Gas Cut – What Is Gas Cutting

Gas cutting refers to the use of gas flame to preheat the cut metal to the ignition point, so that it burns violently in a pure oxygen stream to form molten slag and release a lot of heat. Under the blowing force of high pressure oxygen, the oxidized molten slag is blown off. : The heat released further preheats the next layer of metal to reach the melting point. The gas cutting process of metal is a continuous process of preheating, burning, and blowing slag. Its essence is the process of burning metal in pure oxygen, rather than melting.

Gas cutting is the use of combustible gas and oxygen mixed combustion flame heat to preheat the cutting part of the workpiece to a certain temperature, and then spray a high-speed cutting oxygen stream to violently oxidize the metal and release heat, and use the cutting oxygen stream to blow the molten metal oxide Drop, and realize the cutting method. The gas cutting process of metal is essentially a combustion process of iron in pure oxygen, not a melting process.

Gas cutting utilizes the thermal cutting of flame separation materials produced by the mixed combustion of combustible gas and oxygen, also known as oxygen cutting or flame cutting. During gas cutting, the flame preheats the material to the ignition point at the starting point, and then injects an oxygen stream to oxidize and burn the metal material violently, and the generated oxide slag is blown away by the air stream to form an incision. The purity of the oxygen used for gas cutting should be greater than 99%; the combustible gas is generally acetylene gas, but also petroleum gas, natural gas or coal gas. The cutting efficiency with acetylene gas is the highest, the quality is better, but the cost is higher. Gas cutting equipment is mainly cutting torch and gas source. The cutting torch is a tool that generates gas flame, transfers and regulates cutting heat, and its structure affects the speed and quality of gas cutting. The fast cutting nozzle can increase the cutting speed, make the cut straight and the surface smooth. The manual gas cutting torch uses oxygen and combustible gas cylinders or generators as the gas source. Semi-automatic and automatic gas cutting machines also have cutting torch drive mechanism or coordinate drive mechanism, profiling cutting mechanism, photoelectric tracking or digital control system. The automatic gas cutting machine for mass cutting can be equipped with multiple cutting torches and a computer control system. The metal material to be gas-cut should meet the following conditions:

• ①It can burn violently in pure oxygen, and its ignition point and melting point of slag must be lower than the melting point of the material itself. The slag has good fluidity and is easily blown away by airflow.
• ②The thermal conductivity is small. In the cutting process, the oxidation reaction can generate enough heat to make the preheating speed of the cutting part exceed the heat conduction speed of the material, so as to keep the temperature in front of the incision always higher than the ignition point, so that the cutting will not be interrupted. Therefore, gas cutting is generally only used for low carbon steel, low alloy steel and titanium and titanium alloys. Gas cutting is a commonly used metal thermal cutting method in various industrial sectors. In particular, manual gas cutting is flexible and convenient to use. It is an indispensable process method for sporadic blanking of factories, disintegration of waste products, installation and dismantling.

Gas Cutting Requirements

The equipment used in gas cutting is the same as gas welding except for the cutting torch. The gas cutting process is a preheating-burning-slag blowing process, but not all metals can meet the requirements of this process. Only metals that meet the following conditions can be gas-cut.

• 1. The burning point of metal in oxygen should be lower than its melting point;
• 2. The melting point of metal oxide during gas cutting should be lower than that of metal;
• 3. The combustion of metal in the cutting oxygen stream should be an exothermic reaction;
• 4. The thermal conductivity of the metal should not be too high;
• 5. There are less impurities in the metal that hinder the gas cutting process and improve the hardenability of steel.

Metals that meet the above conditions include pure iron, low-carbon steel, medium-carbon steel, low-alloy steel, and iron. Other commonly used metal materials such as cast iron, stainless steel, aluminum and copper, etc., must use special gas cutting methods (such as plasma cutting, etc.). Gas cutting technology has been widely used in industrial production.

Gas Cutting Method

• Put the guide rail and put the cutting machine on the guide rail. The two ends of the guide rail should be aligned. When cutting a circle, adjust the height of the cutting nozzle and the distance and angle relative to the trolley to ensure continuous cutting in the area of ​​the plate.
• Connect the gas pipe, distinguish the oxygen pipe and the gas pipe.
• Open the oxygen control valve and gas control valve, and confirm the cylinder pressure and output pressure to ensure sufficient gas supply and saving.
• Ignition; turn on the gas and preheat the oxygen, use a lighter to ignite from the side.
• Start cutting; use the preheating flame to heat the starting point (the high-pressure oxygen valve is closed at this time). The preheating time should depend on the metal temperature. Generally, it is heated until the surface of the workpiece is close to melting (the surface is orange-red). At this time, gently open the high-pressure oxygen valve and start gas cutting. If the preheated area cannot be cut off, it means that the preheating temperature is too low. The high pressure oxygen should be turned off to continue preheating. The front end of the preheating flame should be 2 ~ 4mm away from the surface of the workpiece. At the same time, there should be a certain distance between the cutting torch and the workpiece. When gas cutting a workpiece with a thickness of 5-30mm, the torch should be perpendicular to the workpiece; when the thickness is less than 5mm, the torch can be tilted backward by 5-10°; if the thickness exceeds 30mm, the torch can be turned towards Tilt the front 5-10°, when the cutting torch is to be cut through, the cutting torch can be perpendicular to the workpiece until the gas cutting is completed. If the preheated place is cut off, continue to increase the amount of high-pressure oxygen to increase the depth of the incision until it is completely cut.
• When gas cutting steel of different thicknesses, the choice of cutting nozzle and the adjustment of oxygen working pressure have a close relationship with gas cutting quality and work efficiency. For example, if the cutting nozzle is too small to cut thick steel, the cutting work cannot be carried out smoothly because of insufficient oxygen combustion and jetting ability. Even if it is cut off again and again, the quality is bad and the work efficiency is low. Conversely, if a too large cutting nozzle is used to cut thin steel, not only a large amount of oxygen and acetylene will be wasted, but the quality of gas cutting is also not good. So choose the size of the cutting nozzle. The relationship between the pressure of cutting oxygen and the thickness of the metal: Insufficient pressure, not only the cutting speed is slow, but also the slag is not easy to blow off, the incision is uneven, and sometimes it is not cut through; when the pressure is too high, in addition to the increase in oxygen consumption, the metal is also easy Cooling, so that the cutting speed is reduced, the incision is widened, and the surface is rough.

Characteristics of gas cutting

Advantage

• 1. The speed of cutting steel is faster than the blade mobile mechanical cutting process;
• 2. For cutting shapes and cutting thicknesses that are difficult to produce by mechanical cutting, gas cutting can be realized economically;
• 3. The equipment cost is lower than that of mechanical cutting tools;
• 4. The equipment is portable and can be used on site;
• 5. During the cutting process, the cutting direction can be quickly changed within a small radius;
• 6. Quickly cut large metal plates on site by moving the cutter instead of moving the metal block;
• 7. The process can be operated manually or automatically.

Disadvantage

• 1. The dimensional tolerance is significantly lower than that of mechanical tool cutting;
• 2. Although it can also cut easily oxidized metals like titanium, the process is basically limited to cutting steel and cast iron in the industry;
• 3. The preheating flame and the emitted red hot molten slag may cause fire and burn hazards to the operator;
• 4. Fuel combustion and metal oxidation require proper flue gas control and exhaust facilities;
• 5. Cutting high-alloy steel and cast iron requires improvement of the process flow;
• 6.Cutting high-hardness steel may require preheating before cutting and continuing to heat after cutting to control the metallographic structure and mechanical properties of the steel near the edge of the cut.
• 7. Gas cutting is not recommended for large-scale long-distance cutting.