This article summarizes several common plasma cutting methods of stainless steel, aluminum, and carbon steel commonly used in pressure vessels, as well as their process characteristics and cutting parameters, as well as the selection of plasma cutting methods for different materials and cutting requirements.
Keywords: plasma cutting method; process characteristics; working gas; cutting speed; cutting characteristics
Stainless steel, aluminum and carbon steel are commonly used materials for pressure vessels. There are many blanking methods. Plasma cutting has become the most commonly used due to its high efficiency, wide application range, smooth cutting surface, small thermal deformation and suitable for processing various shapes. The blanking method plays an important role in the manufacture of pressure vessels.
Since plasma cutting uses working gas as a conductive medium, it carries heat, melts and processes metal and blows away the molten metal in the incision to achieve the cutting purpose, so different working gases have obvious characteristics, quality, speed and other aspects of plasma cutting. Impact. The following introduces several common plasma cutting methods and their process characteristics and the selection of plasma cutting methods when cutting different materials.
1 Commonly used plasma cutting methods and process characteristics
1.1 Plasma air cutting method
Plasma air cutting method uses dry compressed air as the processing gas. It is mainly used to cut carbon steel, but also can be used to cut stainless steel and aluminum. Since air is mainly composed of nitrogen and oxygen, when cutting carbon steel, the exothermic reaction of oxygen and iron in the incision provides additional heat, and at the same time generates FeO slag with low surface tension and good fluidity, which improves the molten metal in the incision. Fluidity, so not only the cutting speed is faster, but the cutting surface is smoother, the lower edge of the incision is basically free of slag, and the bevel angle of the cutting surface is small. When cutting stainless steel and aluminum, oxygen reacts with chromium and aluminum in stainless steel, and the cutting surface is rough. Generally, this processing method is not used when the cutting surface quality is high.
Plasma air cutting method mainly has the following shortcomings:
(1) There is a nitride layer attached to the cutting surface, and pores will be generated in the weld during welding. Therefore, the cutting edge for welding needs to be polished with a grinding wheel to remove the nitride layer. ?
(2) Due to oxidation, the electrodes and nozzles are easy to wear and have a short service life.
Due to the low cost of compressed air, this cutting method is widely used in the cutting of large quantities of non-welded carbon steel plates.
Under different current intensities, the relationship between common plate thickness and cutting speed is shown in Figure 1.
1.2 Plasma oxygen cutting method
The plasma oxygen cutting method uses oxygen as the working gas, and is mainly used for cutting carbon steel and aluminum. Oxygen has high dissociation heat, good heat carrying capacity, large heat release during particle recombination, and large amount of heat input into cutting, so a higher cutting speed can be obtained. When processing carbon steel, the iron-oxygen reaction in the cutting process provides a large amount of additional heat, which promotes the further increase of the cutting speed.
Compared with the plasma air cutting method, the plasma oxygen cutting method has the following advantages when cutting carbon steel:
(1) The cutting speed is faster;
(2) The cutting surface is smoother and has a metallic luster, especially without a nitride layer, which can be directly used for welding after cutting;
(3) The lower edge of the incision does not stick to slag;
(4) The cutting deformation is small and the accuracy is high.
Plasma oxygen cutting method also has the following shortcomings:
(1) Due to strong oxidation, the electrode wears faster and the service life is short;
(2) The cutting surface has a large bevel angle.
Under different current intensities, the relationship between common plate thickness and cutting speed is shown in Figure 2 and Figure 3.
1.3 Plasma nitrogen cutting method
The plasma nitrogen cutting method uses nitrogen as the working gas and is mainly used for cutting stainless steel. Nitrogen has better heat conduction and heat carrying performance, and the arc column is also longer, so it has better cutting ability. However, the surface quality of the cut is not very good, and there are nitrides on the cut surface. Compared with oxygen, the price of nitrogen is lower, so this cutting method is generally only used for stainless steel blanking that does not require high cutting surface quality and is not directly used for welding.
Under different current intensities, the relationship between common plate thickness and cutting speed is shown in Figure 4.
1.4 Plasma argon-hydrogen cutting method
The plasma argon-hydrogen cutting method uses a mixed gas of argon and hydrogen as the working gas, and is mainly used for cutting stainless steel and aluminum. Argon is easy to ionize and can form a stable plasma arc. In addition to the large atomic weight, the momentum of the plasma flow is also large. The thermal conductivity of hydrogen is good, and the ionized particles emit a high amount of heat when they recombine. The combination of the two can form a stable plasma arc with high energy density and long arc column. It has strong cutting ability. The width of the incision and the bevel of the cutting surface are small, the incision is smooth, and the production of nitrogen oxides during cutting is less. A method with better cutting quality. However, this cutting method uses mixed gas as the working gas, and a mixing device must be added to mix argon and hydrogen well.
Due to the high price of argon and hydrogen, and hydrogen is a dangerous gas, generally this cutting method is mainly used to cut thicker stainless steel and aluminum workpieces that cannot be processed by plasma cutting and require higher incisions, and the use environment meets safety requirements in the case of.
Under different current intensities, the relationship between common plate thickness and cutting speed is shown in Figure 5 and Figure 6.
1.5 Plasma nitrogen water vortex cutting method
The plasma nitrogen water vortex cutting method uses nitrogen as the working gas and is mainly used for cutting stainless steel and aluminum. The working gas forms a vortex flow through the vortex ring, so that the plasma flow is also directed to the workpiece in a vortex manner, so that a cutting edge with a very small bevel angle can be obtained. Around the working gas, there is a treated high-pressure water flow, which greatly increases the arc energy density, forming a plasma arc with extremely high temperature, good stiffness and high flow velocity. In addition, part of the water dissociates into hydrogen and oxygen, which also promotes the cutting process to a certain extent.
Compared with other plasma cutting methods, the plasma nitrogen water vortex cutting method has the following characteristics:
(1) Fast cutting speed;
(2) Good cutting quality. The incision width is small, the cutting surface is smooth, the bevel angle is very small, and the lower edge of the incision is not sticky?
(3) The cutting deformation is small and the accuracy is high;
(4) Long nozzle life;
(5) Suitable for underwater processing, basically no smoke, dust and arc, and low noise.
Under different current intensities, the relationship between common plate thickness and cutting speed is shown in Figure 7 and Figure 8.
2 Plasma cutting method selection when cutting different materials
From the above, there are a variety of plasma cutting methods available for different cutting materials and cutting requirements. The following compares the plasma cutting methods commonly used when cutting stainless steel, aluminum and carbon steel in terms of cutting speed, cutting quality, cutting deformation, pre-welding treatment, and operating costs. For specific operations, suitable cutting methods can be selected according to different cutting requirements. .
21 stainless steel (see Table 1)?
Table 1 Comparison of three plasma cutting methods for cutting stainless steel materials?
Cutting method, cutting speed, cutting quality, cutting deformation, before welding, processing operation cost
Plasma Argon—Hydrogen is slower, better, smaller, no need to be higher
Plasma Nitrogen is faster and poorer and generally needs lower
Plasma nitrogen water vortex is medium, very small, generally not necessary
The cutting speed comparison of several commonly used stainless steel cutting methods is shown in Figure 9. . 2 Aluminum (see Table 2)?
Table 2 Comparison of three plasma cutting methods for cutting aluminum materials?
Cutting method, cutting speed, cutting quality, cutting deformation, before welding, processing operation cost
Plasma Argon—Hydrogen is slower, better, smaller, no need to be higher
Plasma oxygen is faster and worse, generally needs moderate
Plasma nitrogen water vortex is medium, good, very small, generally not required to be low
The cutting speed comparison of several commonly used aluminum cutting methods is shown in Figure 10.
2.3 Carbon steel (see Table 3)?
Table 3 Comparison of two plasma cutting methods for cutting carbon steel materials?
Cutting method, cutting speed, cutting quality, cutting deformation, before welding, processing operation cost
Plasma air is faster, better, smaller, lower demand
Plasma oxygen is fast, small, not high
The cutting speed comparison of several commonly used carbon steel cutting methods is shown in Figure 11.
