Oxy-fuel is a chemical reaction between the steel and pure oxygen to cut the metals having oxides with lower melting points than is base metals. It is a controlled rusting mechanism where preheat flames at around 1800°F are raised to the surface of the metal.
Then the high-pressure stream of pure oxygen is exposed to the heated area to oxidize the steel. The oxygen and preheat are moved together at a constant speed to cut the metal.
Where oxy-fuel is required?
Steel with the composition is less than 3% carbon; the oxy-fuel cutting process is beneficial. Non-ferrous elements such as copper, aluminum, stainless, or brass are not appropriate to cut using the oxy-fuel method. The mild steel consists of 1% manganese, 98% iron, up to 3% of carbon and various other elements. The oxyfuel process is used to cut the elements such as molybdenum, chromium, and nickel.
Oxy-fuel is not simple like it seems due to the various technicalities involved in it. Only experienced machine operators can achieve a quality cut by inculcating all the desired parameters. To make a quality cut, it is essential to understand the factors affecting the quality of oxy-fuel.
Here are the ten factors which affect the quality of oxy fuel cutting significantly:
- Dirty Nozzle
If due to some adhering scale, the fouled nozzle is used in operation, it will lose its parallel form and will not provide a smooth and quality cut. A dirty nozzle can lead to under-cutting, heavy slag, or even proof of pitting. It is essential to clean the nozzle diligently to avoid any bell-mouth or distort to the cutting oxygen bore.
- Cutting speed is too low
Massive gouging of a cut surface is the result of abnormally low cutting speed, which also leads to large globules slang adhering. Both fuel and oxygen are wasted under this condition and challenge the economies of oxy-fuel cost-cutting.
The availability of oxygen is very high when the cutting speed is low. It leads to instability in operations due to the lack of cutting operations at the leading edge. This instability is responsible for causing gouges near the bottom, within the face of the cut. Every cutting action undertakes at the front half of the stream, at the point of contacting metal.
- Cutting speed is too high
Heavy lag is the result of high cutting speed. You can notice the curved lines on the cut surface when the pace of cutting is very high. The bellies and undercuts are noticeable, destroying the flatness as required on the cut face. At extreme speed, the cut at the bottom does not complete due to the severe drag. It will be a severe cut giving no relevance to quality.
- When the nozzle is too near to the surface
When the nozzle is very close to the surface, it leads to the grooves in a cut face. It is due to the buries caused by the preheat flames reaching the inner cones leading to the excessive melting of the edge top. It may result in lost cuts, and the flames can also subject to popping.
- Excess Cutting Oxygen
When the nozzle size is vast, or the cutting oxygen pressure is high, the quality of the cut affects adversely. Nozzles work perfectly within the stipulated range of torch pressures. Any deviation to this causes excessive oxygen pressure leading to the distortion in the oxygen streams after leaving the nozzle. To ensure a quality cut, controlled oxygen pressures are imperative.
- Oily plate
Although the oil has a mild effect on the cut quality, it can still affect the quality of a cut adversely to an extent. The presence of fat leads to the production of smoke and also leads to a slight reduction in the speed when the cutting operation is performed on very thin material.
- Inferior quality plate
Plates with inclusions and laminated plates act as a barrier to the oxidation reaction resulting in the loss of cut and instantaneous rooster. In such cases, the speed should be reduced, and preheat should be increased to effect a quality cut.
Detrimental effects on the overall quality can only be reduced, cannot be eliminated. To penetrate the protective surface of rusty and heave mill scale, stronger preheating is required. Slow cutting speed and heavier preheat are the only remedies to deal with the situation.
- When the nozzle is too far from the surface
When the nozzle is too away from the surface of the work, cutting speed gets lowered. The more significant difference between the surface and the nozzle leads to rounding of the top edge. With right nozzle clearance, the preheat flames should be around 1/4″ above the plate’s top surface.
- Excess Preheat flame
Inexperienced operated try to increase the preheat flames with increased cutting speeds to accelerate the cutting procedure. The increase in the preheat flames lead to the melting of the top edge and results in to slow down of cutting speed eventually.
It merely leads to the wastage of fuel gas and oxygen, affecting the economies of reducing costs. It is crucial to perform the cutting operation within the controlled preheat flame to ensure efficient cut and save fuel.
- Chemical composition of the plate
When the concentration levels are low in the alloying elements such as chromium, silicon, carbon, nickel, and manganese, it also affects the oxy-fuel cutting quality adversely.
The varying concentrations of elements throughout the plate cause erratic operations in the cutting process. These elements act as contaminants which are not easily oxidized and interfere with the smooth formation of slag stream.
What should be the correct cutting technique?
The cut surface should be square and smooth. The kerfs walls must appear parallel. The alignment among the lag lines should be vertical. The slag adhering to the edge’s bottom should be minimal. After adjusting the preheat flames, the top edge should be slightly rounded. These conditions are ideal for the perfect cutting applications, with no further treatment required.