What Is Oxy-fuel Cutting?- Process, Pros, and Cons

What is Oxy-fuel cutting?

One thermal cutting process is called oxy-fuel cutting, wherein a fuel gas or fuel combination (the most common gaseous fuel is acetylene, although propane, MAPP, propylene, and natural gas can also all be used as fuels) is burnt with oxygen in the presence of various metals.

Oxy-fuel welding and oxy-fuel cutting both melt and cut metals using fuel gases (or liquid fuels, such as gasoline or petrol) and oxygen. The first known oxygen-acetylene welding was developed in 1903 by French engineers Edmond Fouche and Charles Picard.

The use of pure oxygen or a combination of pure oxygen and a fuel gas can provide burning temperatures that, at the tip of the flame, can exceed the melting point of metal (for example, the melting point of steel) in an ambient atmosphere.

A common propane/air flame burns at around 2,250 K (1,980 °C; 3,590 °F), a propane/oxygen flame burns at around 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 approximately 3,773 K (3,500 °C; 6,332 °F) allowing for tailored thermal cutting processes to accomplish productive results in varied industrial settings and many specialized applications.

The cutting torch must pre-heat the steel to the ignition temperature at the point of cutting. Once the preheated steel reaches a temperature of about 960°C (this varies depending on the type of alloy) the steel has lost its protective properties against oxygen, and the base metal is still solid.

The pure oxygen can then be directed through the nozzle at the heated area. The fine and high-pressure stream of oxygen will change the pre-heated and unprotected steel into oxidized liquid steel, through an exothermic reaction.

Since the oxidized liquid steel slag has a much lower melting point than steel, the oxygen stream will simply blow liquid oxidized steel slag out of the cavity without any effect on the solid, non-oxidized steel.

This exothermic reaction is continuous and cuts the steel as the torch is slid across the cutting area. To keep the exothermic reaction going, the cutting torch will reheat the steel during the cutting cycle.

This method of cutting can only occur with a metal whose oxides will have a lower melting temperature than the base metal itself. If this does not occur, as soon as the metal oxidizes and turns into an oxide, this terminated the oxidation as a protective crust will develop.

Only mild steel and some low alloys will have the properties outlined above, thus can be cut efficiently using the oxy-fuel process.

How Does the Oxyfuel Cutting Process Work?

Oxy-fuel cutting is a process that entails a chemical reaction between a pure oxygen source and pure steel, yielding iron oxide, essentially rapid, controlled rusting with a preheat flame (1800°F, bright red) to heat the surface or edge of the steel.

Once the steel reaches its ignition temperature, a fine, high pressure stream of pure oxygen is introduced toward the heated region.

As the steel is oxidized and blown away to form a cavity, the preheat flame and oxygen stream both are moved at a constant speed, creating a continuous cut through the steel process.

This cutting process only works on metals where the melting point of the oxide is lower than the melting point of the base metal. For all other metals, as soon as the metal oxidizes it stops the process by forming a protective crust.

Only low carbon steel and some low alloys meet the above conditions from which the oxy-fuel method can be effectively used.

Here are the basics of how it all works:

Step 1: Preheat

In order to begin the cutting process of the steel, the material must be heated to kindling temperature, around 1800°F. At this temperature, the steel will oxidize rapidly with any oxygen that is available.

The pre-heated flames of the oxy-fuel torch provide the heat required to reach the needed kindling temperature. Within the torch, fuel gas is mixed with oxygen and delivered as a flammable mixture.

The nozzle has plenty of holes that are arranged in a circular pattern to best focus the flammable mixture of gas into multiple small jets. The fuel-oxygen mixture ignites outside of the nozzle, where the pre-heat flames exist just outside of the nozzle tip.

Acetylene, propane, natural gas, and several other mixes of gases are the most common fuel gases used in this application.

Adjustments to the fuel-to-oxygen ratio of the flame will be made to produce the greatest temperature within the smallest flame. The smaller the flame, the more concentrated the heat will be in a smaller area at 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

After the cutting oxygen stream has penetrated completely through the plate, a consistent cut can be made by moving at a constant speed and establishing a continuous cut. The molten slag produced at that point is blown out the bottom of the plate.

The heat released by the chemical reaction of the oxygen and steel creates enough heat to preheat the plate just ahead of the cut but is not reliable enough to cut without the preheat flames, therefore the preheat flames stay on while cutting and continue to add heat to the plate as the torch is moved.

This is the basic description, but many other factors 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

• Materials. Oxy-fuel cutting is a process utilized for cutting mild steel. It is important to note that only metals with oxides that have a lower melting temperature than the base metal itself can undergo this process. If this is not true, as soon as the metal oxidizes, the oxidation process will stop, generally because a protective crust forms on the oxidation surface. Only mild steel and certain low alloys fulfill this requirement.
• Wall thickness. Oxy-fuel allows for the cutting of material with a thicker wall than plasma. Plasma can’t cut thicker walls due to the amount of energy to reach the same thickness.
• Cutting Angle. Oxy-fuel cutting allows for cutting of steeper angles, up to 70° (compared to 45° for plasma), due to the focus of the oxygen beam.
• Straight Cuts. If the angle is too steep the plasma beam has a tendency to deflect; however, this can be compensated with automation.
• Cost. Oxy-fuel is more cost-effective than plasma cutting; the cost of investment, consumables, and operational costs are less than plasma cutting. However, the process speeds typically will be slower under 20 mm wall thicknesses (Keeping in mind 3D profiling within the heavy steel industry).

Oxyfuel Cutting Advantages and Disadvantages

In oxyfuel-cutting applications, fuel gas is consumed and oxygen is supplied to produce the cutting flame. These gases are sourced from Messer Cutting Systems, including acetylene, MAPP, propane, and natural gas, along with your related requirements.

Advantages:

• Trait of a straight-edge and high accuracy. 
• Cutting strips along the bevel 
• Pierce mild steel up to 4 inches (101 millimeter) to 5 inches (127 millimeter) thick. 
• Cut steel edge starts, 10 inches (254 millimeter) to 12 inches (305 millimeter) thick. 
• A variety of parts can be produced with more than one torch simultaneously, providing time and labor savings.

Disadvantages:

• Under standard conditions, stainless steel cannot be severed.
• Cutting speeds are slower than plasma cutting.
• Thin materials may not always cut straighter.
• More challenging to create holes smaller than two times the thickness of the steel.

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