CWI Thermal Cutting and Gouging Study Guide (Part A)

Why cutting is on the exam

Most welds start with a cut edge or a gouged groove. The quality of that cut affects fit-up, fusion and cracking, and the inspector checks prepared edges before welding begins.

Cutting is a small share of Part A. The questions tend to ask how each process works and which metals it can cut. They also ask what a bad cut looks like and what metallurgical damage cutting can cause.

Process	How it removes metal	Metals it suits
Oxyfuel cutting (OFC)	Burns (oxidizes) the steel and blows the oxide out	Carbon and low-alloy steels
Plasma arc cutting (PAC)	Melts the metal with a plasma jet and blows it out	Any electrically conductive metal
Air carbon arc cutting (CAC-A)	Melts the metal with a carbon arc and blows it out with compressed air	Mainly used for gouging and weld removal

Oxyfuel cutting

Oxyfuel cutting works by burning, or rapidly oxidizing, the steel.

1. Preheat flames heat the steel to its ignition (kindling) temperature, roughly 1600 degrees F. The flames burn oxygen with a fuel gas such as acetylene.
2. A jet of high-purity cutting oxygen strikes the hot steel, which oxidizes rapidly.
3. The oxygen jet blows the molten oxide out of the kerf.
4. The oxidation reaction itself gives off heat, which helps keep the cut going.

Because the process depends on that oxidation reaction, it works well on carbon and low-alloy steels. It works poorly on metals whose oxides behave differently.

Stainless steel is the main example. Its chromium forms an oxide that melts far above the base metal and does not flow away, so it blocks further cutting. Aluminum and copper are also poor candidates. For those metals, plasma arc cutting is the usual choice, or special methods such as flux injection or powder cutting.

Reading an oxyfuel cut

Cut quality tells you how the cut was made. Typical signs:

• Travel too fast: heavy, sharply curved drag lines, because the bottom of the cut lags behind the top. The cut may also be incomplete, with slag stuck to the bottom edge.
• Travel too slow: melted and rounded top edges, gouging and excessive slag.
• Preheat flame too large: a rounded, melted top edge.
• Tip too far from the plate: a rough, wide kerf.
• Dirty or damaged tip: irregular, wavy cut faces.

AWS C4.1 provides a reference for describing oxygen-cut surfaces, including a plastic replica gauge for comparing roughness.

Plasma arc cutting

Plasma arc cutting (PAC) forces an electric arc through a small constricting orifice. The gas passing through becomes a very hot, fast-moving plasma jet that melts the metal and blows it out of the kerf. Because it melts rather than burns the metal, PAC cuts any electrically conductive metal, including stainless steel and aluminum.

Points to remember:

• It cuts faster than oxyfuel on thinner sections.
• The kerf is narrow and the heat-affected zone is relatively small.
• Some edge bevel and dross depend on settings and speed.
• It produces intense arc radiation, noise and fumes, so eye protection and ventilation matter.

Air carbon arc cutting and gouging

Air carbon arc cutting (CAC-A) melts the metal with an arc between a carbon-graphite electrode and the work. A jet of compressed air blows the molten metal away. It is the usual tool for:

• Back gouging the root of double-sided groove welds to sound metal.
• Removing defective welds for repair.
• Preparing grooves and removing temporary attachments.

For carbon steel, direct current electrode positive (DCEP) with copper-coated electrodes is the normal setup. It gives a stable arc and a clean gouge. Other polarities and AC are used only with special electrodes or metals.

Carbon pickup

If the technique is poor or the air pressure too low, the gouged surface can pick up carbon from the electrode. Carbon-rich layers and leftover deposits can make the weld metal and heat-affected zone (HAZ) hard and crack-sensitive. That is why codes often require gouged surfaces to be ground to bright, clean metal before welding. Inspecting a back-gouged groove for shape, depth and cleanliness is a routine inspector task.

Metallurgical effects of thermal cutting

Any thermal cutting process heats a narrow band of metal at the cut edge. The surrounding cold metal then cools it quickly. On hardenable steels, this can form a hard, brittle layer, sometimes with fine cracks. Ways to reduce it:

• Preheat the steel before cutting, which slows cooling.
• Remove the affected layer by grinding or machining.
• Avoid quenching cut edges with water.

The same reasoning applies to welding: faster cooling means harder structures in hardenable steels. The metallurgy and heat control guide explains why.

What the inspector checks on cut edges

Before fit-up, the inspector looks at prepared edges for:

• Correct bevel angle, root face and dimensions.
• Roughness and gouges within the governing code's limits.
• Slag, scale and other material that must be removed.
• Laminations or other base metal discontinuities exposed by the cut.
• Notches or sharp reentrant corners that could start cracks.

Many codes, including AWS D1.1, set limits on cut-surface roughness, notches and mill laminations found at cut edges. They also say how each must be repaired. Checks before, during and after welding are covered in the weld examination guide.

Safety during cutting and gouging

Cutting throws far more sparks and molten metal than most welding, so fire precautions apply in full. Combustibles within 35 ft should be moved or protected, and a fire watch used when they cannot be. Other hazards to keep in mind:

• Fumes: cutting coated, painted or galvanized steel releases fumes that need local exhaust or respiratory protection.
• Noise: air carbon arc gouging is very loud, so hearing protection is normally needed.
• Radiation: plasma and carbon arcs need suitable filter shades, just like welding arcs.
• Gas equipment: oxyfuel hoses, regulators and cylinders must be in good condition, and acetylene must not be used above 15 psig.

More on ANSI Z49.1 safety rules is in the duties, ethics and safety guide.

Mark J. Curry's tip: when a question describes a cut surface, picture the torch moving. Drag lines curving backward almost always mean the torch was moving too fast.

Work through the free cutting practice questions. To see which Part A topic to study first, take the free CWI diagnostic test.

Sources

The Cutting questions cite these documents, by document and, where we can confirm it, chapter or section. We never reproduce them.

1. AWS D1.1 (1 question)

Found a mistake? Use "Report a problem" under any question, and Mark J. Curry checks it.

Questions? support@cwiready.com