What is Oxy-fuel cutting?
Oxy-fuel welding and oxy-fuel cutting are processes that use fuel gases (or liquid fuels such as gasoline or petrol) and oxygen to weld or cut metals. French engineers Edmond Fouche and Charles Picard became the first to develop oxygen-acetylene welding in 1903.
Pure oxygen, instead of air, is used to increase the flame temperature to allow localized melting of the workpiece material (e.g., steel) in a room environment.
A common propane/air flame burns at about 2,250 K (1,980 °C; 3,590 °F), a propane/oxygen flame burns at about 2,526 K (2,253 °C; 4,087 °F), an oxyhydrogen flame burns at 3,073 K (2,800 °C; 5,072 °F) and an acetylene/oxygen flame burns at about 3,773 K (3,500 °C; 6,332 °F).
Before cutting, the cutting torch has to pre-heat the steel to ignition temperature at the starting point. At this temperature of around 960°C (depending on the type of alloy), the steel has lost protective properties against oxygen and is still solid.
Pure oxygen is then directed through the nozzle at the heated area. This fine and high-pressure oxygen stream changes pre-heated and unprotected steel into oxidized liquid steel by an exothermic reaction.
This slag has a lower melting point than steel, so the oxygen stream can blow the liquid slag out of the cavity without affecting the non-oxidized solid steel. This exothermic reaction is a continuous process and creates a cut as the torch moves. To keep the exothermic reaction working, the cutting torch keeps the steel heated during cutting.
Only metals whose oxides have a lower melting point than the base metal itself can be cut with this process. Otherwise, as soon as the metal oxidizes it terminates the oxidation by forming a protective crust. Only mild steel and some low alloys meet the above conditions and can be cut effectively with the oxy-fuel process.
How Does the Oxyfuel Cutting Process Work?
Oxy-fuel cutting is a chemical reaction between pure oxygen and steel to form iron oxide. It can be described as rapid, controlled rusting. Preheat flames are used to raise the surface or edge of the steel to approximately 1800°F (bright red color).
Pure oxygen is then directed toward the heated area in a fine, high-pressure stream. As the steel is oxidized and blown away to form a cavity, the preheat and oxygen stream are moved at a constant speed to form a continuous cut.
Only metals whose oxides have a lower melting point than the base metal itself can be cut with this process. Otherwise, as soon as the metal oxidizes it terminates the oxidation by forming a protective crust. Only low carbon steel and some low alloys meet the above condition and can be cut effectively with the oxy-fuel process.
Here are the basics of how it all works:
Step 1: Preheat
Before you can start cutting the steel, it has to be heated up to its kindling temperature, about 1800°F. At this temperature, the steel readily reacts with oxygen. The heat is provided by the preheat flames from an oxy-fuel torch. Inside the torch, the fuel gas is mixed with oxygen to create a highly flammable mixture.
A nozzle has multiple holes arranged in a circular pattern to focus the flammable gas mixture into multiple little jets. The fuel-oxygen mixture is ignited outside of the nozzle, and the preheat flames form just outside the nozzle tip.
Commonly used fuel gases include acetylene, propane, natural gas, and a few other mixed gases. By adjusting the fuel-to-oxygen ratio, the flame is adjusted to produce the highest possible temperature in the smallest possible flame. This concentrates the heat in a small area on the surface of the steel plate.
Step 2: Piercing
Once the surface or edge of the plate has reached kindling temperature, a jet of pure oxygen is turned on to begin piercing through the plate. This is called the “cutting oxygen”, and the jet is formed by a single bore in the center of the nozzle.
As the cutting oxygen stream hits the pre-heated steel, the rapid oxidation process begins. This is when the real fun begins. The oxidation process is referred to as an exothermic reaction – it gives off more heat than it takes to get started.
The oxidized steel takes the form of molten slag, and the molten slag has to get out of the way so the oxygen stream can “pierce” all the way through the plate. Depending on how thick the plate is, this can take anywhere from a fraction of a second up to several seconds.
During this time, the cutting oxygen stream is pushing deeper and deeper into the plate, and the molten slag is being blown out of the piercing hole. This can result in a massive geyser of molten steel, or if done properly, a small puddle of slag on top of the plate.
Step 3: Cutting
Once the cutting oxygen stream has made its way all the way through the plate, the torch can start moving at a constant speed, forming a continuous cut. The molten slag formed during this phase is blown out the bottom of the plate.
The heat given off by the chemical reaction between the oxygen and the steel preheats the plate just in front of the cut, but not reliably enough to cut without the preheat flames. So, the preheat flames stay on throughout the cut, adding heat to the plate as the torch moves.
Those are the basics. But there are lots of other factors that affect the quality of the cut edge, including speed, cut oxygen pressure, preheat flame adjustment, cutting height, plate temperature, etc.
Characteristics of oxy-fuel compared to plasma
- Material. Oxy-fuel cutting is used for the cutting of mild steel. Only metals whose oxides have a lower melting point than the base metal itself can be cut with this process. Otherwise as soon as the metal oxidizes it terminates the oxidation by forming a protective crust. Only mild steel and some low alloys meet the above conditions.
- Wall thickness. Oxy-fuel allows the cutting of thicker walled material then plasma. Plasma can’t cut thicker walls because of the huge amounts of energy necessary to reach similar thicknesses.
- Cutting angle. Oxy-fuel allows cutting of steeper angles up to 70° (as compared to 45° with plasma) because of the concentration of the oxygen beam.
- Straight cuts. The plasma beam has the tendency to deflect when the angle is too steep. However, this deflection could be compensated by automation.
- Costs. Oxy-fuel is a more economical solution than plasma cutting. Initial investment costs, consumables and operating costs are all lower than plasma cutting. However, processing speeds are typically lower below a 20 mm wall thickness range (considering 3D profiling in the heavy steel industry)
Oxyfuel Cutting Advantages and Disadvantages
With oxyfuel-cutting applications, fuel gas and oxygen are used to generate the cutting flame. Messer Cutting Systems supplies gases including acetylene, MAPP, propane, and natural gas, and information relative to your requirements.
- Straight-edge quality and high accuracy.
- Bevel strip cutting.
- Pierce mild steel up to 4 inches thick (101 millimeters) to 5 inches (127 millimeters) thick.
- Edge start and cut steel 10 inches (254 millimeters) to 12 inches (304 millimeters) thick.
- With multiple torches, produce multiple parts, reducing time and labor.
- Cannot cut stainless steel under normal circumstances.
- Slower cut speeds compared with plasma cutting.
- Thin material cutting might warp.
- Difficult to produce holes smaller than two times the steel’s thickness.