Standard notation
PAC - Plasma Arc Cutting - plasma arc cutting
Plasma Technology
Plasma is an ionized gas with a high temperature that can conduct electric current. Plasma arc is obtained from the usual in a special device - a plasma torch - as a result of compression and injection of plasma-forming gas in it. There are two schemes:
- plasma-arc cutting and
- cutting of the plasma jet.
 |
| Plasma cutting process |
When cutting a plasma jet arc between the electrode and the tip forming a plasma torch, and the object being processed is not included in the circuit (arc indirect effect). Part of the plasma arc column is removed from the plasma torch in a high-speed plasma jet, the energy which is used for cutting.
Plasma-arc cutting is more effective and widely used for metal working. Cutting of the plasma jet is used less frequently and primarily for the treatment of nonmetallic materials, since they need not be conductive.
A more detailed diagram of the plasma torch for plasma arc cutting is shown in the figure below.
 |
| Plasma Torch |
A pillar of the arc forming the channel fills. In the arc chamber plasma-forming gas is supplied. It is heated by the arc, and ionized by thermal expansion increases the volume of 50-100 times, which causes him to bleed from the plasma torch nozzle at a speed of 2-3 km / c and more. The temperature in the plasma arc can reach 25000-30000 ° C.
 |
| Plasma cutting of metal |
 |
| Nozzles (in section) for plasma cutting - copper (left) and copper with a tungsten insert company Thermacut (right) |
q p = V p · F · γ · c · [(T mp -T 0 ) + q] · 4,19,
where V p - cutting speed (cm / s); F - cross sectional area of the zone of melted metal (cm 2 ); y - metal density (g / cm 3 ) with - specific heat, J / (g · ° C); T Square - the melting point metal (° C); T 0 - temperature of the metal prior to cutting (° C); q - latent heat of melting (° C).
The product of V p · F · γ determines the mass of melted metal per unit time (g / s). For a given thickness of metal has a definite numerical value of the effective thermal capacity q p , below which the cutting is impossible.
The flow rate of the plasma, which removes the molten metal increases with increasing plasma gas flow rate and current strength and decreases with increasing diameter of the plasma torch nozzle. It can reach about 800 m / s at a current of 250A.
Plasma-forming gases
Technological capabilities of plasma cutting process (speed, quality, etc.), as well as the characteristics of the basic units of plasma torches are determined primarily by the plasma-forming medium. Effect of plasma-forming medium on the cutting process:
- by changing the composition of the medium can regulate a wide range of thermal energy released in an arc, because at a certain geometry of the nozzle, and given current composition of the medium determines the field strength of the arc column inside and outside of the nozzle;
- of the plasma-forming medium has the greatest influence on the maximum value of the ratio of current to the diameter of the nozzle that allows you to adjust the current density in the arc, the magnitude of heat flux in the cavity of the cut, and thus, determine the width of cut and cutting speed;
- the composition of the plasma-forming mixture depends on its thermal conductivity, which determines the transmission efficiency of the cut sheet of the heat energy released in the arc;
- in some cases is a very large addition of heat energy released by chemical interaction of plasma-forming environment of the cut metal (it can be comparable with the electrical output of the arc);
- plasma-forming medium in the interaction with the lost wax metal gives the ability to change its viscosity, chemical composition, the magnitude of surface tension;
- choosing the composition of plasma-forming environment, you can create the best conditions for the removal of the molten metal from the cut cavity and prevent the formation of swim up to the lower edges of the cut sheet or making them easily removable;
- the composition of the medium depends on the nature of physico-chemical processes on the walls of the cut and the depth of gas-saturated layer, so that for certain metals and alloys, some of the plasma-forming mixture are not allowed (such as those containing hydrogen and nitrogen in the case of cutting of titanium), the range of acceptable mixtures and narrows with increasing thickness of cut sheets and thermal conductivity of the material.
- the cathode material and the design of the cathode assembly (method of attachment of the cathode in a plasma torch and the intensity of its cooling);
- design of the cooling system of nozzles;
- power supply, as well as its external form of the static characteristics and dynamic properties;
- scheme to control the equipment, because the composition and plasma gas flow completely determine the timeline of forming a working arc.
TABLE. The most common plasma-forming gases
| Gas | The processed metal | ||
Aluminum, copper and their alloys | Corrosion-resistant steel | Carbon and low alloy steels | |
| Compressed air | For harvesting machine cutting | For the cost of manual and machine cutting | |
| Oxygen | It is not recommended | – | For machine cutting of high quality |
Azotno-oxygen mixture | It is not recommended | For cutting machine with high speed | |
| Nitrogen | For the cost of manual and machine cutting | For manual and semi-automatic cutting | – |
Argono-hydrogen mixture | For the cutting edge of high-quality | It is not recommended | |
Technology of plasma cutting of metal
Plasma cutting is economically expedient for processing:
- aluminum and its alloys with thickness up to 120 mm;
- copper thickness of 80 mm;
- alloy and carbon steels up to 50 mm;
- iron thickness of 90 mm.
Cutter have as close to the edge of the cut metal. After pressing the switch the cutter first duty arc is ignited, and then cutting the arc, and the process of cutting. The distance between the surface of the metal being cut and the end of the tip of the cutter should remain constant. Doug should be directed downward and is usually at right angles to the surface of the cut sheet. The cutter is moved slowly along the planned cutting line. The velocity must be adjusted so that the sparks were visible on the reverse side of the cut metal. If they are not visible from the back side, so do not cut through the metal through and through, which may be due to insufficient current, excessive speed, or orientation of the plasma jet is not perpendicular to the surface of the cut sheet.
To obtain a clean cut (with little or no scale, and deformation of the cut metal), it is important to choose the correct cutting speed and the power supply. You can run a few test cuts on a higher current, reducing it if necessary, depending on the speed of movement. At higher currents, or low-speed cutting overheat cut metal, which can lead to the formation of scale.
Plasma arc cutting of aluminum and its alloys, a thickness of 5-20 mm is usually performed in nitrogen, with a thickness of 20 to 100 mm - in the nitrogen-hydrogen mixtures (65-68% 32-35% nitrogen and hydrogen), a thickness over 100 mm - in argon- hydrogen mixtures (35-50% hydrogen) and using plasmatrons with additional stabilization of the arc with compressed air. With manual cutting in the argon-hydrogen mixture to ensure a stable arc of hydrogen should not exceed 20%.
Air-plasma cutting of aluminum, usually used as a separator in the procurement of parts for subsequent machining. Good quality of cut is usually achieved only in thickness and 30 mm at a current of 200 A.
Plasma cutting of copper may be carried out in nitrogen (at a thickness of 5-15 mm), compressed air (for small and medium thickness), argon-hydrogen mixture. As copper has high thermal conductivity and heat capacity for processing it requires more powerful arc than for cutting steel. When the air-plasma cutting edges are formed on the copper is easily removed excess metal (burr). Cutting brass is at a higher rate (20-25%), using the same plasma-forming gases, and copper.
Plasma cutting high-alloy steels is effective only for thicknesses up to 100 mm (thickness is used for large oxygen-Flux cutting). With a thickness of 50-60 mm can be used an air-plasma cutting and hand-cut in nitrogen, at thicknesses of more than 50-60 mm - nitrogen-oxygen mixture.
Cutting of stainless steels up to 20 mm can be carried out in nitrogen, a thickness of 20-50 mm - in the nitrogen-hydrogen mixture (50% nitrogen and 50% hydrogen). It is also possible to use compressed air.
Plasma cutting low-carbon steels is most effective in the compressed air (especially for thicknesses up to 40 mm). At thicknesses above 20 mm can be cut in nitrogen and nitrogen-hydrogen mixtures.
For the cutting of carbon steels using compressed air (usually at thicknesses of 40-50 mm), oxygen, and nitrogen-oxygen mixture.
TABLE. Estimated modes of air-plasma cutting
| Cut the material | Mode Options | ||||||
| Thickness (mm) | The diameter of the nozzle (mm) | Power current (A) | Voltage relation (B) | Flow of air (l / min) | The cut rate (m / min) | The average width of cut (mm) | |
| Aluminum | 5–15 | 2 | 120–200 | 170–180 | 70 | 2–1 | 3 |
| 30–50 | 3 | 280–300 | 170–190 | 40–50 | 1.2–0.6 | 7 | |
| Copper | 10 | 3 | 300 | 160–180 | 40–60 | 3 | 3 |
| 20 | 1.5 | 3.5 | |||||
| 30 | 0.7 | 4 | |||||
| 40 | 0.5 | 4.5 | |||||
| 50 | 0.3 | 5.5 | |||||
| 60 | 3.5 | 400 | 0.4 | 6.5 | |||
| Steel 12Kh18N10T | 5–15 | 3 | 250–300 | 140–160 | 40–60 | 5.5–2.6 | 3 |
| 10–30 | 160–180 | 2.2–1 | 4 | ||||
| 31–50 | 170–190 | 1–0.3 | 5 | ||||
- significantly higher rate of cutting of small and medium thickness;
- Versatility - plasma cutting is used for steel, aluminum and its alloys, copper alloys, iron and other materials;
- accurate and high-quality cuts, and in most cases, eliminated or significantly reduced the subsequent machining;
- efficiency of air-plasma cutting - no need for expensive gases (acetylene, oxygen, propane-butane);
- ability to cut parts of complex shape;
- a very short time burning (with oxygen cutting takes a long preheating);
- Safer, because there is no explosive gas cylinders;
- low levels of pollution.
- Maximum thickness of cut is typically 80-100 mm (oxygen cutting process can be cast, and some steel up to 500 mm);
- more expensive and complicated equipment;
- increased maintenance requirements;
- the angle of deviation from perpendicularity of the cut should not exceed 10-50 º, depending on the thickness of the part (if not greatly extended cut, which leads to a rapid deterioration of consumables);
- there is practically no possibility of using two hand torches connected to a single unit;
- increased noise due to the outflow of gas from the plasmatron at transonic speeds;
- harmful nitrogen allocation (using nitrogen) - a cut to reduce the product is immersed in water.
Plasma cutters - the designs and types
Plasma metal cutting apparatus
Plasma cutting machines
Plasma cutting
Plasma welding
Cutting technology:
No comments:
Post a Comment