear metal over a remaining crack tip and hide it.
A boilermaker is back-gouging a heavy header seam with a 1/4 in. copper-coated carbon electrode on a DC constant-current machine set to DCEP. What is the suggested current range for that electrode?
Where must the compressed air stream be positioned relative to the carbon electrode during air carbon arc gouging, and when is the air turned on?
Using air carbon arc cutting to sever plate rather than gouge a groove, what is the approximate maximum plate thickness that can be severed in one pass with a 3/8 in. electrode, and what travel angle is used?
During gouging, a boilermaker notices a hard, glassy black film in the bottom of the groove. What has occurred and what must be done before welding?