KR100827581B1 - Plasma cutter - Google Patents

Abstract

The plasma cutting device cuts stainless steel in good quality. An inert gas (nitrogen) is used as a plasma gas in the plasma torch 2, and as an assist gas, it is a combustible gas (propane) having a specific gravity that is heavier than air and is reducible, or a mixed gas of the combustible gas (propane) and an inert gas (nitrogen). To supply. The combustible gas (propane) in the assist gas is not supplied in the free flow section and the after flow section, but only in the plasma arc generation section.

Description

Plasma cutting device {PLASMA CUTTER}

The present invention relates to a plasma cutting method for cutting stainless steel and a plasma cutting device for cutting stainless steel by carrying out the method.

In plasma cutting of stainless steel, chromium contained in the stainless steel is oxidized during cutting, and this chromium oxide adheres to the cutting surface due to deterioration of its fluidity, thereby degrading the quality of the cutting surface. As a solution to this problem, the following prior arts are known with respect to the composition of the plasma gas and the assist gas. In the first conventional technique, nitrogen or air is used as the plasma gas, and methane or a mixed gas of methane and air is used as the assist gas (Patent Document 1). Moreover, 2nd prior art uses air or mixed gas of air and nitrogen as a plasma gas, and uses mixed gas containing hydrogen or hydrogen, or hydrocarbon gas as an assist gas (patent document 2).

Patent Document 1: US Patent No. 5,414,236

Patent Document 2: Japanese Patent Application Laid-Open No. 9-295156

Disclosure of the Invention

Problems to be Solved by the Invention

In the first conventional technique, the effect of preventing methane or a mixed gas of methane and air, which is used as an assist gas, from preventing the oxidation of the cut surface can be expected due to the reducing property of methane. However, since methane is lighter in specific gravity than air, when methane is supplied as a shielding gas around a plasma arc, there is a possibility that it is susceptible to the influence of external air flow and wind, and methane gas is easily diffused and cut. There is a fear that the shielding effect on the stainless steel will be affected.

In the second prior art, by using hydrogen, a mixed gas containing hydrogen, or a hydrocarbon gas as the assist gas, the effect of suppressing oxidation of the cut surface can be expected by their reducing properties. However, since hydrogen is lighter in specific gravity than air, when a mixed gas containing hydrogen or hydrogen is supplied as a shielding gas around the plasma arc, there is a possibility that it is easily affected by external air flow or wind, and hydrogen is diffused. It tends to be easy to affect the shielding effect on the cut stainless steel. In fact, according to the tests conducted by the inventors, chromium oxide may be partially attached to the cut surface, and it is difficult to obtain a very good oxide free cut surface. In the market, only a small amount of chromium oxide adheres to the cut surface of the product, and the product value of the product is greatly reduced. For this reason, there is a strong demand for a technique for obtaining a good oxide free cut surface superior to the prior art.

In many plasma cutting plants, not only stainless steel but also mild steel is cut. In the plasma cutting of mild steel, unlike stainless steel, it is preferable to actively oxidize (burn) this and utilize the heat of oxidation for cutting, and therefore, oxygen gas is often used. However, in a factory using such an oxygen gas, there is a strong demand from the user side that the use of hydrogen gas is to be avoided as much as possible. Therefore, the practical use of the prior art that hydrogen gas or its mixed gas is used for the assist gas is difficult.

In addition, when the hydrocarbon gas proposed in the second prior art is used as the shield gas, there is a serious problem that the flowability of the molten metal generated at the time of cutting is worsened and the molten metal adheres to the back of the product. .

It is therefore an object of the present invention to provide a plasma cutting device for cutting stainless steel with good quality, and a cutting method using the plasma cutting device. Moreover, another objective of this invention will become clear from the description of embodiment mentioned later.

Means to solve the problem

The plasma cutting device according to the present invention includes a control device for controlling a gas supply operation, and an inert gas as a plasma gas ejected as a plasma arc from a nozzle of a plasma torch by an instruction from the control device. An assist gas for shielding from outside air is provided with a gas supply means for supplying a flammable gas having a specific gravity that is heavier than air and having a reducing property or a mixed gas of the flammable gas and an inert gas to the plasma torch. According to the plasma cutting device of the present invention, since the plasma gas is an inert gas, the combustible gas having a specific gravity is heavier than that of air and is reducible in addition to that which does not itself cause the oxidation of the cutting surface, or the inert gas By the reducing action and shielding action of the assist gas which is a mixed gas, oxidation prevention and reduction of a cut surface are performed effectively. The action of the plasma gas and the assist gas are combined to obtain high cutting quality.

In a preferred embodiment, nitrogen is employed as the inert gas used as the plasma gas. Moreover, propane gas is employ | adopted as a combustible gas which is heavier than air and has reducibility. As the assist gas, a mixed gas of nitrogen and propane is employed.

In a preferred embodiment, the control means supplies a flammable gas to the plasma arc generating section of the series of preflow sections, the plasma arc generating section and the afterflow section, by the plasma torch, and the combustible gas in the free flow section or the afterflow section. The gas supply means is controlled not to supply the plasma to the torch. Accordingly, there is no fear that the combustible gas used as the assist gas may be released to the outside in large quantities in an unburned state.

In a preferred embodiment, the gas supply means includes a plasma gas line for supplying the plasma gas to the plasma torch and an assist gas line for supplying the assist gas to the plasma torch, and the assist gas line supplies the combustible gas in the vicinity of the plasma torch. It is connected to the confluence point where the combustible gas line and the inert gas line which supplies an inert gas join. Then, in the combustible gas line, gas flow control means for controlling the flow of the combustible gas by the instruction from the control means is disposed near the confluence point. With this configuration, gas flow control such that the combustible gas as described above is supplied in the plasma arc generation section but not in the free flow section or the after flow section can be performed in a timely manner without a large delay time.

According to another aspect of the present invention, there is provided a plasma cutting method comprising: supplying an inert gas to a plasma torch as a plasma gas and combustible gas having a specific gravity heavier than air and reducing gas or a mixed gas of the combustible gas and the inert gas. Supplying as assist gas.

Effects of the Invention

According to the plasma cutting device and method of the present invention, good cutting quality can be obtained in plasma cutting of stainless steel.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of the plasma cutting device which concerns on one Embodiment of this invention.

FIG. 2 is a time chart showing an example of a supply procedure of nitrogen gas and propane gas in the plasma gas and the assist gas in the gas supply process.

* Description of the symbols for the main parts of the drawings *

2: plasma torch

4: electrode

6: nozzle

8: nozzle cap

10: plasma gas passage

12: assist gas passage

14: plasma arc

16: cutting material

18: plasma gas line

20: assist gas line

50: control unit

60: nitrogen source

70: nitrogen source

80: propane

90: confluence

Implement the invention  Best form for

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described based on drawing. In the present embodiment, as described below, an inert gas, for example, nitrogen, is used as a plasma gas, and as a assist gas, a specific gravity is heavier than air and has a reducing property, or a mixed gas of this combustible gas and an inert gas. For example, by using the mixed gas of nitrogen and propane, stainless steel can be cut | disconnected with favorable quality.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows schematic structure of the plasma cutting device which concerns on one Embodiment of this invention.

As shown in FIG. 1, the plasma torch 2 has a multicylindrical shape as a whole, has an electrode 4 at its center position, and a nozzle 6 is covered outside the electrode 4, and the nozzle 6. The outside of the nozzle cap 8 is covered. The gas supply system 17 is connected to the plasma torch 2.

The gas supply system 17 has a plasma gas line 18 for supplying a plasma gas to the plasma torch 2 and an assist gas line 20 for supplying an assist gas to the plasma torch 2. The plasma gas line 18 is connected to a nitrogen source (for example, nitrogen bomb 60). The assist gas line 20 is connected to the confluence point 90 at which the nitrogen gas line 22 for supplying nitrogen gas and the propane gas line 24 for supplying propane gas join at a point near the plasma torch 2. It is. The nitrogen gas line 22 is connected to a nitrogen source (for example, a nitrogen cylinder; 70) (which may be the same as or different from the nitrogen source 60 for plasma gas described above). An electromagnetic valve 30 for starting and stopping the supply of plasma gas is provided to the plasma gas line 18, and an nitrogen valve for starting and stopping the supply of nitrogen gas constituting the assist gas to the nitrogen gas line 22. The solenoid valve 32 is provided in the propane gas line 24 with the solenoid valve 34 for starting and stopping supply of the propane gas which comprises an assist gas, respectively. Between the solenoid valve 34 of the propane gas line 24 and the confluence point 90, the check valve 26 is installed in order to prevent nitrogen from flowing in a propane line. A mechanism for controlling the flow of propane gas such as the solenoid valve 34 and the non-return valve 26 on the propane line 24 is disposed near the confluence point 90.

The gas supply system 17 is connected to a control device 50 for controlling the gas supply operation. In the gas supply process for torch driving which consists of a series of proflow sections, a plasma arc generation section, and an afterflow section, the control apparatus 50 carries out the above-mentioned solenoid valve 30, 32, 34 after FIG. Opening and closing control is performed in the order as described with reference.

In the plasma torch 2, a plasma gas passage 10 is formed between the electrode 4 and the nozzle 6. The plasma gas (nitrogen gas) flows through the plasma gas line 18 from the nitrogen source 60 through the plasma gas line 18 by opening the solenoid valve 30 by the instruction of the control device 50, and thereby the plasma gas passage of the plasma torch 2. It is supplied to (10). In addition, an assist gas passage 12 is formed between the nozzle 6 of the plasma torch 2 and the nozzle cap 8. The assist gas (mixed gas of nitrogen gas and propane gas) opens the nitrogen line 22 by the nitrogen source 70 by opening the solenoid valve 32 and the solenoid valve 34 at the instruction of the controller 50. Nitrogen gas which flowed and propane gas which flowed the propane line 24 from the propane source 80 merge | generate at the joining point 90, are produced | generated, and assist the plasma torch 2 via the assist gas line 20. It is supplied to the gas passage 12 (as described later, in the pro flow section and the after flow section, the assist gas is nitrogen gas only and does not include propane gas).

The nozzle 6 restrains the plasma arc 14 and is a part having the thinnest gas outlet for tightening the plasma gas supplied from the upstream side of the nozzle 6. The plasma gas ejected from the gas ejection port is converted into plasma by an arc discharge between the electrode 4 and the lateral cutting material 16, and becomes a plasma arc 14 of a high speed jet that is sufficiently narrowed and is oriented. It blows toward a to-be-cut material (stainless steel; 16). The stainless steel is cut by the plasma arc 14.

On the other hand, the nozzle cap 8 exists downstream from the said nozzle 6, and has a gas ejection opening whose radius is larger than the gas ejection opening of the nozzle 6, and assist gas between the nozzle cap 8 and the nozzle 6 It is a component which blows out the assist gas flowing from the passage 12 around the plasma arc 14. The assist gas blown out from the assist gas passage 12 does not become plasma but surrounds the plasma arc 14. When the plasma arc 14 cuts the stainless steel, the assist gas functions to shield the plasma arc 14 from the outside air so that the plasma arc 14 is not affected by the external wind or airflow. In addition, the assist gas also has an action of preventing and reducing the oxidation of the stainless steel cut by the plasma arc 14 by the reducing action of propane gas having a strong reducing power contained therein.

By the way, one of the characteristics which is important in the cutting quality of stainless steel is whether or not a cutting surface is an oxide free. This can be judged from the color and metallic gloss of the cut surface observed with the naked eye. If the cut surface is silvery white with no color or dullness and there is sufficient metallic gloss (i.e., the stainless steel is now completely exposed), it can be judged to be a good non-oxidized cut surface. The commodity value of a cut product of stainless steel is drastically lowered only if a part of the cut surface is slightly dull or lightly colored (ie, some chromium oxide is attached). One of the factors that greatly influence the cutting quality is a kind of gas used as the plasma gas and the assist gas.

From the viewpoint of this cutting quality, the composition of the gas used in the plasma cutting device of the present embodiment will be described in detail below.

As the plasma gas, an inert gas containing no oxygen, for example, nitrogen having a substantial volume concentration (molar concentration) of 100% is used. If it is used as a gas containing oxygen (for example, air or a mixed gas of oxygen and another gas), the reducing property of the assist gas is canceled, and it is difficult to obtain an oxide-free cut surface. On the other hand, when pure inert gas, for example, nitrogen gas, having a volume concentration of substantially 100% is used, it is easy to obtain an oxide-free cut surface because it does not counteract the reducibility of the assist gas. From this principle, an inert gas other than nitrogen, such as argon, can also be used as a plasma gas. When nitrogen and argon are compared, nitrogen which can increase the cutting force by increasing the heat amount of the plasma arc is excellent. This is because the heat capacity when nitrogen which is a 2-atomic molecule is made into plasma more than argon which is a monoatomic molecule is large.

As the assist gas, a combustible gas having a reducibility having a heavier specific gravity than an inert gas and air, or a mixed gas of the combustible gas and an inert gas, for example, a mixed gas of nitrogen and propane is used. It is preferable that the volume concentration (molar concentration) of propane in an assist gas is 50% or less, for example, 20% or about 30% can be employ | adopted. The higher the volumetric concentration of propane, the more likely that the molten metal's fluidity deteriorates and that it adheres to the inside of the product. In order to eliminate this problem, the above concentration is preferable. According to the research of the inventors, propane gas is recognized as being optimal as what is included in the assist gas of the plasma cutting of stainless steel, among various reducing gases. The reason for this is that, according to the experiments of the inventors, plasma cutting of the stainless steel was actually performed by using the combination of the plasma gas and the assist gas of the above-described composition. As a result of visual observation of the cut surface, a very good oxide-free cut surface can be obtained. Because it was judged that it was. On the other hand, in the experiments of the inventors, when using a reducing gas other than propane gas, for example, hydrogen gas, as a result of visual observation of the cut surface, some dullness or light color may be attached to the cut surface. It could not be judged that a good anodized cut surface could be obtained when using. The reason why a good oxide-free cutting surface can be obtained by using propane gas in this way is estimated as follows.

First, since propane contains more hydrogen atoms than methane and ethane of hydrogen and hydrocarbons proposed in the above-mentioned prior art, the reduction action of preventing and reducing the oxidation of the cut surface after cutting is more powerful. I guess.

Second, because propane is heavier than air (1.5 times as much as air), the plasma arc 14 can be discharged from external air currents, wind effects, etc., rather than hydrogen, methane, or lighter than air, and ethane almost the same specific gravity as air. It is presumed that the shielding ability to protect and prevent oxygen from outside air from penetrating the cutting point is higher. In the case of gas having a lighter specific gravity than air, the gas tends to diffuse, and the shielding effect of the plasma arc 14 and the cut surface due to the reduction tends to be reduced under the influence of wind and airflow. On the other hand, propane, which has a higher specific gravity than air, is less likely to diffuse, and thus it is easy to maintain a good shielding effect.

Third, propane is a gas that does not liquefy under the supply pressure of the assist gas. In other words, from the viewpoint of a hydrocarbon containing a lot of hydrogen atoms and a heavier specific gravity with respect to air, butane and a hydrocarbon having a higher specific gravity than other propanes and a heavier specific gravity are more excellent than those of propane. . However, in order to supply the assist gas to the plasma torch 2, a gas supply pressure of 1 to 2 kg / cm 2 (absolute pressure, 2 to 3 kg / cm 2) is usually required at the minimum gauge pressure. However, in the case of butane, since the vapor pressure at 25 degreeC is 1.8 kg / cm <2>, there exists a possibility that it may liquefy under the supply pressure of assist gas, and cannot supply as gas. Other hydrocarbon gas having a higher molecular weight cannot be used for the assist gas because the vapor pressure is further lowered and liquified easily. In contrast, propane can be sufficiently supplied as a gas under the supply pressure of the assist gas described above because the vapor pressure at 25 ° C. is 8.5 kg / cm 2.

In addition, propane has the following advantages as compared to other reducing gases. In other words, propane is used as an LP gas (liquefied petroleum petroleum) in comparison with hydrogen, for example, in a gas furnace or the like, and is excellent in safety, availability, and economy. The butanes described above also lag behind propane in availability. Furthermore, since the concentration of propane necessary for obtaining a good anoxic cleavage surface is not as high as described above (this is probably due to the strong reducibility of propane), it can be said that it is also economical in that respect.

For the reason described above, by using a mixed gas of nitrogen and propane as an assist gas, and combining this with a plasma gas using an inert gas, a silvery white non-oxidized cut surface having metallic luster is easily obtained. In addition, propane circulated as LP gas is not pure propane strictly, but a small amount of butane etc. is contained, but since this is not a problem in actuality, LP gas can be used as propane in this invention.

The reason why nitrogen is used as the assist gas is that, in the same way as the plasma gas, since it is an inert gas containing no oxygen, it does not contribute to oxidation of the cut surface itself. From this point of view, an inert gas other than nitrogen, for example, argon, may be employed.

By the way, in the above-described gas supply system 17, even if the pressure of the nitrogen (inert gas) line 22 upstream from the joining point 90 is higher than the pressure of the propane (combustible gas) line 24, The non-return valve 26 prevents nitrogen gas (inert gas) from entering the propane (combustible gas) line 24.

FIG. 2 shows a gas supply process for plasma cutting comprising a series of proflow sections, plasma arc generation sections, and afterflow sections, each of which is executed in the gas supply system 17 by an instruction from the controller 50. It is a figure which shows the supply sequence of gas.

The timing of gas supply differs with each gas as shown in FIG.

At the same time as the start signal is generated in the controller 50, the solenoid valve 30 and the solenoid valve 32 are opened by the instruction from the controller 50, and the plasma of nitrogen gas and nitrogen gas in the assist gas are plasma gases. Supply to torch 2 is started. At this point, the plasma arc 14 has not yet been ignited. The gas supply operation before this arc ignition is called a free flow, and after the solenoid valves 30 and 32 are opened until the gas flow rate in the plasma torch 2 stabilizes to a predetermined value (for example, about 1 to several seconds) ) Is done. In the section of this free flow, propane gas of the assist gas is not supplied to the plasma torch 2. The reason is to prevent unburned propane gas from being released into the plant in large quantities.

At the end of the predetermined free flow period, a plasma power source (not shown) starts operation to ignite the plasma arc 14. The occurrence of the plasma arc 14 is detected (can be detected from the magnitude of the plasma current flowing through the plasma power supply, or from the plasma arc voltage, etc. that the plasma power supply adds between the electrode 4 and the cut material 16). At the same time, the solenoid valve 34 opens by the instruction | indication from the control apparatus 50, and supply of propane in an assist gas is started. Thereafter, while the generation of the plasma arc 14 continues (during the plasma arc generation period), a mixed gas of nitrogen, which is plasma gas, nitrogen, which is an assist gas, and propane, is continuously supplied to puncture or cut the cutting material 16. Thermal processing, etc. are performed. By the way, as already demonstrated, since the solenoid valve 34 is arrange | positioned in the vicinity of the confluence point 90, and the confluence point 90 is arrange | positioned in the vicinity of the plasma torch 2 (that is, a solenoid valve as shown in FIG. 1). Since the gas line from the 34 to the plasma torch 2 is short, the supply of propane is started in a timely manner without being delayed to a problem from the start of the generation of the plasma arc 14.

In the plasma arc generation section, propane is combusted by contacting the high temperature cut surface formed on the high temperature plasma arc 14 and the to-be-cut material 16, and by the reducing action and the shielding action, it is possible to produce a good oxide free cut surface. Contribute. At this time, unburned propane gas is hardly discharged in the factory.

At the end of the plasma arc generation section (at the end of the thermal processing), the plasma current from the plasma power supply is stopped, the plasma arc 14 is extinguished, the extinction of the plasma arc 14 is detected, and the control device 50 As a result, the solenoid valve 34 is closed, and the supply of propane gas in the assist gas is stopped. Thereafter, an operation called an after flow is performed, where only nitrogen of the plasma gas and nitrogen of the assist gas are supplied. The interval of the after flow is the time required (for example, about 1 to several seconds) after the plasma arc is extinguished until the last cutting point cools to some extent and does not oxidize. In the after-flow section, since the supply of propane gas is stopped, unburned propane gas is not discharged into the plant. At the end of the scheduled after-flow section, the start signal disappears, and the solenoid valve 30 and the solenoid valve 32 are closed by an instruction from the controller 50 to supply nitrogen of the plasma gas and nitrogen of the assist gas. Is stopped. By the way, as already demonstrated, since the solenoid valve 34 is arrange | positioned in the vicinity of the confluence point 90, and the confluence point 90 is arrange | positioned in the vicinity of the plasma torch 2 (that is, a solenoid valve as shown in FIG. 1). Since the gas line from the 34 to the plasma torch 2 is short, the supply of propane is not delayed enough to cause a problem from the disappearance of the plasma arc 14 and is stopped in a timely manner.

By the way, the mixed gas of propane and nitrogen remains in the assist gas line 20 of the plasma torch 2 side from the confluence point 90 shown in FIG. 1 at the start time of an after-flow section. However, as described above, since the confluence point 90 is near the plasma torch 2 (that is, the assist gas line 20 is short and the amount of gas therein is small), the remaining propane is reduced by the afterflow. Except for propane line 24 which is discharged from 2) to the outside air and is upstream of the solenoid valve 34, propane does not remain. Propane released at this time is small and does not matter.

By the way, the plasma cutting device of the structure mentioned above can be used not only for stainless steel but also for the use which cuts mild steel. In that case, for example, the nitrogen line 32 of the gas supply system 17 shown in FIG. 1 can be used as a line which supplies air or oxygen which is a retardant gas as an assist gas. As described above, since propane (flammable gas) does not remain in the assist gas line 20 except at the time of cutting of stainless steel, there is no problem even if the retardant gas flows into the nitrogen line 32.

As mentioned above, although one Embodiment of this invention was described, these embodiment is the illustration for description of this invention to the last, and does not limit this invention only to these embodiments. Therefore, this invention can be implemented also in various aspects other than the said embodiment.

Claims (7)

A plasma cutting device for cutting stainless steel, Control means 50 for controlling the gas supply operation; By the instruction from the control means 50, the specific gravity is heavier than air as an assist gas for shielding the plasma arc from outside air as an inert gas as a plasma gas ejected as a plasma arc from the nozzle of the plasma torch 2. And a gas supply means (17) for supplying a reducing combustible gas or a mixed gas of the combustible gas and an inert gas to the plasma torch (2). The method of claim 1, A flammable gas having a specific gravity greater than that of air and having a reducing property is propane, wherein the plasma cutting device is characterized in that the propane. The method of claim 1, The inert gas used as the plasma gas is nitrogen, characterized in that the plasma cutting device. The method of claim 1, And said assist gas is a mixed gas of nitrogen and propane. The method of claim 1, The control means 50 supplies the flammable gas to the plasma torch in the plasma arc generation section among a series of free flow sections, the plasma arc generation section and the after flow section, and supplies the preflow section or the after flow section. In the section, the plasma cutting device characterized in that for controlling the gas supply means not to supply the flammable gas to the plasma torch. The method of claim 5, wherein The gas supply means 17 includes a plasma gas line 18 for supplying the plasma gas to the plasma torch, and an assist gas line 20 for supplying the assist gas to the plasma torch, wherein the assist gas is provided. The line 20 is connected to a confluence point 90 at which the combustible gas line 24 for supplying the combustible gas and the inert gas line 22 for supplying the inert gas join, near the plasma torch, In the combustible gas line 24, near the confluence point 90, gas flow control means 34 for controlling the flow of the combustible gas is arranged by an instruction from the control means 50. Plasma cutting device characterized in that. delete

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