Here is the formula for calculating the cutting speed [3] It is obvious that without increasing the arc current, significantly increase the cutting speed by increasing the arc voltage by the field strength of the arc column E, which is achieved by using molecular plasma-forming gases (N2, H2, O2, etc.) and intense compression column, ie, the increase in current density. Through the use of the vortex system forming the column density of current in the plasma torch nozzle for cutting can be increased up to 100 A/mm2. In this case the cutting speed is increased not only due to the growth capacity of the arc, as by reducing the average width of cut and increase the efficiency of the arc by reducing losses in the edge. Regularities of changes in field strength E and the column of the current density j at changing the composition and the plasma gas flow rate and nozzle diameter were considered earlier in the chapter "Plasma arc and its properties." Thus, with increasing E and j increase both productivity and efficiency of plasma arc cutting process, ie, reduced electricity costs and the loss of melted metal per meter cut. However, increasing the current density can be made to the limit, the limited stability of the plasma torch nozzle and is determined from the conditions of double increasing the thickness of the metal being cut, particularly at thicknesses of more than 60-80 mm, the change in current density in the nozzle is less impact on the performance of cutting, because of the weakening effect compression column, the temperature drop and soon STI plasma jet as the elongation of the arc column. The growth of the arc power by increasing the arc length is not sufficient. Therefore, there is a sharp rise in the need for current Id. With an increase in current increases and decreases the width of the cutting efficiency of the power of the arc, so that, despite the increase in power with increasing thickness of the metal, the cutting speed decreases and the cutting quality deteriorates [2]. Increasing the arc current has its limits, determined from the conditions of permissible thermal load on the cathode plasma torch. Currently, the limiting thickness of stainless steel, which can be cut, up to 150 mm, aluminum - 250 mm. Flame cutting carbon steel far exceeds these limits. In practice, the maximum current in the plasma cutting does not exceed 1000 A, arc voltage - 300, the cutting speed ~ 5 m / min. Cutting quality and speed are determined not only the energy parameters of the plasma arc, but also to a large extent thermophysical, chemical and metallurgical properties of plasma-forming gases. Recently, a plasma-forming environments using natural or artificial mixture, representing different combinations of the four main gases: argon, Ar, nitrogen, N2, H2 hydrogen and oxygen, O2. Due to the high heat capacity of hydrogen has the highest heat content at relatively low temperature plasma, and due to its high thermal conductivity allows to obtain the best conditions for heat transfer capacity of the plasma column in the metal, ie, the maximum rjn. Therefore, at the same power arc cutting speed in hydrogen and in mixtures based on it is higher than in other gases. Plasma jet on the basis of hydrogen retains high energy of the gas at a maximum length of the arc. Therefore, cutting in hydrogen mixtures is most advisable to apply for high-alloy steels of large thickness and high heat of metals such as copper and aluminum [2]. The use of hydrogen provides a clean cut surface. The economically most advantageous use of low-cost hydrogen gases: ammonia, consisting of 75% H2 and 25% N2, or so-called "mixed" gas, which is the feedstock for ammonia synthesis. In contrast to pure hydrogen ammonia explosion-proof, cheap. Consumption of hydrogen mixtures depends on the current cutting and is 2-4 m / h However, the hydrogen-containing mixtures have a significant disadvantage. Due to the high thermal conductivity of hydrogen, even at relatively low powers violated the thermal and electrical insulation of the plasma torch nozzle of the arc column, which leads to the destruction of the nozzle. Normal operation of the nozzle by using hydrogen plasma-forming gas is ensured only when they were added at least 20% argon. It is believed that due to thermal diffusion of argon, as a much more severe compared with the hydrogen gas accumulates in the walls of the nozzle and having a relatively low thermal conductivity elektroi provides thermal protection for the nozzle. Unfortunately, this protection is not entirely reliable, since the slightest deviation from the column axis of the nozzle leads to the destruction of the latter. Argon gas is scarce and expensive, used and transported in cylinders. Therefore, its use even as an admixture reduces the efficiency of the cutting process in hydrogen-containing mixtures. When cutting carbon steel, stainless steel and aluminum, the average thickness of commonly used technical nitrogen. The quality of plasma arc cutting for nitrogen is somewhat worse, and the rate is significantly lower than in hydrogen mixtures (due to less heat nitrogen plasma). In addition, there is a noticeable increase in nitrogen content of the melted layer of the cutting edge to a depth of 0.15 mm. Technical nitrogen is cheap, but its use in most cases is also associated with the need to use containers that are not always convenient. The simplest and most economical is created at the Institute of Electric them. Paton SSR apparatus which - zirconium cathode plasma torch with a special design - runs on compressed air. The air is oxygen-containing gas (78% N2 and 21% O2). As shown in Table. 2, in its heat content is close to the nitrogen. The maximum thermal conductivity of its Hschah (at T = 7000 ° K) higher than the Xmax of hydrogen (at T = 3800 ° K). Therefore plasma cutting jet has a higher concentration of energy walkie-talkie and a higher gi than hydrogen. In the presence of oxygen in the plasma of the heat goes into the cavity of the cutting, not only from the arc, but also because of the heat transfer resulting from the oxidation of iron. In addition, the presence of oxygen there is a significant decrease at the lower edge of the cut sheet. The rate of air-plasma cutting of steel in the 1.5 - 2.5 times higher than the cutting speed when using nitrogen as the plasma gas. Quality of cut is increased. Studies have shown that when more than in the air, the percentage of oxygen in the mixture of oxygen and nitrogen can slightly increase the cutting speed. However, from an economic point of view, the use of an artificial mixture is inappropriate. According to preliminary calculations plasma cutting carbon steel up to 50 mm and a half to two times less expensive than gas-oxygen. The quality of the cut in the air-plasma cutting of the above, almost completely eliminated burr, reduces strain when cutting thin sheet material and completely eliminates the need for gas cylinders. Air-plasma cutting can be successfully used for cutting non-ferrous alloys, but the purity of the cut surface is somewhat lower than the cutting of hydrogen mixtures. When the thickness of copper over 40 - 50 mm, 80-100 mm aluminum over cutting speed due to the decrease can not compete with the sharp in hydrogen mixtures. The ideal plasma-forming medium, representing a successful and cost effective combination of hydrogen with oxygen is water. Despite the lengthy studies to determine the feasibility of using water for cutting torches with water stabilization of the arc due to the complexity and unreliability of the present time has not yet found wide industrial applications. Least cost-effective plasma-forming gas is expensive and maloentalpiyny argon. However, due to the low voltage arc, he still finds wide application in hand-cut separation of non-ferrous and alloy steels for small and medium thickness, a single gas or a mixture of technical nitrogen.
See also:
Plasma Welding
Plasma Welding Introduction
Plasma Welding Technique
Microplasma Welding
Gases for plasma processing of materials
Separation of plasma jet cutting
Compression of the arc
The energy properties of the plasma arc
Rationalization of plasma welding
Plasma welding and spraying
The plasma melting and remelting
Plasmatron. Requirements for plasmatron
Plasmatron. Schemes, classification
Classification by type of electrode plasma torches
Classification of torches by the nature of the current
Structure of the plasmatron basic units
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