9.6 Thermal Cutting & Gouging: Oxyfuel, Plasma Arc and Air Carbon Arc

Why Cutting Sits in the Welding Body of Knowledge

Thermal cutting determines joint preparation quality, and joint preparation quality determines weld quality. AWS D1.1 governs thermally cut edges as strictly as it governs welds, with limits on surface roughness and notch depth and a requirement to remove damaged material. A welding engineer specifies the cutting process, the edge quality, and the post-cut conditioning.

Oxyfuel Gas Cutting (OFC)

The Chemistry

Oxyfuel cutting is not melting. It is controlled rapid oxidation. A preheat flame raises a starting point on the steel to its kindling temperature — approximately 870 degrees Celsius (1600 degrees Fahrenheit), a bright red heat — and a high-purity oxygen jet is then released. The iron burns:

$3\,\text{Fe} + 2\,\text{O}_2 \rightarrow \text{Fe}_3\text{O}_4 + \text{heat}$

The reaction is strongly exothermic and supplies most of the energy that sustains the cut; the preheat flame mainly maintains the leading edge. The oxygen jet simultaneously blows the molten oxide slag out of the kerf.

The Three Conditions for a Metal to Be Oxyfuel Cuttable

1. The metal must oxidise exothermically in an oxygen jet.
2. The oxide melting point must be below the metal melting point, so the oxide flows out of the kerf instead of forming a refractory crust.
3. The metal's thermal conductivity must be low enough that the cut zone stays above kindling temperature.

Carbon and low-alloy steels satisfy all three. Three important families do not:

Stainless steel can be forced with powder injection or a waster plate, but the accepted engineering answer is to use plasma arc cutting instead.

Fuel Gases

Acetylene has the highest primary-cone flame temperature and therefore the fastest pierce, which is why it dominates manual and repair cutting; the hydrocarbon fuels carry more total heat in the secondary envelope and suit long mechanised cuts.

Cut Quality

Drag lines on the cut face record the relationship between travel speed and oxygen flow. Vertical, evenly spaced drag lines indicate correct speed; heavy trailing drag lines indicate excessive travel speed and risk an incomplete cut at the bottom. Excessive preheat rounds the top edge; insufficient oxygen purity leaves adherent dross. AWS D1.1 sets roughness and notch-depth limits on thermally cut edges and requires that gouges beyond the limit be repaired.

Plasma Arc Cutting (PAC)

Plasma arc cutting uses a constricted transferred arc between a tungsten or hafnium electrode and the workpiece. The orifice constricts the plasma column, raising its temperature and velocity dramatically; the jet melts the metal and blows it out of the kerf by momentum rather than by chemical reaction.

Because the mechanism is melting and ejection rather than oxidation, plasma cuts any electrically conductive metal: stainless steel, aluminum, copper, nickel alloys and carbon steel alike. That is its decisive advantage over oxyfuel.

Two metallurgical cautions apply. Plasma cutting with nitrogen or air can nitride the cut face on stainless and duplex steels, and the nitrogen-enriched layer should be removed before welding. On aluminum, the cut face oxidises immediately and must be cleaned before welding as with any other preparation method.

Air Carbon Arc Gouging (CAC-A)

Air carbon arc gouging strikes an arc between a copper-coated carbon-graphite electrode and the work, melting the metal, then blows the molten pool clear with a compressed air jet issuing from the holder at approximately 80 to 100 psi. It removes metal rather than joining it.

Principal applications:

• Backgouging the root of a double-sided groove weld to sound metal before welding the second side.
• Defect removal — excavating cracks, slag and porosity for repair.
• Removing temporary attachments, lugs and run-off tabs.
• Bevelling heavy sections where a machined edge is not required.

The Carbon Pickup Problem

The graphite electrode leaves a thin carbon-enriched layer on the gouged surface. If a weld is deposited directly onto it, that carbon dissolves into the weld metal, raising local hardenability and producing a hard, crack-prone deposit. Codes therefore require the gouged surface to be ground clean to bright metal before welding. The same requirement arises if the air jet is inadequate or lags the arc: the pool is not cleared, carbon is trapped, and an irregular gouge profile results.

Air carbon arc gouging is also the noisiest and among the highest fume-generating operations in a fabrication shop, and it produces intense arc radiation. Local exhaust ventilation and full protective equipment are mandatory under ANSI Z49.1.

Exam Trap 1: "Oxyfuel cutting melts the steel." It does not. Oxyfuel cutting burns the steel in an oxygen jet, and the exothermic reaction supplies most of the cutting energy. This is exactly why it cannot cut stainless steel, aluminum or copper.

Exam Trap 2: Welding directly over an air carbon arc gouge. The carburized layer must be ground away first. Skipping that step is one of the most common causes of unexplained hardness and cracking in repair welds.

Exam Trap 3: Choosing oxyfuel for stainless because it is cheaper. Chromium oxide is refractory and forms a protective crust. Plasma arc cutting is the correct process for stainless, and no adjustment of oxygen pressure will change the oxide chemistry.