When plasma arc cutting metal materials, argon, nitrogen, hydrogen, oxygen or their mixed gases can be used as the cutting gas. Select the appropriate gas type according to the type and thickness of the material to be cut and the cutting process conditions. See Table Selection of Common Gases for Plasma Arc Cutting and Table Applicability of Various Gases in Plasma Arc Cutting for the selection and applicability of commonly used gases for plasma arc cutting.
Argon is a monatomic gas with a large atomic weight, low 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 good for preventing electrodes and nozzles from burning. When pure argon is used as the cutting gas, the no-load voltage is low, but its thermal conductivity is poor, the thermal conductivity is small, and the arc column is short, so it is not suitable for cutting workpieces with large thickness. Helium in particular is expensive and is therefore not usually used alone.
The ionization potential of nitrogen gas is also lower, but its atomic weight is smaller than that of argon gas. It is a diatomic gas. It absorbs the largest heat when its molecules decompose, and has better heat conduction and heat carrying properties. In addition, the nitrogen plasma arc has a long arc column and a large cutting capacity. , so it is often used alone as a 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 the metal at high temperature, and has a strong corrosion 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 produced during cutting.
Hydrogen has the smallest atomic weight, good thermal conductivity, and absorbs a large amount of decomposition heat during decomposition. Therefore, pure hydrogen is not suitable for forming a stable plasma arc, so hydrogen is usually not used alone as a cutting gas. In addition, hydrogen is reductive, which helps to improve the quality of the cut surface.
Oxygen is a diatomic gas with high heat of dissociation and good heat carrying capacity. It can be used as a working gas alone because it puts a lot of heat into the workpiece during cutting. It is oxidizing, especially when cutting iron-based metals, both the melting process of high-temperature plasma arc and the exothermic process of iron-oxygen combustion occur, increasing heat and accelerating the cutting process. However, the general tungsten electrode will be burned quickly, so special electrode materials and torch structures need to be used.
Air is a mixture of nitrogen and oxygen etc. The air contains about 80% nitrogen and about 20% oxygen. Its main characteristics are close to those of nitrogen, and it has some characteristics of oxidation. It is the most widely used working gas. But it has both nitrogen and oxygen deficiencies.
Argon, nitrogen, and hydrogen are used in combination of any two gases, and they learn from each other and each exerts its own strengths. When using hydrogen, you must pay attention to the use of safety issues. In addition to paying attention to pipelines, joints, valves, etc. – must not leak, you should also pay attention to closing them in time after cutting. When using nitrogen-hydrogen mixed gas for cutting, in order to make arc ignition easier, nitrogen gas is generally passed first, and then the hydrogen valve is opened after ignition of the arc. After cutting, the hydrogen valve should be closed first.
During 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 the electrode material, the material with strong electron emission ability, small work function and small electrode burning loss during cutting should be selected. Practice has proved that the burning loss is still quite serious when tungsten with high melting point is used as the electrode. The thoriated tungsten electrode made by adding a small amount of oxide needles to tungsten has a smaller burning loss than pure tungsten and stable arc. However, the thorium tungsten electrode contains oxidized needles with a mass fraction of 1.2% to 2.0%. Since thorium is a radioactive element, it is harmful to manufacturers and users, and it is not used in China at present. In recent years, cerium tungsten electrodes (with a mass fraction of cerium oxide of 3%) have been widely produced and used in China. The electrode of this material has better electron emission ability and burning resistance than thoriated tungsten, and the tip of the electrode can still maintain a sharp point after burning, which is helpful for maintaining long-term stable cutting and arc compression, and improving Cutting efficiency is favorable. At the same time, the cerium tungsten electrode is not radioactive, which is beneficial to the 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: pencil shape and mosaic structure. In addition to the properties of the electrode material, the diameter and shape of the electrode also affect the burning of the electrode and the stability of the arc. The electrode tip should not be too sharp or too blunt. If the tungsten is too sharp, it is easy to burn, and if it is too blunt, the cathode spots will easily drift, which will affect the stability of cutting, and even produce double arcs or burn out the nozzle. The pencil electrode is shown in Figure Pencil electrode tip shape. There are also some units that grind the electrode into a pointed shape, burn off the pointed end after burning, and naturally form a most suitable electrode shape.
The mosaic structure electrode is composed of pure copper base and electron-emitting electrode metal, and its structure is shown in Figure Shape and structure of direct water-cooled mosaic electrodes.The electrode metal uses cerium tungsten, yttrium tungsten alloy, zirconium and hafnium, etc., and usually adopts direct water cooling, which can withstand large working current and reduce electrode loss.