What Is Oxy-Fuel Cutting?

Oxy-fuel cutting is one of the oldest and most cost-effective thermal cutting processes for steel — capable of cutting thicknesses from 0.5 mm to 250 mm with minimal capital investment and no requirement for electrical power at the cutting point. Understanding how the process works, what materials it can and cannot cut, how different fuel gases compare, and what controls cut quality is essential for anyone operating or specifying oxy-fuel cutting equipment.

What Is Oxy-Fuel Cutting?

Oxy-fuel cutting — also called flame cutting, torch cutting, or burning — is a thermal cutting process that uses a combination of pure oxygen and a fuel gas to cut steel. The process is sometimes described as "rapid, controlled rusting" because it works by rapidly oxidising the steel rather than simply melting it. A preheat flame raises the steel surface to its ignition temperature, then a high-pressure jet of pure oxygen is directed onto the heated area. The steel oxidises rapidly and the molten oxide (slag) is blown away by the oxygen jet, creating a continuous cut as the torch moves forward.

This is fundamentally different from plasma or laser cutting, which melt the material. Oxy-fuel cutting is a chemical process — the oxidation reaction is exothermic, meaning it releases more heat than it consumes, which is what allows it to sustain itself through thick steel plate. For a comparison of oxy-fuel with plasma cutting, see our article on what is plasma arc cutting.

What Materials Can Be Oxy-Fuel Cut?

Oxy-fuel cutting works only on metals whose oxides have a lower melting point than the base metal itself. This limits the process to:

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

On metals where the oxide has a higher melting point than the base metal — including stainless steel, aluminium, copper, and cast iron — the oxide layer either forms a protective crust that prevents further oxidation, or the material melts and flows away before a clean cut can be established. For cutting these materials, plasma cutting or laser cutting is required.

Carbon content also affects oxy-fuel cuttability. As carbon content increases above approximately 0.3%, the cut quality decreases and the risk of hardening in the cut edge and HAZ increases. Very high carbon steels and many alloy steels are difficult or impractical to oxy-fuel cut without pre- and post-heat treatment.

Oxygen: Purity and Nozzle Design

The purity of the cutting oxygen directly affects cutting speed and cut quality. Oxygen purity must be no less than 99.5% — even small reductions in purity significantly reduce cutting speed and increase oxygen consumption. At 99% purity, cutting speed can reduce by 10–15% and oxygen consumption increases substantially.

Nozzle design is equally important. The cutting oxygen jet must remain clean and coherent as it exits the nozzle — any turbulence, contamination, or air entrainment disrupts the oxidation reaction and produces a rough, irregular cut face. The central bore must be smooth, undamaged, and correctly sized for the plate thickness and cutting speed. For nozzle selection guidance, see our article on oxy-fuel torch tip and nozzle design.

Fuel Gas Comparison

The choice of fuel gas affects preheat flame temperature, heat distribution, piercing time, cutting speed, and edge quality. The most common options are:

For detailed guidance on preheat flame design for different fuel gases, see our article on oxy-fuel torch tip preheat design.

The Oxy-Fuel Cutting Process: Three Steps

Step 1: Preheat

The preheat flame raises the steel surface or edge to its kindling (ignition) temperature — approximately 980°C (1,800°F) for mild steel. Inside the torch, fuel gas is blended with oxygen to create the preheat mixture, which is directed through multiple small holes arranged around the central cutting oxygen bore in the nozzle tip. The fuel-to-oxygen ratio can be adjusted during preheating to produce the highest possible temperature in the smallest possible flame area, concentrating heat on the cut start point and minimising the time to reach ignition temperature.

Preheat time varies with plate thickness, surface condition, and ambient temperature. Cold plate, surface rust, scale, or coatings all increase preheat time. Heavy preheat nozzles are available for these conditions. See our nozzle selection guide for more on preheat capacity selection.

Step 2: Piercing

Once the steel has reached kindling temperature, the cutting oxygen valve is opened to begin piercing. The cutting oxygen jet — delivered through the central bore of the nozzle — contacts the preheated steel and initiates rapid oxidation. This exothermic reaction generates sufficient additional heat to sustain itself, without the need for continued preheat input to drive the reaction forward.

The molten oxide (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 thick plate. A correctly executed pierce produces a small, controlled pool of slag on the plate surface. An incorrect approach — torch held too low, insufficient preheat, or cutting oxygen opened too quickly — produces a geyser of molten slag that can damage the nozzle and create an irregular start for the subsequent cut.

Step 3: Cutting

Once the oxygen jet has penetrated through the full thickness of the plate, the torch moves forward at a constant, controlled speed to produce the continuous cut. The cutting tip and the oxygen pressure set at the regulator control the depth of cut — matched to the plate thickness being cut.

As the torch moves forward, the preheat flames maintain the steel just ahead of the cut at or above kindling temperature, ensuring the exothermic oxidation reaction continues uninterrupted. The slag is blown continuously out of the bottom of the cut kerf by the oxygen jet.

Cut quality is determined by the interaction of several variables:

• Travel speed — too fast produces a lagging drag line and incomplete cut; too slow produces excessive HAZ and a rounded top edge
• Cutting oxygen pressure — must be matched to nozzle size and plate thickness; too high causes turbulence and a rough cut face; too low fails to blow slag clear
• Preheat flame adjustment — must be sufficient to maintain ignition temperature without overheating the cut edge
• Cutting height — the distance from nozzle tip to plate surface affects both gas coverage and cut quality
• Plate temperature — cold plate requires more preheat; plate already hot from previous cuts may require reduced preheat

For CNC oxy-fuel cutting applications, where these variables are controlled automatically, see our ESAB cutting automation range and COLUMBUS nesting software for plate optimisation.

Related Articles

• Oxy-Fuel Torch Tip/Nozzle Design and Selection
• Oxy-Fuel Torch Tip/Nozzle Preheat Design
• What Is Plasma Arc Cutting?
• Nozzle Selection Guide
• ESAB Cutting Automation Equipment