Oxy-Fuel Cutting: Process, Cut Quality and Applications
March 16, 2022
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Oxy-Fuel Cutting: Process, Cut Quality and Applications

Oxy-fuel cutting has been a cornerstone of industrial steel cutting since ESAB's involvement in the process dating back to 1907. It remains the most cost-effective process for cutting carbon steel — capable of handling thicknesses from 0.5 mm to 250 mm, suitable for both manual and fully automated CNC applications, and uniquely able to be combined with plasma or waterjet cutting on the same part. This guide covers how the process works step by step, what determines cut quality, and the range of applications where oxy-fuel cutting is the right choice.

For a more detailed look at fuel gas selection, nozzle design, and the variables that affect cut quality, see our companion articles on oxy-fuel cutting technique, oxy-fuel torch tip and nozzle design, and preheat design for different fuel gases.

What Is Oxy-Fuel Cutting?

Oxy-fuel cutting is a chemical process — not a melting process. A mixture of fuel gas and oxygen produces a preheat flame that raises the steel to its ignition temperature, then a high-pressure jet of pure cutting oxygen is directed onto the heated area. The steel oxidises rapidly, forming molten iron oxide (slag) that is blown away by the oxygen jet. The reaction is exothermic — it releases more heat than it consumes — which allows it to sustain itself through the full thickness of the plate as the torch moves forward.

This process is sometimes described as "rapid, controlled rusting" — an accurate description of the chemistry. It is fundamentally different from plasma or laser cutting, which melt the material. For a comparison with plasma cutting, see our article on what is plasma arc cutting.

Common fuel gases include acetylene, propane, natural gas, MAPP, and propylene. The choice of fuel gas affects flame temperature, piercing speed, and edge quality. Acetylene produces the highest flame temperature and fastest piercing; natural gas is lowest cost but slowest. For full fuel gas comparison, see our oxy-fuel cutting technique guide.

Which Metals Can Be Oxy-Fuel Cut?

Oxy-fuel cutting works only on metals whose oxides have a lower melting point than the base metal itself. When this condition is met, the oxidation reaction continues unimpeded through the full thickness of the plate. When it is not met — as with stainless steel, aluminium, or copper — the oxide layer either forms a protective crust that halts further oxidation, or the base metal melts before the oxide can form properly.

In practice, oxy-fuel cutting is limited to:

  • Low-carbon and mild steel
  • Some low-alloy steels

As carbon content increases above approximately 0.3%, cut quality decreases and HAZ hardening becomes an increasing concern. Very high carbon steels and many alloy steels require preheat before cutting and controlled cooling after cutting to prevent edge cracking. For stainless steel, aluminium, and non-ferrous metals, plasma cutting is the appropriate process.

The Oxy-Fuel Cutting Process: Three Steps

Step 1: Preheat

The preheat flame raises the steel surface to its kindling (ignition) temperature — approximately 980°C (1,800°F) for mild steel. Inside the torch, fuel gas is mixed with oxygen to create the preheat mixture, directed through multiple small holes arranged around the central cutting oxygen bore in the nozzle tip. Adjusting the fuel-to-oxygen ratio during preheating concentrates the maximum heat on the smallest possible area, minimising preheat time and the extent of the heat-affected zone at the cut start.

Preheat time depends on plate thickness, surface condition, and ambient temperature. Rusty, scaled, or coated surfaces and cold ambient conditions all increase preheat time. For cutting these conditions, heavy preheat nozzles are available. Always ensure the oxygen purity is no less than 99.5% — lower purity significantly reduces cutting speed and cut quality. For nozzle preheat design guidance, see our article on oxy-fuel torch tip preheat design.

Step 2: Piercing

Once the steel has reached kindling temperature, the cutting oxygen valve is opened to begin piercing. The pure oxygen jet — delivered through the central bore of the nozzle — contacts the preheated steel and initiates rapid oxidation. The exothermic reaction is self-sustaining once started: it generates more heat than it consumes, allowing the jet to work progressively deeper into the plate without continuous external heat input.

Molten slag is blown downward and out of the growing pierce hole by the oxygen jet. Piercing time ranges from a fraction of a second on thin plate to several seconds on heavy plate. A correctly executed pierce produces a small, controlled pool of slag on the plate surface. An incorrect approach — insufficient preheat, cutting oxygen opened too quickly, or torch held too close — produces a geyser of molten slag that can damage the nozzle and create an irregular start to the subsequent cut.

Step 3: Cutting

Once the oxygen jet has pierced through the full plate thickness, the torch moves forward at a controlled, constant speed to produce the continuous cut. Preheat flames maintain the steel just ahead of the cut at or above ignition temperature, ensuring the exothermic oxidation reaction continues uninterrupted. Molten slag is blown continuously out of the bottom of the cut kerf by the oxygen jet.

The principal variables controlling cut quality during this phase are:

  • Travel speed — too fast produces angled drag lines and incomplete penetration; too slow produces an excessively wide kerf, heavy HAZ, and a rounded top edge
  • Cutting oxygen pressure — must match nozzle bore size and plate thickness; too high causes turbulence and a rough cut face; too low fails to clear slag and can extinguish the reaction
  • Preheat flame adjustment — must maintain ignition temperature just ahead of the cut without overheating the top edge
  • Cutting height — the stand-off distance from nozzle tip to plate surface affects gas coverage, kerf width, and cut squareness
  • Plate temperature — pre-warmed plate (from previous cuts or preheat) requires reduced preheat input; cold plate requires more

For CNC oxy-fuel cutting, these variables are controlled automatically by the machine controller and cutting software. For manual cutting, operator skill and consistent technique are the determining factors.

What Makes a High-Quality Oxy-Fuel Cut?

A quality oxy-fuel cut has five defining characteristics. Each can be used to diagnose what is going wrong when cut quality is poor:

  • Square top corner with minimum radius — a rounded or melted top edge indicates excessive preheat flame or too-slow travel speed
  • Flat cut face, no undercut — concavity or undercutting on the cut face indicates incorrect cutting height or oxygen pressure
  • Cut face square with the top surface — a bevelled cut face indicates torch angle error or nozzle misalignment
  • Clean surface with near-vertical drag lines — drag lines angled sharply backwards indicate travel speed too fast or cutting oxygen pressure too low. Near-vertical drag lines indicate correct speed and pressure balance
  • Minimal bottom slag, easily removed by scraping — heavy, adherent slag that requires grinding indicates travel speed too fast, oxygen pressure too low, or insufficient preheat

Applications of Oxy-Fuel Cutting

Oxy-fuel cutting is one of the most versatile cutting processes for steel, used across a wide range of industrial applications:

  • Plate edge preparation for welding — one of the most common applications; oxy-fuel cutting produces groove and bevel profiles on plate edges ready for butt welding. For weld joint design guidance, see our welding consumables selection guide
  • Automated precision contour cutting — on CNC cutting machines, oxy-fuel cutting produces accurate shapes and profiles on steel plate at low cost per metre of cut. It can be combined with plasma cutting on the same CNC machine — oxy-fuel for thicker sections, plasma for thinner or for improved edge quality. For nesting and plate optimisation, see COLUMBUS CAD/CAM nesting software
  • Field repair and construction site cutting — oxy-fuel's minimal equipment requirements (torch, gas cylinders, regulator) make it the preferred process for remote or site cutting where electrical power is unavailable or impractical. See our range of ESAB oxy-fuel cutting equipment
  • Manual rough severing and scrap cutting — fast, economical removal of scrap steel and rough separation of large sections
  • Flame hardening — controlled application of oxy-fuel heat to harden steel surfaces selectively
  • Metallising — oxy-fuel flame used as the heat source for thermal spray coating applications

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