JP2000312973A - Plasma cutting method, device and plasma torch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce dross adhesion while preventing burning of cutting portions. SOLUTION: This plasma torch 1 jets a plasma arc 43 of orifice gas containing richer oxygen than air against a work 41, jets a secondary gas curtain 45 including poorer oxygen than air to the surrounding of the plasma arc 43, and jets a third gas curtain 47 including richer oxygen than air to the surrounding of the secondary gas curtain 45. Thereby, the secondary gas curtain 45 including poor oxygen makes heat source distribution of the plasma arc 43 sharp so as to prevent burning. The third gas curtain 47 including rich oxygen promotes oxidation of dross immediately after cutting so as to reduce adhesion of dross.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cutting technique using a plasma arc, and more particularly to an improvement in a technique for cutting while supplying a gas curtain around the plasma arc in order to obtain good cutting quality.

[0002]

2. Description of the Related Art An oxygen plasma cutting method for cutting using a plasma arc using an oxygen gas or a gas containing a large amount of oxygen as a working gas (plasma gas) is a method for cutting a low alloy steel or a low carbon steel (mild steel). Suitable for cutting. In this oxygen plasma cutting method, a technique for improving cutting quality (particularly reducing dross adhesion) by forming a curtain of a secondary gas having a component more oxygen-rich than air around the plasma arc is known. Have been. Since the forefront of the cutting in the plasma cutting method is a place slightly deviated from the plasma arc (that is, a place where the plasma arc involves the surrounding atmosphere), the cutting is performed by covering the periphery of the plasma arc with an oxygen-rich atmosphere. This is because the oxygen concentration at the location is increased and the oxidation of the molten metal is promoted, and the fluidity of the molten metal is increased and dross is easily removed.

A prior art of this kind is disclosed in Japanese Unexamined Patent Publication No.
No. 2928 discloses an oxygen plasma cutting method in which a curtain of pure oxygen gas is formed around a plasma arc using pure oxygen as a working gas. But,
In the method using this pure oxygen curtain, the combustion reaction of the work becomes too active, and there arises a problem that a cutting groove becomes too wide or a cut surface becomes rough (burning occurs).
Therefore, this method can be applied only to a thin plate having a thickness of at most about 1 mm where the roughness of the cut surface does not cause much problem,
Therefore, it has not been put to practical use.

In order to solve this problem, Japanese Patent Application Laid-Open Publication No.
No. 8793 and Japanese Patent Application Laid-Open No. 7-51861 disclose a plasma cutting method in which a secondary gas supplied around an arc is used.
A mixed gas of oxygen and nitrogen (or oxygen and air) is used. Here, the ratio of oxygen in the secondary gas (flow rate ratio, that is, volume ratio) is 40% to 9 according to Japanese Patent Application Laid-Open No. 6-508793.
0% (preferably 2/3) and 60% in JP-A-7-51861, all of which have a lower oxygen concentration than pure oxygen but are more oxygen-rich than air (oxygen ratio about 20%). .

[0005]

In the oxygen plasma cutting method using the above-mentioned mixed gas as a secondary gas, the oxygen ratio of the mixed gas is set as large as possible within a range where burning does not occur, and cutting is performed as much as possible. This will improve the quality (especially the reduction of dross adhesion). However, in order to prevent burning, the composition of the mixed gas must be finely adjusted in accordance with cutting conditions such as the plate thickness and the cutting speed, and the adjustment is complicated.

In order to reduce the dross adhesion, it is desirable to promote the combustion reaction by oxygen. However, since the amount of oxygen supplied to the cut groove is limited to a range where burning does not occur, the combustion reaction by oxygen is suppressed. It cannot be used under the best conditions and dross adhesion cannot be completely eliminated.

Accordingly, an object of the present invention is to provide a plasma cutting technique capable of effectively reducing dross adhesion while preventing burning.

Another object of the present invention is to provide a plasma cutting technique which does not require fine adjustment of the gas composition even when cutting conditions such as a plate thickness and a cutting speed change.

[0009]

SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a plasma cutting method comprising: forming a plasma arc using a working gas containing oxygen; Providing a secondary gas stream having a concentration to form a secondary gas curtain;
Providing a tertiary gas stream having a higher oxygen concentration than the secondary gas around the secondary gas curtain to form a tertiary gas curtain.

According to this plasma cutting method, it is possible to reduce dross adhesion while preventing burning. The reason is presumed to be (although not necessarily confirmed) as follows. In other words, the secondary gas curtain with low oxygen concentration formed around the plasma arc indicates that the heat source (the high temperature and high oxygen concentration region where the work can be melted and cut) by the plasma arc greatly expands outside the plasma arc. Since the distribution of the heat source is suppressed to be sharp, the burning is prevented and a sharp sharpness is obtained. In addition, the tertiary gas curtain with high oxygen concentration around the secondary gas curtain promotes the oxidation of dross existing on the cut surface immediately after cutting, and enhances the fluidity and peelability of dross,
Dross adhesion is reduced. As a result, good cutting quality is obtained. Furthermore, because the secondary gas curtain prevents burning, even if cutting conditions such as plate thickness and cutting speed change, there is no longer any need to fine-tune the gas composition in view of burning, and the oxygen concentration of the tertiary gas or The oxygen flow rate can be increased to an optimum value for the purpose of eliminating dross adhesion.

In a preferred embodiment, the working gas is air, a mixed gas containing at least 20% by volume of oxygen (eg, a mixed gas of oxygen and a non-oxidizing gas (eg, nitrogen), a mixed gas of oxygen and air, etc.). ) Or oxygen gas. As the secondary gas, air, a mixed gas containing 20% or less by volume of oxygen, or a non-oxidizing gas containing no oxygen (for example, nitrogen) is used. As the tertiary gas, a mixed gas containing oxygen in a volume ratio of 20% or more (for example, a mixed gas of oxygen and a non-oxidizing gas (for example, nitrogen), a mixed gas of oxygen and air, or the like) is used. By using a gas having such an oxygen concentration, the object of preventing the burning described above and reducing the dross adhesion can be particularly effectively achieved.

In another preferred embodiment, the tertiary gas curtain is formed not in the entire periphery of the secondary gas curtain but behind the cutting direction of the secondary gas curtain. Even if you do this,
Since the oxygen-rich tertiary gas is supplied to the cutting groove immediately after cutting, dross adhesion can be reduced.

A plasma cutting apparatus according to a second aspect of the present invention includes a plasma torch and a gas supply system for supplying a working gas, a secondary gas, and a tertiary gas to the plasma torch. The plasma torch has a nozzle for injecting a plasma arc of a working gas, a secondary gas curtain forming mechanism for forming a secondary gas curtain by supplying a secondary gas flow around the plasma arc injected from the nozzle, and a secondary gas curtain. A tertiary gas curtain forming mechanism is provided to supply a tertiary gas flow around or behind the secondary gas curtain formed by the gas curtain forming mechanism to form a tertiary gas curtain.

A plasma cutting torch according to a third aspect of the present invention includes a nozzle having a working gas passage, a nozzle orifice communicating with the working gas passage and injecting a plasma arc of the working gas, a secondary gas passage, A secondary gas curtain forming mechanism having a secondary gas ejection port that communicates with the secondary gas passage and ejects a secondary gas flow around the plasma arc to form a secondary gas curtain; and a tertiary gas passage. A tertiary gas curtain forming mechanism that forms a tertiary gas curtain by supplying a tertiary gas flow around the secondary gas curtain or behind the cutting direction in communication with the tertiary gas passage.

According to the plasma cutting apparatus or the plasma cutting torch of the present invention, a three-layer gas flow having a double gas curtain of a secondary gas curtain and a tertiary gas curtain is formed around the plasma arc of the working gas. can do. Various variations can be considered for the combination of the working gas, the secondary gas, and the composition of the tertiary gas supplied to the plasma cutting device or the plasma cutting torch.As an example, a gas having a relatively high oxygen concentration is used as the working gas. A gas having a low oxygen concentration can be used as the secondary gas, and a gas having a relatively high oxygen concentration can be used as the tertiary gas. Then, as described above, the effect of preventing burning and reducing dross adhesion can be obtained. Further, by using the plasma arc as a swirl flow and the secondary gas curtain as a swirl flow also swirling in the same direction, the effect that the bevel angle of the cut surface can be adjusted to a desired value by adjusting the swirl intensity of the secondary gas is obtained. Can be At this time, it is possible to select whether the tertiary gas curtain is also turned or not.
By swirling the tertiary gas, it is possible not only to further assist the operation of bevel angle adjustment by swirling the secondary gas, but also to improve the uniformity of the tertiary gas curtain.

Further, the plasma cutting apparatus or the plasma cutting torch of the present invention can be used not only for the oxygen plasma cutting method but also for the cutting of a material such as stainless steel whose cutting quality is deteriorated by oxidation. The present invention can also be applied to a plasma cutting method used for gas. In plasma cutting stainless steel, a plasma arc using a non-oxidizing working gas such as nitrogen or argon is covered with a non-oxidizing secondary gas curtain such as nitrogen, and the secondary gas curtain cuts out the cut location. Shield from the air to prevent oxidation of the cut surface. However, since there is some outside air entrained in the secondary gas curtain (especially when the secondary gas is swirled to adjust the bevel angle as described above), an extremely large amount of secondary gas is used. It is difficult to sufficiently shield the cut surface from the outside air unless the air flows. In contrast, the plasma cutting device or plasma cutting torch of the present invention further includes a tertiary gas curtain outside the secondary gas curtain, so that both the secondary gas and the tertiary gas curtain are non-oxidizing gas curtains such as nitrogen. By doing so, a sufficient shielding effect can be obtained with the double non-oxidizing gas curtain with a smaller shielding gas amount than before. In particular, when the secondary gas curtain is swirled to adjust the bevel angle as described above, a tertiary gas curtain exists outside the swirling secondary gas curtain, and outside air is supplied to the secondary gas curtain. By avoiding direct contact, the shielding effect is particularly large when compared with the prior art.

Thus, the effect of forming the double gas curtain of the secondary gas curtain and the tertiary gas curtain outside the plasma arc is a special one that cannot be expected from the prior art.

[0018]

FIG. 1 shows a schematic sectional structure of a plasma torch used in a plasma cutting method according to an embodiment of the present invention.

The plasma torch 1 has a generally multi-cylindrical shape as a whole, and has a substantially cylindrical electrode 3 at the center thereof.
A substantially cylindrical nozzle 5 covers the outside of the electrode 3, a substantially cylindrical first nozzle cap 7 covers the outside of the nozzle 5, and a substantially cylindrical nozzle 5 covers the outside of the first nozzle cap 7. A second nozzle cap 9 of shape is covered. The two nozzle caps 7, 9 are electrically insulated from the nozzle 5.

The electrode 3 has a cooling water passage 11 through which cooling water passes, and a tip portion of the electrode 3 serving as an ignition point of the plasma arc is made of a material having a high melting point capable of withstanding the high heat of the plasma arc, such as hafnium or zirconium. It has a heat-resistant insert 19 made of.

A working gas passage 13 is formed between the electrode 3 and the nozzle 5, and the working gas is supplied from a base end side of the torch into the working gas passage 13 by a working gas supply system (not shown) and is directed to a tip of the torch. Flowing towards. An annular working gas swirler 21 is fitted in the middle of the working gas passage 13. When passing through the working gas swirler 21, the working gas flow becomes a swirling flow. A nozzle orifice 3 having a sufficiently small diameter is provided at the tip of the nozzle 5 for narrowing the plasma arc sufficiently narrow and jetting the jet into a jet stream.
One is open. The nozzle 5 also has a cooling water passage for cooling it, but is not shown in FIG.

A secondary gas passage 15 is formed between the nozzle 5 and the first nozzle cap 7, and the secondary gas is introduced into the secondary gas passage 15 from the base end of the torch by a secondary gas supply system (not shown). It is supplied and flows toward the torch tip. An annular secondary gas swirler 2 is provided in the middle of the secondary gas passage 15.
The secondary gas flow is swirling when passing through the secondary gas swirler 23. The swirling direction of the secondary gas swirling flow is the same as the swirling direction of the working gas swirling flow. First
At the tip of the nozzle cap 7, a secondary gas ejection port 33 from which a secondary gas is ejected is opened. The diameter of the secondary gas outlet 33 is larger than the diameter of the nozzle orifice 31. That is, the secondary gas outlet 33 is an annular opening surrounding the nozzle orifice 31.

A tertiary gas passage 17 is formed between the first nozzle cap 7 and the second nozzle cap 9, and tertiary gas is supplied into the tertiary gas passage 17 from the torch base end by a tertiary gas supply system (not shown). It flows toward the tip of the torch. An annular tertiary gas swirler 25 is fitted in the middle of the tertiary gas passage 17, and the tertiary gas flow becomes a swirling flow when passing through the tertiary gas swirler 25. 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 ejection port 35 from which a tertiary gas is ejected is opened at the tip of the second nozzle cap 9. The diameter of the tertiary gas jet 35 is larger than the diameter of the secondary gas jet 31. That is, the tertiary gas outlet 35 is an annular opening surrounding the secondary gas outlet 33.

Here, the nozzle 5, the first nozzle cap 7
There are three types of cylindrical components, each of which surrounds the electrode 3 and has a gas outlet, namely, a second nozzle cap 9 and a second nozzle cap 9. In this specification, the terms “nozzle” and “nozzle cap” are used. Note that is used to refer to components that have 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” ) Are all turned into plasma and are called working gas or plasma gas in that sense. On the other hand, the “nozzle cap”
It has a gas outlet located downstream (outside) of the "nozzle" and larger in diameter than the gas outlet (nozzle orifice) of the nozzle, and supplies the gas to the gas outlet from the gas passage between the nozzle cap and the nozzle The generated gas does not become plasma, but flows along the plasma arc as a shielding gas and surrounds the plasma arc. The secondary gas and the tertiary gas in the present embodiment function as a shielding gas.
This is not to be confused with, for example, a conventional plasma torch having a “nozzle” having a multiplex structure disclosed in JP-A-9-239545 and JP-A-10-314951, and it is important to understand the embodiments of the present invention. Is important.

Now, in the plasma torch 1 having the above structure, the swirling flow of the working gas flowing near the tip of the electrode 3 is turned into plasma here, and is passed through the nozzle orifice 31 so as to be sufficiently narrowed down. It becomes a plasma arc of the current and gushes toward the torch. From the secondary gas outlet 33, the secondary gas swirl flow is ejected toward the torch toward the outer periphery of the plasma arc to form a secondary gas curtain on the outer periphery of the plasma arc. Tertiary gas outlet 3
From 5, the tertiary gas swirling flow is jetted toward the torch ahead on the outer periphery of the secondary gas curtain, forming a tertiary gas curtain on the outer periphery of the secondary gas curtain. In this manner, a gas flow having a three-layer structure is formed in which the outer periphery is surrounded by the secondary gas curtain and the outer periphery is surrounded by the tertiary gas curtain around the plasma arc of the working gas.

Here, when 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.
Preferably, the working gas is a gas having an oxygen concentration equal to or higher than air, such as air, pure oxygen gas, a mixed gas of oxygen and nitrogen, wherein the oxygen ratio is higher than air (oxygen ratio about 20%), or oxygen. A mixed gas of air and air is used. The tertiary gas is also a gas having an oxygen concentration higher than air, for example, pure oxygen gas, a mixed gas of oxygen and nitrogen having an oxygen ratio higher than air (oxygen ratio of about 20%), or a mixed gas of oxygen and air Etc. are used. The working gas and the tertiary gas may have the same composition, or may have different compositions. On the other hand, in the secondary gas, a gas whose oxygen concentration is lower than air,
For example, a non-oxidizing gas such as pure nitrogen, air, or a mixed gas of oxygen and nitrogen having an oxygen ratio lower than air (oxygen ratio about 20%) is used. Here, the term “ratio” is used to mean a gas flow ratio, that is, a volume ratio.

FIG. 2 shows the change in the oxygen ratio of the three-layer gas flow in the case where pure oxygen gas is used as the working gas and the tertiary gas and pure nitrogen gas is used as the secondary gas in the radial direction from the center of the plasma arc. The distance is schematically shown on the horizontal axis. As shown, although the plasma arc has a high oxygen concentration, the oxygen concentration temporarily drops below the air outside the air (secondary gas curtain), and the oxygen concentration again becomes higher outside the air (the tertiary gas curtain). ). As can be seen from the above description, using pure oxygen gas as the working gas and tertiary gas and using pure nitrogen gas as the secondary gas is merely an example, and a combination of gases having other compositions is also possible. . It is desirable that the working gas and the tertiary gas contain oxygen at or above the oxygen ratio of air (20%) indicated by the dotted line in FIG. 2, and the secondary gas contains oxygen at no more than the oxygen ratio of air or contains no oxygen at all. It is desirable. One preferable example of other gas composition combinations is that the working gas is oxygen and nitrogen (or oxygen and air) having an oxygen ratio of about 70 to 95%.
, The secondary gas is pure nitrogen gas, and the tertiary gas is pure oxygen gas. In this combination, since the working gas is a mixed gas having an oxygen ratio of about 70 to 95%, there is an advantage that compared with the case of using pure oxygen gas, the cutting power is maintained and the consumption of the electrode is reduced. can get. In addition, during cutting, a gas that satisfies the above conditions is used, but when forming a pilot arc before starting cutting,
When the arc is extinguished after the cutting is completed, it is particularly desirable to use a gas having a lower oxygen concentration than the above conditions or a gas containing no oxygen, for example, a nitrogen gas, in order to suppress electrode consumption.

FIGS. 3 and 4 are a cross-sectional view and a plan view, respectively, showing the state of the cut portion when cutting the steel plate 41 using the plasma torch 1. As shown in FIGS.

As described above, the outer periphery of the oxygen-rich plasma arc 43 ejected from the plasma torch 1 is covered by the secondary gas curtain 45 having a low oxygen concentration, and the outer periphery is covered by the oxygen-rich tertiary gas curtain 47. In the illustrated example, the plasma torch 1 cuts the steel plate 41 while moving from left to right with respect to the steel plate 41 in the figure, and a cutting groove 51 is formed after the plasma arc 43. Here, the cutting front 53 of the steel plate 41 is
But not outside or near the outer edge of the plasma arc 43. The reason is that the frontmost cutting surface 53 precedes the plasma arc 43 due to radiation from the plasma arc 43 and heat conduction of the steel plate 41. In the present embodiment, the plasma arc 43 is surrounded by the secondary gas curtain 45 having a low oxygen concentration, and in the region of the secondary gas curtain 45 having a low oxygen concentration at a low temperature, the combustion reaction is suppressed and the cutting phenomenon proceeds. The distribution of the heat source (the high-temperature and high-oxygen-concentration region where the metal of the steel plate 41 can be melted and cut) by the plasma arc 43 is reduced by the distribution of the plasma arc 43 region surrounded by the secondary gas curtain 45 and its vicinity. It becomes a sharp shape limited to only. By the way, when the plasma arc is surrounded by an oxygen-rich atmosphere than air as in the prior art,
The oxygen-rich atmosphere promotes the oxidation of the metal, which supplies enough energy to melt the metal to a certain distance from the plasma arc, unnecessarily widening the heat source distribution and causing the burning. Becomes In the present embodiment, the heat source distribution is reduced by the presence of the secondary gas curtain 45 having a low oxygen concentration.
Since the cutting groove 51 becomes sharp along the shape of 5, the cutting groove 51 does not unnecessarily widen greatly (that is, burning is suppressed), and a sharp sharpness is obtained.

Furthermore, in this embodiment, since the tertiary gas curtain 47, which is richer in oxygen than air, covers the outer periphery of the secondary gas curtain 45, the cut surface 55 immediately after cutting enters the tertiary gas curtain 55. This promotes oxidation of dross remaining on the cut surface 55 and its vicinity. Dross promotes oxidation, lowering surface tension and increasing fluidity,
It is easily blown off by the power of the tertiary gas jet, thereby reducing dross adhesion. Further, 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 45 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 cutting groove 51 without being restricted by the burning. Accordingly, dross adhesion can be more effectively reduced as compared with the related art in which the oxygen concentration is suppressed under the limitation of burning. As a result, it is possible to obtain a high cut quality, that is, a preferable cut surface with no burning and little dross adhesion.

Even if the cutting conditions such as the plate thickness and the cutting speed change, it is sufficient to simply increase the oxygen concentration (oxygen flow rate) of the tertiary gas in order to prevent dross from adhering. There is no need for subtle adjustments in relation to burning.

Further, in this embodiment, the plasma arc 4
3 is a swirling flow, and the secondary gas curtain 45 and the tertiary gas curtain 47 on the outer circumference thereof are swirling flows that swirl in the same direction as the swirling flow of the plasma arc 43. In this case, the secondary gas curtain 45 and the tertiary gas curtain 47
The bevel angle of the cut surface can be adjusted over a large variable range by adjusting the swirl strength (flow rate). That is, as shown in FIG. 4, when the turning direction is clockwise when the cutting position is viewed from the torch, as the turning strength of the secondary gas curtain 45 and the tertiary gas curtain 47 increases, the plasma arc 43 moves in the cutting direction. Cut surface 5 on the right side in the cutting direction
The bevel angle of 7R changes from a plus value (an angle between the upper surface of the steel plate 41 and the cut surface) to an obtuse angle, to zero (the same angle is a right angle), and further to a minus value (the same angle is an acute angle). Conversely, when the turning direction is counterclockwise when the cutting portion is viewed from the torch, as the turning strength of the secondary gas curtain 45 and the tertiary gas curtain 47 increases, the plasma arc 43 tilts leftward in the cutting direction. Therefore, the bevel angle of the left cut surface 57L in the cutting direction changes from a positive value to zero and further to a negative value. 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. In this embodiment, in order to obtain the bevel angle adjusting function, both the secondary gas curtain 45 and the tertiary gas curtain 47 are swirled, but only one of them may be swirled. That is, for example, only the secondary gas may be swirled in the same direction as the plasma arc 43, and the tertiary gas may be a non-swirl jet. However, when the tertiary gas is swirled, the operation of adjusting the bevel angle by the secondary gas can be further maintained, and the uniformity of the tertiary gas curtain 47 can be improved.

FIGS. 5 and 6 show a modification of the above-described embodiment.

As shown in FIG. 5, the plasma torch 1 according to this modification is different from the above-described embodiment in the structure for forming the tertiary gas curtain 47. That is, the tertiary gas pipe cover 61 is attached to the outer surface of the first nozzle cap 7 on the rear side in the cutting direction, so that a pipe-shaped tertiary gas passage 63 is formed. The tertiary gas jet is supplied from the gas jet port 65 of the tertiary gas passage 63 to the cutting groove 51 behind the secondary gas curtain 45 in the cutting direction. Therefore, the tertiary gas curtain 47 is formed only on the rear side of the secondary gas curtain 45 in the cutting direction. Figure 3, already
According to the same principle as described with reference to FIG. 4, burning is prevented by the secondary gas curtain 45 having a low oxygen concentration, and dross adhesion is reduced by the tertiary gas curtain 47 having a high oxygen concentration.

As another application of the plasma torch 1 having the structure shown in FIG. 1, there is an application for cutting stainless steel.

In the cutting of mild steel, as described above, for the purpose of optimizing the combustion reaction between iron and oxygen, a working gas having a high oxygen concentration, a secondary gas having a low oxygen concentration, and a tertiary gas having a high oxygen concentration are used. A combination of gases is used. On the other hand, in cutting stainless steel, it is necessary to suppress the combustion reaction due to oxygen. That is, when chromium, which is one of the alloy elements of stainless steel, is oxidized, chromium oxide that inhibits the fluidity of the molten metal is generated, resulting in deterioration of cutting quality. Therefore, in the plasma cutting of stainless steel, in order to prevent oxidation, a mixed gas mainly composed of nitrogen containing no oxygen and added with argon or hydrogen is usually used as a working gas. Here, the reason for adding hydrogen is to eliminate the influence of oxygen in the air. In cutting this stainless steel, the torch 1 having the structure shown in FIG.
It is preferable that the working gas, the secondary gas, and the tertiary gas are all non-oxidizing gases such as nitrogen and argon. Conventionally, a non-oxidizing secondary gas curtain has been used to prevent the adverse effects of oxygen in the air, but by further providing a non-oxidizing tertiary gas curtain on the outside,
Since the secondary gas curtain is less likely to come into direct contact with the outside air, and the cutting atmosphere can be more effectively shielded from the air, the adverse effect of oxygen in the air can be reduced more reliably. Compared to the conventional method of shielding only with a secondary gas curtain, the method of shielding with a double curtain of secondary gas and tertiary gas requires a shielding gas flow rate necessary to obtain the same shielding effect. Less is needed. Furthermore, when a gas containing hydrogen is used as the tertiary gas that directly contacts air, oxidation can be more effectively prevented. According to this method, the same effect can be obtained with a smaller gas amount than when the tertiary gas is supplied over the entire circumference.

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

[Brief description of the drawings]

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

FIG. 2 is an explanatory diagram schematically showing an example of a gas composition of a three-layer gas flow of a working gas (plasma arc), a secondary gas, and a tertiary gas, with a horizontal axis representing a radial distance from the center of the plasma arc. .

FIG. 3 is a cross-sectional view of a cut portion of the steel sheet, showing a state where the steel sheet 41 is cut using the plasma torch 1 of FIG. 1;

FIG. 4 is a plan view of a cut portion of the steel sheet showing a state when the steel sheet 41 is cut using the plasma torch 1.

FIG. 5 is a cross-sectional view of a torch and a cut portion of a steel plate showing a state when cutting a steel plate 41 using a modification of the plasma torch 1 of FIG. 1;

FIG. 6 is a plan view of a cut portion of the steel sheet, showing a state when cutting the steel sheet 41 using the plasma torch 1 according to the modification of FIG. 5;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Plasma torch 3 Electrode 5 Nozzle 7 1st nozzle cap 9 2nd nozzle cap 13 Working gas passage 15 Secondary gas passage 17 Tertiary gas passage 21 Working gas swirler 23 Secondary gas swirler 25 Tertiary gas swirler 31 Nozzle orifice 33 Secondary gas outlet 35 Tertiary gas outlet 41 Steel plate (work) 43 Plasma arc 45 Secondary gas curtain 47 Tertiary gas curtain 53 Forefront of cutting 55 Location where dross oxidation is promoted 57R Cutting surface on the right side in cutting direction 57L Facing in cutting direction Cut left side

Claims (9)

[Claims] 1. A method for cutting a workpiece by a plasma arc, comprising: forming the plasma arc using a working gas containing oxygen; and providing a second gas having a lower oxygen concentration than the working gas around the plasma arc. Supplying a secondary gas flow to form a secondary gas curtain; and supplying a tertiary gas flow having a higher oxygen concentration than the secondary gas around the secondary gas curtain to form a tertiary gas curtain. A plasma cutting method comprising: 2. The working gas is air, a volume ratio of 20%.
The plasma cutting method according to claim 1, wherein the mixed gas contains oxygen or oxygen gas. 3. The secondary gas is air, a volume ratio of 20%.
The plasma cutting method according to claim 1 or 2, wherein the mixed gas contains the following oxygen or a non-oxidizing gas containing no oxygen. 4. The plasma cutting method according to claim 1, wherein the tertiary gas is a mixed gas containing oxygen at a volume ratio of 20% or more, or an oxygen gas. 5. A step of supplying the tertiary gas flow to a cutting groove behind the secondary gas curtain in a cutting direction, instead of forming the tertiary gas curtain around the secondary gas curtain. 2. The plasma cutting method according to 1. 6. A plasma torch, a gas supply system for supplying a working gas, a secondary gas, and a tertiary gas to the plasma torch, wherein the plasma torch is configured to spray a plasma arc of the working gas; A secondary gas curtain forming mechanism for supplying a secondary gas flow around a plasma arc injected from a nozzle to form a secondary gas curtain, and a secondary gas curtain formed by the secondary gas curtain forming mechanism. And a tertiary gas curtain forming mechanism for forming a tertiary gas curtain by supplying a tertiary gas flow around or behind the cutting direction. 7. The plasma of claim 6, wherein the working gas contains oxygen, the secondary gas has a lower oxygen concentration than the working gas, and the tertiary gas has a higher oxygen concentration than the secondary gas. Cutting device. 8. The working gas is air, a volume ratio of 20%.
The above-mentioned mixed gas containing oxygen, or oxygen gas, The secondary gas is air, a non-oxidizing gas containing no more than 20% oxygen by volume or oxygen, and the tertiary gas is 7. The plasma cutting device according to claim 6, wherein the plasma cutting device is a mixed gas or oxygen gas containing oxygen at a volume ratio of 20% or more. 9. A nozzle having a working gas passage, a nozzle orifice communicating with the working gas passage and injecting a plasma arc of the working gas, a secondary gas passage, and the plasma arc communicating with the secondary gas passage. A secondary gas curtain forming mechanism having a secondary gas ejection port for ejecting a secondary gas flow around the periphery to form a secondary gas curtain, a tertiary gas passage, and the secondary gas communicating with the tertiary gas passage. A tertiary gas curtain forming mechanism for forming a tertiary gas curtain by supplying a tertiary gas flow around the curtain or behind the cutting direction.