Abstract: This paper introduces the types, application status and new progress of plasma cutting technology, and compares the cutting efficiency, cutting quality and cutting cost of three kinds of thermal cutting methods. Under the premise of the same cutting quality, from the viewpoint of cutting efficiency and cutting cost, when the thickness of carbon steel is 30 to 50 mm or less, plasma cutting is suitable, and when the thickness is above this thickness, gas cutting is appropriate.
Key words : plasma cutting; efficiency; quality; cost In recent years, thermal cutting technology has developed rapidly, especially plasma cutting technology, which has made great progress in cutting torch structure design, life expectancy of consumables, and control, and expanded its Application surface. Nowadays, many companies in China use both gas cutting and plasma cutting, and individual companies also use laser cutting. Under what conditions, what is the method of cutting, what is the reason, how to make the work more optimized, this article comprehensively analyzes this issue.
1 Types of thermal cutting and their application
The so-called hot cutting method mainly refers to gas cutting, plasma cutting, and laser cutting. Conventional gas cutting relies on the oxidation reaction of metals to cut, so the cutting speed is limited by the metal oxidation reaction rate. Plasma cutting relies on electric energy to melt metal for cutting. Laser cutting relies on light energy to melt metal for cutting. These two cutting methods can increase the cutting speed by increasing the cutting energy. In the field of gas cutting, in addition to cutting with oxygen and gas, the recent successful research in Germany using liquid oxygen and gas to achieve high-energy cutting has greatly improved the cutting efficiency, but the cutting machine is expensive, even in industrialized countries. There are also few applications. Therefore, the gas cutting refers to the former, and specifically refers to the automatic gas cutting machine. 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 hydrogen plasma cutting, and the like. Now the cutting current of air plasma cutting machine is below 120A, and the manual cutting machine of 30~70A is the majority. In Japan, the number of air plasma cutting machines and automatic gas cutting machines sold in the market has been comparable [1].
Oxygen plasma cutting machine is mainly used for thick plate cutting in shipyards, and its cutting current is mainly 350-500A. There are fewer users of this kind of cutting machine. In Japan, only more than 100 sets are manufactured in the year [1]. Air plasma cutting machine is easy to generate nitrogen gas in the weld seam when the submerged arc welding is directly performed due to the nitrogen nitride nitride cutting surface in the air. The oxygen plasma cutting machine overcomes this problem. The argon hydrogen plasma cutting machine is specially used for cutting aluminum and thick plate stainless steel, and its cutting current is mainly 500-1000A.
2 Development status of plasma cutting technology
The development of plasma cutting technology is to cut stainless steel, aluminum and other materials. However, until now, plasma cutting technology has been used to cut carbon steel. The main reason is that the plasma cutting speed is fast, the cutting conditions are easy to set, and it is more suitable for automation and unmanned operation. The cutting process of gas cutting requires frequent adjustment of the flame, the cutting speed is slow, and the preheating time limits the cutting productivity, which is not suitable for automation and unmanned operation.
The key to air or oxygen plasma cutting is the consumable life in the cutting torch. Due to the oxidizing properties of air or oxygen, the electrodes and nozzles in the plasma cutting torch must be resistant to arcing temperatures in an oxidizing atmosphere. Therefore, the current electrode uses a high melting point tantalum material and is embedded in copper in a special processing method. The nozzle adopts a double-layer airflow structure, which has the advantages of improved airflow directivity, speed, ion gas purity, cooling, etc., which protects the nozzle and further compresses the arc. In addition, in order to reduce the electrode consumption, the cutting gas does not use pure oxygen when starting the arc, and uses a mixture of nitrogen and oxygen, and adopts a small air flow speed, and switches to oxygen after the arc is started. In addition, the contact of the nozzle with the cutting material during the cutting process will also damage the nozzle, which is to work on the height control of the cutting torch. In short, the life of the electrode and nozzle is much higher than in the past, and the oxygen plasma cutting current has reached 500A. According to the literature [2], 400A oxygen plasma cutting, each electrode can work for 3.5h, each nozzle can work. 4.8h, which is much better than the data presented in [3].
3 Comparative analysis of hot cutting method
3.1 Cutting efficiency
From the point of view of cutting productivity, it is necessary to see not only the maximum thickness of the cutting but also the corresponding cutting speed. Figure 1 summarizes the relationship between the cutting speed of three thermal cutting methods and the thickness of the cutting plate. The cutting material is carbon steel [1].
Compared to the cutting efficiency of plasma cutting and CO 2 laser cutting, the 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. A high-power laser cutting machine equivalent to an oxygen plasma cutting efficiency of 300 A or more has not yet been released. Therefore, from the perspective of cutting efficiency, when cutting carbon steel of 50 mm or less, the plasma cutting efficiency is the highest, and when cutting carbon steel of 50 mm or more, the cutting efficiency of liquid oxygen cutting is the highest.
3.2 Cutting quality
Table 1 compares the cutting surface roughness, cutting accuracy, cutting width, and heat affected zone size of gas cutting, plasma cutting, laser cutting [1], which represent the cutting quality. As long as the cutting method and cutting conditions are correctly selected, the rounding of the edge of the cutting surface and the slag of the lower edge can be overcome, so these two items are not listed.
In terms of the roughness of the cut surface, plasma cutting is generally preferred. As long as special attention is paid, laser cutting is not inferior to plasma cutting.
Laser cutting is optimal in terms of cutting accuracy and slit width. The accuracy of gas cutting and plasma cutting can be guaranteed if the accuracy and motion characteristics of the running mechanism with the cutting torch can be guaranteed.
The width and deformation of the heat-affected zone produced by cutting have a great relationship with the cutting speed. The cutting speed of gas cutting is slow, the heat-affected zone is wide, and the deformation is also large. Generally, the narrower the slit is, the faster the cutting speed is, the smaller the heat input to the material to be cut is, and the deformation caused by the cutting is small. The results of the test and comparative analysis are that the deformation of the laser cutting is the smallest, the deformation of the plasma cutting is second, and the deformation of the gas cutting is the largest.
3.3 Cutting costs
The cost of purchasing cutting equipment depends on the production volume. The larger the production volume, the smaller the relative cost of purchasing equipment. But the cutting cost is proportional to the production. Figure 2 compares the cutting costs of gas cutting, plasma cutting, and laser cutting. The cutting cost includes replacement parts cost, gas cost, electricity cost, and labor cost. The cutting cost is the unit length cutting cost.
Figure 1 Comparison of cutting efficiency of three thermal cutting methods
Figure 2 Comparison of cutting costs of three thermal cutting methods
Table 1 Cutting quality of three kinds of thermal cutting methods
| Cutting method | Cutting surface roughness / μm | Cutting accuracy / mm | Cutting width/degree | Heat affected zone width | Cutting speed |
| laser cutting | 20~100m | 0.05~0.1/mm | 0.3 to 0.6 mm | small | fast |
| Plasma cutting | 30~100 | 0.5 to 1.0 | 1.0 | small | fast |
| gas cutting | 50 | 0.5 to 1.0 | 1.0 | Big | slow |
If the cutting cost is determined by cutting, the cutting of the thick plate is advantageous for gas cutting, and the cutting of the thin plate is advantageous by laser cutting or plasma cutting, and the cost of plasma cutting is always lower than the cost of laser cutting.
The corresponding cutting thickness of the intersection of the laser cutting and gas cutting cost curves is 15-20 mm, and the corresponding cutting thickness of the intersection of the plasma cutting and gas cutting cost curves is 30-50 mm.
4 Conclusion
In China, laser cutting applications are rare, and plasma cutting is developing rapidly. Although the main purpose of purchasing plasma cutting machines is to cut stainless steel, aluminum and other materials, some companies may have realized the advantages of plasma cutting for cutting carbon steel. In this situation, it is necessary to raise awareness and promote application.
About the author : Hang Zhengxiang (1961-), male, Shenyang, Liaoning, associate professor of Shenyang University of Technology, doctoral student.
Author: Hang fight Xiang Fu Qin Sheng Shenyang Institute of Materials Science and Engineering, Shenyang 110023
Ma Xuezhi, Ma Rong, Shenyang University of Technology, Welding Equipment Company, Shenyang 110023, China
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