The oxyfuel gas process cuts low-carbon steel and low-alloy steel reliably, but it cannot cut most other metals. This limitation comes down to chemistry: the process works by burning metal in a stream of pure oxygen, and only certain metals burn the right way. Carbon steel with a carbon content between 0.1% and 0.3% is the sweet spot, and mild steel is the most commonly cut material by far.
How the Process Actually Works
Oxyfuel cutting is not melting. A fuel gas flame (acetylene, propane, or similar) preheats the metal to its ignition temperature, then a jet of high-purity oxygen hits the surface. The iron in the steel reacts with that oxygen and combusts, releasing enough heat to sustain the reaction as the torch moves forward. The molten iron oxide (slag) blows out the bottom of the cut, leaving a clean kerf behind.
This chain reaction only works when four conditions are met. The metal must actually combust in an oxygen stream. Its ignition temperature must be lower than its melting point, so it catches fire before it turns to liquid. The slag produced must flow freely out of the cut. And the metal’s thermal conductivity must be low enough that heat stays concentrated at the cutting zone rather than dissipating into the surrounding material. Low-carbon steel checks all four boxes. Most other metals fail at least one.
Metals That Cut Well
Plain carbon steel and low-alloy steel are the primary materials for oxyfuel cutting. Within that category, mild steel (roughly 0.1% to 0.3% carbon) gives the cleanest, most predictable cuts. The iron burns aggressively in the oxygen jet, the slag runs freely, and the heat-affected zone stays manageable.
Thickness is almost a non-issue with this process. Oxyfuel handles plate steel from fractions of an inch up to extreme thicknesses that no other thermal cutting method can touch. At a handful of industrial facilities worldwide, oxyfuel torches cut through steel slabs up to 10 feet deep. For plates in the 4- to 12-inch range, oxyfuel is the go-to process, and even piercing through 8 or 10 inches of solid steel is routine for experienced operators.
Medium-carbon steels (up to about 0.3% carbon) still cut, though the higher carbon content increases the risk of hardening along the cut edge. Preheating the plate before cutting and slowing down the travel speed help reduce cracking in these grades.
Metals That Cannot Be Cut
Several common metals fail the basic requirements of oxyfuel cutting, each for a different reason.
- Aluminum melts at a lower temperature than its oxide layer. The protective aluminum oxide skin has a melting point far above the base metal, so the aluminum turns to liquid and pools before the oxide ever burns away. The oxygen jet has nothing to react with.
- Stainless steel contains high levels of chromium, which creates an extremely sticky, viscous slag during cutting. That slag won’t flow out of the kerf. It clogs the cut, insulates the metal from the oxygen stream, and stops the reaction. Some shops use chemical flux or iron powder injected into the cut to help carry the slag away, but these are workarounds rather than true oxyfuel cutting.
- Copper and copper alloys fail for the same reason as aluminum: the oxide’s melting point sits above the base metal’s melting point. Copper also conducts heat so rapidly that maintaining a localized hot spot is nearly impossible.
- Cast iron presents a similar oxide problem. Grey cast iron’s oxide melts at a higher temperature than the iron itself, and the high carbon content (typically 2% to 4%) further disrupts the combustion reaction. The graphite flakes in grey cast iron also interfere with a clean cut.
Titanium is an interesting exception. It burns violently in oxygen and can technically be severed with an oxyfuel torch, but the reaction is so aggressive and difficult to control that it’s not a practical cutting method for production work.
Where Alloying Elements Cause Trouble
Even within the family of steels, certain alloying elements degrade oxyfuel cut quality or make cutting impossible altogether. Chromium, nickel, molybdenum, and carbon all reduce the ability of oxygen to sever the material, each with its own threshold. A steel with 2% or more chromium starts to form resistant oxides. High-nickel alloys behave similarly.
This means that as you move from plain carbon steel toward more heavily alloyed grades, oxyfuel becomes progressively less effective. A low-alloy structural steel with small amounts of manganese or vanadium cuts fine. A tool steel loaded with chromium and molybdenum does not. Before attempting to cut any unfamiliar alloy, it’s worth reviewing the specific composition, the same way you would before heat treating it.
Choosing the Right Fuel Gas
The fuel gas you pair with oxygen affects preheat speed and pierce time but doesn’t change which metals you can cut. Acetylene and propane are the two most common choices, and they perform differently at different stages of the cut.
Acetylene produces a hotter, more concentrated flame. Its primary flame cone delivers roughly 18,890 kJ per cubic meter, compared to 10,433 kJ for propane. That translates to pierce times about one-third as long as propane, which matters when you’re starting cuts on thick plate or making lots of pierces on a multi-part nest. The higher flame speed (7.4 m/s versus 3.3 m/s for propane) also concentrates heat in a smaller area, reducing distortion and narrowing the heat-affected zone.
Propane pierces much more slowly, but once the cut is established and the oxygen jet is doing the work, cutting speeds are about the same as acetylene. Propane costs less per unit of heat and is easier to store, which makes it popular for mechanized cutting operations where the torch stays in continuous motion and piercing is less frequent.
Cut Quality Factors
Getting a clean oxyfuel cut depends on more than just choosing the right metal. Oxygen purity is critical: even small drops in purity slow the reaction and leave a rougher edge. The condition of the torch tip, the steadiness of travel speed, and the surface condition of the plate all play a role. Scale, rust, and paint on the steel surface can disrupt the preheating stage and cause irregular cuts.
The American Welding Society publishes surface roughness standards for oxygen-cut edges, using physical replica guides that show four levels of finish quality. What counts as acceptable depends entirely on the application. A cut edge that gets welded over doesn’t need to be as smooth as one that serves as a finished surface. Shops typically define their own acceptance criteria based on what the part will be used for.
For metals that fall outside oxyfuel’s range, plasma cutting and laser cutting handle stainless steel, aluminum, copper, and other non-ferrous materials effectively. Plasma in particular is the natural alternative when the chemistry of the metal won’t support an oxyfuel reaction.