JP2004160555A - Plasma cutting device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce wear of a nozzle or an electrode in oxygen plasma cutting. <P>SOLUTION: Hafnium or zirconium is used for an electrode material, and pure nitrogen gas is used as a working gas in forming pilot arc. A mixture gas of oxygen and nitrogen at 70-95 mol% (desirably 80 mol% or more) oxygen density and 30-5 mol% (desirably 5-10 mol% or more) nitrogen density is used as the working gas during cutting. Pure nitrogen gas is used as the working gas to nitride the electrode after cutting is completed and arc is eliminated. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a cutting technique using a plasma arc, and more particularly to an improvement in a technique for reducing the consumption of electrodes and nozzles in a plasma cutting method using a working gas containing oxygen.

  The plasma cutting method using a working gas (plasma gas) containing oxygen is suitable for cutting low alloy steel or low carbon steel (mild steel). In this plasma cutting method, since an electrode in a plasma torch is exposed to oxygen, it is known that hafnium is optimal as an electrode material (a material of a heat-resistant insert attached to an electrode tip) (Patent Document 1). This is because hafnium, when it becomes an oxide, has a melting point of about 2800 to 2900 ° C. and exhibits excellent heat resistance. Almost all plasma cutting machines currently in practical use using oxygen or air as a working gas use hafnium for electrodes. Further, zirconium has almost the same properties as hafnium, and thus is suitable as an electrode material in an oxidizing atmosphere.

  By the way, in plasma cutting, a nozzle and an electrode are consumed with arc generation. Although there are many causes of wear of the nozzle and the electrode, the following is a description of the wear caused by the arc itself when an oxidizing working gas is used.

  First, when the pilot arc (between the electrode and the nozzle) is first ignited, the electrode and the nozzle are consumed. At the moment when the pilot arc is ignited, the surface temperature of the hafnium of the electrode material instantaneously rises from room temperature to a high temperature exceeding 3000 ° C., and at that time, the hafnium is rapidly consumed. In this transient state, even if the time is about 0.1 second, the consumption of hafnium may correspond to about 1 minute of the subsequent high-temperature stable state of hafnium. Further, while the pilot arc (between the electrode and the nozzle) exists, the nozzle is in the same condition as the work at the time of cutting, and is melted in an oxidizing atmosphere.

  The second is the consumption of the electrode when the main arc (between the electrode and the work) is established and cutting is being performed. During cutting, the hafnium of the electrode is stable at a high temperature, so the consumption rate is lower than at the time of pilot arc ignition, but the consumption proceeds, and the hafnium is gradually consumed so as to shorten the candle.

  In order to solve the problem of electrode and nozzle consumption due to this arc, Patent Document 2 discloses that the oxygen concentration at the start (not only during the pilot arc ignition but also during the period when the main arc is established and piercing is performed). Discloses a plasma cutting start method in which an oxygen / nitrogen mixed gas of 70 to 10 mol% is flowed as a working gas, and thereafter, is switched to oxygen gas. Further, Patent Literature 3 discloses a plasma cutting start method in which nitrogen gas is flown at the time of pilot arc ignition, and thereafter, the gas is switched to oxygen gas. As described above, the consumption of the hafnium electrode or the nozzle during the pilot arc ignition can be reduced by lowering or eliminating the oxygen concentration of the working gas during the pilot arc ignition.

  Further, Patent Literature 4 discloses a technique for reducing electrode consumption at the end of cutting by lowering the oxygen concentration of the working gas to 95 mol% or less at the end of cutting.

JP-B-49-8622 JP-A-61-92782 JP-A-3-25864 (U.S. Pat. No. 487,747) Japanese Patent Publication No. 1-9112

  As described above, Patent Documents 2 and 3 disclose at the start of cutting, and Patent Document 4 at the end of cutting, in order to reduce the consumption of electrodes or nozzles, reduce the oxygen concentration of the working gas or purify pure nitrogen. Gas is used. However, any of the conventional techniques uses a pure oxygen gas as the working gas during cutting (Patent Documents 2 and 3) or a working gas having an extremely high oxygen concentration of 95 mol% or more. (Patent Document 4). Therefore, the consumption of the hafnium electrode during cutting cannot be effectively suppressed.

  Thus, in the prior art, in plasma cutting with an oxidizing working gas using a hafnium electrode, the reason for using a pure oxygen gas or a working gas having a high oxygen concentration comparable to that during cutting is considered to be as follows. .

  First, even if pure oxygen gas is used (although there is a problem of high running cost due to frequent electrode replacement), the life of the hafnium electrode can be several hours or more unless it is a plasma having a particularly large current. Become. On the other hand, if the oxygen concentration of the working gas is reduced in order to reduce electrode consumption, the cutting quality is accordingly reduced. Deterioration of cutting quality is a problem that cannot be neglected. Therefore, even if a high running cost is allowed, pure oxygen gas is used to obtain the best cutting quality.

  Secondly, if it is assumed that a mixed gas obtained by adding nitrogen to oxygen, for example, instead of pure oxygen as a working gas, in a conventional working gas supply system, each gas flow rate changes during cutting, so that The oxygen / nitrogen mixture ratio cannot be maintained at the optimum value during cutting. That is, the conventional gas supply / system adopts one of the following two methods to set the gas flow rate to the target value. The first method is to set the gas pressure supplied to the torch by the pressure reducing valve to a target value. The second is a method of setting a flow rate to a target value with a needle valve and a flow meter. In either method, the gas flow is maintained at the target value only when the pressure loss in the torch is constant. However, during cutting, the nozzle is degraded and the electrodes are consumed, so that the pressure loss in the torch changes, and the gas flow rate also deviates from the target value. Specifically, as the pressure loss in the torch decreases as the cutting progresses, the gas flow rate gradually increases. For example, in some cases, the flow rate is increased three times at the end of cutting compared to the new state at the start of cutting. As described above, since the gas flow rate changes during cutting, the oxygen concentration of the working gas cannot be kept constant at an optimum value. Therefore, during cutting, pure oxygen gas is used as the working gas.

  However, if the consumption of the electrodes can be reduced, there is nothing better than that. In particular, in the case of oxygen plasma with a large current of about 300 A, the life of the hafnium electrode is reduced to about 2 hours, so that it is not possible to neglect an increase in running cost and a decrease in work efficiency due to frequent electrode replacement.

  Therefore, an object of the present invention is to provide a novel working gas supply technique for reducing the consumption of nozzles and electrodes in plasma cutting.

  It is another object of the present invention to provide a novel gas supply system for maintaining a constant working gas composition during plasma cutting.

  In the plasma cutting device according to the first aspect of the present invention, when the pilot arc is ignited (preflow section), nitrogen gas, air, or a mixed gas having a higher nitrogen concentration than air is used as a working gas, and the electrode and the nozzle are connected to each other. After the pilot arc has shifted to the main arc, a mixed gas containing oxygen and nitrogen is supplied as a working gas while the workpiece is being cut using this main arc (main flow section). . The mixed gas supplied during cutting has an oxygen concentration of 70 to 95 mol% and a nitrogen concentration of 30 to 5 mol%. In particular, it is desirable that the oxygen concentration is 80 mol% or more and the nitrogen concentration is 5 to 10 mol% or more.

  According to this plasma cutting device, the working gas having a high nitrogen concentration is used at the time of pilot arc ignition, so that the consumption of electrodes and nozzles is reduced, and during cutting, a mixed gas containing oxygen and nitrogen is used instead of pure oxygen. Since the gas is used as the working gas, the consumption of the electrode is suppressed. Furthermore, according to experiments and studies by the inventor, in plasma cutting, the oxygen purity of the working gas does not have a critical influence on the cutting quality as conventionally thought, and therefore, the oxygen is added by adding nitrogen. Even if the concentration is slightly lowered, electrode consumption can be reduced without substantially lowering the cutting quality. In particular, when the oxygen concentration is 70 mol% or more, the decrease in cutting quality can be neglected, and when the nitrogen concentration is 5 mol% or more, particularly when the electrode material is hafnium or zirconium, good electrode consumption close to pure nitrogen is obtained. A reduction effect is obtained.

  As the electrode material, a metal material having a higher melting point of an oxide and a nitride than a pure metal is preferable. In a preferred embodiment, hafnium, zirconium or an alloy thereof is used. Hafnium and zirconium are oxides having a melting point of 2800 ° C. or more, and nitrides further increase the melting point to 3000 ° C. or more. When such an electrode material is used, the plasma cutting method of the present invention exhibits particularly excellent effects. Because a mixed gas containing oxygen and nitrogen is used during cutting, the electrode surface becomes nitride and has a high melting point, and the durability is further increased.

  In a preferred embodiment, the gas mixture supplied during cutting has an oxygen concentration of 70-95 mol% and a nitrogen concentration of 30-5 mol%. In particular, it is desirable that the oxygen concentration is 80 mol% or more and the nitrogen concentration is 5 to 10 mol% or more. When the oxygen concentration is 70 mol% or more, the decrease in cutting quality can be neglected, and when the nitrogen concentration is 5 mol% or more, particularly when the electrode material is hafnium or zirconium, a favorable electrode consumption reduction effect close to pure nitrogen is obtained. Is obtained.

  In a preferred embodiment, even after the cutting is completed and the main arc is to be extinguished (post flow section), nitrogen gas, air, or a mixed gas having a higher nitrogen concentration than air is supplied as the working gas. I do. Thereby, particularly when the electrode material is hafnium or zirconium, the electrode surface ends with a nitride having an extremely high melting point, so that electrode consumption when the pilot arc is ignited in the next cutting is suppressed.

  The plasma cutting apparatus according to the second aspect of the present invention uses hafnium, zirconium, or an alloy thereof as an electrode material, and has an oxygen concentration of 70 to 95 mol% and a nitrogen concentration of main flow section during cutting. A mixed gas of 30 to 5 mol% is supplied as a working gas. According to this plasma cutting device, during cutting, the electrode becomes a nitride having an extremely high melting point and is stabilized, so that electrode consumption is reduced. Also, good cutting quality can be maintained.

  In a preferred embodiment, the gas supply system includes a gas flow path for flowing gas from the gas source to the plasma torch, a flow control valve for setting a gas flow rate provided in the gas flow path, and a flow control valve. A constant differential pressure valve that acts to keep the differential pressure between the front and rear constant.

  According to this gas supply system, the nozzle loss of the plasma torch at the start of cutting or during cutting (for example, the nozzle diameter is expanded by a pilot arc) or the pressure loss in the torch (that is, resistance to gas flow) is reduced due to electrode wear. Even if the gas flows more easily, the differential pressure across the flow control valve is kept constant, so that the gas flow is kept constant. Therefore, in the past, if the nozzle of the plasma torch deteriorated or the electrode was worn out, the gas flow rate increased and the cutting quality deteriorated, so the nozzle and the like had to be replaced immediately. Even if the electrode is worn out to some extent, the gas flow rate does not increase, so that the cutting quality is not remarkably reduced as compared with the related art, so that replacement of consumables such as nozzles can be delayed. That is, the life of the consumable part is substantially extended.

  In a preferred embodiment, the gas supply system is a gas supply system for supplying a mixed gas of a plurality of gases from a plurality of gas sources to a plasma torch, wherein each gas flows from the plurality of gas sources to the plasma torch. And a plurality of single gas flow paths that merge into a mixed gas flow path upstream of the plasma torch, and a flow control valve provided in each single gas flow path to set a flow rate of each gas. And a constant pressure differential valve that acts to keep the differential pressure across the flow control valves constant.

  According to the gas supply system of the mixed gas, in addition to the advantages of the gas supply system described above, there is obtained an advantage that each gas concentration of the mixed gas can be kept constant during cutting. If this gas supply system is used as a supply system for a mixed gas containing oxygen and nitrogen in the above-described plasma cutting method or apparatus, the concentration of oxygen and nitrogen can be maintained at an optimum value throughout the cutting, so The effect of reducing electrode consumption while maintaining the cutting quality can be guaranteed.

  In a preferred embodiment, the supply system of the nitrogen / oxygen mixed gas having the above-described configuration extends from the oxygen gas source and the nitrogen gas source, and the nitrogen gas supply system further extends separately from the nitrogen gas source. . The nitrogen gas supply system and the nitrogen / oxygen mixed gas supply system are configured so that the nitrogen gas supply system is automatically blocked when the nitrogen / oxygen mixed gas supply system is opened upstream of the plasma torch. So they are joining. With this configuration, for example, when shifting from the pilot arc to the main arc, it is possible to switch from the nitrogen gas as the preflow to the nitrogen / oxygen mixed gas as the main flow simply by opening the supply system of the nitrogen / oxygen mixed gas. it can.

  In a preferred embodiment, a pressure reducing valve for setting an upper limit of the supply pressure to the plasma torch is further provided downstream of the constant pressure differential valve of the gas supply system configured to keep the flow rate constant by the constant pressure differential valve. ing. With this, when the gas supply is started to start the arc, not the excessive gas pressure from the gas source but a moderately low gas pressure set by the pressure reducing valve is applied to the plasma torch, and the arc ignition is stabilized. And the gas flow rate after the arc ignition stabilizes in a short time, so that cutting can be started immediately.

  FIG. 1 shows a schematic configuration of a plasma cutting apparatus according to one embodiment of the present invention.

  The plasma torch 1 has a substantially cylindrical shape as a whole and has a substantially cylindrical electrode 3 at the center thereof, and a tip portion of the electrode 3 serving as a plasma arc generating point can withstand high heat of the plasma arc. It has a heat-resistant insert 5 made of a high melting point material, for example, hafnium (or zirconium or an alloy thereof). Hereinafter, the heat resistant insert 5 made of hafnium is referred to as a “hafnium electrode”. It is in this hafnium electrode 5 that electrode wear occurs. Here, the melting point of hafnium itself is about 2200 ° C., but when it becomes oxide (HfO 2), the melting point rises to about 2800 to 2900 ° C., and when it becomes nitride (HfN), the melting point further rises to about 3300 ° C. (Zirconium also has similar characteristics). The characteristic that the melting point of oxides and nitrides increases to about 3000 ° C. In particular, the characteristic that nitrides have higher melting points than oxides is used to reduce electrode consumption, as described later.

  A substantially cylindrical nozzle 7 covers the outside of the electrode 3, and a working gas passage 6 is formed between the electrode 3 and the nozzle 7. A working gas 8 is supplied from a working gas supply system (not shown) into the working gas passage 6 from the base end side of the nozzle and flows toward the tip of the nozzle. At the tip of the nozzle 7, a nozzle orifice 9 having a sufficiently small diameter is opened to narrow the plasma arc sufficiently narrow and jet it toward the jet stream.

  A workpiece 10 to be cut is arranged at the front of the torch 1. A negative terminal of a plasma power supply 11 for supplying a plasma current is connected to the electrode 3, and a positive terminal is connected to the work 10. Further, a plus terminal of the plasma power supply 11 is connected to the nozzle 7 via the switch 13.

  The procedure for starting the cutting is roughly as follows. First, the working gas 8 flows through the working gas passage 6 at a predetermined flow rate and pressure, and the switch 13 is closed to apply a high voltage from the power supply 11 between the electrode 3 and the nozzle 7. Then, the insulation of the working gas 8 between the electrode 3 and the nozzle 7 is broken, and a pilot arc 15 is formed. The working gas 8 is turned into plasma by the energy of the pilot arc 15, and the plasma gas is ejected forward from the nozzle orifice 9. At the same time, the pilot arc 15 is pushed by the plasma gas flow, passes through the nozzle orifice 9 and connects to the work 21 (transition arc 17). At this time, the switch 13 is opened in response to the current 19 flowing through the work 10, and a main plasma arc 21 is established between the electrode 3 and the work 10. Subsequently, the cutting of the work 10 is started by the main arc 21.

  FIG. 2 shows an example of a method of supplying a working gas to the torch 1 from the start to the end of cutting, together with a change in arc current.

  As shown in FIG. 2, the working gas flow is divided into three types: “pre-flow”, “main flow”, and “post-flow”. The preflow is performed in a section from immediately before the pilot arc is ignited until the pilot arc shifts to the main arc (hereinafter, referred to as “pilot arc ignition”). The main flow is caused to flow in a section where cutting is being performed after the pilot arc has shifted to the main arc (hereinafter, “during cutting”). The post flow is caused to flow in a section (hereinafter, referred to as “at the end of cutting”) from immediately before the main arc is extinguished after the end of cutting to a point at which the temperature of the hafnium electrode falls to a temperature at which the oxidation reaction does not occur after the main arc is extinguished. .

  As shown in FIG. 2, during pilot arc ignition, pure nitrogen gas is supplied to the torch 1 as a preflow at a relatively small flow rate or low pressure. During cutting, a mixed gas of oxygen and nitrogen is supplied to the torch 1 at a relatively large flow rate or high pressure as a main flow. The oxygen concentration in this mixed gas is 70 to 95 mol% (preferably 80 mol% or more), and the nitrogen concentration is 30 to 5 mol% (preferably 5 to 10 mol%). When the cutting is completed, pure nitrogen gas is supplied at a relatively low flow rate or low pressure as a post flow immediately before the main arc is extinguished.

  At the time of pilot arc ignition, since the pilot arc is formed in pure nitrogen gas substantially free of oxygen, the consumption of the nozzle and the hafnium electrode is reduced. At the end of cutting, the flow of pure nitrogen gas causes the surface of the hafnium electrode to end in a state of nitride with a high melting point, so when the pilot arc is ignited to start the next cutting, the hafnium electrode is consumed. Is suppressed.

  In addition, during cutting, a mixed gas obtained by adding nitrogen to oxygen is used, so that consumption of the hafnium electrode is reduced. As described above, since the melting point of hafnium is higher in nitride than in oxide, the consumption of the hafnium electrode is effectively reduced by adding nitrogen to the working gas. According to the experiment of the inventor, if the nitrogen concentration in the working gas is set to about 10 to 5 mol% as a boundary area and the nitrogen concentration is lower than the boundary area, the consumption of the hafnium electrode increases critically, but if the nitrogen concentration is higher than the boundary area. As a result, a favorable electrode consumption suppressing effect comparable to that of pure nitrogen gas was obtained. On the other hand, regarding the cutting quality, according to the experiment of the inventor, in the plasma cutting, the oxygen purity is not a critical factor as conventionally thought, and the oxygen concentration is 70 mol% or more (preferably 80 mol% or more). Then, it was found that a good cutting quality that can withstand practical use can be ensured. (By the way, in laser cutting or gas cutting, for example, the cutting quality is greatly different at oxygen concentrations of 99.90 mol% and 99.99 mol%. The high purity critically affects the cutting quality.) Therefore, by using a mixed gas having an oxygen concentration of 70 to 95 mol% (preferably 80 mol% or more) and a nitrogen concentration of 30 to 5 mol% (preferably 5 to 10 mol%), it is possible to reduce electrode consumption. The two objectives of good cutting quality can be compatible.

  Based on the above principle, various working gas supply methods can be used in addition to the working gas supply method shown in FIG. 3 to 8 show examples of such another working gas supply method.

  In the method of FIG. 3, the composition of the working gas supplied to the torch is the same as the method of FIG. 2, but the flow of the gas in the working gas supply system is slightly different. That is, in the method of FIG. 2, when shifting from the preflow to the main flow, the pure nitrogen gas is stopped and the oxygen / nitrogen mixed gas is started, and when shifting from the main flow to the post flow, the reverse gas switching is performed ( A gas supply system shown in FIG. 10 described below is used). In contrast, in the method of FIG. 3, pure nitrogen gas is continuously flowed from the preflow to the main flow and the post flow, and pure oxygen gas is also flowed only during the main flow, so that the pure oxygen gas and the pure nitrogen gas are separated. Mixing is performed on the upstream side of the torch (using a gas supply system shown in FIG. 9 described later).

  In the method of FIG. 4, a mixed gas of nitrogen and oxygen flows in any of the preflow, main flow, and postflow. The concentrations of oxygen and nitrogen in the main flow are the same as in the method of FIG. 2, that is, the oxygen concentration is 70 to 95 mol% (preferably 80 mol% or more), and the nitrogen concentration is 30 to 5 mol% (preferably 5 to 10 mol%). %). On the other hand, in the preflow and the postflow, a working gas having a nitrogen concentration equal to or higher than the main flow, preferably a working gas having a nitrogen concentration equal to or higher than that of air (nitrogen concentration is about 80 mol%) is flown.

  In the method of FIG. 5, air (nitrogen concentration is about 80 mol%) is flowed in the preflow and postflow, and an oxygen / air mixed gas having the same oxygen / nitrogen concentration as the main flow in FIG. 2 is flowed in the main flow.

  In the method of FIG. 6, the gas composition supplied to the torch is the same as that of FIG. 5, but while continuing to flow air in the preflow, the main flow and the post flow, pure oxygen gas is flown only in the main flow, Mix pure oxygen gas and air upstream of the torch.

  In the method of FIG. 7, pure nitrogen gas is flowed in the preflow and post flow, and the same oxygen / air mixed gas as in the main flow of FIG. 5 is flowed in the main flow.

  In the method of FIG. 8, air (nitrogen concentration is about 80 mol%) flows in the preflow and postflow, and the same oxygen / nitrogen mixed gas as in the main flow of FIG. 2 flows in the main flow.

  In any of the above methods, when the pilot arc is generated and when the cutting is completed, the working gas containing a large amount of nitrogen is flown, so that the consumption of the hafnium electrode and the nozzle is reduced. During the cutting, a working gas containing a large amount of oxygen and a small amount of nitrogen is used, so that the consumption of the hafnium electrode can be reduced without substantially lowering the cutting quality. What is important here is that the nitride of the hafnium electrode has a higher melting point than that of the oxide, so that the hafnium electrode is actively nitrided during cutting or at the end of cutting. Based on the new finding that it is not a critical factor for quality, the oxygen concentration during cutting is slightly reduced to the extent that it does not substantially affect the cutting quality, and nitrogen is added instead.

  FIG. 9 shows a configuration of a working gas supply system used in the working gas supply method shown in FIG. FIG. 10 shows a configuration of a working gas supply system used in the working gas supply method shown in FIG.

  The working gas supply system shown in FIG. 9 includes a constant flow rate oxygen supply system 30 that always supplies a constant flow of pure oxygen gas, and a constant flow rate nitrogen supply system 50 that always supplies a constant flow rate of pure nitrogen gas. Oxygen and nitrogen from the two constant flow gas supply systems 30 and 50 can be mixed by the mixer 45 and supplied to the torch 1. At the most downstream part of each of the constant flow rate oxygen supply system 30 and the constant flow rate nitrogen supply system 50, pressure reducing valves 43 and 63 for setting the upper limit of the supply pressure to the torch 1, and the respective systems are opened and closed downstream thereof. Solenoid valves 45 and 65 are provided. The constant flow nitrogen supply system 50 remains open throughout the preflow, mainflow and postflow sections. The constant flow rate oxygen supply system 30 is opened only in the section of the main flow.

  The working gas supply system of FIG. 10 includes a constant concentration mixed gas supply system 70 that constantly supplies an oxygen / nitrogen mixed gas having a constant oxygen / nitrogen concentration to the torch 1, and a lower pressure than the constant concentration mixed gas supply system 70. And a nitrogen supply system 80 for supplying a small flow of pure nitrogen gas. At the most downstream part of each of the constant concentration mixed gas supply system 70 and the nitrogen supply system 80, pressure reducing valves 75 and 81 for setting the upper limit of the supply pressure to the torch 1, and each system is opened and closed downstream thereof. Solenoid valves 77 and 83 are provided. The constant-concentration mixed gas supply system 70 and the nitrogen supply system 80 are connected to the torch 1 in such a manner that the non-return valve 83 blocks the nitrogen supply system 80 when the constant-concentration mixed gas supply system 70 is opened. ing. The nitrogen supply system 80 remains open throughout the preflow, mainflow and postflow sections. The constant concentration mixed gas supply system 70 is opened only in the section of the main flow.

  The constant-concentration mixed gas supply system 70 has a constant flow rate oxygen supply system 30 and a constant flow rate nitrogen supply system 50 similar to those shown in FIG. Are mixed in a mixer 71 and supplied through a pressure reducing valve 75.

  Now, in the working gas supply system shown in FIGS. 9 and 10, one point to be noted is the configuration of the constant flow rate gas supply systems 30 and 50. That is, pressure reducing valves 33 and 53, pressure gauges 35 and 55, flow meters 37 and 57, needle valves (flow regulating valves) 39 and 59, and constant differential pressure valves 41 and 61 are arranged in this order from the gas cylinders 31 and 51 downstream. ing. The constant differential pressure valves 41 and 61 have their pressure monitors connected to the inlets of the flow meters 37 and 57 and the outlets of the differential pressure valves 41 and 61, respectively. Acts to maintain a constant differential pressure. Therefore, even if the pressure loss of the torch 1 downstream of each system changes during cutting, a constant flow rate is maintained because the differential pressure before and after the needle valves 39 and 59 is constant. Oxygen and nitrogen concentrations are maintained at appropriate values. Therefore, the effects of reducing the electrode consumption during cutting and maintaining the cutting quality as described above are guaranteed. Conventionally, if the nozzles and the like deteriorate, the overall gas flow rate increases and the cutting quality deteriorates. Therefore, it is necessary to replace the nozzles. Since the gas flow rate is kept constant, there is little reduction in cutting quality, and therefore, nozzle replacement can be delayed. That is, the consumables such as the nozzles have a substantially longer life.

  The second point to be noted is that the pressure reducing valves 43, 63, 75 are provided downstream of the constant pressure difference valves 41, 61 of the constant flow gas supply systems 30, 50, and the upper limit value of the supply pressure to the torch 1 is appropriately adjusted. Is set to a low value. That is, without such a pressure reducing valve, when the gas supply is stopped before starting the arc, the outlet pressure of the gas supply system rises to the original supply pressure (for example, about 10 atm at the outlet pressure of the pressure reducing valves 33 and 53). As soon as the solenoid valve is opened to start the arc, the high gas pressure is instantaneously applied to the torch 1, making it difficult to ignite the arc or time until the gas flow becomes stable after the arc is ignited. This causes a problem that the cutting cannot be performed during that time. However, in the present embodiment, since the maximum pressure applied to the torch 1 is set to an appropriate low value by the pressure reducing valves 43, 63, and 75 downstream of the constant differential pressure valves 41 and 61, stable arc ignition is possible. In addition, since the gas flow rate becomes stable immediately after the ignition of the arc, cutting can be started immediately.

  FIG. 11 shows a configuration example of another plasma torch to which the plasma cutting method according to the present invention can be applied.

  The plasma torch 101 is generally a multi-cylindrical shape as a whole, has a substantially cylindrical electrode 103 at the center thereof, and a substantially cylindrical nozzle 105 covers the outside of the electrode 103, and the outside of the nozzle 105 A first nozzle cap 107 having a substantially cylindrical shape covers the first nozzle cap 107, and a second nozzle cap 109 having a substantially cylindrical shape covers the outside of the first nozzle cap 107. The two nozzle caps 107, 109 are electrically insulated from the nozzle 105.

  The electrode 103 has therein a cooling water passage 111 through which cooling water passes, and has a heat-resistant insert 119 made of hafnium or zirconium at a tip portion where a plasma arc is generated.

  A working gas passage 113 is formed between the electrode 103 and the nozzle 105. The working gas is supplied into the working gas passage 113 from the base end side of the torch by a working gas supply system (not shown) and flows toward the tip of the torch. . An annular working gas swirler 121 is fitted in the middle of the working gas passage 113, and the working gas flow becomes a swirling flow when passing through the working gas swirler 121. At the tip of the nozzle 105, a nozzle orifice 131 having a sufficiently small diameter is opened so that the plasma arc can be narrowed sufficiently and jetted toward a jet stream. The nozzle 105 also has a cooling water passage for cooling it, but is not shown in FIG.

  A secondary gas passage 115 is formed between the nozzle 103 and the first nozzle cap 107. The secondary gas is supplied into the secondary gas passage 115 from the base end side of the torch by a secondary gas supply system (not shown). Flow toward the tip of the torch. An annular secondary gas swirler 123 is fitted in the middle of the secondary gas passage 115, and the secondary gas flow becomes a swirling flow when passing through the secondary gas swirler 123. The swirling direction of the secondary gas swirling flow is the same as the swirling direction of the working gas swirling flow. At the tip of the first nozzle cap 107, a secondary gas ejection port 133 from which a secondary gas is ejected is opened. The diameter of the secondary gas outlet 133 is larger than the diameter of the nozzle orifice 131. That is, the secondary gas outlet 133 is an annular opening surrounding the nozzle orifice 131.

  A tertiary gas passage 117 is formed between the first nozzle cap 107 and the second nozzle cap 109, and the tertiary gas is supplied into the tertiary gas passage 117 from the base end side of the torch by a tertiary gas supply system (not shown). Flow towards the tip. An annular tertiary gas swirler 125 is fitted in the middle of the tertiary gas passage 117, and the tertiary gas flow becomes a swirling flow when passing through the tertiary gas swirler 125. The swirling direction of the tertiary gas swirling flow is the same as the swirling direction of the working gas swirling flow. A tertiary gas jet 135 from which a tertiary gas is jetted is opened at the tip of the second nozzle cap 109. The diameter of the tertiary gas jet 135 is larger than the diameter of the secondary gas jet 31. That is, the tertiary gas jet 135 is an annular opening surrounding the secondary gas jet 133.

  Here, there are three types of cylindrical components, each of which includes the nozzle 105, the first nozzle cap 107, and the second nozzle cap 109, each of which surrounds the electrode 103 and has a gas outlet. It should be noted that the terms "nozzle" and "nozzle cap" are used to refer to components having completely different roles. That is, the “nozzle” is a component having the narrowest gas ejection port for restraining and narrowing the plasma arc, and the gas supplied from the upstream side (inside) of the “nozzle” is ) Are all turned into plasma and are called working gas or plasma gas in that sense. On the other hand, the “nozzle cap” is located on the downstream side (outside) of the “nozzle”, has a gas outlet larger in diameter than the gas outlet (nozzle orifice) of the nozzle, and has a gas outlet between the nozzle cap and the nozzle. The gas supplied from the passage to the gas outlet is not converted into plasma, but surrounds the plasma arc as a shielding gas. The secondary gas and the tertiary gas in the present embodiment function as a shielding gas. This is important for understanding the present embodiment without being confused with, for example, a plasma torch having a “nozzle” having a multiplex structure disclosed in JP-A-9-239545 and JP-A-10-314951. .

  Now, in the plasma torch 101 having the above structure, the swirling flow of the working gas which has flowed to the vicinity of the tip of the electrode 103 during the cutting is turned into plasma here, and the high-speed jet flow narrowed sufficiently through the nozzle orifice 131 And gushes toward the torch. From the secondary gas outlet 133, the secondary gas swirl jets toward the torch ahead on the outer periphery of the plasma arc to form a secondary gas curtain on the outer periphery of the plasma arc. From the tertiary gas outlet 135, the tertiary gas swirl jets toward the front of the torch toward the outer periphery of the secondary gas curtain to form a tertiary gas curtain on the outer periphery of the secondary gas curtain. In this manner, a three-layer gas flow is formed in which the secondary gas curtain surrounds the outer periphery of the plasma gas of the working gas and the tertiary gas curtain surrounds the outer periphery.

  Here, a gas having a composition as illustrated in FIGS. 2 to 8 is used as the working gas. The secondary gas includes a gas having an oxygen concentration of less than air, for example, a non-oxidizing gas such as pure nitrogen, air, or a mixed gas of oxygen and nitrogen having an oxygen concentration of 20 mol% or less. used. As the tertiary gas, a gas having an oxygen concentration equal to or higher than air, for example, a pure oxygen gas, a mixed gas of oxygen and nitrogen having an oxygen concentration of 20 mol% or more, or a mixed gas of oxygen and air is used. .

  FIG. 2 schematically illustrates an example of the oxygen concentration of the working gas, the secondary gas, and the tertiary gas during cutting, with the horizontal axis representing the distance in the radial direction from the center of the plasma arc. The example of FIG. 2 shows a case where an oxygen / nitrogen (or oxygen / air) mixed gas having an oxygen concentration of 90 mol% is used as the working gas, pure nitrogen gas is used as the secondary gas, and pure oxygen gas is used as the tertiary gas. . As shown in the figure, the plasma arc (working gas) has a high oxygen concentration, but the oxygen concentration temporarily drops below air (secondary gas curtain) outside, and further the oxygen concentration becomes higher again than air outside. (Tertiary gas curtain). Thus, during cutting, assuming that the oxygen concentration of the working gas is N1, the oxygen concentration of the secondary gas is N2, and the oxygen concentration of the tertiary gas is N3, the conditions of N1> N2 and N2 <N3 are satisfied. The composition of each gas is chosen.

  When the gas composition during cutting is selected in this manner, the plasma arc is surrounded by the secondary gas curtain having a low oxygen concentration, so that the combustion reaction is suppressed in the region of the secondary gas curtain having a low oxygen concentration at a low temperature, and the cutting phenomenon proceeds. Because it disappears, the distribution of the heat source by the plasma arc (the high temperature and high oxygen concentration area where the metal of the workpiece can be melted and cut) becomes sharp along the shape of the plasma arc surrounded by the secondary gas curtain. Therefore, the problem of burning in oxygen plasma cutting is reduced, and sharpness is obtained. In addition, since the outer periphery of the secondary gas curtain is covered by the tertiary gas curtain richer in oxygen than air, the tertiary gas promotes oxidation of dross remaining on the cut surface immediately after cutting and in the vicinity thereof. The dross is easily blown off by the force of the tertiary gas jet because the surface tension is reduced and the fluidity is increased when the oxidation is promoted, and thus the dross is reduced. Also, even if some dross adheres to the cut surface and remains, it can be easily peeled off due to the progress of oxidation. In particular, since the burning is prevented by the secondary gas curtain having a low oxygen concentration, the oxygen concentration of the tertiary gas can be increased and a large amount of oxygen can be supplied to the cut groove of the work without being restricted by the burning. As a result, a high cutting quality with no burning and little dross adhesion can be obtained.

  Further, in the torch 101 of FIG. 11, the plasma arc is swirled, and the secondary gas curtain and the tertiary gas curtain around the plasma arc are swirled in the same direction as the swirling flow of the plasma arc. In this case, the bevel angle of the cut surface can be adjusted over a large variable range by adjusting the turning strength (flow rate) of the secondary gas curtain and the tertiary gas curtain. By having this bevel angle adjusting function, not only the cutting quality in terms of burning and dross can be improved, but also a desired bevel angle (typically 0 degrees) can be obtained, and the effect of further improving the cutting quality can be obtained. can get.

  Although the embodiments of the present invention have been described above, these embodiments are merely examples for describing the present invention, and are not intended to limit the present invention to only these embodiments. Therefore, the present invention can be implemented in various modes other than the above-described embodiment.

FIG. 1 is a diagram showing a schematic configuration of a plasma torch used in a plasma cutting method according to an embodiment of the present invention.

6 is a time chart showing a first example of a method of supplying a working gas from the start to the end of cutting along with a change in arc current.

6 is a time chart showing a second example of a method for supplying a working gas flow from the start to the end of cutting.

9 is a time chart showing a third example of a method for supplying a working gas flow from the start to the end of cutting.

9 is a time chart showing a fourth example of a method for supplying a working gas flow from the start to the end of cutting.

9 is a time chart showing a fifth example of a method for supplying a working gas flow from the start to the end of cutting.

9 is a time chart showing a sixth example of a method of supplying a working gas flow from the start to the end of cutting.

9 is a time chart showing a seventh example of a method for supplying a working gas flow from the start to the end of cutting.

FIG. 4 is a system diagram showing a configuration of a working gas supply system used in the working gas supply method shown in FIG. 3.

FIG. 3 is a system diagram showing a configuration of a working gas supply system used in the working gas supply method shown in FIG. 2.

FIG. 4 is a cross-sectional view showing another plasma torch that can be used in the plasma cutting method according to one embodiment of the present invention.

FIG. 4 is an explanatory diagram schematically showing a gas composition example of a working gas (plasma arc), a secondary gas, and a tertiary gas during cutting, with a horizontal axis representing a distance in a radial direction from a center of the plasma arc.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Plasma torch 3 Electrode 5 Heat resistant insert (hafnium electrode)
6 Working gas passage 7 Nozzle 8 Working gas 9 Nozzle orifice 15 Pilot arc 17 Transition arc 21 Main arc 31, 51 Gas cylinder (gas source)
33, 53, 43, 63, 75, 81 Pressure reducing valve 39, 69 Needle valve (flow control valve)
41, 61 Constant differential pressure valve 45, 65, 77, 83 Solenoid valve 85 Check valve

Claims (12)

In an apparatus for cutting a workpiece by a plasma arc,
A plasma torch with electrodes and nozzles,
A gas supply system for supplying a working gas between the electrode and the nozzle of the plasma torch,
Igniting a pilot arc between the electrode and the nozzle, then establishing a main arc between the electrode and the workpiece, maintaining the main arc while cutting the workpiece, when the cutting of the workpiece is finished A plasma power supply for extinguishing the main arc,
Has,
The gas supply system,
(1) Supplying nitrogen gas, air, or a mixed gas having a higher nitrogen concentration than air to the preflow section when the pilot arc is ignited,
(2) supplying a mixed gas containing oxygen and nitrogen to the section of the main flow during cutting the work;
A plasma cutting apparatus, wherein the mixed gas supplied to the section of the main flow has an oxygen concentration of 70 to 95 mol% and a nitrogen concentration of 30 to 5 mol%. The plasma cutting apparatus according to claim 1, wherein the electrode is made of hafnium, zirconium, or an alloy thereof. The gas supply system,
(3) Supplying a nitrogen gas, air, or a mixed gas having a higher nitrogen concentration than air to a post flow section subsequent to the main flow section;
The plasma cutting device according to claim 1. The gas supply system,
A gas source,
A gas flow path for flowing gas from the gas source to the plasma torch,
A flow control valve for setting a gas flow rate, provided in the gas flow path,
The plasma cutting apparatus according to claim 1, further comprising: a constant pressure difference valve provided in the gas flow path to maintain a pressure difference before and after the flow rate control valve constant. The gas supply system,
Multiple gas sources;
A plurality of single gas flow paths that flow the respective gases from the plurality of gas sources to the plasma torch, and merge into an upstream of the plasma torch to form a mixed gas flow path,
A flow control valve provided in each single gas flow path for setting a flow rate of each gas,
The plasma cutting apparatus according to claim 1, further comprising: a constant pressure difference valve provided in each single gas flow path and acting to maintain a pressure difference before and after each flow rate control valve constant. The gas supply system,
Further comprising an additional single gas flow path for supplying a single gas from one gas source to the plasma torch;
Upstream of the plasma torch, the additional single gas flow path and the mixed gas flow path merge in a form in which gas from one of the flow paths is selectively supplied to the plasma torch,
The plasma cutting device according to claim 5. The plasma cutting apparatus according to claim 4, further comprising a pressure reducing valve downstream of each of the constant differential pressure valves for setting an upper limit of a supply pressure to the plasma torch. In an apparatus for cutting a workpiece by a plasma arc,
A plasma torch with electrodes and nozzles,
A gas supply system for supplying a working gas between the electrode and the nozzle of the plasma torch,
Igniting a pilot arc between the electrode and the nozzle, then establishing a main arc between the electrode and the workpiece, maintaining the main arc while cutting the workpiece, when the cutting of the workpiece is finished A plasma power supply for extinguishing the main arc,
Has,
The electrode is made of hafnium, zirconium or an alloy thereof,
A plasma cutting device, wherein the gas supply system supplies a mixed gas having an oxygen concentration of 70 to 95 mol% and a nitrogen concentration of 30 to 5 mol% to a section of a main flow while cutting the work. The gas supply system,
A gas source,
A gas flow path for flowing gas from the gas source to the plasma torch,
A flow control valve for setting a gas flow rate, provided in the gas flow path,
7. The plasma cutting apparatus according to claim 6, further comprising: a constant differential pressure valve provided in the gas flow path, the constant differential pressure valve acting to maintain a constant differential pressure before and after the flow control valve. The gas supply system,
Multiple gas sources;
A plurality of single gas flow paths that flow the respective gases from the plurality of gas sources to the plasma torch, and merge into an upstream of the plasma torch to form a mixed gas flow path,
A flow control valve provided in each single gas flow path for setting a flow rate of each gas,
7. The plasma cutting apparatus according to claim 6, further comprising: a constant differential pressure valve provided in each single gas flow path and acting to keep a differential pressure before and after each flow control valve constant. The gas supply system,
Further comprising an additional single gas flow path for supplying a single gas from one gas source to the plasma torch;
Upstream of the plasma torch, the additional single gas flow path and the mixed gas flow path merge in a form in which gas from one of the flow paths is selectively supplied to the plasma torch,
The plasma cutting device according to claim 10. The plasma cutting apparatus according to claim 9, further comprising a pressure reducing valve downstream of each of the constant pressure differential valves for setting an upper limit of a supply pressure to the plasma torch. .

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