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When plasma arc cuts metal materials, argon, nitrogen, hydrogen, oxygen or their mixtures can be used as cutting gases. According to the type and thickness of the material to be cut, and the cutting process conditions, select the appropriate gas type. The selection and applicability of common gases for plasma arc cutting are shown in Table Selection of Common Gases for Plasma Arc Cutting and Table 2-12.
Argon is a monatomic gas with large atomic weight, small thermal conductivity and low ionization potential, so it is easy to form a plasma arc with high ionization degree and good stability. Argon is an inert gas, which is beneficial to prevent electrode and nozzle burnout. When pure argon is used as the cutting gas, the no-load voltage is low, but its heat carrying capacity is poor, the thermal conductivity is small, and the arc column is short, so it is not suitable for cutting workpieces with large thickness. Argon, in particular, is expensive and is therefore not usually used alone.
The ionization potential of nitrogen is also lower, but the atomic weight is smaller than that of argon. It is a diatomic gas. It absorbs more heat during molecular decomposition, and has better thermal conductivity and heat-carrying properties. Used alone as working gas. However, because the atomic weight is smaller than that of argon, the power supply is required to have a high no-load voltage.
Nitrogen will react with metal at high temperature, and has a strong erosive effect on the electrode. Especially in the case of high gas pressure, it is advisable to add argon or hydrogen. In addition, when nitrogen is used as the working gas, the cutting surface will be nitrided, and more nitrogen oxides will be generated during cutting.
Hydrogen has the smallest atomic weight, good thermal conductivity, and absorbs a lot of decomposition heat during decomposition. Therefore, pure hydrogen is not suitable to form a stable plasma arc, so hydrogen is usually not used as cutting gas alone. In addition, hydrogen has reducibility, which helps to improve the quality of the cut surface.
Oxygen is a diatomic gas with high dissociation heat and good heat-carrying properties. It puts a lot of heat into the workpiece during cutting, so it can be used alone as a working gas. It has oxidizing properties, especially when cutting iron-based metals, not only the melting and cutting process of high temperature plasma arc, but also the exothermic process of ferrite combustion, which increases the heat and can speed up the cutting process. However, the general tungsten electrode will be burnt out quickly, so special electrode material and torch structure must be used.
Air is a mixture of nitrogen and oxygen, etc. Air contains about 80% nitrogen and about 20% oxygen. Its main characteristics are close to nitrogen, and it has some oxidative characteristics. It is the most widely used working gas. But it has both nitrogen and oxygen deficiencies.
Argon, nitrogen and hydrogen can be mixed with any two gases, and they complement each other and exert their own strengths. When using hydrogen, attention must be paid to the use of safety issues. In addition to paying attention to pipelines, joints, valves, etc., which must not leak, it should also be closed in time after cutting. When using nitrogen-hydrogen mixed gas for cutting, in order to make arc ignition easy, nitrogen gas is generally passed first, the arc is ignited, and then the hydrogen valve is opened. After cutting, the hydrogen valve should be closed first.
When plasma arc cutting, DC positive connection is usually used, that is, the electrode is connected to the negative electrode, and the workpiece is connected to the positive electrode. When selecting electrode materials, materials with strong electron emission capability, small work function and small electrode burnout during cutting should be selected. Practice has proved that using tungsten with high melting point as the electrode, its burning loss is still quite serious. The thorium tungsten electrode made by adding a small amount of thorium oxide to tungsten has a very small burning loss and stable arc than pure tungsten. However, the thorium tungsten electrode contains thorium oxide with a mass fraction of 1.2% to 2.0%. Since thorium is a radioactive element, it has certain hazards to manufacturers and users, and is currently not used in China. In recent years, cerium tungsten electrodes (containing cerium oxide with a mass fraction of 3%) have been widely produced and used in China. The electrode of this material has better electron emission capability and better resistance to burnout than thorium tungsten. After the electrode is burnt, the tip of the electrode can still maintain a pointed tip, which is useful for maintaining long-term stable cutting and maintaining arc compression. cut
Efficiency is beneficial. At the same time, the cerium tungsten electrode has no radioactivity, which is beneficial to the labor protection of the operator. Therefore, cerium tungsten electrodes should be used as much as possible.
There are two types of electrodes for plasma arc cutting: pen-shaped and mosaic. In addition to the properties of electrode materials, electrode diameter and shape also affect electrode burnout and arc stability. The electrode tip should not be too sharp or too blunt. Too sharp tungsten is very easy to burn, too dull and the cathode spot is easy to drift, which affects the stability of cutting, and even produces double arcs or burns the nozzle. The pencil electrode is shown in Figure Pen electrode tip shape. There are also some units that grind the electrode into a pointed shape, and burn the pointed end after burning to naturally form a most suitable electrode shape.
The damascene structure electrode is composed of pure copper base and electron-emitting electrode metal. Its structure is shown in Figure 2-38. The electrode metal uses cerium-tungsten, yttrium-tungsten alloy, zirconium and hafnium, etc., usually by direct water cooling, which can withstand large the working current and reduce the electrode loss.