A high stand-off also reduces melting of the upper edges of the cut. However, if the stand-off becomes too large, big drops of molten slag can form on the upper surface of the material just ahead of the oxygen jet. It these fall down into the kerf then high ripple edges are formed on the cut surface.
Starting the cut
The main difficulty when starting a cut is preheating the lower part of the material to the temperature range where the cutting operation can be carried out. If the start of the cut fails, (i.e. the cutting oxygen simply gouges the material) then it is often very difficult to restart in the same position.
In practice, as the material thickness increases it becomes more difficult to preheat the lower part of the material. To start a cut away from the edge in a plate of >130 mm thickness, it is frequently necessary to use either additional direct heating of the lower face, or to drill a start hole, essentially creating an edge start. Where piercing is carried out, care must be taken to ensure that the nozzle is not damaged, by using a technique such as withdrawal piercing. For an edge start, a number of different procedures can be used to achieve the preheat:
- The whole length of the flame can be used to preheat the vertical side of the material.
- A manual torch can be used to enhance preheating of the vertical side, particularly at the bottom of the plate.
- The cutting oxygen valve can be opened slowly and the torch motion started when slag flows down the whole side of the material. The slag helps to preheat and initiate the combustion of the material.
Cutting speed
A good quality cut will have an edge that is perpendicular at the top and bottom surface of the plate, a slag that does not adhere and drag lines that are shallow and nearly vertical. The speeds suggested by this program are typical of those achieved for good quality cuts. Cuts made only for separating pieces of metal do not need the same accuracy and smoothness. These severance quality cuts can be carried out at higher speeds, but have rough drag lines that slope sharply in a direction opposite to that of torch travel, a kerf with concave walls and slag that tends to stick to the bottom of the cut.
It should also be noted that significantly reducing the cutting speed below that for a quality cut may also lead to difficulties, such as a melted upper edge of the cut and also notches in the cut at irregular intervals.
Kerf widths
The kerf widths tend to increase with the plate thickness, because both the cutting oxygen ducts of the nozzles and the cutting oxygen flow increase with the plate thickness. The following table shows typical kerf widths with acetylene and propane nozzles:
| Plate thickness (mm) |
Kerf width (mm) |
| 100 |
3–6 |
| 300 |
6–12 |
| 500 |
10–20 |
Effects of alloying elements on flame cutting of steelThe composition of the material being cut can effect the oxyfuel cutting process. Alloys where
Al > 8-10% or
Cr > 10%, high Mo-W steels and cast iron containing up to 4% C can require a special technique, such as metal-powder cutting. Steels containing 14% Mn and 1.5% C and high red hardness W steel, along with those where
C > 0.25% or
Si > 4% (with high C and Mn) can require preheat.
In general, problems are not encountered below the following proportions of alloys elements:
| Aluminium (<8-10%) |
Carbon (<0.25%) |
| Chromium (<5%) |
Molybdenum (<5%) |
| Nickel (<3% with <0.25%C) |
Silicon (<4%) |
| Tungsten (<14%) |
|
The amounts of manganese, sulphur, phosphorous and vanadium usually found in steel have no effect on flame cutting, although it should be noted that high sulphur slows cutting speed and produces sulphur dioxide fumes.
Troubleshooting
This section of the program provides assistance in overcoming the typical problems associated with oxyfuel cutting.
You will need to examine the profile of the cut surface, noting particularly:
- The shape of the top edge;
- The shape of the bottom edge;
- The contour of the cut surface;
- Whether the cut surface is tapered;
- Whether undercut is present.
Profile 1
The top of the cut is both sharp and clean, the drag lines are almost invisible, producing a smooth side. Oxide is easily removed, the cut is square and the bottom edge is clean and sharply defined. Drag lines should be vertical for profiles. A small amount of drag is allowed on straight cuts.
This is characteristic of a good cut.
Profile 2
Melting has caused the top edge to become rounded. Gouging is pronounced at the bottom edge, which is also rough. Scale on the cut face is difficult to remove.
Recommendation: Increase the cutting speed, or increase the oxygen pressure.
Profile 3
The top edge may be either sharp or beaded. Undercut is present at the top of the cut face. The drag lines have excessive backward drag, which may result in the cut not being properly severed at the end. The bottom edge is slightly rounded.
Recommendation: Reduce the cutting speed.
Profile 4
Excessive rounding and melting of the top edge. Undercut has been caused by the oxygen stream opening out.
Recommendation: Reduce the stand-off distance (separation) between the nozzle and the plate.
Profile 5
Heavily beaded and rounded top edge, otherwise of good appearance.
Recommendation: Increase the stand-off distance (separation) between the nozzle and the plate.
Profile 6
The edge has a regular bead. The kerf is wider at the top with undercutting just beneath it.
Recommendation: Reduce the oxygen pressure.
Profile 7
Due to excessive heat, the preheat flame has caused the top edge to melt and become rounded. The kerf tapers from just below the top edge to the bottom of the cut face.
Recommendation: Reduce the size of the preheat flame.
Profile 8
The surface is rough and of poor appearance. The top of the cut face looks like a 'folded curtain' with beads on the lower edge. The flat face has vertical runs. Slag adheres to the underside. This is typical of a dirty nozzle.
Recommendation: Either clean the oriface of the nozzle carefully with the correct cleaning tool, or replace the nozzle.
Cutting Terminology
- Oxygen purity
- The purity of the cutting oxygen can have a significant effect upon the cutting speed.
The standard industrial grade oxygen is 99.5% pure. An increase in the oxygen purity will allow an improvement in the cutting speed, although the additional cost of the oxygen may make it uneconomic. Oxygen of a lower purity would reduce the cutting speed.
- Kerf width
- Kerf angle
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