ESAB Knowledge center.
Oxy-fuel torch preheat and its proper adjustment
Flame Cutting/Oxygen Burning requires some source of intense heat to get the plate hot enough to be cut/burned. This source of heat is referred to as the “preheat flame”. The whole plate does not need to be heated, not even the bottom, just that portion slightly ahead of the cutting reaction. There are some plate materials that must be “preheated” before cutting to keep them from cracking during cutting. Typically materials with Carbon Levels above .25% fall in this category. This is NOT the “preheat” we are discussing. Some of these materials require “preheating” up to 600 °F. There are also some materials that require “preheating” to achieve a quality cut.
Numerous fuels are used for the fuel/Oxygen mixture called Preheat and each has some feature to make it attractive. Up to about 1970 Acetylene was the primary fuel used but its escalating cost, the growth of plasma, and the aggressive marketing of other fuels have caused it to no longer be a primary choice particularly where plasma is also available. The most common fuels today are natural gas and propane. There are also some mixtures of natural gas or propane with some additive that claims to make them hotter. If in fact these gases are hotter it is of little value because cutting speeds are limited by preheat capability (temperature) only in thinner materials (1/8 -1/2 inches). In thicker materials the cutting speed is limited by the capability of the cutting Oxygen stream, not the preheat, so higher flame temperatures are of little or no value.
As previously stated the purpose of the preheat gases is to raise the temperature of the plate, just ahead of the cut, so a high heat transfer rate from the flame to the plate is required. The factors the operator can control to achieve this transfer of heat, are 1) the flame temperature 2) the quantity of preheat gases used and 3) the tip to work distance. The flame temperature is controlled by adjusting the O/F Ratio. The chart below illustrates how the temperature is affected by this ratio. The quantity of preheat can be adjusted by raising or lowering the preheat pressures to the torch. High pressure will result in high flowrates, i.e. a high velocity preheat. Low pressures will result in low flowrates/velocity. The operator can adjust the O/F ratio by looking at the inner cone of the flame. When this inner cone is the shortest length, but still has a very bright appearance this is the hottest flame temperature. This procedure can be used regardless of the total flow of gases.
The quantity of preheat required varies depending on the operation to be conducted. For a pierce start a very high flowrate (at the correct O/F ratio) is required followed by a significantly lower flowrate (again at the right O/F ratio) for the continuing cutting operation. These preheat conditions are often referred to as Hi Preheat and Lo Preheat. A Hi Preheat that is not high enough will result in pierce times of 20-120 seconds. Pierce time being the time the Hi Preheat is on prior to initiation of the Cutting Oxygen. With proper control of the Hi Preheat the pierce time on ½ to 6 inch material can consistently be 10 sec or less. If the operation is beveling then the Hi preheat condition is required at all times.
A properly adjusted preheat setting results in a sharp top edge and a slag free and sharp bottom edge. Sometimes if the mill scale is very heavy a higher preheat will be required to break the mill scale loose while keeping the leading edge of the cut properly heated. Too much preheat will result in a rounded top edge as well as a significant increase in slag. Often times this increase in slag will result in a cut that will not drop from the base plate. Sometimes the slag is so tenacious the part must be beat out of the plate with a sledge hammer.
The actual recommended quantity (CFH) of fuel and Oxygen varies with the tip manufacturer and tip purpose. In general for practical shape cutting the quanties required are Natural Gas 10-20 CFH and 18-36 CFH Oxygen, and Propane 5-10 CFH and 23-45CFH Oxygen. Flowrates for Propylene are about the same as Propane but with a little less Oxygen. Preheat Quantities vary somewhat depending on plate surface conditions, material composition, operator capability, and other factors to numerous to be mentioned. Flame cutting is as much an art as it is a science.