Perform Plasma Arc Cutting and Gouging

Performing Plasma Cutting and Gouging

Introduction to the Process

Plasma cutting is a thermal cutting process that uses a constricted electric arc to ionize a gas, creating a plasma capable of melting and blowing away metal. This process is used on all conductive metals, including carbon steel, stainless steel, aluminum, copper, and their alloys. For the Red Seal exam, you must understand the physics of the process, adjustment parameters, equipment, operating techniques, applicable safety standards, and gouging applications.

Plasma gouging is a variant of cutting where the arc is angled to remove a layer of metal from the surface, creating a groove or chamfer, without necessarily penetrating through the workpiece. This technique is used to prepare weld joints, eliminate weld defects, or remove existing welds.

Physical Principles of Plasma

Plasma Arc Formation

The plasma arc is created when a gas (compressed air, nitrogen, argon, hydrogen, or mixtures) passes through a constricted nozzle orifice. The electric arc established between the electrode (negative) and the workpiece (positive) heats the gas to extreme temperatures of 15,000 °C to 30,000 °C. At these temperatures, the gas dissociates and ionizes, forming a conductive plasma.

The principle of constriction is fundamental: by forcing the arc through a small orifice, the current density and gas velocity are increased. The resulting plasma jet is extremely concentrated, allowing for fast, narrow cuts with minimal heat-affected zone (HAZ).

Cutting Gas and Shielding Gas

Two types of gases are used in plasma torches:

Gas Type	Role	Common Gases	Applications
Plasma gas	Forms the plasma, melts and blows away metal	Compressed air, nitrogen (N₂), argon (Ar), hydrogen (H₂)	Cutting all metals
Shielding gas	Protects the nozzle and cools the workpiece, improves cut quality	Oxygen (O₂), air, CO₂, Ar/H₂ mixtures	Improves cut quality on stainless steel and aluminum

Gas selection table by metal:

Metal	Plasma Gas	Shielding Gas	Typical Thickness
Carbon steel	Compressed air or O₂	Air or O₂	3 mm to 50 mm
Stainless steel	N₂ or Ar/H₂	N₂ or air	3 mm to 40 mm
Aluminum	N₂ or Ar/H₂	N₂ or air	3 mm to 40 mm
Copper and alloys	N₂	N₂	2 mm to 25 mm

Polarity and Electrical Connections

The electrode is connected to the negative terminal (straight polarity, DCEN) in most plasma torches. The workpiece is connected to the positive terminal. This polarity ensures long electrode life and a stable arc. Some high-current torches use reversed polarity for special applications, but this is rare.

Operating Modes: Pilot and Cutting

Pilot arc: a low-intensity arc is established between the electrode and the nozzle, creating an ionized path. This pilot arc is used to initiate the cutting arc without contact with the workpiece. It is maintained until the torch approaches the workpiece, at which point the main arc is established.

Cutting arc: the main arc transfers from the electrode to the workpiece. The current is much higher (20 A to 800 A depending on the equipment). The pilot arc automatically extinguishes during cutting.

Plasma Cutting Equipment

System Components

A plasma cutting system includes:

22.Power source (rectifier): provides direct current (DC) with constant voltage or drooping characteristics. The open-circuit voltage (OCV) ranges from 200 V to 400 V depending on the model.

23.Plasma torch: contains the electrode, nozzle, gas diffuser, shield, and handle.

24.Gas control unit: regulates plasma gas and shielding gas flow rates.

25.Air compressor or gas cylinders: supply gas under pressure.

26.Work cable (return): connected to the workpiece, must be in good condition and of adequate size.

27.Cooling system: for high-power torches, water cooling (closed loop with antifreeze).

Torch Consumable Components

Consumables are parts that wear out and must be replaced regularly:

Component	Function	Signs of Wear
Electrode	Emits the arc, contains a hafnium or tungsten insert	Deep crater, erosion of the insert
Nozzle	Constricts the arc, directs the gas	Oval or enlarged orifice, contamination
Diffuser	Distributes gas evenly	Blockage, deformation
Shield	Protects the nozzle, guides shielding gas	Chips, deformation
Retaining ring	Holds consumables in place	Wear, damaged threads

Preventive Maintenance

Check the condition of consumables before each use.

Clean threads and contact surfaces with a non-metallic brush.

Replace the electrode when the hafnium insert is pitted deeper than 1.5 mm.

Replace the nozzle if the orifice is deformed or enlarged by more than 10%.

Purge gas lines to remove moisture and contaminants.

Check the work cable and connections to prevent current losses.

Cutting Parameters

Cutting Current

Current is the primary parameter that determines the maximum cutting thickness. The general rule is approximately 4 A per millimetre of thickness for carbon steel. For example, to cut a 12 mm plate, approximately 48 A is required. However, this rule varies depending on the type of metal and the desired cut quality.

Practical formula: I = 40 × √(t) where I is the current in amperes and t is the thickness in millimetres. This formula provides an approximation for carbon steel.

Cutting Speed

Cutting speed is determined by metal thickness, current, and desired cut quality. A speed that is too slow produces a wide cut with excessive dross on the underside. A speed that is too fast produces an angled cut with irregular striations and lack of penetration.

Signs of incorrect speed:

Problem	Probable Cause	Correction
Excessive dross on underside	Speed too slow	Increase speed
Irregular striations, angled cut	Speed too fast	Reduce speed
Unstable arc, blowout	Torch-to-workpiece distance too great	Reduce distance
Double arcing (arc between nozzle and workpiece)	Worn nozzle or distance too small	Replace nozzle, adjust distance

Torch-to-Workpiece Distance

The ideal distance between the nozzle (or shield) and the workpiece is generally 1.5 mm to 6 mm depending on the torch type and current. Modern torches with shields can be dragged directly on the workpiece (drag cutting mode). For torches without a shield, the distance must be maintained constant at approximately 3 mm.

Cutting Angle

The torch angle relative to the workpiece should be 90° for a perpendicular cut. For angled cuts (bevels), the angle is set according to the weld joint specifications. An incorrect angle produces an uneven bevel and poor cut quality.

Gas Pressure and Flow Rate

The recommended gas pressure is specified by the torch manufacturer. Pressure that is too low produces an unstable arc and poor cut quality. Pressure that is too high blows away molten metal and creates excessive spatter. The flow rate is typically 100 to 200 L/min for plasma gas depending on power.

Cutting Techniques

Manual Cutting

Manual cutting requires a steady, consistent technique:

55.Position the torch perpendicular to the workpiece.

56.Initiate the pilot arc approximately 10 mm from the workpiece.

57.Bring the torch down to the cutting distance (1.5 to 6 mm).

58.Move the torch at a constant speed along the cutting line.

59.Maintain constant distance and angle throughout the cut.

60.Release the trigger at the end of the cut to extinguish the arc.

Guided Cutting

For straight, precise cuts, use a straightedge guide or a motorized carriage. The guide maintains constant distance and angle, improving cut quality and reducing operator fatigue.

Hole Cutting (Plasma Piercing)

To pierce a hole in a plate, angle the torch at approximately 30° from vertical to initiate the arc, then gradually straighten the torch to vertical once the metal is pierced. This technique prevents spatter from damaging the nozzle.

Cutting Grating or Expanded Metal

Reduce the current by approximately 20 to 30% and increase cutting speed to avoid excessively melting the thin sections.

Underwater Cutting

Underwater cutting is used to reduce noise, fumes, and UV radiation. The torch is equipped with a special shield and the workpiece is submerged to a depth of 50 to 75 mm. Cut quality is slightly lower, but the working environment is improved.

Plasma Gouging

Gouging Principle

Plasma gouging involves removing metal from the surface by angling the torch at 30° to 45° relative to the workpiece. The plasma arc melts the metal and the gas blows it out of the groove. Groove depth is controlled by travel speed, torch angle, and current.

Gouging Applications

Weld joint preparation (bevels, U-grooves, or V-grooves).

Elimination of weld defects (porosity, cracks, lack of fusion).

Removal of existing welds for repair.

Surface descaling (removal of metal layers).

Creating grooves for root welding.

Gouging Parameters

Parameter	Typical Value	Effect
Current	60 to 80% of maximum cutting current	Determines groove depth and width
Torch angle	30° to 45°	The smaller the angle, the deeper the groove
Travel speed	300 to 600 mm/min	The slower the speed, the deeper the groove
Torch-to-workpiece distance	3 to 6 mm	Must be constant for a uniform groove

Gouging Technique

81.Set the current to approximately 70% of the torch's maximum current.

82.Angle the torch at 35° relative to the surface.

83.Initiate the pilot arc and bring the torch toward the workpiece.

84.Move the torch at a constant speed while maintaining angle and distance.

85.Observe groove formation: it should be smooth and uniform.

86.For deep grooves, make several successive passes.

Gouging Quality Control

A good-quality gouging groove exhibits:

A rounded bottom, without sharp edges.

Smooth side walls, without craters or overhangs.

Constant width along the entire length.

Absence of residual carbon (if compressed air is used) or oxides.

Standards and Safety

Applicable Canadian Standards

Plasma cutting is a welding and thermal cutting process. The following standards apply:

CSA W117.2 — "Safety in Welding, Cutting, and Allied Processes": this standard is the primary reference for operator safety. It covers personal protective equipment, ventilation, fire prevention, and electrical hazards.

Canadian Electrical Code, Part I (C22.1): applies to the electrical installation of welding equipment. Relevant rules concern wiring, grounding, and circuit protection.

CSA B149.1 — "Natural Gas and Propane Code": applies if combustible gases are used for preheating or drying, although plasma uses inert gases or air.

Hazardous Products Regulations (WHMIS): applies to the handling of compressed gases and cutting fumes.

Specific Plasma Hazards

Hazard	Source	Protection
UV and infrared radiation	Intense plasma arc	Welding helmet with shade 8 to 12 lens, covering clothing
Fumes and gases	Molten metal, plasma gas	Local ventilation, source extraction, respirator if necessary
Noise	Plasma jet, compressor	Hearing protection (plugs or muffs)
Electric shock	High open-circuit voltage (200-400 V)	Insulating gloves, insulating mat, equipment in good condition
Burns	Molten metal spatter, hot torch	Leather clothing, gloves, face shield
Ultrasonic noise	Some high-frequency torches	Hearing protection

Essential Safety Rules

Never touch the workpiece or torch while the equipment is energized.

Check the condition of cables and connections before each use.

Use a helmet with the appropriate lens shade (minimum shade 8 for plasma).

Wear dry welding gloves in good condition.

Ensure adequate ventilation to remove fumes (ozone, nitrogen oxides, metals).

Keep flammable materials away from the work area (minimum 10 m or protect with screens).

Never look at the arc without protection, even briefly.

Follow manufacturer's instructions for consumable maintenance.

Ventilation and Fumes

Plasma cutting fumes contain metal oxides (iron, chromium, nickel, aluminum) and gases (ozone, nitrogen oxides). CSA W117.2 requires local exhaust ventilation at the source or sufficient general ventilation. For metals containing chromium (stainless steel), additional measures are required (mandatory source extraction).

Practical Calculations and Adjustments

Cutting Current Calculation

For carbon steel, use the empirical formula:

I = 40 × √(t)

Where:

I = current in amperes

t = metal thickness in millimetres

Example: To cut a 16 mm plate:

I = 40 × √(16) = 40 × 4 = 160 A

Cutting Speed Calculation

The approximate cutting speed for carbon steel can be estimated by:

V = 5000 / I (in m/min)

Where I is the current in amperes.

Example: For a current of 160 A:

V = 5000 / 160 = 31.25 m/min (too fast for manual use, indicating that a lower current would be more appropriate for manual cutting).

In practice, manual speed is 300 to 800 mm/min depending on thickness and current.

Gas Consumption

Plasma gas consumption is approximately:

Q = 0.1 × I (in L/min)

Where I is the current in amperes.

Example: For 160 A, Q = 0.1 × 160 = 16 L/min (this is a low estimate; actual values range from 50 to 200 L/min depending on the torch).

Maximum Cutting Thickness

The maximum cutting thickness for a given torch is specified by the manufacturer. In general, the maximum cutting capacity is approximately 25 mm per 100 A for carbon steel with a compressed air torch. High-definition (HD) torches can cut up to 80 mm with 400 A.

Cut Quality Control

Quality Criteria

A good-quality plasma cut exhibits:

A smooth, perpendicular cut face (cut angle < 3°).

Little or no dross on the underside.

Minimal and constant kerf width.

A narrow heat-affected zone (HAZ).

No notches or craters on the cut face.

Common Defects and Corrections

Defect	Cause	Correction
Adherent dross	Speed too slow, current too high	Increase speed, reduce current
Easily removable dross	Speed too fast, insufficient gas	Reduce speed, increase gas flow
Angled cut face	Torch tilted, speed too fast	Keep torch perpendicular, reduce speed
Deep striations	Speed too fast, current too low	Reduce speed, increase current
Notches on the face	Worn nozzle, distance too great	Replace nozzle, reduce distance
Carbon deposit	Contaminated compressed air (oil, moisture)	Use a dehydrator filter, purge lines
Unstable arc	Worn electrode, incorrect gas pressure	Replace electrode, adjust pressure

Consumable Inspection

Inspect consumables after each use:

Electrode: the hafnium insert must be clean and without a deep crater. A crater deeper than 1.5 mm indicates excessive wear.

Nozzle: the orifice must be round and clean. An oval or enlarged orifice indicates wear and produces a distorted arc.

Shield: check for cracks and deformation.

Diffuser: verify that the gas passage orifices are not obstructed.

Industrial Applications

User Industries

Plasma cutting is used in:

Metal fabrication: cutting sheet metal, plates, tubes, and profiles.

Shipbuilding: cutting heavy plates, gouging defects.

Oil and gas industry: joint preparation, pipe cutting.

Repair and maintenance: weld removal, part repair.

Demolition: cutting metal structures.

Plasma Advantages and Limitations

Advantages	Limitations
Cuts all conductive metals	High equipment cost
High cutting speed	Consumables require regular replacement
Narrow HAZ compared to oxy-fuel	Significant noise and fumes
No preheating required	Dangerous open-circuit voltage
Underwater cutting possible	Cut quality inferior to laser for thin thicknesses
Gouging possible without equipment change	Requires a compressed air source or gas cylinders

Comparison with Oxy-Fuel Cutting

Criterion	Plasma	Oxy-Fuel
Cuttable metals	All conductive metals	Carbon steel only
Maximum thickness	Up to 80 mm (standard), more with special equipment	Up to 300 mm and more
Cutting speed (≤ 25 mm)	Faster	Slower
Cut quality (≤ 25 mm)	Superior	Inferior
Operating cost	High (consumables, electricity)	Moderate (fuel gases)
HAZ	Narrow	Wide
Surface preparation	Not demanding	Clean surface required (paint, rust)

Pitfalls to Avoid

165.Confusing polarity: the electrode is negative (DCEN) in most plasma torches. Reversed polarity quickly destroys the electrode.

166.Neglecting consumable maintenance: worn consumables produce poor cut quality and damage the torch.

167.Using incorrect gas pressure: pressure that is too low or too high degrades cut quality and consumable life.

168.Ignoring compressed air quality: moisture and oil in compressed air contaminate the plasma and reduce consumable life.

169.Forgetting grounding: poor current return (work cable) causes stray arcs and electric shock hazards.

170.Looking at the arc without protection: plasma UV radiation is intense and can cause severe eye burns (flash).

171.Confusing gouging and cutting: gouging uses a 30° to 45° angle and does not penetrate through the workpiece.

172.Neglecting ventilation: plasma fumes are toxic, especially on stainless steel (hexavalent chromium).

173.Using excessive current for the thickness: this produces a wide cut, excessive dross, and rapid consumable wear.

174.Not checking the work cable: a damaged or poorly connected work cable causes arc instability and shock hazards.

Summary

Plasma cutting uses a constricted electric arc to ionize a gas and create a plasma jet at very high temperatures (15,000 to 30,000 °C).

The process cuts all conductive metals: carbon steel, stainless steel, aluminum, copper, and alloys.

The main parameters are: current (I), cutting speed, torch-to-workpiece distance (1.5 to 6 mm), torch angle (90° for straight cuts), and gas pressure.

The approximate cutting current for carbon steel is I = 40 × √(thickness in mm).

Consumables (electrode, nozzle, diffuser, shield) must be inspected and replaced regularly to maintain cut quality.

Plasma gouging uses a 30° to 45° angle and is used to prepare joints, eliminate defects, and remove welds.

CSA W117.2 is the primary reference for welding and cutting safety in Canada.

The main hazards are: UV radiation, toxic fumes, noise, electric shock, and burns.

Plasma is faster and more versatile than oxy-fuel cutting for thicknesses ≤ 50 mm, but more expensive in consumables.

Cut quality is controlled by cut angle, presence of dross, kerf width, and the condition of the cut face.

To succeed on the Red Seal exam, master the adjustment parameters, calculation formulas, safety standards, and consumable troubleshooting techniques. Practice with gouging and understanding the differences between plasma gases and shielding gases are frequently assessed topics.