Oxy-fuel cutting (or oxygen cutting) is a thermal process that uses the combustion of a preheated ferrous metal in a stream of pure oxygen to separate it or gouge it. Unlike welding, this process does not primarily melt the metal: it oxidizes it (burns it). This distinction is fundamental for the Red Seal exam.
Oxy-fuel gouging (or arc-air gouging is a different process, but oxy-fuel gouging with a torch uses the same principle as cutting, with a lower angle of attack and a reduced oxygen flow rate to remove a layer of metal from the surface.
The candidate must master the physical principles, adjustment parameters, speed calculations, applicable safety standards, and typical defects associated with this process.
Physical Principles of Oxy-Fuel Cutting
Conditions Required for Cutting
For a metal to be cut by oxy-fuel combustion, three conditions must be met simultaneously:
9.The ignition temperature of the metal must be lower than its melting point. For mild steel, the ignition temperature is approximately 870 °C to 900 °C, while the melting point is approximately 1,500 °C. This difference of over 600 °C allows combustion in the solid state.
10.The oxide formed must have a melting point lower than that of the base metal. Iron oxide (Fe₃O₄) melts at approximately 1,370 °C, which is below the melting point of steel (≈1,500 °C). The liquid oxide is then ejected by the oxygen jet.
11.The oxidation reaction must be exothermic and sufficiently energetic. The combustion of iron in oxygen releases approximately 257 kJ/mol, which helps sustain the reaction once initiated.
Cuttable and Non-Cuttable Metals
Metal
Cuttable by Oxy-Fuel?
Reason
Mild steel (C < 0.30%)
**Yes**
Conditions met
Medium-carbon steel (0.30–0.60%)
Yes with preheat
Possible hardening
High-carbon steel (> 0.60%)
Difficult
Hardening and cracking
Cast iron
No (practically)
Melting point ≈ ignition point; refractory graphite
Stainless steel
No
Chromium oxide (Cr₂O₃) melts at ≈ 2,260 °C, higher than the metal's melting point
Aluminum
No
Al₂O₃ oxide melts at ≈ 2,072 °C; metal melts at 660 °C
Copper and alloys
No
Thermal conductivity too high; refractory oxides
Frequent exam trap: you are often asked why stainless steel cannot be cut with oxy-fuel. The correct answer is that chromium oxide forms a refractory protective layer that prevents further oxidation, not that stainless steel "melts too fast."
Chemical Reactions Involved
Preheating uses the combustion of a fuel gas (acetylene, propane, propylene, MAPP) with oxygen:
The total heat available at the cutting point is the sum of the preheat heat and the exothermic heat from the oxidation of iron. In practice, approximately 70% of the total heat comes from the combustion of the iron itself once cutting has been initiated.
Equipment and Gases
Comparison of Fuel Gases
Property
Acetylene
Propane
Propylene
MAPP
Flame temperature (with O₂)
3,160 °C
2,828 °C
2,896 °C
2,927 °C
Flame speed (m/s)
7.0
3.9
4.6
4.5
Heat of combustion (kJ/m³)
57,000
101,000
87,000
89,000
O₂/gas ratio (neutral combustion)
1.1:1
4.3:1
4.7:1
3.3:1
O₂/gas ratio (cutting)
1.3:1
5.0:1
5.5:1
4.0:1
Density (kg/m³)
1.17
2.01
1.91
1.95
Key points for the exam:
Acetylene produces the hottest and fastest flame; it is the only gas whose secondary combustion (with air oxygen) is hotter than its primary combustion (with pure oxygen).
Acetylene is unstable above 15 psi (103 kPa) absolute pressure; it must never be used above this pressure. Acetylene regulators are calibrated accordingly.
Propane has a slower flame speed; it requires a torch with a larger injector and a longer preheat time.
MAPP (methylacetylene-propadiene) is a substitute for acetylene for cutting, with a slightly lower temperature but better storage safety.
Regulators and Hoses
Oxygen regulators have right-hand threads (clockwise); fuel gas regulators have left-hand threads (counter-clockwise). This distinction is a classic exam question.
Oxygen hoses are green with right-hand fittings; fuel gas hoses are red with left-hand fittings.
Hoses must comply with CSA W117.2 (standard on safety in welding, cutting, and allied processes).
The recommended minimum hose length is 6 meters; the maximum length should not exceed 30 meters without increasing the diameter.
Hoses must be protected against sparks, flames, oil, and grease. Oil and oxygen under pressure form an explosive mixture.
Cutting Torch
The cutting torch consists of:
A handle with two adjustment valves (preheat oxygen and fuel gas)
A cutting lever that controls the cutting oxygen
A head with a central orifice for cutting oxygen and peripheral orifices for the preheat mixture
Interchangeable tips sized according to the thickness of the metal
The choice of tip depends on the thickness of the plate. A tip that is too small produces a slow cut and excessive slag; a tip that is too large wastes gas and produces a rough cut.
Flame Adjustment
Flame Types
Flame Type
O₂/C₂H₂ Ratio
Characteristics
Use
Carburizing (excess gas)
< 1.1:1
Elongated white feather, lower temperature
Preheating steel, welding aluminum
Neutral
1.1:1
Clean white cone, no feather
Cutting mild steel
Oxidizing (excess O₂)
> 1.1:1
Short cone, bluish flame, characteristic noise
Cutting steel at higher speed, but excessive oxidation
For cutting, the preheat flame must be neutral or slightly oxidizing. A carburizing flame deposits carbon on the cut edge, which locally hardens the steel and makes cutting difficult.
Adjustment Procedure
49.Open the preheat oxygen valve half a turn.
50.Open the fuel gas valve and ignite immediately with a friction lighter (spark lighter). Never use matches or a gasoline lighter.
51.Adjust the fuel gas flow until the flame stops producing black smoke.
52.Gradually increase the preheat oxygen until a clean, well-defined white cone appears (neutral flame).
53.For a slightly oxidizing flame, add a little oxygen until the cone shortens by about 10%.
Flashback
A flashback (backfire) is a phenomenon where the flame goes out with a sharp pop, sometimes accompanied by a small explosion in the torch. The main causes are:
Clogged or damaged tip
Gas pressure too low or too high
Incorrect preheat distance
Overheating of the torch head
Incorrect gas mixture
In the event of a flashback, immediately close the fuel gas valve, then the oxygen valve. Allow the torch to cool before investigating the cause.
Cutting Techniques
Work Preparation
65.Clean the surface: remove paint, rust, scale, oil, and grease over a width of at least 25 mm on either side of the cutting line.
66.Mark the cutting line: use chalk or a thermal marker.
67.Position the workpiece: the cutting line must be accessible; provide space beneath the workpiece for slag evacuation.
68.Preheat the edge: bring the edge of the workpiece to ignition temperature (cherry red, ≈ 870 °C) before activating the cutting oxygen lever.
Torch Angle
Steel Thickness
Torch Angle from Vertical
< 6 mm
15° to 20° forward (in the direction of the cut)
6 to 25 mm
90° (perpendicular)
25 to 50 mm
90° with a slight 5° angle backward
> 50 mm
90° with oscillatory movement if necessary
Preheat Distance
The distance between the tip and the workpiece surface should be 3 to 6 mm for thin steel and 6 to 10 mm for thick steel. The white cone of the flame should almost touch the surface without touching it.
Cutting Speed
Cutting speed is critical. A correct speed produces a drag line (striations) of 5 to 10% of the plate thickness.
Too slow: the drag line becomes irregular, the edges melt, slag adheres to the bottom of the cut.
Too fast: the cut does not completely penetrate the workpiece, the drag line is excessive, the oxygen jet deflects.
Practical speed calculation: for mild steel, the approximate speed is:
v = 12 / e
where v is the speed in mm/s and e is the thickness in mm. For a 12 mm plate, v = 12 / 12 = 1 mm/s (60 mm/min). This formula gives an order of magnitude; tip manufacturers provide more precise tables.
Cutting Shapes and Holes
Holes: drill a starting hole (or use a torch with a piercing orifice); preheat a central point, then activate the cutting oxygen while slightly tilting the torch to blow away the molten metal.
Circles: use a circular guide (torch compass) fixed at the center.
Straight lines: use a linear guide (straightedge or carriage).
Bevel cutting: tilt the torch to the required angle (typically 30° for a 60° included weld bevel).
Oxy-Fuel Gouging
Principle
Oxy-fuel gouging consists of removing a layer of metal from the surface using a cutting torch with a 30° to 45° angle of attack and a reduced cutting oxygen flow rate. The metal is preheated, then the cutting oxygen is activated; the jet blows away the oxidized metal, creating a groove (gouge).
Applications
Removal of defective welds (weld repair)
Preparation of chamfers and bevels
Removal of rivets or bolts
Surface descaling before welding
Opening grooves for root welding
Gouging Parameters
Parameter
Recommended Value
Torch angle
30° to 45° from the surface
Preheat distance
3 to 5 mm
Groove depth
2 to 5 mm per pass
Groove width
6 to 12 mm depending on the tip
Cutting oxygen flow rate
50 to 70% of normal cutting flow rate
Comparison with Arc-Air Gouging
Characteristic
Oxy-Fuel Gouging
Arc-Air Gouging
Heat source
Oxy-fuel flame
Electric arc
Metal removed
Oxidized then blown away
Melted then blown away
Applicable metals
Carbon steel only
All conductive metals (including stainless steel and cast iron)
Removal speed
Moderate
Fast
Surface quality
Groove with slag
Cleaner groove
Carburization risk
Possible if carburizing flame
Carbon electrode may leave deposits
Exam trap: oxy-fuel gouging is not suitable for stainless steel because oxidation does not occur properly. For stainless steel, use arc-air gouging or grinding.
Applicable Standards and Codes
CSA W117.2 — Safety in Welding, Cutting, and Allied Processes
This standard is the primary reference for cutting safety operations in Canada. Essential points:
Clause 6.2: Compressed gas cylinders must be stored upright and secured firmly.
Clause 6.4: Oxygen and fuel gas cylinders must be separated by at least 3 meters or by a half-hour fire-rated wall.
Clause 7.3: Hoses must be checked regularly for leaks (with soapy water solution, never with a flame).
Clause 8.1: A fire extinguisher must be available within 15 meters of the work station.
Clause 9.2: Adequate ventilation must be provided to remove cutting fumes (iron oxides, ozone, nitrogen oxides).
Canadian Electrical Code, Part I
Although primarily applicable to electrical installations, this code applies when cutting is performed near electrical equipment. Rule 8-200 requires that work near energized conductors be performed by qualified personnel with appropriate protections.
CSA B149.1 — Natural Gas and Propane Code
This code applies to fixed fuel gas installations. For cutting with mobile cylinders, the requirements of CSA W117.2 take precedence. The candidate must know that propane used in fixed installations must comply with CSA B149.1, particularly for piping and regulators.
Hazardous Products Regulation (WHMIS)
Cutting fumes contain metal oxides classified as hazardous. The candidate must know:
The Safety Data Sheet (SDS) must be available in the workplace.
Iron oxides (Fe₂O₃, Fe₃O₄) have a time-weighted average exposure limit (TWAEV) of 5 mg/m³.
Wearing appropriate respiratory protection is required if ventilation is insufficient.
Calculations and Numerical Parameters
Gas Consumption
The cutting oxygen consumption for a plate of thickness e (mm) and speed v (mm/min) is approximately:
Q = 0.03 × e × v
where Q is in L/min. For a 20 mm plate cut at 300 mm/min:
Q = 0.03 × 20 × 300 = 180 L/min
Recommended Gas Pressures
Thickness (mm)
Tip (number)
Cutting O₂ Pressure (psi)
Preheat O₂ Pressure (psi)
Acetylene Pressure (psi)
3 – 6
0
20 – 25
20
3 – 5
6 – 12
1
25 – 30
20 – 25
3 – 5
12 – 25
2
30 – 40
25 – 30
4 – 6
25 – 50
3
40 – 50
30 – 35
5 – 7
50 – 100
4
50 – 60
35 – 40
6 – 8
Note: Acetylene pressures must never exceed 15 psi (103 kPa) due to the risk of explosive decomposition.
Thermal Expansion
Cutting introduces localized heat that causes differential expansion. For steel, the coefficient of linear expansion is α = 12 × 10⁻⁶ /°C. For a 1-meter workpiece heated by 100 °C, the expansion is:
ΔL = α × L × ΔT = 12 × 10⁻⁶ × 1,000 × 100 = 1.2 mm
This expansion can cause distortion if the workpiece is not properly clamped or if the cutting sequence is poorly planned.
Cutting Defects and Corrections
Defect
Probable Cause
Correction
Melted (rounded) top edge
Speed too slow; excessive preheat
Increase speed; reduce preheat
Bottom edge with adhering slag
Speed too slow; insufficient cutting O₂ pressure
Increase speed; increase O₂ pressure
Excessive drag (deep striations)
Speed too fast; tip too small
Reduce speed; use a larger tip
Non-perpendicular cut
Torch tilted; clogged tip
Check the angle; clean the tip
Impossible to restart the cut
Carburized edge; scale
Grind the edge; use a neutral flame
Slag projected forward
Cutting O₂ pressure too high
Reduce cutting O₂ pressure
Hardened edges
Carburizing flame; rapid cooling
Use a neutral flame; preheat more
Safety — Golden Rules
132.Check for leaks with soapy water, never with a flame.
133.Open the oxygen valve slightly before lighting to purge the hose.
134.Light the fuel gas first, then add oxygen.
135.Close the fuel gas first when shutting down.
136.Never use oil or grease on oxygen fittings.
137.Purge the hoses before changing a cylinder.
138.Use a friction lighter; never matches or a lighter.
139.Wear cutting goggles with tinted lenses (shade 4 or 5) and leather gloves.
140.Ventilate to remove fumes; use local exhaust ventilation if necessary.
141.Inspect hoses before each use; replace any damaged hose.
Pitfalls to Avoid
143.Confusing cutting and melting: oxy-fuel cutting oxidizes the metal, it does not melt it. This distinction is fundamental.
144.Forgetting that stainless steel cannot be cut with oxy-fuel: chromium oxide forms a protective barrier. Use plasma or arc-air gouging.
145.Using acetylene above 15 psi: risk of explosive decomposition. Regulators are calibrated to limit this pressure.
146.Reversing the threads: oxygen right-hand, fuel gas left-hand. An assembly error can cause a dangerous mixture.
147.Neglecting cutting speed: an incorrect speed produces defects; the drag line must be 5 to 10% of the thickness.
148.Confusing carburizing and oxidizing flames: a carburizing flame carburizes the steel and hardens the cut edges.
149.Forgetting the shutdown sequence: close the fuel gas first, then the oxygen. The reverse can cause a flashback.
150.Not knowing exposure limits: iron oxide fumes have a TWAEV of 5 mg/m³.
151.Ignoring CSA W117.2: it is the reference safety standard in Canada for welding and cutting.
152.Using a tip unsuitable for the thickness: a tip that is too small or too large compromises cut quality.
Summary
Oxy-fuel cutting relies on the exothermic oxidation of iron in the solid state, made possible by the difference between the ignition temperature (≈ 870 °C) and the melting point (≈ 1,500 °C) of mild steel.
Three conditions must be met: ignition temperature lower than the melting point, fusible oxide, exothermic reaction.
Mild steel is the only metal commonly cut; stainless steel, aluminum, copper, and cast iron cannot be cut with oxy-fuel.
Acetylene provides the hottest flame (3,160 °C) but is limited to 15 psi; propane and MAPP are alternatives with different characteristics.
The preheat flame must be neutral or slightly oxidizing for cutting.
Cutting speed is optimal when the drag line represents 5 to 10% of the plate thickness.
Oxy-fuel gouging uses a 30° to 45° angle and a reduced oxygen flow rate; it is not suitable for stainless steel.
CSA W117.2 governs the safety of cutting operations in Canada; the Canadian Electrical Code, Part I (Rule 8-200) applies near electrical installations.
Gas consumption and thermal expansion calculations are typical exam questions.
Cutting defects (slag, excessive drag, melted edges) are almost always due to inappropriate speed or pressure.
The candidate must be able to explain the physical principle, adjust a torch, choose parameters according to thickness, and identify defects with their corrections. Mastery of safety standards and exposure limits is also essential to pass the Red Seal exam.
