When cutting metal materials with plasma arc, argon, nitrogen, hydrogen, oxygen or their mixed gases can be used as cutting gas. Select the appropriate gas type according to the type and thickness of the material to be cut and the cutting process conditions. The selection and applicability of commonly used gases for plasma arc cutting are shown in Table-Selection of commonly used gases for plasma arc cutting and Table-Applicability of various gases in plasma arc cutting.
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 useful in preventing electrodes and nozzles from burning. When pure argon is used as the cutting gas, the no-load voltage is low, but it has poor heat carrying properties, low thermal conductivity, and short arc columns, making it unsuitable for cutting thicker workpieces. Nitrogen in particular is expensive and therefore not usually used alone.
Nitrogen also has a lower ionization potential, but its atomic weight is smaller than that of argon. It is a diatomic gas. It absorbs more heat when the molecules are decomposed, 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. Therefore, it is often used as a working gas alone. However, because the atomic weight is smaller than that of argon, the power supply is required to have a high no-load voltage.
Nitrogen reacts with metal at high temperatures and has a strong corrosive effect on the electrode. Especially in situations where the gas pressure is high, argon or hydrogen should be added. 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 and good thermal conductivity. It absorbs a large amount of decomposition heat during decomposition. Therefore, pure hydrogen is not suitable to form a stable plasma arc. Therefore, hydrogen alone is usually not used as a cutting gas. In addition, hydrogen has reducing properties and helps improve the quality of the cutting surface.
Oxygen is a diatomic gas with high dissociation heat and good heat-carrying properties. It invests a lot of heat into the workpiece during cutting, so it can be used as a working gas alone. It has oxidizing properties. Especially when cutting iron-based metals, both the high-temperature plasma arc melting and cutting process and the iron-oxygen combustion exothermic process occur, which increases heat and accelerates the cutting process. However, ordinary tungsten electrodes will be burned quickly, so special electrode materials and cutting torch structures are required.
Air is a mixture of nitrogen, oxygen, etc. Air contains about 80% nitrogen and about 20% oxygen. Its main characteristics are close to those of nitrogen, and it also has some oxidizing characteristics. It is the most widely used working gas. But it has the disadvantages of both nitrogen and oxygen.
Any two gases among argon, nitrogen and hydrogen are mixed and used. They can complement each other and each can exert its own specialties. When using hydrogen, you must pay attention to safety issues. In addition to ensuring that 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 through first, and then the hydrogen valve is opened after igniting the arc. After cutting is completed, the hydrogen valve should be closed first.