GB1562014A

GB1562014A - Arc welding

Info

Publication number: GB1562014A
Application number: GB537977A
Authority: GB
Original assignee: V N I PROEKTNOKONSTRUKT I TEKH
Current assignee: V N I PROEKTNOKONSTRUKT I TEKH
Priority date: 1977-02-09
Filing date: 1977-02-09
Publication date: 1980-03-05

Description

(54) IMPROVEMENTS IN AND RELATING TO ARC WELDING (71) We, VSESOJUZNY NAUCHNO ISSLEDOVATELSKY, PROEKTNOKON STRUKTORSKY I TEKHNOLOGICHESKY INSTITUT ELEKTROSVAROCHNOGO OBORUDOVANIA, of Litovskaya utilisa 10, Leningrad, U.S.S.R., a Corporation organised and existing under the laws of the Union of Soviet Socialist Republics, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to methods of plasma arc welding.

According to the invention there is provided a method of plasma arc welding of a workpiece using a plurality of continuously burning direct current arcs, comprising the steps of either pro ducing all the arcs between nonconsumable electrodes and the workpiece or producing some arcs between nonconsumable electrodes and the other arc or arcs between one or more consumable electrodes and the workpiece, feeding a plasma forming gas to the arcs burn ing from nonconsumable electrodes, supplying a shielding medium to the area of the weld, arranging the arcs so that adjacent arcs have opposite polarities, and controlling the current of the arcs and the distance between them so that the ratio of the product of the currents of each two adjacent arcs to the distance between them is not less than 6.103 A2 1cm.

According to the invention there is further provided a method of progressively welding a workpiece using a plurality of direct-current plasma arcs, comprising the steps of arranging a plurality of parallel arcs in a row with altern ate arcs of one polarity and intervening arcs of the opposite polarity, producing relative dis placement between the row of arcs and the workpiece along the line of the proposed weld, and controlling the currents of the arcs so that the ratio of the product of the currents of each two adjacent arcs to the distance between the axes of these arcs is greater than 6.103A2/cm.

Methods of arc welding according to the invention will now be described by way of example, with reference to the accompanying diagrammatic drawings in which; Figure 1 is a side elevation of welding system for performing one of the methods for welding steels; Figure 2 is a side elevation of another welding system for performing another one of the methods for welding steels and coppers; Figure 3 is a side elevation of another welding system for performing yet another one of the methods for welding aluminium or its alloys; and Figure 4 is a side elevation of a further welding system for performing a further one of the methods for welding aluminium or its alloys.

The welding system shown in Figure 1 is arranged to produce three arcs for plasma welding of steels.

The welding system of Figure 1 is arranged to produce an elongate welded joint between two members of a workpiece 7, by progressively displacing the system relative to the workpiece along the line of the proposed joint. The system employs three non-consumable electrodes 3, 6 and 4 arranged in series along the line of the proposed joint and electrically insulated from one another. Three A.C. to D.C.

converters are coupled to a common A.C. power supply to form respective D.C. power sources 1, 5 and 2. The negative output terminals of sources 1 and 2, the positive output terminal of the source 5 and the two members of the workpiece 7 are all grounded. The negative output terminals of the sources 1 and 2 are connected to respective electrodes 3 and 4 while the positive output terminal ofthe source 5 is connected to the electrode 6.

When the electrodes 3, 6 and 4 are energised, they produce separate arcs 12, 13 and 14.

Plasma-shaping nozzles 8, 9 and 10 provided with central channels 11 serve to stabilize respective arcs 12, 13 and 14. The plasma-forming nozzles 8, 9 and 10 are electrically insulated from one another. A high frequency discharger 15 is connected between each nonconsumable electrode and its corresponding plasma-forming nozzle for contactless starting of welding arcs.

The arcs 12, 13 and 14 are supplied through respective passages 16, 17, 18 with a plasmaforming gas from respective gas bottles 19, 20, 21. A shielding gas is supplied through a passage 23 from a cylinder 24 to a shielding nozzle assembly 22 common to all three arcs. When flux is used as a shielding medium, it is supplied from a hopper 25 directly to the workpiece 7.

If filler material is used for welding, a feed mechanism 26 is provided to supply filler material 27 to the arcs.

A welding operation is performed in the following sequence. Firstly, arcs 12 and 14 are initiated to burn from the electrodes 3 and 4 to the workpiece 7. The initial current for each arc 12 and 14 is selected to ensure stable burning of the arcs so that the maximum deviation caused by electro-magnetic interaction between the two electric arcs from respective electrode axes is less than 7 Secondly, the reversed polarity arc 13 is started with the same current and the three-arc system becomes electro-magnetically balanced.

The currents of the arcs 12, 13, 14 are simultaneously raised to the operating level. At the same time or after a predetermined delay, the system is moved with respect to the workpiece 7 at a welding speed, for example, in the direction opposite to that indicated by the arrow in Figure 1.

It is possible that all welding arcs are started simultaneously. However, in this case electromagnetic interaction between the arcs increases and the permissible starting current of each arc should be lowered as compared to the previous stepwise starting of welding arcs, which deteriorates stability of plasma arc burning at light currents.

Welding with nonconsumable electrodes may need the supply of a filler material to thicken the joint depending on the requirements set to the welded joint. The filler material is electrically non-conductive. The filler material is melted directly in the burning zone of any of the welding arcs 12, 13, 14. The resultant welding pool is protected by a shielding gas flux, or a combination of the gas and flux.

In the welding system shown in Figure 2, parts similar to those in Figure 1 are similarly referenced. The welding system of Figure 2 uses both consumable and non-consumable electrodes to effect the welding of steel or copper parts.

As shown in Figure 2, the negative terminals of two direct current sources 1 and 2 are respectively connected to nonconsumable electrodes 3 and 4. The positive terminal of a direct current source 5 is connected via a lead 28 to a consumable electrode 29 which is arranged to be fed towards the workpiece by a mechanism 30. The nonconsumable electrodes 3 and 4 and the consumable electrodes 29 are electrically insulated from one another. The positive terminal of the power sources 1 and 2 and the negative terminal of the power source 5 are connected to a workpiece 31.

Two plasma-forming nozzles 8 and 10 each provided with central channel 11 act to stabilize the arcs 12 and 14 produced at respective electrodes 3 and 4. A nozzle 32 serves to supply a shielding gas directly to the zone of burning of an arc 33 produced at the consumable electrode 29. This shielding gas is similar in composition to that of the shielding gas for the arcs 12 and 14. The plasma-forming nozzles 8, 10 and the shielding nozzle 32 are electrically insulated from one another. Three high-frequency dischargers 15 are connected one between each nonconsumable electrode 3, 4 and its corresponding nozzle 8, 10 and one between the consumable electrodes 29 and the workpiece 31. When energised, the dischargers 15 effect contactless initiation of the arcs 12, 14,33.

The plasma arcs 12 and 14 are supplied with a plasma-forming gas from respective bottles 21 and 19 through respective conduits 16 and 18.

The shielding gas is supplied to the arcs 12 and 14 through a common shielding nozzle 22, the gas being fed to the nozzle 22 through a passage 23 from a gas cylinder 24. The shielding gas which is supplied directly to the zone of the consumable electrode 29 is fed through a passage 34 from a gas cylinder 35. Where the flux is to be used as the shielding medium, it is supplied from the hopper 25 and discharged directly on to the workpiece 31.

A welding operation is started in the following sequence. Firstly, arcs 12 and 14 are initiated between the nonconsumable electrodes 3 and 4 and the workpiece 31. The initial current for the arcs 12 and 14 is selected so that the arcs are stable and any electromagnetic interaction which takes place does not displace the welding arcs from the axes of their respective electrodes 3 and 4 by more than 70 The arc 33 is then started by causing a highfrequency discharge between the consumable electrode 29 and the workpiece 31, and the feed mechanism 30 for driving the consumable electrode 29 towards the workpiece is switched ON. In a modification, the arcs can be initiated by contacting workpiece with the electrodes.

Devices are used to limit the magnitude of the resultant short-circuit current. After all arcs are initiated, the multi-arc system is electromag netically balanced and the current is increased to its operating value simultaneously on all three arcs 12, 14 and 33. At the same time or after a short interval, the arcs are displaced with respect to the workpiece 31 at a welding speed, for example in the direction opposite to that indicated by the arrow in Figure 2.

In a modification, all the welding arcs 12, 14 and 33 are started simultaneously by the highfrequency dischargers 15. In another modification, the arc 33 between the consumable electrode 29 and the workpiece 31 is started by contacting the workpiece with an electrode and using a device to limit the magnitude of the short-circuit current.

In the welding system where the arc is first started between the consumable electrode and the workpiece by short circuit, welding programming devices are used to ensure stable burning of the welding arc with light starting currents.

In the welding system shown in Figure 3, parts similar to those in Figure 1 are similarly referenced.

As shown in Figure 3, the welding system, which is arranged to weld aluminium or its alloys, has two direct current sources 1 and 2, the positive terminals of which are connected to respective nonconsumable electrodes 3 and 4, and another direct current source 5, the negative terminal of which, is connected to the nonconsumable electrode 6. The negative terminals of the sources 1 and 2 and the positive terminal of the source 5 are connected to the workpiece 7.

In the welding system shown in Figure 4, parts similar to those in Figure 2 are similarly referenced. The system of Figure 4 is arranged to weld aluminium or its alloys. The only difference in the systems of Figures 2 and 4 is that the connection polarities between the sources 1, 2 and 5 and the electrodes 3, 4 and 29 are reversed.

In order to achieve satisfactory welding using any of the welding systems of Figures 1 to 4, a welding pool must be established and maintained.

In order to achieve this condition, the arcs should be located fairly close to one another.

The ratio of the product of the currents of adjacent arcs to the distance between the arcs should be > 6.103A2/cm, that is Il -I2 = 6.103A2/cm, where Il and I2 are the Q currents of adjacent arcs and Q is the distance between the axes of those arcs.

The value 6.103A2/cm is the minimum permissible value and is determined by pecu liarities of welding of small thicknesses.

EXAMPLE I The welding of a composite beam from two channel beams will now be described. Beam operational conditions require that one of butt joints should be reinforced but that the other butt joint should have no seam reinforcement.

The joints are single-side welded, one-pass and square butt. The welded material has the following composition, in percentage terms by weight.

C 0.12; Si = 0.17-0.37; Mn = 1An1.80; C 0.30; Ni 0.30; Cu 0.30; S = 0.04; P = 0.035; the rest = Fe. The welded edges are 8mm thick.

Welding of the joint without reinforcement is performed using the welding system of Figure 1 employing three nonconsumable electrodes.

The leading and trailing arcs are of the same polarity, while the middle arc is of the reversed polarity.

Carbon dioxide (CO2) is used as the plasmaforming gas for the two outer arcs, and argon (Ar) as the plasma-forming gas for the middle arc. Consumption of the CO2 plasma forming gas in the two outer arcs is equal to 260 /mum.

The middle arc consumes 200 Q/min of Ar plasma-forming gas.

Carbon dioxide (CO2) is used as the shielding medium. Consumption of the CO2 shielding gas amounts to 2,000 /her. Flux used as the shielding medium has the following composition in 5b: SiO2 =41.0-44; MnO = 34.0-38.0; CaO 6.5;Mg0 = 5.0-7.5;A203 < 4-5; CaF2 =4.05.5;Fe2O3 < 2.0; S < 0.15; P < 0.12.

Employment of flux improves the final appearance of the weld.

The operating current of the leading arc is equal to 1,150 A, the operating current of the trailing arc is equal to 980 A, the operating current of the middle arc is equal to 780 A.

The distance between the axis of the middle arc and those of the two outer arcs is 37mm.

The lower of the two ratios of the product of the currents of two adjacent arcs to the dis, tance between them has a value of 20.7 103 A2 /cm.

The speed of welding is 390 m/hr. The removal of slag and the appearance of the weld are improved by using a mixture C02 + 2 as the plasma-forming gas for the two outer arcs.

Other parameters of welding remain unchanged.

The welded joint is formed by melting the welded material. In order to compensate for metal and other losses, a filler material having the following composition in percentage terms by weight is used C = 0.05-0.11;Mn = 1.80-2.10;Si = 0.7-0.95; Cr < 0.20;Ni < 0.25;S < 0.025;P < 0.030; the rest = Fe.

Welding of the reinforced butt joint is per formed using the welding system of Figure 1 which has two nonconsumable electrodes and one consumable electrode. The arcs between the nonconsumable electrodes and the work piece are of the same polarity while the arc between the consumable electrode and the work-piece is of the reversed polarity. Compo sitions and expenditures of the plasma-forming gas and the shielding media are similar to those used for welding the unreinforced butt joint.

The diameter of the consumable electrode is 3mm.

The composition of the consumable electrode, in percentage terms by weight is: C # 0.10; Mn = 1.40-1.70; Si = 0.65-0.85; Cr# 0.20; Ni < N 0.25; S < 0.025;P < 0.030; the rest = Fe.

The operating current of the leading arc is 1,1 50A the operating current of the trailing arc is 980A and the operating current of the middle arc burning between the consumable electrode and the workpiece (beam) is 850A.

The distance between the axis of the consumable electrode and the axis of each of the other arcs is 37mm.

The lower of the two ratios of the product of the currents of two adjacent arcs to the distance between their axes is 22.5 103A2/cm.

The speed of welding is 440m/hr.

The clearance between the welded edges should be within 1 to 1.5mm, when the arcs are started between nonconsumable electrodes and the workpiece (beam) and other arcs are started between consumable electrodes and the workpiece (beam).

EXAMPLE 2 The welding of vessels of corrosion-resisting steel having a thickness of 20mm will now be described. The production process provides for single-sided weld, one-pass, square butt or bevelled joints. With square butt welding the weld cannot be reinforced.

The composition of the material to be welded, in percentage by weight terms is: C < 0.08;Cr= 17.0-l9.0;Ni=9.0-ll.0; Si < 0.80; Mn 6 1.20; S # 0.020; P # 0.035; the rest = Fe.

Welding of a square butt joint is performed using the welding system of Figure 1 with three nonconsumable electrodes. The leading and trailing arcs have the same polarity, whereas the middle arc has a reverse polarity.

Argon is used as the plasma-forming gas for all three arcs. Each arc consumes 180 l/hr of the plasma-forming gas.

Argon is also used as the shielding medium and its consumption for this purpose amounts to 1,200 Q/hr.

The operating current of the leading arc is 1,100 A, the operational current of the trailing arc is 950 A and the operating current for the middle arc is 730 A.

The distance between the axis of the middle arc and the axis of each of the other two arcs is 37mm. The lower of the two ratio of the product of the current of two adjacent arcs to the distance between them is 18.7 103A2/cm.

The welding speed is 55 m/hr.

In a modification, the welding speed is increased by 50 to 60% and a mixture of 95% by weight of Ar + 5% by weight of H2 is used as the plasma-forming gas for the leading and trailing arcs. Other parameters of the process remain unchanged.

If carbon dioxide (CO2) is used as the plasma-forming gas for the leading and trailing arcs, the speed of welding can be increased by 1.5 to 2 times; all other parameters remain the same.

If carbon dioxide is used as the plasma forming gas for the leading and trailing arcs as well as for the shielding medium, the welding speed may be increased by 2 to 2.5 times, but the appearance of the weld suffers.

The appearance of the weld can, however, be improved by protecting the weld metal by flux, even though carbon dioxide is used as the plasma-forming gas for the leading and trailing arcs. In this case the consumption of the CO2 plasma-forming gas at the leading and trailing arcs is increased, as well as the depth of fusion, by up to 20% without changing other parameters of the process.

The composition of the flux in percentage terms by weight is as follows: SiO2 = 19.0-24.0; MnO < N 0.5; CaO = 3.0-9.0; MgO = 9.0-13.0;Al2O3 = 27.0-32.0;Na2O and K2O = 2.0-3.0; CaF2 = 25.0-33.0; Fe2O3 = 1.0; S # 0.08; P # 0.05.

The weld joint is formed in this case by fusion of the welded material only.

The welding of the bevelled joint is per formed using the welding system of Figure 2.

The composition of and the consumption of the plasma-forming gas and the shielding media are similar to those encountered in the square butt welding.

The diameter of the consumable electrode is Smm.

The composition of the consumable electrode, in percentage terms by weight is: C # 0.08; Mn = 1.0-2.0; Si = 0.3-0.8; Cr = 18.0-20.0; Ni = 9.0-11.0; Ti = 0.5-0.8; S # 0.018; P # 0.025; Mo = 2.0-3.0;the rest = Fe.

The operating current for the leading arc is 1,100 A, the operating current for the trailing arc is 950 A, and the operating current for the middle arc is 790 A.

The distance between the axis of the middle arc and that of each of the other arcs is 37mm.

The lower of the two ratios of the product of the current of two adjacent arcs to the distance between their axes is 20.3 10 A/cm.

The welding speed is equal to 67m/hr.

The speed of welding can be increased by up to 4 to 6 times if active gases or gas mixtures of inert and active gases are employed as the plasma-forming gases, as well as if flux is used as the shielding medium.

EXAMPLE 3 The welding of crystallizer moulds for vacuum arc and electroslag melting will now be described. The operating conditions of moulds used for vacuum arc melting are such that they are preferably made of chromium copper. The operating conditions of moulds used for electroslag melting are such that they are preferably made of copper. The welded edges are 40mm thick.

The welding of moulds employed for vacuum arc melting is performed using the welding system of Figure 1. Square butt welding is employed. The welded material has the following composition, in percentage terms by weight Cr = 0.F0.7; Fe < N 0.06; Pb < N 0.005; Zn 0.015; Mn 0.002; Si Q 0.05;P O.Ol;the rest Cu.

Argon is used as the plasma-forming gas for all three arcs. Consumption of the plasmaforming gas in each arc amounts to 260 Qlhr.

The shielding medium is argon and its consumption for shielding purposes is 900 Qlhr.

The operating current of the leading arc is 1,100 A, the operating current of the trailing arc is 950 A and the operating current of the middle arc is 730 A.

The distance between the axis of the middle arc and each of the other two arcs is 37mm.

The lower of the two ratios of the product of the currents of two adjacent arcs to the distance between the axes is 18.7 103A2/cm.

The speed of welding is 11.0 m/hr. In a modification, nitrogen (N2) is used as the plasma-forming gas for the leading and trailing arcs and for the shielding gas. In this case the speed of welding is doubled without altering any of the welding parameters.

In another modification, a mixture of 30% by weight of Ar and 70% by weight He is used as the plasma-forming medium for all three arcs and argon is used as the weld shielding.

Instead, the shielding medium can be a flux having the following composition, in percentage terms by weight: SiO = 30.0-32.0; A1203= 20.0-22.0; MnO = 2.5-3.5; CaF2 = 20.0-24.0; MgO= 16.0-l8.0;CaO= 5.0-6.5; FeO2 +Fe203 < N 1.0; S # 0.15; P # 0.1.

The weld is formed using the basic (welded) material and a filler material. The composition of the filler material should be the same as the basic (welded) material.

The welding of bevelled joints of moulds employed for electroslag melting is performed using the welding system of Figure 2.

The compositions and consumptions of the plasma-forming gas and the shielding media are similar to those used for welding by the welding system of Figure 1.

The diameter of the consumable electrode is 5mm.

The composition of the consumable electrode in percentage terms by weight is: Si=3.5;Mn= l.5;Zn= 1.0; Fe # 0.03; Pb # 0.03 P 0.05; Ni O.1;Bi 0.002;the rest = Cu.

The operating current of the leading arc is 1,100 A, the operating current of the trailing arc is 950 A and the operating current of the middle arc is 790 A.

The distance between the axis of the middle arc and each axis of the other two arcs is 37mm.

The lower of the ratios of the product of the cur rents of two adjacent arcs to the distance between their axes is 20.3 103A2/cm. The speed of welding is equal to 15 m/hr. The speed of welding can be increased when active gases, such as N2, or a mixture of inert gases such as Ar and He are employed as the plasma-forming and shielding media, as well as when active and inert gases and their mixtures are combined with flux. In this case the speed of welding can be increased by up to four or six-fold.

EXAMPLE 4 The welding of cryogenic equipment with 30mm thick edges will now be described. The welding operation is performed using the welding system of Figure 3. The composition of the welded material is as follows, in percentage terms by weight: Cu < N 0.02;Mg < N 0.05;Mn < N 0.025; Fe < N 0.03 ZZn # 0.3; Ti # 0.1; the rest= Al.

The plasma-forming gas for all three arcs is a mixture of argon and helium (Ar + He). The consumption of argon in the mixture is 45 Q/hr and the consumption of helium in the mixture is 135Q/hr. Argon is also used as the shielding medium. Consumption of argon for shielding the resulting molten pool is 1,800 Qilir.

The operating current of the leading arc is 820 A, the operating current of the trailing arc is 710 A, and the operating current of the middle arc is 430 A.

The distance between the axis of the middle arc and the axes of each of the other two arcs is 32mm. The lower of the two ratios of the product of the currents, of two adjacent arcs to the distance between the axes is 9.5-103A2/cm.

The speed of welding is equal to 20 m/hr.

The welded joint is formed by melting the basic metal.

The spattering and other losses are compensated for by a wire of the following composition, in percentage by weight terms: Al = 99.97; Fe = 0.015; Si = 0.010; Cu = 0.005.

The speed of welding can be increased by 15 to 20% by introducing an active gas (for example oxygen) mixed with argon (Ar + 02) into the plasma-forming medium of the leading and trailing arcs. All other parameters remain the same.

Where the plasma-forming medium for all the arcs is a mixture of Ar + He, the weld metal can be shielded by a flux having the following composition, in percentage terms: KC1 = 50; NaCl = 28; LiCl = 14; NaF = 8.

The flux acts to dissolve oxide films and refines the metal, but an additional operation of clearing residual flux using warm water is required.

When the plasma-forming medium for all the arcs comprises argon alone and the weld is shielded by a flux having the above described composition, a layer of reduced thickness can be used. Instead the flux can be applied as a paste of the same consumption.

Welding of cryogenic equipment to provide bevelled edges is performed using the welding system of Figure 4. The composition and thickness of the material is similar to that used in Figure 3.

There are two nonconsumable electrodes.

There is one consumable electrode.

Argon is employed as the plasma-forming gas for the leading and trailing arcs. Consumption of argon is 220 /hr. An inert shielding medium of argon is used. Consumption of argon is 1,800 Q/hr. The operating current of the leading arc is 820 A, the operating current of the trailing arc is 760 A and the operating current of the middle arc is 520 A.

The distance between the axis of the middle arc and the axis of each of two other arcs is 32mm.

The lower of the two ratios of the product of the currents of two adjacent arcs to the distance between their axes is 12.3'103A2/cm.

The speed of welding is 38 m/hr. The welding joint is formed using the melt of the basic metal and a consumable electrode having the same consumption. In a modification, a mixture Ar + He is used as the plasma-forming gas and the speed of welding is increased by 30% with all the other parameters of the process remaining unchanged.

A mixture of Ar + O or a flux is employed as the shielding medium. If argon is used as the plasma-forming gas for the leading and trailing arcs and Ar + O as the shielding medium, the speed of welding is increased and a fine spray metal is transferred between the consumable electrode and the workpiece.

If argon or a mixture Ar + He combined with the flux are used as the plasma-forming gas, oxide-free compounds can be obtained. An additional operation of clearing flux residue is required by washing the workpiece in warm water..

Standard fluxes and pastes developed and used for submerged metal-arc welding, submerged melt welding by consumable and nonconsumable electrodes and gas-shielded welding by a nonconsumable electrode can be used as a shielding medium in the described example. High silica manganic fluxes are preferable for welding of carbon and low-alloy steels and low silica fluxes are preferable for welding heavily alloyed steels and copper.

Other fluxes and pastes based on chlorous and fluorous lop salts can be used.

WHAT WE CLAIM IS: 1. A method of plasma arc welding of a workpiece using a plurality of continuously burning direct-current arcs, comprising the steps of either producing all the arcs between nonconsumable electrodes and the workpiece or producing some arcs between nonconsumable electrodes and the workpiece and the other arc or arcs between one or more consumable electrodes

Claims

**WARNING** start of CLMS field may overlap end of DESC **. There are two nonconsumable electrodes. There is one consumable electrode. Argon is employed as the plasma-forming gas for the leading and trailing arcs. Consumption of argon is 220 /hr. An inert shielding medium of argon is used. Consumption of argon is 1,800 Q/hr. The operating current of the leading arc is 820 A, the operating current of the trailing arc is 760 A and the operating current of the middle arc is 520 A. The distance between the axis of the middle arc and the axis of each of two other arcs is 32mm. The lower of the two ratios of the product of the currents of two adjacent arcs to the distance between their axes is 12.3'103A2/cm. The speed of welding is 38 m/hr. The welding joint is formed using the melt of the basic metal and a consumable electrode having the same consumption. In a modification, a mixture Ar + He is used as the plasma-forming gas and the speed of welding is increased by 30% with all the other parameters of the process remaining unchanged. A mixture of Ar + O or a flux is employed as the shielding medium. If argon is used as the plasma-forming gas for the leading and trailing arcs and Ar + O as the shielding medium, the speed of welding is increased and a fine spray metal is transferred between the consumable electrode and the workpiece. If argon or a mixture Ar + He combined with the flux are used as the plasma-forming gas, oxide-free compounds can be obtained. An additional operation of clearing flux residue is required by washing the workpiece in warm water.. Standard fluxes and pastes developed and used for submerged metal-arc welding, submerged melt welding by consumable and nonconsumable electrodes and gas-shielded welding by a nonconsumable electrode can be used as a shielding medium in the described example. High silica manganic fluxes are preferable for welding of carbon and low-alloy steels and low silica fluxes are preferable for welding heavily alloyed steels and copper. Other fluxes and pastes based on chlorous and fluorous lop salts can be used. WHAT WE CLAIM IS:

1. A method of plasma arc welding of a workpiece using a plurality of continuously burning direct-current arcs, comprising the steps of either producing all the arcs between nonconsumable electrodes and the workpiece or producing some arcs between nonconsumable electrodes and the workpiece and the other arc or arcs between one or more consumable electrodes and the workpiece, feeding a plasmaforming gas to the arcs burning from nonconsumable electrodes, supplying a shielding medium to the area of the weld, arranging the arcs so that adjacent arcs have opposite polarities, and controlling the current of the arcs and the distance between them so that the ratio of the product of the currents of each two adjacent arcs to the distance between them is not less than 6.103A2/cm.

2. A method according to Claim 1, wherein the shielding medium comprises an active gas.

3. A method according to Claim 2 wherein the active gas is carbon dioxide.

4. A method according to Claim 1, wherein the shielding medium is an inert gas.

5. A method according to Claim 4, wherein the inert gas is argon.

6. A method according to Claim 1, wherein the shielding medium is a mixture of active and inert gases.

7. A method according to any preceding Claims, wherein the shielding medium comprises flux.

8. A method according to any preceding Claim, wherein the plasma-forming gas is an active gas.

9. A method according to Claim 8 wherein the plasma-forming gas is carbon dioxide.

10. A method according to any one of Claims 1 to 7 when the plasma-forming gas is an inert gas.

11. A method according to Claim 10 wherein the plasma-forming gas is argon.

12. A method according to any one of Claims 1 to 7, wherein the plasma-forming gas is a mixture of active and inert gases.

13. A method according to any preceding Claim, wherein the composition of the or each consumable electrode is the same as that of the workpiece.

14. A method according to any one of Claims 1 to 12 wherein the composition of the or each consumable electrode is different from that of the workpiece.

15. A method of welding low-alloy steel workpieces according to Claim 1, wherein the plasma-forming gas used for arcs of one polarity and the shielding medium are both carbon dioxide, the plasma-forming gas for arcs of the opposite polarity is argon and only nonconsumable electrodes are employed.

16. A method of welding low alloy steel workpieces according to Claim 1, wherein the plasma-forming gas for arcs of one polarity is a mixture of carbon dioxide and oxygen, the plasma-forming gas for the arcs of the opposite polarity is argon, and only nonconsumable electrodes are employed.

17. A method of welding stainless steel workpieces according to Claim 1, wherein only nonconsumable electrodes are employed and the plasma-forming gas for all electrodes and the shielding medium is argon.

18. A method of welding stainless steel workpieces according to Claim i,wileleiil the plasma-forming gas for arcs of one polarity is a mixture of argon and hydrogen, the plasmaforming gas for arcs of the opposite polarity and the shielding medium is argon, and only nonconsumable electrodes are employed.

19. A method of welding stainless steel

workpieces according to Claim 1, wherein the plasma-forming gas for arcs of one polarity is argon while the plasma-forming gas for the arcs of opposite polarity and the shielding medium are both carbon dioxide, and only nonconsumable electrodes are employed.

20. A method of welding low-alloy steel and stainless steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity is carbon dioxide, the plasmaforming gas for the arcs of opposite polarity is argon, the shielding medium is a flux and only nonconsumable electrodes are employed.

21. A method of welding copper workpieces according to Claim 1, wherein the plasmaforming gas for all the arcs and the shielding medium is argon, and only nonconsumable electrodes are employed.

22. A method of welding copper workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of one polarity arcs and the shielding medium is nitrogen, the plasmaforming gas for the arcs of opposite polarity is argon and only nonconsumable electrodes are employed.

23. A method of welding copper and aluminium workpieces according to Claim 1, wherein the plasma-forming gas for all the arcs is a mixture of argon and helium, the shielding medium is argon and only nonconsumable electrodes are employed.

24. A method of welding copper and aluminium workpieces according to Claim 1, wherein the plasma-forming gas for all the arcs is a mixture of argon and helium, the shielding medium is flux, and only nonconsumable electrodes are employed.

25. A method of welding aluminium workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of one polarity is a mixture of argon and oxygen, the plasma-forming gas for the arcs of opposite polarity and the shielding medium is a mixture of argon and helium and only nonconsumable electrodes are employed.

26. A method of welding aluminium workpieces according to Claim 1 , wherein the plasmaforming gas for all the arcs is argon, the shielding medium is a mixture of argon and a flux, and only nonconsumable electrodes are employed.

27. A method of welding low-alloy steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity and the shielding medium is carbon dioxide, and both consumable and nonconsumable electrodes are employed.

28. A method of welding low-alloy steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity is a mixture of carbon dioxide and oxygen, the shielding medium is carbon dioxide, the arcs of the opposite polarity are produced with con sumable electrodes and the arcs of said one polarity are produced with nonconsumable electrodes.

29. A method of welding of stainless steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity and the shielding medium is argon, the arcs of the opposite polarity are produced with consumable electrodes and the arcs of said one po polarity are produced with nonconsumable electrodes.

30. A method of welding stainless steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity is a mixture of argon and hydrogen, the shielding medium is argon, the arcs of opposite polarity are produced with consumable electrodes and the arcs of the opposite polarity are produced with nonconsumable electrodes.

31. A method of welding stainless steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of said one polarity and the shielding medium is carbon dioxide, the arcs of opposite polarity are produced with consumable electrodes and the arcs of said one polarity are produced with nonconsumable electrodes.

32. A method of welding stainless and lowalloy steel workpieces according to Claim 1, wherein the plasma-forming gas for the arcs of one polarity is carbon dioxide, the shielding medium is a flux, the arcs of opposite polarity are produced by consumable electrodes and the arcs of said one polarity are nonconsumable electrodes.

33. A method of welding copper workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of one polarity and the shielding medium is argon, the arcs of opposite polarity are produced with consumable electrodes and the arcs of said one polarity are prodiced with nonconsumable electrodes.

34. A method of welding copper workpieces according to Claim 1, wherein the plasma- form forming gas for the arcs of said one polarity and the shielding medium is nitrogen, the arcs of opposite polarity are produced with consumable electrodes and the arcs of said one polarity are produced with nonconsumable electrodes.

35. A method of welding copper workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of one polarity is a mixture of argon and helium, the shielding medium is argon, the arcs of opposite polarity are produced with consumable electrodes and the arcs of said one polarity are produced with nonconsumable electrodes.

36. A method of welding copper workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of said one polarity is a mixture of argon and helium, the shielding medium is a flux, the arcs of OppOsite nr?ledty are produced with consumable electrodes and the arcs of said one polarity are produced with nonconsumable electrodes.

37. A method of welding aluminium workpieces according to Claim 1, wherein the plasmaforming gas for arcs of opposite polarity and the shielding medium is argon, the arcs of one polarity are produced with consumable electrodes and the arcs of opposite polarity are produced with nonconsumable electrodes.

38. A method of welding aluminium workpieces according to Claim 1 wherein the plasmaforming gas for the arcs of opposite polarity is a mixture of argon and helium, the shielding medium is argon, the arcs of said one polarity are produced with consumable electrodes and the arcs of opposite polarity are produced with nonconsumable electrodes.

39. A method of welding aluminium workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of opposite polarity is argon, the shielding medium is a mixture of argon and oxygen, the arcs of one polarity are produced with consumable electrodes, and the arcs of opposite polarity are produced with nonconsumable electrodes.

40. A method of welding aluminium workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of said opposite polarity is a mixture of argon and helium, the shielding medium is a flux, the arcs of said one polarity are produced with consumable electrodes and the arcs of opposite polarity are produced with nonconsumable electrodes.

41. A method of welding aluminium workpieces according to Claim 1, wherein the plasmaforming gas for the arcs of opposite polarity is argon, the shielding medium is a mixture of argon and a flux, the arcs of said one polarity are produced with consumable electrodes and the arcs of said opposite polarity are produced with nonconsumable electrodes.

42. A method of progressively welding a workpiece using a plurality of direct-current plasma arcs, comprising the steps of arranging a plurality of parallel arcs in a row with alternate arcs of one polarity and intervening arcs of the opposite polarity, producing relative displacement between the row of arcs and the workpiece along the line of the proposed weld, and controlling the currents of the arcs so that the ratio of the product of the currents of each two adjacent arcs to the distance between the axes of these arcs is equal to or greater than 6.103A2/cm.

43. A method of welding according to any one of the preceding claims and substantially as described hereinbefore with reference to the accompanying drawings.