Reasons for slag hanging at the bottom when oxygen c utting carbon steel

When cutting carbon steel with oxygen, slag hangs down at the bottom (the lower edge of the cutting surface). This is a very common phenomenon, professionally known as “slag hanging” or “slag adhesion”. The main constituents of this slag are high-melting-point oxides and impurities, such as iron oxide and silicates, generated during the cutting process. The root cause of slag formation is that the temperature and gas flow at the lower edge of the cut are insufficient to completely blow away the molten oxides, causing them to cool and adhere to the bottom of the cut.

The specific reasons can be analyzed from the following aspects:

Improper cutting parameter settings (the most common cause)

1. The cutting oxygen pressure is inappropriate:

• Insufficient pressure: The blowing force of the gas stream is inadequate to completely remove the molten slag from the bottom of the cut, resulting in slag drooping and solidifying into “nodule-like” slag attachments.
• Excessive pressure: Although the gas flow is high, it can result in an overly wide and rough cut. Moreover, the cooling effect of the gas stream may cause the lower part of the cut to cool prematurely, effectively “locking” some molten slag at the edge.

2. The cutting speed does not match:

• The speed is too fast: the oxygen flow is lagging, and the bottom metal cannot be fully oxidized and blown off. The molten metal and oxide that have not been completely reacted are dragged and accumulated at the bottom.
• The speed is too slow: the incision is excessively burned, the heat input is too large, a large amount of slag is produced, which exceeds the blowing capacity of the oxygen flow, and a large amount of accumulation under the action of gravity.

3. Improper preheating flame energy rate:

• The flame is too strong (acetylene/gas is too much): the upper edge of the incision is melted and collapsed, resulting in more slag, and the lower edge temperature may be too high, the slag fluidity is too good, and it is not easy to be blown off.
• The flame is too weak: it cannot provide enough heat to maintain the cutting reaction in the direction of the thick plate, resulting in insufficient oxidation of the lower metal, forming a viscous slag that is difficult to blow away.

cutting equipment and gas problems

1. Cutting mouth problem:

• The model does not match: the cutting nozzle number is too small, and the oxygen flow is insufficient; the cutting nozzle number is too large, and the airflow is disordered.
• Blocked or damaged: the internal passage of the cutting nozzle (especially the oxygen hole) is burr, worn or partially blocked by splashes, resulting in deformation of the oxygen flow (not pure cylindrical), bifurcation, and weakening of the slag blowing force.
• Improper distance from the workpiece: too high, the preheating efficiency is low, and the airflow diffuses; too low, it is easy to block the cutting nozzle and cause tempering.

2. Gas purity and pressure:

• Low oxygen purity: industrial oxygen (generally 99.5%) if containing nitrogen and other impurities, will reduce the oxidation reaction efficiency, resulting in more viscous slag.

cut material problem

1. Material composition: when the content of carbon, silicon, manganese, phosphorus, sulfur and other elements in carbon steel is high:

• High carbon content: the burning point is improved, the fluidity is poor, and the slag is viscous.
• High content of silicon and manganese: it will form high melting point silicate and manganate oxides, which are viscous and more difficult to be blown away.

2. Surface condition: Heavy rust, paint, oxide skin or moisture on the surface of the steel plate will absorb heat, interfere with preheating, and introduce more impurities into the molten pool, increasing the amount and viscosity of slag.

3. Material temperature: cutting in a cold environment, the initial temperature of the steel plate is low, the heat loss is fast, and the lower part of the incision is easy to cool and solidify.

Operational Technical Issues

1. Torch angle: Failure to keep the cutting nozzle perpendicular to the surface of the workpiece during cutting (when cutting in a straight line), or improper control of the trailing amount (when cutting in a curve) will affect the discharge path of slag.

2. Unstable operation: uneven speed and jitter during hand-held cutting, resulting in intermittent cutting process.

Solution and suggestion

1. Optimize cutting parameters:

• Select the correct cutting nozzle number according to the thickness of the steel plate, and set the oxygen pressure and preheating flame according to the cutting parameter table.
• Adjust the cutting speed: make the cutting sparks from the lower part of the incision vertically downward (or slightly backward), at this time the most appropriate speed.

2. Check and maintain equipment:

• Clean or replace the cutting nozzle to ensure smooth airflow and regular shape.
• Check the gas pipeline and pressure reducing valve to ensure that the pressure is stable and the oxygen purity is up to standard.
• Maintain a proper cutting nozzle height (usually about 1/10 of the thickness of the steel plate, and refer to the equipment instructions).

3. Pretreatment of workpieces:

• Remove rust, paint and dirt from the vicinity of the cutting line as much as possible.
• In cold environments, local preheating of steel may be considered.

4. Improve operational skills:

• Maintain a smooth, uniform cutting movement.
• For thick plates or difficult-to-cut materials, a slight forward inclination (the head of the cutting nozzle is inclined in the direction of movement) can be used to help blow off the slag, but it should be noted that this may cause a certain slope of the cut.

Summary

Bottom hanging slag is usually not caused by a single cause. You can perform systematic troubleshooting in the order of Parameters> Equipment> Materials> Operations. The highest priority checks are usually cutting oxygen pressure, cutting speed and cutting nozzle status. Through fine adjustment, it is possible to control the hanging slag to the extent that it is very little or easy to remove. If the slag has been produced, usually with a hammer, angle grinder and other tools can be easily removed.