GB1566192A

GB1566192A - Method of treating argonarc cathode

Info

Publication number: GB1566192A
Application number: GB2817/77A
Authority: GB
Original assignee: V N I P KONSTRUKT I TEKHNOLOG
Current assignee: V N I P KONSTRUKT I TEKHNOLOG
Priority date: 1977-02-14
Filing date: 1977-01-24
Publication date: 1980-04-30
Also published as: FR2380683B1; FR2380683A1

Description

(54) METHOD OF TRFATINO ARGON-ARC CATHODE (71) We, VSESOJUNZY NAUCHNO-ISSLE fX)VATELSKY PROEKTNOKONSTRUKTORSKY I ThKHNOLOGICHESKY INSTITUT ELEKTROS VAROCHNOGO OBORUDOVANIA, a Russian corporate ibody of Litovskaya ulitsa, 10, Leningrad, USSR., do hereby declare the invention, for which we pray that a patent nny 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 the field arc ale and plasma treatment of metals, and more panticularly, to a method of treating an electrode intended for operation as the cathode of an electric argon arc. The invention can be used in plasma and electricarc metallurgy, and may also be employed in treating the cathodes of electron-beam devices.

The principal object of the invention is to develop a method of treating electrodes which ensures an extension of the range of working currents and good stabilization of the arc in all attitudes in space over the entire range of current control, a low heat flow to the electrode and a reduction of the cathode voltage drop.

While endeavouring to attain these objeots the inventors discovered an unexpected effect which is graphically illustrated by the tollowing examples.

t1. An electrode known in the prior art was taken, which consisted of a tungsten holder with an active insert d yttrium oxide, the diameter of the electrode being 4 mm.

When this electrode was used as a cathode of an argon arc a spatial stabilization of the arc was obtained, with the working current lowered to a minimum value of 20 Amperes (A).

The electrode was further treated in the electric arc for a short period in oxygen.

The unexpected effect discovered by the in ventons is that after the above treatment the lower limit of the range of stable working currents during operation as a cathode in argon dropped a hundred-fold to 0.2 A, and the cathode voltage drop substantially decreased over the entire range of currents.

2. An electrode known in the prior art which consisted of a 4Hmm diameter zirconium holder with an active insert of zir cerium nitride, was tested as the cathode of an argon arc.

The tests showed that this electrode operated in the range of currents not exceeding 300 A and did not ensure the stabilization of the arc in all the positions in space within the entire range of the current control.

This electrode was further treated as an arc electrode in an atmosphere containing nitrogen.

The unexpected effect which was discovered is that after treatment in an arc in the atmosphere of nitrogen the cathode with an active in.sert of zirconium nitride began operating in argon in the range of working currents from 50 to 500 A. The stabilization of the arc was preserved during multiple variations in the working current in both directions-from 50 to 500 A and from 500 to 50 A.

3. After arc treatment in nitrogen of a cathode, consisting of a 4-mm diameter hafnium holder with an active insert of cerium oxide the upper limit of the range of working currents increased to 700 A, the heat losses in the electrode decreased considerably and the stabilization of the arc improved.

At the same time the maximum permissible magnitude of the current at the hafnium cathode with an active insert of cerium oxide without treatment in nitrogen was 5 times lower.

The unexpected effect was that the treatment in a nitrogen arc of the hafnium cathode with an active insert of cerium oxide imparted new properties to the electrode.

It was established that arc treatment in oxygen of cathodes containing oxides of different elements on the working surfaces, and treatment in nitrogen of cathodes containing nitrides on the working surfaces, as well as arc treatment both in nitrogen and oxygen of cathodes containing oxides or nitrides on the active surfaces, is conducive to a considerable increase in the emission activity, decrease in the work function of electrons and the localization of the cathode spot This makes it possible to considerably expand the range of working currents, reduce the heat losses in the electrode and the cathode voltage drop.

The unexpected effect discovered by the inventors served as a basis for developing a method of treating electrodes which ensured the attainment of said objects.

According to the present invention there is provided a method of treating an electrode intended for operation as the cathode of an electric arc in an argon atmosphere or as the cathode of an electron beam device, wherein the active working surface of said electrode incorporates at least one substance selected from the group comprising rare-earth metals, yttrium, alkaline-ear.th metals, elements of Group IV A of the periodic system and compounds thereof, taken both separately and in combination, said method comprising treating the electrode by forming an electric arc for which the electrode being treated functions as a cathode, and supplying to the cathode zone of the arc a mixture d argon with either oxvgen or nitrogen to form with the or a said substance a compound having a work function of not more than three electron volts.

The invention also provides a method for continuous operation of an electrode treated by the method in the preceding paragraph as the cathode of an electric arc in argon, wherein nitrogen is fed regularly to the cathode area of the arc, the addition of nitrogen to argon being repeatedly carried out for a period which does not exceed 1/100 of the period of operation of the electrode in pure argon.

The treatment in reactive gases of cathodes of which the active working surfaces contain elements from the group comprising rare-earth metals, yttrium, alkaline-earth elements and elements of Group IV A of the periodic system, ensures an increase in the emission activity a decrease in the work function of the electrons and localization of the cathode spot, which results in an extension of the useful range of working currents and a reduction of heat flow to the electrode. At the same time a considerable reduction of the cathode voltage drop is ensured.

A great advantage of cathodes which have been treated in a reactive gas is an improvement in the stabilization of the arc and its localization in space over the entire range of the working current control.

In addition, cathodes treated according to this method have a longer service life in an argon arc as compared with untreated cathodes.

The invention will be better understood from the following description of an embodiment given with reference to the accompanying drawings, wherein: Figure 1 shows an apparatus for treating electrodes in a reactive gas, by a method according to the invention Figure 2 is an electrode intended for operation in organ and which has in accordance with the invention been treated as the cathode of an electric arc while in a reactive gas; and Figure 3 shows comparative current-voltage characteristics of an electrode treated according to the invention and of electrodes known in the art.

The treatment of an electrode intended for operation in argon as the cathode of an electric arc is carried out in a reactive gas.

Such cathodes have the feature that their active working surfaces that contact the arc contain at least one element selected from the group comprising rare-earth metals, yttrium, alkaline-earth elements, elements of Group IV A of the periodic system and compounds thereof, taken both separately and in combination. The treatment is carried out under the condition that the electrode being treated functions as the cathode of an electric arc. There is fed to the cathode region of the arc a mixture of argon with a reactive gas which, together with one or more of the rare-earth metals, yttrium, alkaline-earth elements, elements of Group IV A of the periodic system and compounds thereof, taken both separately and in combination that is included in the electrode, forms a compound having a work function of not more than three electronvolts. The period of treatment of the electrode need not exceed 100th of the expected period of operation of the electrode as the cathode of an argon arc. The electrodes are preferably treated with currents not exceeding the upper limit of the range of working currents, with the volumetric concentration of the reactive gas in the mixture being from 0.1 to loo%.

The method of treating electrodes is as follows.

An electric arc 1 (Figure 1) is excited in argon between an electrode 2 being treated, connected to the negative pole of a supply source 3 and an anode 4 connected to the positive pole. The current of the arc is established at a value not exceeding the maximum working current.

A controlled amount of argon is supplied from a cylinder 5 via a flowmeter 6 and enters the arc 1 through a nozzle 7. Then a valve 8 is opened and a reactive gas is supplied together with argon to the region of the arc 1 near the cathode. This gas also is fed to the nozzle 7 from a cylinder 9 through a flow meter 10. The volumetric concentration of the reactive gas in the mixture with argon is established in the range from 0.1 to 100%. The reactive gas entering the region of the arc near the cathode forms, together with one of the elements present on the active working surface of the cathode in the zone d contact with the arc, a compound with a work function of not more than three electron-volts. After a period which is at least 100 times less than the period of operation of the electrode in argon, the valve 8 closes. Thereafter, the supply source 3 is cut off, the arc 1 is extinguished and the supply of argon is stop ped. The electrode 2 has thus been treated in accordance with the invention and is ready for use as a cathode of an arc or plasma device.

It is preferable to use nitrogen as a reactive gas during treatment of electrodes intended for operation as cathodes in argon at high currents (over 500 A).

It is advisable to use oxygen as a reactive gas in treatment of electrodes intended for operation in argon at low currents (from 0.1 A).

Periodic re-treatment of electrodes in the process of continuous operation in an argon arc may be carried out in the following manner. An electrode treated in a reactive gas as above described and thus ready for opera- tion is taken, an arc is excited in argon and a required technological process is conducted in argon for a required period. Then, without interrupting the process, there is fed to the region of the arc near the cathode a sms11 amount of a reactive gas (not less than 0.1 ,fo) together with argon, for a short period (not exceeding 100th of the period of the technological process). Then the supply of the reactive gas is stopped and the technological process is continued in an argon atmosphere. Repeated supply of the reactive gas is periodically carried out in a similar manner.

To illustrate the method of treating electrodes in a reactive gas, several examples are given.

EXAMPLE 1 An electrode was taken, consisting of a tungsten rod 1 Figure 2) having a diameter of 4 ma a length of 45 mm, and a blind cyllndrical hole having a depth of 4 mm and a diameter of 1.5 mm and which was filled with cerium oxide to form an active insert 12.

The preliminary treatment of the electrode in the arc in an oxygen containing atmosphere was carried.out in the following manner. The treatment was conducted under the conditions described in relation to Figure 1, so that the electrode being treated was functioning as a cathode. The arc 1 was excited from the supply source 3 with a current of 50 A in argon whose consumption was 0.3 g/sec. After 10 seconds, when the active insert 12 (Figure 2) was sufficiently hot, oxygen was fed (a 10% addition of oxygen to argon) for one second. After one second the supply of oxygen was stopped.

After this treatment with oxygen this electrode underwent tests in an arc in argon for 8 hours. The tests showed that the treated electrode ensured the stabilization of the arc and its localization in space during operation in argon in the current range of from 5 to 200 A; the working current could be varied in both directions.

Multiple striking and extinguishing of the arc at currents of from 5 to 200 A also showed that the electrode operated in argon under the conditions of a steady stabilization of the arc.

EXAMPLE 2 A cylindrical rod electrode made of titanium with a diameter of 4 mm was provided with an active insert of samarium oxide. Samarium oxide was pressed into a blind hole in the end face of the electrode, the depth of the hole being 4 mm and its diameter 1.5 mm.

The electrode was treated in an arc containing nitrogen. For this purpose the electrode 2 (Figure 1) was connected to the negative pole of the supply source 3. The arc 1 was produced to the electrode for 5 seconds with a current of 20 A in a mixture of argon and nitrogen. The argon consumption was 0.3 glsec, and the volumetric concentration of nitrogen in the mixture was 20%.

Thereafter, the nitrogen supply was stopped. The electrode was tested in argon.

The maximum permissible current amg- nitude at the electrode was 300 A. The test of the electrode in argon was carried out under the following conditions: first the arc was excited with a current of 20 A, then the current intensity was smoothly increased to a certain magnitude and the arc was extinguished. The next excitation was also done at 20 A. The current was adjusted from 20 to 300 A by steps of 20 A with the subsequent extinguishing of the arc, i.e. the arc was extinguished at currents of 20, 40, 60 .300 A. The two-hour test showed that the electrode was working in argon under the conditions of a steady stabilization of the arc in the range of the current control in both directions from 40 to 300 A.

EXAMPLE 3 The electrode consisted of a tungsten holder with a diameter of 4 mm and a length of 30 mm with an active insert of yttrium oxide filled in a blind hole in the end face of the tungsten rod with a depth of 4 mm and a diameter of 1.5 mm. The electrode 2 (Figure 1) was connected to the negative pole of the supply source 3, and an arc was struck to the electrode in argon with a current intensity of 5 A for 10 seconds, after which time oxygen was supplied, the volumetric concentration of oxygen in the mixture being 0.1%. The treatment in the mixture of argon with oxygen was conducted in the arc for 30 seconds, then the oxygen supply was cut off. Tests carried out in argon showed that the electrode ensured the stabilization of the arc even when the current was reduced to 0.2 A. The maximum exciting current of the arc was 0.3 A.

The cathode voltage drop over the entire range of currents was considerably reduced.

The test of the cathode which has thus been treated in an arc containing oxygen were carried out together with a similar cathode which had not been pretreated. At the same time tests were conducted of cathodes of thoriated tungsten for microplasma burners and cathodes of yttriated and lanthanated tungsten. The rod cathodes of thoriated, lanthanated and yttriated tungsten with a diameter of 2.5 mm were taperturned at an angle of 20 to 300.

In the process of testing the range of working currents, the minimum exciting current of the arc and voltage-current characteristics were determined.

The tests showed the following: the minimum exciting current of the arc on a tungsten electrode with an insert of an yttrium oxide was 5 amps, on thoriated tungsten 3A, and on lanthanated and yttriated tungsten 3 A.

But the minimum exciting current of the arc on a tungsten electrode with an insert of yttrium oxide which had been treated in oxygen was 0.3 A.

The stabilization of the arc and its localization in space on electrodes from thoriated, lanthanated and yttriated tungsten were dis- turbed when the current was reduced to 1.8 A, whereas there was no disturbance of the stabilization of the arc on the electrode treated by the method of the invention when the current was reduced to 0.2 A.

The comparative voltage-current characteristics of all the electrodes which had undergone testing are shown in Figure 3,.

wherein the values of voltage drops in the arc in volts are shown on the Y-axis, and the intensity of the current in amperes are shown on the X-axis. The voltage-current characteristics 13 of the electrode which had been treated in oxygen was considerably lower than those of electrodes of thoriated tungsten 14, lanthanated tungsten 15 and yttriated tungsten 16. The voltage drop on the arc for a cathode treated in an oxygen arc with low currents was half that of the cathodes known in the art, a feature of substantial importance in designing supply sources.

EXAMPLE 4 This example illustrates a method ofperiodic re-treatment of an electrode during its continuous operation in forming an arc in argon in the process of welding copper with normal polarity. The electrode was a tungsten rod with a diameter of 4 mm and a length of 45 mm. In the end face of the tungsten rod there was drilled a blind hole into which an insert of hafnium with a diameter of 1.5 mm and a length af 5 mm was pressed.

The arc was excited in argon on an expendable strip During the burning of the arc on the strip with a current of 200 A a mixture of argon with nitrogen (20% of nitrogen) was fed to the cathode region for 5 to 8 seconds. When the arc approached the beginning of the working section of the weld the supply of nitrogen was stopped, the current was increased to 1,000 A and the welding was conducted in argon. After passing the weld and when the arc again passed to the expendable strip, the current was again reduced to 200 A and 20% of nitrogen was again fed for 5 to 8 seconds.

The addition of 20% of nitrogen when the arc was on the expendable strip and the cathode was moving from one welded joint to another (5 to 8 seconds) ensured reliable operation of the electrode in argon and a strict localization of the arc during welding.

The method of periodic treatment ensured a 20-fold extension of the time during which the electrode was suitable for opera- tion in argon. After 10 hours of operation the electrode showed no noticeable deterioration and ensured reliable operation unith currents from 1,000 to 1,200 A.

WHAT WE CLAIM IS:- 1. A method of treating an electrode intended for operation as the cathode d an electric arc in an argon atmosphere or as the cathode of an electron beam device, wherein the active working surface of said electrode incorporates at least one substance selected from the group comprising rare-earth metals, yttrium, alkaline-earth metals, elements of Group IV A of the periodic system and compounds thereof, taken both separately and in combination,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. the conditions of a steady stabilization of the arc in the range of the current control in both directions from 40 to 300 A. EXAMPLE 3 The electrode consisted of a tungsten holder with a diameter of 4 mm and a length of 30 mm with an active insert of yttrium oxide filled in a blind hole in the end face of the tungsten rod with a depth of 4 mm and a diameter of 1.5 mm. The electrode 2 (Figure 1) was connected to the negative pole of the supply source 3, and an arc was struck to the electrode in argon with a current intensity of 5 A for 10 seconds, after which time oxygen was supplied, the volumetric concentration of oxygen in the mixture being 0.1%. The treatment in the mixture of argon with oxygen was conducted in the arc for 30 seconds, then the oxygen supply was cut off. Tests carried out in argon showed that the electrode ensured the stabilization of the arc even when the current was reduced to 0.2 A. The maximum exciting current of the arc was 0.3 A. The cathode voltage drop over the entire range of currents was considerably reduced. The test of the cathode which has thus been treated in an arc containing oxygen were carried out together with a similar cathode which had not been pretreated. At the same time tests were conducted of cathodes of thoriated tungsten for microplasma burners and cathodes of yttriated and lanthanated tungsten. The rod cathodes of thoriated, lanthanated and yttriated tungsten with a diameter of 2.5 mm were taperturned at an angle of 20 to 300. In the process of testing the range of working currents, the minimum exciting current of the arc and voltage-current characteristics were determined. The tests showed the following: the minimum exciting current of the arc on a tungsten electrode with an insert of an yttrium oxide was 5 amps, on thoriated tungsten 3A, and on lanthanated and yttriated tungsten 3 A. But the minimum exciting current of the arc on a tungsten electrode with an insert of yttrium oxide which had been treated in oxygen was 0.3 A. The stabilization of the arc and its localization in space on electrodes from thoriated, lanthanated and yttriated tungsten were dis- turbed when the current was reduced to 1.8 A, whereas there was no disturbance of the stabilization of the arc on the electrode treated by the method of the invention when the current was reduced to 0.2 A. The comparative voltage-current characteristics of all the electrodes which had undergone testing are shown in Figure 3,. wherein the values of voltage drops in the arc in volts are shown on the Y-axis, and the intensity of the current in amperes are shown on the X-axis. The voltage-current characteristics 13 of the electrode which had been treated in oxygen was considerably lower than those of electrodes of thoriated tungsten 14, lanthanated tungsten 15 and yttriated tungsten 16. The voltage drop on the arc for a cathode treated in an oxygen arc with low currents was half that of the cathodes known in the art, a feature of substantial importance in designing supply sources. EXAMPLE 4 This example illustrates a method ofperiodic re-treatment of an electrode during its continuous operation in forming an arc in argon in the process of welding copper with normal polarity. The electrode was a tungsten rod with a diameter of 4 mm and a length of 45 mm. In the end face of the tungsten rod there was drilled a blind hole into which an insert of hafnium with a diameter of 1.5 mm and a length af 5 mm was pressed. The arc was excited in argon on an expendable strip During the burning of the arc on the strip with a current of 200 A a mixture of argon with nitrogen (20% of nitrogen) was fed to the cathode region for 5 to 8 seconds. When the arc approached the beginning of the working section of the weld the supply of nitrogen was stopped, the current was increased to 1,000 A and the welding was conducted in argon. After passing the weld and when the arc again passed to the expendable strip, the current was again reduced to 200 A and 20% of nitrogen was again fed for 5 to 8 seconds. The addition of 20% of nitrogen when the arc was on the expendable strip and the cathode was moving from one welded joint to another (5 to 8 seconds) ensured reliable operation of the electrode in argon and a strict localization of the arc during welding. The method of periodic treatment ensured a 20-fold extension of the time during which the electrode was suitable for opera- tion in argon. After 10 hours of operation the electrode showed no noticeable deterioration and ensured reliable operation unith currents from 1,000 to 1,200 A. WHAT WE CLAIM IS:-

1. A method of treating an electrode intended for operation as the cathode d an electric arc in an argon atmosphere or as the cathode of an electron beam device, wherein the active working surface of said electrode incorporates at least one substance selected from the group comprising rare-earth metals, yttrium, alkaline-earth metals, elements of Group IV A of the periodic system and compounds thereof, taken both separately and in combination,

said method comprising treating the electrode by forming an electric arc for which the electrode being treated functions as a cathode, and supplying to the cathode zone of the arc a mixture of argon with either oxygen or nitrogen to form with the or a said substance a compound having a work function of not more than three electron vomits.

2. A method according to claim 1.

wherein a holder for an active insert of the electrode contains tungsten and a mixture of argon with oxygen is fed to the cathode zone of the arc.

3. A method according to claim 2, wherein the volumetric concentration of oxygen in the mixture is in the range of 0.1 to 10 percent

4. A method according to claim 1, wherein an aotive insert of the electrode contains at least one element of Group IV A of the periodic system and/or a compound thereof and a mixture of argon with nitrogen is fed to the cathode zone of the arc.

5. A method according to claim 4, wherein the volumetric concentration of nitrogen in the mixture is in the range of 0,1 to 20 percent.

6. A method for continuous operation of an electrode treated by the method of any of the preceding claims as the cathode of an electric arc in argon, wherein nitrogen is fed regularly to the cathode area of the arc, the addition of nitrogen to argon being repeatedly carried out for a period which does not exceed 1/100 of the period of operation of the electrode in pure argon.

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