WO1999053734A1

WO1999053734A1 - Torch and method for electric arc welding and cutting

Info

Publication number: WO1999053734A1
Application number: PCT/FR1999/000791
Authority: WO
Inventors: Michel Delzenne; Régis Augeraud; Eric Salassa
Original assignee: La Soudure Autogene Francaise
Priority date: 1998-04-09
Filing date: 1999-04-06
Publication date: 1999-10-21
Also published as: FR2777214B1; FR2777214A1; AU3041299A

Abstract

The invention concerns a method for electric arc processing using a torch comprising a tungsten electrode and a main nozzle equipped with a main gas outlet orifice, which consists in: supplying said torch with at least a non-oxidizing gas, said non-oxidizing gas coming into direct contact with at least part of the tungsten electrode; forming at least one main non-oxidizing gas jet containing at least part of said non-oxidizing gas in ionized state and the electric arc, said main non-oxidizing gas jet coming out through said main nozzle main orifice; supplying, downstream of said main nozzle main outlet, said main non-oxidising gas jet with at least an oxidizing gas flux, so as to obtain a total gas jet containing an at least partial mixture of said non-oxidizing gas and said oxidizing gas.

Description

TORCH AND METHOD OF ELECTRIC ARC CUTTING OR WELDING

The present invention relates to a torch and a method of working with an electric arc, in particular a plasma cutting method, using, on the one hand, a tungsten electrode and, on the other hand, a non-plasma-producing gas. oxidant and an oxidizing plasma gas. Plasma cutting of sheet metal and other metallic structures is a technique widely used in the industry.

In general, a plasma arc cutting device comprises a direct current electrical source, which is connected, on the one hand, to a plasma cutting torch and, on the other hand, to the workpiece; the negative pole of the current source being connected to the electrode of the plasma torch, also called cathode, and the positive pole of said current source being connected to the part to be cut, which acts as anode.

The establishment of an electric arc between the electrode of the plasma torch and the workpiece is done in particular by means of a plasma gas which undergoes ionization at least inside the plasma torch and, more precisely, at least in the space separating the nozzle from the electrode equipping said torch.

The ionization of the plasma gas then being continued in the jet or column of plasma gas exiting through the orifice of the nozzle of the plasma torch; said gas jet also ensuring the transfer of the arc 2

electric to the metal piece to be cut. This is known as a transferred plasma arc process.

In general, to form the groove and obtain the desired cutting profile, it is customary to maintain the plasma torch, during its operation at a substantially constant distance from the workpiece and to move said torch relative to said workpiece also at a substantially constant speed. Currently, there are two main types of plasma torches with transferred arc capable of being used in a plasma cutting process, namely single-gas or single-flow torches, and bi-gas or double-flow torches.

More precisely, a single-gas torch implements a single stream of plasma gas, intended to allow the establishment of an electric arc between the electrode and the piece to be cut, to ensure the stabilization of the arc root on the electrode and to generate the process of cutting the part to be cut by thermal, kinetic and / or chemical effects combined with the plasma jet proper.

In this case, the quality of the cut obtained is mainly a function of the choice of the plasma gas used taking into account the nature of the material to be cut, the mode of distribution of said gas in the plasma chamber, as well as other parameters conditioning the geometry of the plasma gas jet, such as its flow speed and the distribution of temperature zones.

A single-gas torch is described in particular in document US-A-4977305.

Furthermore, the operating principle of a bi-gas or double-flow torch is identical to that of a single-gas torch, but usually also includes one or more additional gas circuits making it possible to distribute at least one second gas, in general, peripherally with the plasma jet coming out of the nozzle.

Although this additional gas produces additional kinetic and chemical effects making it possible, in certain applications, to improve the quality of the cuts, this additional gas participates little or not at all in the formation of the plasma jet, due to very weak ionization, or even almost zero.

In some cases, the addition of an additional gas peripherally to the plasma jet generates a thermal action to stabilize the central vein of said plasma jet by cooling the outer peripheral layer of the latter and concentrating the energy in it. soul of the plasma jet. Similar plasma torches are described in particular in documents US-A-3575568 and US-A-5558786.

In general, these two types of plasma torches, namely mono or bi-gas, are used with plasma gases both of the oxidizing type and of the non-oxidizing type.

However, in all cases, care should be taken to ensure correct compatibility of the plasma gas with the nature of the metal constituting the electrode of the torch.

Thus, when the torch is used to use a non-oxidizing gas, such as nitrogen or an argon / hydrogen, nitrogen / hydrogen or nitrogen / argon / hydrogen mixture, a tungsten electrode is usually used.

Such a combination of tungsten electrode / non-oxidizing gas makes it possible to obtain an increased service life of said electrode.

Plasma torches using a non-oxidizing gas flow are conventionally used to cut stainless steels and light alloys. In addition, when the torch is a bi-gas torch, the second gas can be of the same family as the gas 4

plasma or a different gas, such as carbon dioxide or a hydrocarbon, neutral gas or neutral gas mixture, an active gas.

Conversely, for oxidizing gas torches, tungsten electrodes are to be avoided because of their rapid deterioration in the presence of the oxidizing gas.

In fact, the presence of oxygen in the gas results in the formation of an oxide with a low sublimation point on the tungsten electrode and generates accelerated destruction of it.

Hence, it is customary to use with this type of torch a zirconium or hafnium electrode, which exhibits a clearly more satisfactory behavior in the presence of oxygen than the tungsten electrodes. However, the fact remains that the lifetime of a zirconium or hafnium electrode in the presence of oxidizing gas is significantly shorter than that of a tungsten electrode in the presence of non-oxidizing gas, and this, the lower the intensity of the cutting current.

Thus, for intensities greater than 150 A, the lifetime of a zirconium or hafnium electrode can be approximately 4 to 10 times less than that of a tungsten electrode. However, it is known that oxidizing gases, more particularly oxygen, are of great interest for cutting, in particular structural steels because they provide unparalleled cutting qualities and performance. However, given the wear problem of the aforementioned zirconium and hafnium electrodes, industrial applications are generally limited to intensities less than or equal to 300 A.

In certain applications, a second gas, such as a mixture of nitrogen and oxygen, can be distributed around the periphery of the jet of oxidizing gas to avoid aspiration. 5

of ambient air, improving the stability of the anode foot of the arch in the groove being formed and thus the resulting quality of cut.

In other words, it is currently very difficult to ensure, with an oxidizing gas and a hafnium electrode, an operation of a torch and of a plasma cutting process of a duration greater than or equal to that obtained with a non-oxidizing gas and a tungsten electrode; this being all the more true as the intensity of current implemented is strong.

The object of the present invention is therefore to alleviate the above-mentioned problems, that is to say to obtain, whatever the intensity of the current used, an operating time of a working torch at electric arc, such as a plasma cutting torch or TIG welding torch, using an oxidizing gas substantially comparable to that obtained with a torch equipped with a tungsten electrode and using a non-oxidizing gas, and this, by conserving or improving performance in terms of quality and / or cutting speed, in particular of structural steels, and / or quality of the weld obtained.

The present invention therefore relates to a method of working with an electric arc using a torch comprising a tungsten electrode and a main nozzle provided with a main gas outlet orifice, in which:

said torch is supplied with at least one non-oxidizing gas, said non-oxidizing gas coming into direct contact with at least part of the tungsten electrode,

- At least one main jet of non-oxidizing gas is formed containing at least part of said non-oxidizing gas in the ionized state and the electric arc, said main jet of non-oxidizing gas leaving in the form of a plasma jet by l main opening of said main nozzle, - one feeds, downstream of the outlet orifice of said main nozzle, said main jet of non-oxidizing gas with at least one stream of at least one oxidizing gas, so as to obtain a total jet of gas containing a mixture at least partially of said non-oxidizing gas and said oxidizing gas.

Depending on the case, the method of the invention comprises one or more of the following characteristics:

the proportions of said oxidizing gas and of said non-oxidizing gas are adjusted so as to prevent any entry of oxidizing gas into said main nozzle and / or any contact of said mixture with the tungsten electrode;

- The proportions of said oxidizing gas and of said non-oxidizing gas are adjusted, so as to obtain cutting edges of the part to be cut substantially free of burrs;

the proportions of oxidizing gas and of non-oxidizing gas are adjusted by acting on at least one parameter chosen from the site of injection of the non-oxidizing gas, the site of injection of the oxidizing gas, the pressure of the oxidizing gas, the non-oxidizing gas pressure, oxidizing gas flow and non-oxidizing gas flow;

the non-oxidizing gas is chosen from nitrogen, argon, helium, hydrogen and their mixtures; the oxidizing gas is chosen from oxygen, air, carbon dioxide, ozone and their mixtures;

- it is chosen from plasma cutting, plasma welding, plasma marking, plasma reloading and plasma spraying processes or TIG welding processes.

The invention also relates to an electric arc work torch, such as a plasma cutting torch or a TIG (Tungsten Inert Gas) welding torch, comprising: - a tungsten electrode, 7

means for supplying non-oxidizing gas,

means for supplying oxidizing gas,

means for forming a main jet of gas containing at least the non-oxidizing gas and the electric arc,

- a main nozzle fitted with a main outlet for the main gas jet,

distribution means making it possible to supply, downstream of the outlet orifice of said main nozzle, said main jet of non-oxidizing gas with at least one oxidizing gas, so as to obtain a total jet of gas containing a mixture at least partially of said non-oxidizing gas and said oxidizing gas.

Preferably, the torch according to the invention comprises a second nozzle also provided with a gas outlet orifice, the gas outlet orifices of said two nozzles being coaxial.

According to another aspect, the invention also relates to the use of a plasma torch according to the invention for cutting steel or an iron-containing material, such as in particular non-alloyed or low-strength structural steels allies.

The present invention will now be described in more detail with reference to the accompanying figures, given by way of illustration, but not limitation.

FIG. 1 represents a diagram of the plasma cutting technique with transferred arc.

More specifically, in this FIG. 1 is shown, in partial longitudinal section, the active end of a plasma cutting torch 1 comprising an electrode 2 and a nozzle 10 provided with a main orifice 13 for ejecting the plasma jet 7.

A direct current electric source 5 and connected, by its negative pole, to the electrode 2 or cathode of the plasma torch 1 and, by its positive pole, 8

to the workpiece 12 serving as an anode, for example a steel sheet.

A gas 3 is introduced into the plasma chamber 9 separating the electrode 2 from the nozzle 10 from the torch 1; the introduction of gas which may be laminar or vortex.

After ionization of gas 3 in the plasma chamber

9, the electric arc 6 is transferred by means of the jet of ionized plasma gas 7 between the electrode 2 and the piece to be cut 12, and a cutting groove 8 is then formed within the piece 12 to be cut.

Figures 2 and 3 show, for their part, diagrams similar to that of Figure 1, of a single-gas plasma torch and a bi-gas plasma torch, respectively.

Thus, FIG. 2 schematizes the operating mode of a single-gas torch according to the prior art, according to which a gas 3 of non-oxidizing type, for example nitrogen or argon, is used in combination with a tungsten electrode 2.

As mentioned above, the combination of a tungsten electrode 2 and a non-oxidizing gas makes it possible to increase the resistance of the electrode to the stresses of the plasma arc 6, that is to say to increase the lifetime of the electrode 2, due to low wear thereof.

However, the quality of the cut obtained and / or the cutting speed cannot be considered to be really satisfactory, in particular when cutting non-alloyed or low-alloyed structural steel.

Furthermore, FIG. 3 represents a bi-gas plasma torch, also called a double-flow torch, comprising as above an electrode 2, a nozzle

10 provided with a main outlet 13 for the outlet of the main gas jet 7 and separated from the electrode 2 by a plasma chamber 9 into which the non-oxidizing gas 3 is introduced.

According to FIG. 3, the torch 1. also carries a second nozzle or nozzle forming a sleeve around at least part of the main nozzle 10, within which a second nozzle 14 is formed an orifice 19 coaxial with the orifice 13 carried by the nozzle 10, so as to allow the main jet of plasma gas 7, the electric arc 6 and a secondary gas 4 introduced into the space 15 separating the main nozzle 10 from the second nozzle 14 to pass.

When the gas 3 and the secondary gas 4 are of the non-oxidizing type, the electrode 2 is made of tungsten; the bi-gas torch then has the drawbacks of the mono-gas torch of FIG. 2.

Conversely, when the plasma gas 3 and the secondary gas 4 are of the oxidizing type, the electrode 2 is then made of zirconium or hafnium.

However, as mentioned above, the lifetime of an electrode 2 with a hafnium or zirconium insert may be approximately 4 to 10 times less than that of a tungsten electrode 2 associated with a non-oxidizing gas, but tends to have better performance in terms of quality of cut, in particular when cutting structural steel.

FIGS. 4 and 5 represent diagrams, in partial longitudinal section, of the active end of a plasma torch 1 according to the present invention provided with a tungsten electrode 2 carried by an electrode holder 17 in copper for example, said electrode being cooled by heat exchange with a cooling fluid supplied by a dip tube 21.

A non-oxidizing plasma gas 3, such as nitrogen or argon, is introduced into the plasma chamber 9 separating the electrode 2 from the main nozzle 10, then 10

ejected from the main outlet 13 of the main plasma jet 7 and the electric arc 6.

According to the embodiment of FIG. 4, the main nozzle 10 comprises a first internal wall 23 and a second external wall 24 delimiting an intra-nozzle space 25 in which an oxidizing gas travels

4, such as oxygen or air.

This intra-nozzle space 25 opens downstream of the orifice 13 of the main nozzle 10, so as to allow at least partial mixing of the jet 7 of ionized non-oxidizing plasma gas with the oxidizing gas 4.

In other words, this intra-nozzle space 25 channels the oxidizing gas 4 towards the column formed by the ionized plasma gas 7 and the electric arc 6. By doing this, a plasma jet formed of a mixture of gases is obtained. oxidant and non-oxidizing gas having good cutting performance and, in addition, any harmful entry of oxidizing gas, such as oxygen, within the plasma chamber 9, that is to say any contact between the oxidizing gas 4 and the tungsten electrode 2.

FIG. 5 is similar to FIG. 4, but this time represents an embodiment according to which, the intra-nozzle space 25 conveying and channeling the oxidizing gas 4 is delimited, on the one hand, by a first wall 23 carried by the electrode 2 and, on the other hand, a second external wall 24 constituted by the main nozzle 10.

As before, the distribution of oxidizing gas is carried out downstream of the main orifice 13 of the main nozzle and towards the column 6, 7 of a plasma jet composed of the non-oxidizing plasma gas 7 ionized within the plasma chamber 9 and of the electric arc 6 taking root on or near the electrode 2 in tunσstene. 11

In general, the embodiments shown in FIGS. 4 and 5 exploit the advantages of non-oxidizing plasma gas torches, namely a high lifetime of the tungsten electrode 2, and oxidizing gas torches, know how to obtain performance, particularly in terms of quality and high cutting speed, in particular when cutting non-alloyed or low-alloyed structural steels.

In other words, the present invention makes it possible to separate, on the one hand, the functions of arc generation and stabilization of the cathode root of said electric arc and, on the other hand, generation of the cutting process of the material.

However, in order to guarantee the longevity or lifetime of the tungsten electrode 2, any direct contact between the flow of oxidizing gas 4 and said electrode 2 should be avoided.

To do this, care is taken that the ratio of the flow rates of non-oxidizing plasma gas 3 and oxidizing gas 4 is such that the quantity of non-oxidizing gas is sufficiently low compared to the quantity of oxidizing gas so as not to penalize cutting performance, since the useful gas used to guarantee said cutting performance is mainly oxidizing gas.

The injection sites for the non-oxidizing gas 3 and the oxidizing gas 4 and the cross-sections of the passages for the said gases in the plasma chamber 9 and the intra-nozzle space 25, respectively, are chosen and dimensioned. the flow of non-oxidizing gas 3 and to favor the flow of said oxidizing gas 4.

For an embodiment of two-stage nozzle 10, as shown in FIG. 4, it is then necessary to limit, for the first stage, the passage section and the length of the ejection channel from the main orifice 13 to maximum admissible but taking care 12

to avoid destruction of said ejection channel from the main orifice 13 by parasitic electric arcs, whereas, for the second stage, a large passage section of the column of plasma gas and of the oxidizing gas flow is chosen, so as to obtain a sufficient flow rate to evacuate the molten metal and avoid burrs on the workpiece; the first stage being delimited by the first wall 23 and the second stage being delimited by the second wall 24. The constriction of the electric arc is then ensured by a proportion chosen between the section and the length of the ejection channel, where it results in a good quality of cut and an absence or almost absence of parasitic electric arc. In addition, the plasma torch according to the invention makes it possible to obtain a jet of plasma gas formed by a gaseous mixture in a chosen proportion but by dispensing with the use of any gas mixer. In addition, a plasma torch according to the invention makes it possible to practice cutting under an oxidizing gas, such as oxygen, without limiting the intensity of the current of the electric arc used, that is to say in making it possible to work under higher cutting current intensities than under conventional conditions, for example for current intensities greater than 300 A, or even greater than 450 A, without penalizing the productivity of the process which allows cut steel sheets with a thickness of up to 25 to 100 mm, for example.

The transferred electric arc plasma torch according to the present invention can be of the manual or automatic type, and can be applied to any cutting, welding, marking or plasma projection operation. In addition, use is preferably made of a non-oxidizing gas containing a small proportion of impurities of 13

oxygen and / or water vapor type, so as to avoid wear or deterioration of the tungsten electrode by said impurities. Advantageously, the non-oxidizing gas contains less than 20 ppm of oxygen and less than 80 ppm of water vapor, as impurities. Maintaining a low impurity content of said non-oxidizing gas can be carried out through the use of non-oxidizing gas of sufficient purity to guarantee said contents and / or through the implementation of sufficient sealing of the materials. (supply lines, torch, etc.) and / or the arrangement on the means for supplying non-oxidizing gas with purification means making it possible to remove said impurities H ₂ O and / or 0 ₂ .

Claims

14 Claims

1. A method of working with an electric arc using a torch comprising a tungsten electrode and a main nozzle provided with a main gas outlet orifice, in which:

- At least one main jet of non-oxidizing gas is formed containing at least a portion of said non-oxidizing gas in the ionized state and the electric arc, said main jet of non-oxidizing gas exiting through said main orifice of said nozzle main,

- is fed, downstream of the main outlet of said main nozzle, said main jet of non-oxidizing gas with at least one stream of at least one oxidizing gas, so as to obtain a total jet of gas containing a at least partial mixing of said non-oxidizing gas and said oxidizing gas.

2. Method according to claim 1, characterized in that the proportions of said oxidizing gas and of said non-oxidizing gas are adjusted so as to prevent any entry of oxidizing gas or ambient air into said main nozzle.

3. Method according to one of claims 1 or 2, characterized in that the proportions of said oxidizing gas and said non-oxidizing gas are adjusted, so as to obtain cutting edges of the workpiece substantially free of burrs.

4. Method according to one of claims 1 to 3, characterized in that the proportions of oxidizing gas and non-oxidizing gas are adjusted by acting on at least one parameter chosen from the site of injection of the oxidizing gas, the non-oxidizing gas injection site, oxidizing gas pressure, non-oxidizing gas pressure, oxidizing gas flow and non-oxidizing gas flow. 15

5. Method according to one of claims 1 to 4, characterized in that the non-oxidizing gas is chosen from nitrogen, argon, helium, hydrogen and their mixtures.

6. Method according to one of claims 1 to 4, characterized in that the oxidizing gas is chosen from oxygen, air, ozone, carbon dioxide and their mixtures.

7. Method according to one of claims l to 6, characterized in that it is chosen from the plasma cutting processes, plasma welding, plasma marking, plasma reloading and plasma spraying or the TIG welding processes.

8. Torch (1) comprising: - a tungsten electrode (2),

- means for supplying non-oxidizing gas (3),

- means for supplying oxidizing gas (4),

means (9, 23) making it possible to form a main jet (7) of gas containing at least part of said non-oxidizing plasma gas (3) and said electric arc (6),

a main nozzle (10) provided with a main outlet (13) for exit from said main jet (7) of plasma gas,

- distribution means (24, 25) making it possible to supply, downstream of the main outlet (13) of said main nozzle (10), said main jet (7) of non-oxidizing gas with at least one oxidizing gas (4), so as to obtain a total jet (4 ′) of plasma gas containing an at least partial mixture of said non-oxidizing gas (3) and said oxidizing gas (4).

9. Torch according to claim 8, characterized in that it comprises a second nozzle (14) comprising a second orifice (19) for the gas outlet, said main orifice (13) and second orifice (19) being coaxial.

10. Use of a plasma torch (1) according to one of claims 8 or 9 for cutting a 16

steel part, such as a non-alloyed or low-alloyed structural steel part.