While cutting with acetylene is more common, oxyfuel cutting with MAPP gas can be used with equally good results as well. Cutting with MAPP gas is similar to cutting with acetylene gas in combination with oxygen.
Cutting With MAPP Gas
1. The right combination of parameters such as preheating flame, oxygen pressure, torch angle, torch speed, plate quality, tip size, etc. is important to obtain good quality of cutting with MAPP gas.
2. In a manner similar to acetylene gas, different types of flames can be obtained by varying the proportion of MAPP gas and oxygen.
How To Obtain Different Types Of Flames With MAPP Gas?
All three types of flames, viz oxidizing flame, neutral flame, and carburizing flame can be obtained by varying the proportion of MAPP gas and oxygen gas, in a manner similar to oxy-acetylene operation.
Light up the torch, and increase the preheat oxygen till the yellow colored flame turns blue. Some yellow color feathers still remain at the end of the preheat cone. This flame is a slightly carburizing flame.
A slight turn of the oxygen valve on the torch causes the yellow feathers to vanish. The blue color preheat cone turns deep blue, and has a sharp profile. This flame is a neutral flame. It remains neutral even with a slight further increase in preheat oxygen.
A further turning of the oxygen valve makes the color change from blue to a lighter blue. The preheat cone becomes longer, and is accompanied by an increase in sound. The glare of the flame is not so sharp; the flame is easier to look at. This is called an oxidizing flame.
In general, the preheat cones produced with MAPP-oxygen combination are 1.5 times longer than their oxy-acetylene counterparts, when using the same tip type. However, when a two-piece tip is used, the situation is reversed. The preheat cone produced by MAPP-oxygen is much shorter when two piece tip is used.
The following table lists the ratio of oxygen to MAPP gas required to obtain different types of flames.
Type of Flame Ratio of oxygen to MAPP gas
Highly carburizing 2.0 to 1.0
Slightly carburizing 2.3 to 1.0
Neutral flame 2.5 to 1.0
Oxidizing flame 3.0 to 1.0
Highly oxidizing flame 3.5 to 1.0
Table 1: Oxyfuel ratios to obtain different flame types
Which Flame Should Be Used For Cutting With MAPP Gas?
Neutral flame is most commonly used for cutting operations. However, sometimes the oxidizing and carburizing flames are also used. When a stack is required to be cut, or a square edge at the top is required, a carburizing flame produces better results.
When a slightly carburizing flame is used, slag formation is minimal. So it is used to stack cut light material.
Oxidizing flame should not be used for this purpose, because lot of slag tends to be produced in the kerf which welds the plates together. It is difficult to separate slag welded plates once the cutting is completed. The whole point of cutting gets defeated.
A ‘high-low’ style of cutting is used by some operators. In this, the operator starts with a moderately oxidizing flame. The oxidizing flame has higher temperatures than flame types. So, it is possible to obtain a fast start to the oxyfuel cutting operation. This is called the ‘high’ position.
Once the start is obtained, the operator brings the flame to neutral, and continues with the cutting work. This is called the ‘low’ position.
However, it should be kept in mind that it is enough to use a moderately oxidizing flame for the fast starting. Very oxidizing flame should not be used for a faster start. The extra oxygen does not cause a substantial increase in combustion temperature, instead – the extra oxygen contributes to cooling the flame. Extra oxygen also contributes to oxidation of the metal surface.
This, instead of providing a faster start, in fact slows down the start to the cutting pass.
Right Pressure Of Oxygen For Cutting
The pressure should be varied to find the correct pressure for a application. Begin with a low-volume soft flame first. Then increase the pressure a little so that a visible oxygen cutting stream appears.
Low pressures do not yield effective stingers for cutting. Short stingers are obtained which break up at the end. These stingers cannot be used for cutting.
Increase the pressure gradually. The stinger suddenly becomes coherent and strong. The length increases substantially. This pressure is the right pressure for cutting operation. The length and quality of stinger remains intact over fairly wide range of pressures. However, pressure should not be increased too much either.
If pressure is increases to too high, the stinger returns to being short, which breaks at the ends, and is not suitable for cutting work.
