In recent years, the rapid development of thermal cutting technology, especially plasma cutting technology, has made great progress in the design of the cutting torch structure, the improvement of the life of consumables, and control, and has expanded its application surface. At present, many enterprises in China use gas cutting and plasma cutting, and individual companies also use laser cutting. Under what conditions, what cutting methods are used, what is the reason, and how to make the work more optimal, Wuhan Baoba comprehensively analyzes this issue in this article.
1 Types of Hot Cutting Methods and Their Applications The commonly referred to as hot cutting methods mainly include gas cutting, plasma cutting, and laser cutting. Conventional gas cutting relies on the oxidation reaction of metal, so the cutting speed is limited by the speed of metal oxidation reaction. Plasma cutting relies on electric energy to melt the metal for cutting. Laser cutting relies on light energy to melt the metal for cutting. Both cutting methods can increase the cutting speed by increasing the cutting energy. In the field of gas cutting, in addition to using oxygen and gas for cutting, recently successfully used liquid oxygen and gas to achieve high-energy cutting methods in Germany, greatly improving the cutting efficiency, but the cutting machine is expensive, even industrial advanced countries in the production There are also few applications. Therefore, the former refers to gas cutting and specifically refers to automatic gas cutting machines. In the field of plasma cutting, depending on the type of plasma gas, it can be divided into air plasma cutting, oxygen plasma cutting, argon plasma cutting, and the like. At present, the cutting current of the air plasma cutting machine is below 120A, and the manual cutting machine of 30 to 70A accounts for the majority. In Japan, the market for air plasma cutting machines and automatic gas cutting machines is quite similar.
Oxygen plasma cutting machine is mainly used for cutting heavy plates in shipyards, and the cutting current is mainly 350 to 500A. This type of cutting machine has fewer users. In Japan, it only manufactures more than 100 [1]. The air plasma cutting machine is easy to produce nitrogen holes in the weld because of the nitrogen nitride cutting surface in the air. Oxygen plasma cutters overcome this problem. Argon-Hydrogen plasma cutting machine is specially used for cutting aluminum and thick stainless steel. Its cutting current is mainly 500 to 1000A. 2 Development of plasma cutting technology Plasma cutting technology is to cut stainless steel, aluminum and other materials. However, since its development, plasma cutting technology has been used to cut carbon steel. The main reason for this is that the plasma cutting speed is high and it is easy to set cutting conditions, which is more suitable for automation and unmanned operation. The gas cutting process requires frequent adjustment of the flame, the cutting speed is slow, and the preheating time limits the cutting productivity. In contrast, it is not suitable for automation and unmanned operation.
The key technology for air or oxygen plasma cutting is the life of consumables in the cutting torch. Due to the oxidizing properties of air or oxygen, the electrodes and nozzles in the plasma torch must be resistant to the high temperatures of the arc in the oxidizing atmosphere. Therefore, current electrodes use high-melting germanium materials and are embedded in copper with special processing methods. The nozzle adopts a double-layer air flow structure, which has the advantages of improving the air flow directionality, speed, ion gas purity, cooling, etc., protecting the nozzle and further compressing the arc. In addition, in order to reduce the electrode consumption, the cutting gas does not use pure oxygen but uses a mixed gas of nitrogen and oxygen at the time of arc starting, and adopts a relatively small gas flow rate, which is switched to oxygen after arcing. In addition, the contact of the nozzle with the cutting material during the cutting process will also damage the nozzle, which requires efforts on the height control of the cutting torch. In short, the life of the electrode and nozzle is greatly improved than in the past, and the corresponding maximum oxygen plasma cutting current has reached 500 A. According to the literature [2], 400A oxygen plasma cutting, each electrode can work for 3.5 hours, each nozzle can work At 4.8h, the data presented in [3] is greatly improved.
3 Comparative analysis of the thermal cutting method 3.1 Cutting efficiency From the perspective of cutting productivity, not only the maximum thickness of the cutting but also the cutting speed should be considered.
Comparing the cutting efficiency of plasma cutting and CO2 laser cutting, 50A air plasma cutting efficiency is equivalent to 1.5kW laser cutting efficiency, while the former of 70A and 150A is equivalent to the latter of 2kW and 6kW. High-power laser cutting machines with an oxygen plasma cutting efficiency of 300 A or more are not yet available. Therefore, from the perspective of cutting efficiency, when cutting carbon steel below 50 mm, the plasma cutting efficiency is the highest. When cutting carbon steel above 50 mm, liquid oxygen cutting has the highest cutting efficiency.
3.2 Cutting quality comparison Gas cutting, plasma cutting, laser cutting cutting surface roughness, cutting accuracy, cutting seam width, heat affected zone size, these factors represent the cutting quality. As long as the cutting method and the cutting conditions are properly selected, the edge rounding on the cutting surface and the slag at the bottom edge can be overcome, so these two items are not included in the table.
In terms of cutting surface roughness, plasma cutting is generally preferred. With special attention, laser cutting is not inferior to plasma cutting.
Laser cutting is optimal in terms of cutting accuracy and cutting seam width. The accuracy of gas cutting and plasma cutting can be guaranteed if the accuracy and movement characteristics of the walking mechanism equipped with the cutting torch can be guaranteed.
The width, deformation and cutting speed of the heat-affected zone produced by cutting have a great relationship. The cutting speed of gas cutting is slow, the width of heat affected zone is wide, and the deformation is also large. The narrower the general cutting gap, the faster the cutting speed, the smaller the heat input to the material being cut, and the smaller the deformation caused by cutting. As a result of tests and comparative analysis, the laser cutting has the smallest deformation, the plasma cutting has the second largest deformation, and the gas cutting has the largest deformation.
3.3 Cutting costs The cost of purchasing cutting equipment depends on the production volume. The greater the production volume, the smaller the relative cost of purchasing equipment. However, cutting costs are proportional to the amount of production. Compared with gas cutting, plasma cutting and laser cutting cutting costs, cutting costs include replacement parts costs, gas charges, electricity costs, labor costs. The cutting cost is the cutting cost per unit length. If the stereotype analysis of cutting costs, cut thick plate with gas cutting is beneficial, cutting thin plate laser cutting or plasma cutting is favorable, and the plasma cutting cost is always lower than the laser cutting cost.
The corresponding cutting thickness at the intersection of the laser cutting and gas cutting cost curves is 15 to 20 mm, and the corresponding cutting thickness at the intersection of the plasma cutting and gas cutting cost curves is 30 to 50 mm.
4 Conclusion In China, there are few laser cutting applications, and plasma cutting is developing rapidly. Although the main purpose of companies currently purchasing plasma cutting machines is to cut stainless steel, aluminum and other materials, some companies may have realized that plasma cutting is used to cut carbon steel. In this situation, it is necessary to raise awareness and promote application.
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