Selection of cutting process parameters of the hot

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Selection of cutting process parameters of NC plasma cutting machine

first, cutting current: it is the most important cutting process parameter, which directly determines the thickness and speed of cutting, that is, cutting ability. Effects: 1. The cutting current increases, the arc energy increases, the cutting capacity increases, and the cutting speed increases accordingly; 2. The cutting current increases, the arc diameter increases, and the arc thickens, making the notch wider; 3. If the cutting current is too high, the nozzle heat load will increase, the nozzle will be damaged prematurely, and the cutting quality will naturally decline, or even normal cutting will not be possible. Therefore, the cutting current and the corresponding nozzle should be correctly selected according to the thickness of the material before cutting

II. Cutting speed: the optimal cutting speed range can be selected according to the equipment description or determined by test. Due to the thickness of materials, different materials, high and low melting point, thermal conductivity and surface tension after melting, the cutting speed also changes correspondingly. Main manifestations: 1. Moderately improving the cutting speed can improve the quality of the incision, that is, the incision is slightly narrower, the surface of the incision is smoother, and the deformation can be reduced at the same time. 2. The cutting speed is too fast, which makes the cutting linear energy lower than the required value. The jet in the cutting seam cannot quickly blow off the molten cutting melt immediately, resulting in a large amount of drag. With the slag hanging on the incision, the surface quality of the incision decreases. 3. When the cutting speed is too low, because the cutting place is the anode of the plasma arc, in order to maintain the stability of the arc itself, the anode spot or anode area must find the place of conducting current near the slit nearest to the arc, and at the same time, it will transfer more heat to the radial direction of the jet. Therefore, the incision is widened, and the molten materials on both sides of the incision gather and solidify at the bottom edge, forming slag that is not easy to clean, Moreover, the upper edge of the incision forms a fillet due to excessive heating and melting. 4. When the speed is very low, the arc will even go out because the notch is too wide. It can be seen that good cutting quality and cutting speed are inseparable

III. arc voltage: Generally speaking, the normal output voltage of the power supply is the cutting voltage. Plasma arc cutting machines usually have high no-load voltage and working voltage. When using gases with high ionization energy such as nitrogen, hydrogen or air, the voltage required to stabilize the plasma arc will be higher. When the current is constant, the increase of voltage means the increase of arc enthalpy and cutting ability. If the enthalpy value is increased and the diameter of the jet is reduced, if the stress continues to increase to point C and the gas flow rate is increased, faster cutting speed and better cutting quality can often be obtained

IV. working gas and flow: working gas includes cutting gas and auxiliary gas. Some equipment also requires arc starting gas. Usually, the appropriate working gas should be selected according to the type, thickness and cutting method of cutting material. Cutting gas should not only ensure the formation of plasma jet, but also ensure the removal of molten metal and oxide in the incision. Excessive gas flow will take away more arc heat, making the length of the jet shorter, leading to the decline of cutting ability and instability of the arc; Too small gas flow will make the plasma arc lose its due straightness and make the cutting depth shallow, and it is also easy to produce slag; Therefore, the gas flow must be well matched with the cutting current and speed. Nowadays, most plasma arc cutting machines rely on gas pressure to control the flow, because when the gun aperture is certain, controlling the gas pressure also controls the flow. The gas pressure used for cutting materials with a certain thickness is usually selected according to the data provided by the equipment manufacturer. If there are other special applications, the gas pressure needs to be determined through the actual cutting test. The most commonly used working gases are argon, nitrogen, oxygen, air, H35, argon nitrogen mixture, etc

1. Argon hardly reacts with any metal at high temperature, and argon plasma arc is very stable. Moreover, the nozzle and electrode used have a high service life. However, the voltage of argon plasma arc is low, the enthalpy is not high, and the cutting ability is limited. Compared with air cutting, its cutting thickness will be reduced by about 25%. In addition, in argon protected environment, the surface tension of molten metal is larger, which is about 30% higher than that in nitrogen environment, so there will be more slag hanging problems. Even if the mixture of argon and other gases is used for cutting, there will be a tendency of slag sticking. Therefore, pure argon is rarely used for plasma cutting alone

2. Hydrogen is usually used as an auxiliary gas to mix with other gases. For example, the famous gas H35 (the volume fraction of hydrogen is 35%, and the rest is argon) is one of the gases with the strongest plasma arc cutting ability, which is mainly beneficial to hydrogen. Because hydrogen can significantly increase the arc voltage, the hydrogen plasma jet has a high enthalpy. When mixed with argon, the cutting ability of the plasma jet is greatly improved. Generally, argon + hydrogen is often used as cutting gas for metal materials with a thickness of more than 70mm. If water jet is used to further compress argon + hydrogen plasma arc, higher cutting efficiency can also be obtained

3. Nitrogen is a commonly used working gas. Under the condition of high power supply voltage, nitrogen plasma arc has better stability and higher jet energy than argon. Even when cutting materials with high viscosity of liquid metal such as stainless steel and nickel base alloy, the amount of slag hanging on the lower edge of the incision is small. Nitrogen can be used alone or mixed with other gases. For example, nitrogen or air are often used as working gases in automatic cutting. These two gases have become the standard gases for high-speed cutting of carbon steel. Sometimes nitrogen is also used as the starting gas for oxygen plasma arc cutting

4. The new reactor is planned to deliver oxygen to the plant next year, which can improve the speed of cutting low-carbon steel materials. When cutting with oxygen, the cutting mode and flame cutting are very imaginable. High temperature and high-energy plasma arc makes the cutting speed faster, but it must be combined with the use of high-temperature oxidation resistant electrode, and at the same time, the impact protection of the electrode during arc starting is carried out to prolong the service life of the electrode

5. The air contains nitrogen with a volume fraction of about 78%, so the slag hanging caused by air cutting is very imaginable as when cutting with nitrogen; The air also contains about 21% oxygen by volume. Because of the existence of oxygen, the speed of cutting low carbon steel materials with air is also very high; At the same time, air is also the most economical working gas. However, when air cutting is used alone, there will be problems such as slag hanging, notch oxidation, nitrogen increase, etc., and the low service life of electrodes and nozzles will also affect the work efficiency and cutting cost. 5、 Nozzle height: refers to the distance between the nozzle end face and the cutting surface, which constitutes a part of the entire arc length. As the power supply with constant current or steep drop characteristics is generally used for plasma arc cutting, the current changes little after the nozzle height increases, but it will increase the arc length and lead to the increase of arc voltage, so as to improve the arc power; But at the same time, it will also increase the arc length exposed to the environment and the energy loss of the arc column. In the case of the combined action of the two factors, the effect of the former is often completely offset by the latter, which will reduce the effective cutting energy and reduce the cutting capacity. It usually shows that the blowing force of the cutting jet is weakened, the residual slag at the lower part of the incision is increased, and the upper edge is over melted, resulting in rounded corners, etc. In addition, considering the shape of plasma jet, the jet diameter expands outward after leaving the muzzle, and the increase of nozzle height will inevitably lead to the increase of notch width. Therefore, it is beneficial to choose a nozzle height as small as possible to improve the cutting speed and cutting quality. However, when the nozzle height is too low, it may cause double arc phenomenon. Using ceramic external nozzle can set the nozzle height to zero, that is, the end face of the nozzle directly contacts the cut surface, and a good effect can be obtained

VI. cutting power density: in order to obtain the plasma arc cutting arc with high compressibility, the cutting nozzle adopts smaller nozzle aperture, longer hole length and enhanced cooling effect, which can increase the current passing through the effective section of the nozzle, that is, the power density of the arc. But at the same time, compression also increases the power loss of the arc. Therefore, the effective energy actually used for cutting is smaller than the power output, and its loss rate is generally between 25% and 50%. Some methods, such as water compression plasma arc cutting, will have a higher energy loss rate. This problem should be considered when designing cutting process parameters or economic accounting of cutting costs

for example: the thickness of metal plates used in industry is mostly less than 50mm. Within this thickness range, conventional plasma arc cutting will often form large upper and small cuts, and the upper edge of the cut will also lead to the reduction of the dimensional accuracy of the cut and increase the subsequent processing. When oxygen and nitrogen plasma arc are used to cut carbon steel, aluminum and stainless steel, when the plate thickness is within the range of 10~25mm, the thicker the material is, the better the perpendicularity of the end edge is, and the angle error of the cutting edge is 1 ~ 4 degrees. When the plate thickness is less than 1mm, the notch angle error increases from 3 ~4 degrees to 15 ~25 degrees with the decrease of the plate thickness. It is generally believed that this phenomenon is caused by the imbalance of heat input of plasma jet on the cutting surface, that is, the release of plasma arc energy in the upper part of the cutting is more than that in the lower part. This imbalance of energy release is closely related to many process parameters, such as the degree of plasma arc compression, cutting speed and the distance from the nozzle to the workpiece. Increasing the compression degree of the arc can lengthen the high-temperature plasma jet and form a more uniform high-temperature area. At the same time, increasing the speed of the jet can reduce the width difference between the top and bottom of the notch. However, the over compression of the conventional nozzle often causes the double arc phenomenon. The double arc will not only lose the electrode and nozzle, making the cutting process impossible, but also lead to the decline of the cutting quality. In addition, too large cutting speed and too large nozzle height will cause the increase of the width difference between the top and bottom of the incision

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