Air carbon arc gouging (CAC-A) is one of the workhorses of the structural fabrication shop — fast, portable, and effective for opening root passes, removing defective weld metal, and preparing heavy plate for repair welding. AWS D1.1 permits its use, but the surface it leaves behind is not automatically weld-ready. The carbon and oxidized metal that remain after gouging are real contamination risks, and the groove geometry after the cut must still meet the fit-up requirements for the applicable weld joint design. CWIs who skip the post-gouge surface check are setting up the shop for fusion defects and potentially rejectable welds.

What CAC-A is and why shops use it

CAC-A uses a copper-coated carbon electrode, direct current reverse polarity (DCRP), and a continuous jet of compressed air directed behind the arc to simultaneously melt and blow away base metal. The result is a U-shaped or V-shaped groove cut at high speed — faster than grinding on thick plate and more controllable than oxy-fuel gouging on complex contours.

The most common structural applications are:

  • Back gouging CJP groove welds — exposing the root pass from the second side for inspection and remelting before the fill passes on the opposite face begin
  • Joint repair — removing a length of weld containing a crack, pore cluster, or incomplete fusion indication before repair welding
  • Initial joint opening — particularly on built-up shapes where the joint geometry is cut rather than machined
  • Removing temporary welds — strongbacks, erection cleats, and burn bars are sometimes removed by gouging rather than mechanical cutting, especially when flush removal is required

Shops that have been using CAC-A for years often treat it as a routine step, which leads to inspection gaps: because the process is fast and familiar, the post-gouge surface check gets compressed into a quick look rather than a methodical inspection.

AWS D1.1 requirements for surface condition after CAC-A

AWS D1.1:2025 does not prohibit CAC-A but requires a clean, sound surface before welding on the prepared groove. The carbon electrode deposits a carbon-rich residue on the molten surface as the arc progresses. That residue is visible as black or blue-gray discoloration; when it remains in the joint, it prevents proper fusion in the first weld pass and increases local carbon content, raising HAZ hardness.

The requirement: the gouged surface must be ground smooth and free of carbon deposits before welding begins. Grinding removes the discoloration layer and confirms that the parent metal beneath is sound. On carbon steel, a 60-grit or finer angle grinder produces a clean bright metal surface in a single systematic pass. The goal is no visible black carbon marks and no ragged metal from the gouge edge.

The groove profile after grinding must also conform to the applicable joint detail. Back-gouging a CJP groove weld to expose the root — a common step in double-sided groove weld fabrication — requires a groove that is wide enough for the electrode to reach the bottom and clean enough to verify complete root fusion before filling begins. If the gouge is too shallow or irregular, the inspector is looking at a groove that will hide incomplete fusion no matter how good the subsequent welding is.

See: Back gouging and CJP groove weld documentation under AWS D1.1

Carbon pickup risk by base metal type

Carbon pickup from CAC-A surface contamination matters most on steel that is sensitive to carbon-induced hardening in the heat-affected zone.

Low-carbon steel (A36, A572 Grade 50): Low baseline carbon content and relatively forgiving HAZ hardness response. Grinding the contaminated layer is still required, but the risk of significant hardness elevation from residual surface carbon is lower than for alloy or Q&T grades.

High-strength low-alloy steel (A588, A709 Grade 50W, A913 Grade 65): Higher carbon equivalents mean the HAZ is more sensitive to rapid cooling and carbon introduction. For these steels, grinding to clean bright metal before welding is not optional — it is the only way to prevent hard zones in the HAZ that can initiate hydrogen-assisted cold cracking during the cooling cycle after welding.

Quenched and tempered steel (A514, A517): Some fabricators and Q&T steel producers' guidelines recommend against CAC-A on these materials, substituting oxy-fuel gouging or grinding only. The Q&T temper is inherently sensitive to thermal cycle; adding carbon contamination from an unground gouge compounds the risk. Where CAC-A is used on A514, the WPS should specify minimum grinding depth after gouging and require a Brinell hardness check on a representative coupon before production use.

See: Hydrogen cracking prevention and WPS documentation

WPS and procedure documentation for CAC-A

CAC-A is a preparation method, not a welding process in the AWS D1.1:2025 sense — it does not appear in the essential variable tables of Table 6.6, and a change in joint preparation method is not by itself an essential variable that requires WPS requalification. However, the WPS or its companion shop traveler should document:

  • Method of joint preparation — noting that CAC-A is used establishes the surface condition expectation that the CWI will verify before welding
  • Required post-gouge grinding — specify that the surface must be ground to clean bright metal and the groove profile must be verified before the preheat inspection
  • Base metal restrictions — if the WPS covers A514 or other Q&T grades, note any restrictions or additional verification steps (hardness check, minimum grinding depth)
  • Preheat relative to gouging sequence — on hardenable steels, preheat may be applied before gouging, not just before welding, to reduce the rate of cooling in the HAZ left by the arc

Shops that handle repair welding regularly should have a repair WPS that explicitly addresses CAC-A preparation. General production WPSs covering initial joint fabrication may not address it. A repair welding procedure that is silent on how the repair cavity was prepared leaves the CWI without a documented standard to verify against.

See: Repair weld WPS documentation under AWS D1.1

CWI inspection checkpoints after CAC-A

Before the first weld pass is deposited on a CAC-A prepared surface, the CWI should verify:

Surface condition:

  • No visible black or blue-gray carbon deposit on the groove faces or groove root
  • Groove surfaces are smooth — not ragged or stepped from erratic arc travel
  • No visible oxidation scale remaining in the groove, especially at the root of a back-gouged CJP weld

Groove geometry:

  • Width and depth conform to the applicable joint detail — either the prequalified dimensions from Annex B or the dimensions established during PQR testing
  • Root access for the electrode: for back-gouged CJP groove welds, the groove must allow the electrode to reach and fuse to the deepest point without bridging
  • Groove angle and included angle are within the WPS range if groove geometry is a controlled variable

Preheat verification:

  • Preheat must be at or above the minimum required by the WPS before the first weld pass, independent of whether residual heat from gouging is still present in the part. A contact pyrometer or temperature-indicating crayon at the required distance from the groove confirms compliance. Heat from gouging does not substitute for preheat verification at the weld joint.

See: Preheat and interpass temperature on a WPS

Common production inspection findings

The most frequent CWI findings after CAC-A preparation:

Residual carbon deposit not ground out. The welder starts filling before the grinder finishes, or the grinder takes one pass rather than systematically covering the full groove. MT or PT on the first pass sometimes reveals fine linear porosity running in the plane of the gouge — a signature of carbon pickup in the weld metal.

Uneven groove profile. An inexperienced operator chases the gouge in a wandering path rather than a steady line, leaving a groove with variable width and depth. Filling an irregular groove produces inconsistent pass thickness and increases the risk of lack-of-fusion at the narrow sections. The remedy is grinding the groove to a consistent profile before inspection sign-off.

Preheat not confirmed after gouging. The part was preheated before gouging, but the gouge pass took long enough that the surface temperature dropped below the required minimum. The first weld pass is deposited into a part that is below preheat temperature. This is common on large weldments where the gouging zone is far from the preheat temperature measurement point.

Over-gouging into sound base metal. Too aggressive a pass — especially when removing a repair cavity — takes more base metal than necessary. On cyclically loaded members or fatigue-classified connections, the final surface profile after grinding must still conform to the smoothness requirements for the applicable fatigue category.

Documenting CAC-A as a distinct step in the fabrication sequence — with its own CWI inspection sign-off before welding begins — keeps the quality record complete and gives the QC manager a defensible audit trail. WPS Welding tracks procedure documentation, inspection hold points, and PQR records together so nothing in the sequence gets treated as a background step.

Rule library based on AWS D1.1:2025; verify against your governing edition.