Theory of Oxy-Fuel Gas Cutting
Oxy-Fuel gas cutting is functional for use with low carbon and low alloy steels, with a carbon content generally restricted to 1/10 to 3/10 of 1%. The various alloy elements found in steel affect the ability of the oxygen to cut the metal. Elements such as manganese, silicon, phosphorus, and sulfur have very little effect in normally found levels. Other elements such as chromium, nickel, molybdenum, and carbon generally reduce the ability of oxygen to sever the material up to the various limits of each.
Prior to attempting to flame cut a piece of material, you must study it and consider the elements it contains as well as the combinations of these elements, just as you would if you studied the metal for physical properties for heat treatment, flame hardening, etc.
A cutting torch diverts the oxygen and mixes part of it with the fuel gas to create the preheat flame, forming the ring of flame around the cutting tip. This preheat flame will reach a temperature of 4400°F to 6000°F, depending on the fuel gas used as well as the ratio of oxygen to fuel gas.
In order to start a chemical reaction, the metal must be raised to the kindling temperature, which in mild carbon steel is approximately 1600°F. At this point the metal will reach a bright orange color and sparks will be noticed on the top edge. When the kindling temperature is reached, the cutting oxygen lever is opened and high purity oxygen is introduced.
As the oxygen chemically combines with the iron exothermically (exothermic reaction), the result is generally referred to as the "cutting jet." The cutting jet is always in the center of the tip and instantly starts a rapid oxidation of the steel through the depth of the cut. A tremendous amount of heat is liberated when the high purity oxygen unites with the steel during this reaction.
If carried through to completion, you have three balanced chemical equations as a result of this reaction:
- Fe + O --- FeO + Heat (63,800 cal) First Reaction
- 3Fe + 2O2 --- Fe3O4 + Heat (267,800 cal) Second Reaction
- 2Fe + 3/2 O2 --- Fe2O3 + Heat (196,800 cal) Third Reaction
The third reaction occurs, to some extent, in heavier section cutting, with the first and second predominating. Theoretically it takes 4.6 cubic feet of oxygen to oxidize one pound of steel completely to ferrous oxide (Fe3O4). In practical cutting operations the amount of oxygen used is less because not all of the iron is completely oxidized to ferrous oxide. This set amount of oxygen is the constant required to flame cut metal no matter what fuel gas is used for the preheat function.
As the iron is oxidized it starts to flow; some of the material adjacent to the iron oxides is melted and also flows, due only to the intense heat that is liberated in the chemical reaction. The removal of all the metal is based in a large part on the velocity and coherency of the high purity oxygen cutting jet.