US3825789A

US3825789A - Vacuum arc devices with hard, ductile, ferrous electrodes

Info

Publication number: US3825789A
Application number: US00375133A
Authority: US
Inventors: L Harris
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1973-06-29
Filing date: 1973-06-29
Publication date: 1974-07-23
Anticipated expiration: 1991-07-23
Also published as: DE2431058A1; GB1480285A; CA1028388A; JPS5033466A

Abstract

Vacuum arc devices utilized to protect electric circuit elements against very high transient currents and voltages and exhibiting high recovery voltage and high voltage holdoff strength include primary arc electrodes fabricated from hardenable ferrous material exhibiting high hardness, ductility, and high tensile strength. Such vacuum arc devices are superior to similar devices utilizing copper or cuprous alloy electrode materials because of increased mechanical strength and increased voltage holdoff strength and higher current interruption capacity.

Description

United States Patent [1 12/1964 Lee 6.; "ZOO/144 8 Harris *July 23, 1974 54] VACUUM ARC DEVICES WITH HARD, 3,281,563 10/1966 W aterton 200/144 13 DUCTILE, R S ELECTRODES Lakeuchi et al. 2024 arrls [75] Inventor: Lawson P. Harris, Scotia, N.Y. [73] Assignee: General Electric Company, Primary Exarriiner-James W. Lawrence Schenectady, NY. {Assistant Examiner-Wm. l-l. Punter 1 Notice: The portion of the term of this 1" 'g f 'l or f if cquillaro; Joseph patent subsequent to Oct. 30, 1990,. I C0 Ju ms Zaskahc y I has been disclaimed. 1 ;[57] ABSTRACT 22 Filed: June 29, 1973 6 l 4 1 Vacuum are devices ut111zed to protect electnc circuit [21] Appl. No.: 375,133 elements against very high transient currents and volt- I ages and exhibiting high recovery voltage and high 52 us. Cl. 200/144 B, 313/233, 313/311 Wage l Strength include Primary 51 Int. Cl H0lh 1/02 HOlj 1/02 .tmdes fabl'lcated from hardenable ferrous matena' Q [58] Field v 313/233 200/144 B hibiting high hardness, ductility, and high tensile strength. Such vacuum are devices are superior to sim- [56] Refeiences Cited 1 ilar devices utilizing copper or cuprous alloy electrode 1 4 materials becauseof increased mechanical strength I l 582 330 271525 Z IT P T 3 3nd I and increased voltage holdoff strength and higher currac l 2,391,459 12/1945 Hensel 313/311 mm; Interruption Capacty 3,163,734

- 14 Claims, 3 Drawing Figures PATENIEflmzamu 3.825.789

. SHEET 2 OF 3 gig. 2

PAIENTENJLZSISH SHEET 3 OF '3 c- HARD DUCTILE STEELS 0- 304 STAINLESS STEEL B- BERYLLIUM A- COPPER LF T O O O O O O O m 6 5 4 3 2 8 IO I2 l4 l6 I8 20 22 '24 TIME AFTERCURRENT ZERO, psec FERROUS ELECTRODES v cUuM ARC DEVICESWITH HARD, DUCTILE,

RELATED lNVENTlONS This inventionis related to the invention described and claimed. in my co-pending application Ser. No.

236,278, filed Mar; 20, ,l972,of which' this application is a continuation-in-partthe disclosure of which is incorporated herein by reference thereto.

BACKGROUND OF THE INVENTION The present invention relates to improved vacuum are devices. More particularly,the invention relates to such improveddevices as vacuum gaps, triggerablevacuum gaps, vacuum switches and the like. i

In the art of vacuum 'arc devices,.arcs are struck by the separation of a pair of arc electrodes or by the infusion of condensible conducting medium such as an electron-ion plasmaintoan evacuated chamber, generally evacuatedto a pressure of at least Torr with a very high voltage applied across thearc electrodes.

' Ideally, once the arches-been struck, specie from the arc cathode is removed or boiled off therefrom to produring a relatively short period of time as, for example, a portion of a half cycle of an alternating current voltage at 60 Hz frequency; Should this .valu e be very low so that a too sparse quantity of conducting specie is evolved, the phenomenon known as chopping may 'occur. Chopping is a phenomenon that occursdue to vapor starvation andis characteristic of materials with too high a boiling point or too low a vapor pressure, of the arc electrode materials..When chopping occurs, as a positive half-cycle of an alternating voltage apupon the value of vapor pressure evolves a quantity of conducting species so as to allow for a high current are proac'hes current zero and as the current begins to fall,

current is suddenly extinguished, causing a high voltage transient to be reflected back intothe system protected by the device, potentially causingdamage, particularly in inductive circuits.

While the chopping phenomenon maybe avoided I readily by the provision of high vapor pressure materials as, for example, one ormoreof those'materials set forth in =U.S. Pat. Nos. 2,975,255 Lafferty and 2,975,256 Leeand Cobine, anover-plenteous supply of conducting specie. may have deleterious effects. Thus, for example, when the current zero, alluded. to above, is reached, the supply of conducting specie present can be too great such that the rate of condensation thereof and the recovery of a dielectric voltage holdoff strength may not occurat a sufficient rateto preclude a restrike of the are when the opposite half-cycle of an operation difficult,if not impossible. a

alternating voltage is applied'between the arc elec rent cycle. Such electrodes in accord with the invention Still another disadvantage of high vapor pressure materials as vacuum arc electrodes is that those materials tend'tobe'mechanically soft and may readily be deformed or rapidly eroded by the boiling away of a tooplenteous supply of conduction; specie during arcing.

On the other end of the spectrum, if the arc electrode materialis of a much lower vapor pressure material, as

' for example molybdenum, tungsten, andother refractory metals which avoid the erosion and welding problems presented by high vapor pressure material, current chopping can occur. Even if current chopping does not occur, or is not of serious consequences, a lirn' itation of usefulness under'some circumstances is imposed by the thermionic emission characteristics of such refractory materials. Thus, for example, during a half cycle of alternating current at which a given are electrode functions as an anode, anode spots tend to occur by the bunching of current footpoints and portionsof the anode electrode become exceptionally hot.

Thus, the choice of arc electrode materials for vacuum are devices isa tradeoff between conflicting requirements and must be made with due care.

Accordingly, it is an object of the present invention to provide vacuum arc'devices possessing improved arc electrode. materials which have ideal vapor pressure and boiling point characteristics so as to avoid current chopping or interruption failure due to excessive thermionic emission on one hand, or interruption failure or poor life due to excessive metal vapor evolution on the other hand. i Still another object of the invention is to provide vacuum are devices possessing improved mechanical strength which facilitate high voltage recovery strength and higher current interruption than devices of the prior art,

Yet another object of the present invention is to provide improved vacuum are devices suitable for holding of the prior art.

Yet another object of the present invention is to provide vacuum are devices having arc electrode materials which are relatively inexpensive and which 'may be readily processed by conventional techniques for the production of high quality devices at relatively low cost.

BRIEF DESCRIPTIONOF THE INVENTION Briefly stated, in accord with one embodiment of the present invention,,vacuum are devices in accord therewith include an hermetically evacuatedenvelope, evac-- and to interrupt the same rapidly upon the cessation of current therethrough as, for example, by the occurrence of a normal current zero on an alternating curare fabricatedof a gas-free, hardened, ductile, ferrous metal which exhibits all 'of the qualities of freedom from all gases and gas-forming impurities incompatible with such high vacuum, high hardness, reasonable ductility, and a characteristic of high voltage recovery a strength and high rate-of voltage recovery strength.

The novel features characteristic of the invention are set forth in the appended claims. The invention itself,

. together with further objects and advantages thereof,

may bestbe understood by referenc'to the following detailed description taken in connection with the ap-, pended drawings in which:

FIG. 1 is a schematic, vertical, cross-sectional view of a vacuum arc device constructed in accord with the present invention; 1

FIG. 2 is a schematic, vertical, cross-sectional view of an alternative embodiment of a device constructed .in

In'FIG. 1, a triggerable vacuum gap device represented generally as 10, comprises an hermetically sealed evacuable'envelope 11 evacuated to a pressure of IO Torr orless. Envelope llcomprises a cylindri- .cal insulating sidewall member 12 supporting at respective ends thereof a pair of

endwall members

13 and 14. Upper endwall member 13 is connected with cylindrical sidewall member12 bya conventional insulator-tometal seal flange 15 soldered, brazed, or. otherwise affixed to endwall 13 and embedded in insulating side wall 12. A similar insulator-to-metal seal flange 15 is used to connect inhermetic seal bottom endwall member 14 to cylindrical sidewall member 12. A pair of primary arcelectrodes l7 and 18 defining an interelectrode gap 16 are respectively disposed in fixed mechanical and electrical conductive relationship with

respective endwall members

13 and 14. Arc electrode member 17 is supported by an electrode support member 19 from endwall member 13 and electrode member 18 is supported from bottom endwall member 14 by an electrode support assembly 20. Arc electrode support 19 is comprised of a central core 21 of copper or cuprous alloy, for example, of high electrical conductivity which is abutted into and affixed with a good electrical conductive joint to arc electrode 17. Arc electrode 17 is fabricated from a gas-free, hard and ductile ferrous ma 1 terial, as is described with greater particularity hereinafter. An arc support sheath member 22, also fabricated from a gas-free, hard, ductile, ferrous material may surroundcentral-core member 21. In like fashion, arc support assembly comprises a cuprous or copper, for example, high electrical conductivity core member 23 which may be surrounded by a ferrous sheath member 24. Copper or cuprous core member 23 is embedded into ferrous arc electrode member 18 in

electrode members

17 and 18 by the application of an electricpulse through trigger anode lead 28 to trigger anode 27 to cause a breakdown of trigger gap 28 and the infusion of electron ion plasma into primary arcing gap 16. Trigger electrode 26 is shown herein without detail. Details of suitable trigger assemblies may, for example, be found in U.S. Pat. Nos. 3,087,092, 3,465,192 and 3,465,205.

A shield assembly 31 which may, for example, coniprise'a large primary shield member 32 in the form of a cylindrical member depending from upper endwall member 13 and extending approximately two-thirds to three-quarters of the distance between endwall members l3 and 14 encompassing the area surrounding-the arc'gap l2 and a second cylindrical shield member 33 upwardly depending from lower endwall member 14 and terminating short of shield member 32 leaving a gap 35 by a distance which is large as compared with arcing gap 16 surrounds the arc electrodes and protects the insulating integrity'of insulating sidewall 12. A deflector shield member 34 in the form of a foreshortened truncated conical member depends from lower arc electrode support assembly 20 and has an angle with respect to the normal to the axis of the arc electrodesupport members which is directed to a point along shield member 32 above gap 35, such that conduction specie emanated from arcing gap 16 is deflected by shield member 34 to shield member 32 without allowing a line-of-sight passage to gap 35 from arcing gap 12 such that conduction species emanating from an arc between the primary arc electrodes are deflected to shield member 32 short of gap 35. A distance between

shield members

34 and 32, like. the distance between

shield members

32 and 33, is large as compared with the interelectrode gap 16. While the illustrated shield arrangement is shown by way of example, it should be appreciated that other shield arrangements may be utilized in devices in accord with the invention.

In operation, a high voltage of, for example, 30 kilovolts, is placed across arc

electrode support assemblies

19 and 20 as, for example by connecting. an appropriate lug at screw connector 36 and a similar connector (not shown) on the end of arc electrode support member 20. Such high voltage may be across an electrical device as, for example, a transformer or a capacitor which is to be protected against voltage transients and the vacuum gap and it may be used to short such transients by applying a pulse as, for example, by means of a voltage source represented schematically by battery and switch 37, between trigger anode lead 29 and base member 13 which is electrically connected to are electrode 18.- Alternatively, a sample of the voltage between the primary arc electrodes may be applied between the trigger anode and arc electrode 18. When such a trigger voltage is applied, trigger gap 28 breaks down and an electron ion pulse is injected into gap 12 instituting the initiation of an electric arc which is thereupon carried between

arc electrodes

17 and 18 until the occurrence of current zero, at which time the arc is extinguished. Alternatively, the gap 16 may itself breakdown and no trigger electrode assembly need be included.

As is indicated hereinbefore, in the long and tortuous development of vacuum are devices, those skilled in the art have worked toprovide vacuum arc electrodes of materials having intermediate vapor pressures and boiling points to provide arc conduction specie for such devices. A necessary criterion for such arc electrode material .is that it may be vacuum processed to remove therefrom all sorbedgas es or gas-forming constitutents which, upon the formation of a highcu'rrent arc, can cause the evolution of gaseous contaminants which would permanently affect the maintenance of a steadystate equilibrium pressure arthe order of Torr or less. A further considerationhas been the seeking of arcing'material whichjexhibits good chopping characteristicsyn-amely, the ability to hold the current as a cycle of current alternation uponwhichan are has been struck approaches zero without having the current abruptly terminated from a rel'ativelyhigh value to zero with the creation of a high voltage transient. For these reasons, among others, the art has concentrated upon the use of copperand cuprous alloys of vacuum arc de- Furthermore, ithas been an unwritten folklore that theuse of any ferromagnetic materials inarcing electrodes in vacuum are devices would'result' in unpredictable and deleterious magnetic effects and is,therefore, to be avoided. Additionally, such materials have generally been avoided in devices in which any substantial steady-state current is to be conducted because of the Y lesser electrical conductivity of ferrous materials than cupr'ous materials. For these reasons, innovators workample, the degassification of ferrous materials is relatively simple. Initial de'gassification of the bulk material may be provided by vacuum-melting, a technique of relatively recentcommercial use, not available until recently for vacuum are devices or the fabrication of ferrous electrodesfor use therein. Such vacuum-melting,

particularly of the consumable electrode type vacuum melting wherein each portion of a ferrous bar from which the finally utilized ferrous electrode is formed, is at one point the arcing point of a vacuum arc and each portion of the arc electrode source material is melted under vacuumconditions to cause the removal of all gas and gas-forming constituents before formation of the electrode is an inexpensive methodof preparing highly gas-free metal for vacuum arc electrodes. Once the electrodes have been formed from such material, the surface degassification of adsorbed gaseous mate- 'rial isquite simple. Ferrous materials, as with cuprous materials, may be degassified of adsorbed gases merely by assembling them in the vacuum arc device andiduring normal'bakeout, subjecting-them to thetemperature and time of bakeout necessary to degassify the other materials in the device. Due to their high temperature characteristics, such bakeout temperatures are not limited thereby.

Of greatest importance and bearing upon the utility and advantages of ferrous arc electrode materials in accord with the present invention, I have found that the use of hard and ductile gas-free arc electrode materials results in the attainment of a high dielectric recovery characteristic and high electrical breakdown strength as well as a more rapid rate of recovery of such high electrical breakdown strength than any cuprous material. This characteristic greatly facilitates a substantial increase in voltage rating of devices in accord with the invention as compared with prior'art'devices utilizing conventional electrode materials.

adequate to sustain the current conduction during the relatively short arcing periods that high currents are carried. Thus, l-havefound that high currents of the order of kiloampere arcs and arcingtimes of the order of 'onecycle of power frequency alternating current do not provide any problems, evenconsidering the higher electrical resistivit of ferrous materials than copper, and like-materials. This is true at this time because present vacuum arc device technology has overcome the early (e.g., 1950-4960) problems relating to problems of sufficient arcing specie to sustain .an arc, avoiding unacceptable chopping currents and avoidingthe formation of destructive arc footpoints, particularly anode spots. The present problems facing innovators in the vacuum are an relate largely to extending the magnitude of voltage which maybe safely impressed between arcing members and increasing the currents which maybe interrupted and thereby tenninated. This is particularly true in the case of vacuum gap devices wherein'a fixed gap exists and no steady-state electrical conductionof the main current is provided, but current is conducted only for a very short, time during arcing under fault voltage conditions. r M

Since thechopping characteristics of iron .and of ferrous materials in general arecompatible with vacuum are use, and so long as ferrous materials are not used as extended conductors for steady-state conduction of high currents, there is thus no electrical'disadvantage totheir proper use in vacuum arc devic'esQl find, however, that there are decided advantages. Thus, for ex-' from 15 to 30 weight percent nickel. Somesuch steels -While numerous ferrous materials are useful-in the production of vacuum are devices in accord with'the present invention, 1 find that certain hardenable steels having martensitic grain structure and certain precipi I tated'inclusions are particularly useful. Many of these steels may be machined while relatively soft'in condition and then heat-treated to cause an increase in hardness and yield strength and are ideally suited for devices in accord with the present invention.

One general grouping of such steels is well known and generally identified as age hardening or precipitation hardening .steels. In general, these steels are ,martensitic steels obtained by quenching rapidly an austenitic steel to obtain dispersion of impurities and small grain size. Hardening is effectuated by the presence of impurities, which also contribute to the necessary ductility. The hardening may result from a heat treatment generally performed at a relatively low temperature (as compared with the temperature of the original quenching), as for example 900-l ,000F. Some such steels are iron-nickel maraging steels (generally containing also. minor quantities of molybdenum and cobalt, and sometimestitanium) with are described in an article entitled l 8 percent Nickel Maragi ng Steel by Decker, Eash and Goldman, published in the Transactions of the ASM, pp. 58-76, I962. Certain specific maraging steels which have been found suitable for use in the invention are available from Vanadium Alloy Steel Co., Latrobe, Pennsylvabetween nia, and are described in a brochure copyrighted by that firm in 1966 entitled VASCOMAX 200-250-300-350 which brochure lists many specific compositions of suitable maraging steels.

Maraging steels are normally manufactured by vacuummelting practices and are relatively free of dissolved gases as commercial steels go. Developmental tests have verified that the gas content of these steels is acceptable for use in arc electrodes in vacuum are devices. As purchased commercially, maraging steels may typically have a hardness of approximately 30Rockwell C and exhibit a yield strength of approximately 100,000 psi, approximately six times that of copper. In

this condition'the'steels are, nevertheless, readily main excess of 250,000 psi with remarkably high ductility in excess of 10 percent of 2 inches length and standard crosssection. .Such characteristics are uniquely adapted for the formation of electrode assemblies in vacuum are devices since the bakeout time of such devices may be sufficient to cause appropriate hardening of a maraging steel. Thus, 'for example, I havev constructed vacuum devices in accord with the present invention utilizing a'particular maraging steel known as VASCOMAX 300 CVM to form arc-electrode members exhibiting the foregoing mechanical characteristics. In one illustrative instance, VASCOMAX 300 CVM maraging steel having a 30 Rockwell C hardness was machined to provide arc electrode members of vacuum arc electrode assemblies in accord with the present. invention, assembled, and the device was baked-out for 24 hours at 550C. After such treatment, the steel was found to exhibit a hardness of approximately 41 Rockwell C and a corresponding yield strength of 1'50,000.psi, at least ten times stronger than conventional vacuum processed O FHCcopper generally utilized in such devices, and a ductility in excess-of 10 percent. As is mentioned hereinbefore, for short term arcing characteristics, the steels of the invention are comparable to copper insofar as arcing currentcarrying capacity is concerned. Developmental tests of VASCOMAX 300 steel utilized as described herein have shown an ability to carry without deleterious effect a current density of approximately 225 amperes/cm over the surface thereof for a half cycle of arcing, which is equal to or better than the same value as maybe carried by copper electrodes.

Other hardened or hardenable. steels from which ferrous arc-electrodes for use indevices in accord with the erally hard, having tensile strengths in excess of 150,000 and usually'in excess-of'200,000 psi, are easily outga ssed and have ductility of the order of 10 percent elongation of standard 2 inches long standard test section. Many other precipitation-hardened stainless, austenitic and martensitic steels may be found.

One specific precipitation-hardened stainless steel possessingcharacteristics ideally adapted for use in devices in accord with the invention is identified as Carpenter Custom 455 and is available from Carpenter Technology Corportion, Reading, Pennsylvania. This steel contains approximately 12 percent chromium, 8.5 percent nickel, 1.2 percent titanium, 2.25 percent copper, less than 1 percent each of carbon, manganese and columbium, balance iron. Its tensile strength is in excess of 225,000 psi, readily outgassed and has high ductility as represented by a 5-10 percent elongation in a standard 2 inches long test section of standard cross section.

Still another class of uniquely adapted hardened ferrous materials or alloys well adapted for use in devices in accord with the invention are the so-called TRIP steels (Transformation Induced Plasticity). Such steels are normally au'stenitic, but their composition places them near the phase line so that warm working causes a transformation to martensite under stress at service temperatures. .A description of TRIP steels may be found in an article appearing in preprint No.-UCRL 18609 (Lawrence Radiation Laboratory), University of California, November 1968, by W. W. Gerberich and entitled Metastable Austenitic Steels with Ultrahigh Strength and Toughness.- See also TRANS. ASM 60, 252 (1967), The Enhancement of Ductility in High Strength Steels by Zackey et al.

In general, another class of ferrous alloy materials which may be'utilized in accord with the present invention is-that of carbon alloy steels. These steels differ from maraging steels, and other hardenable steels which are often'soft after a quench from a high temperature in excess of 1500F and hardened by later exposure to intermediate temperatures of the order of 900F-1200F that correspond reasonably well to bakeout temperatures of vacuum are devices in accord with the present invention-Carbon alloy steels, on the other hand, as is epitomized for example by VASCO- J-ET 1000-CVM, a carbon alloy steel available from Vanadium Alloy Steel Co., Latrobe, Pennsylvania, and comprising approximately 0.40 percent carbon, 5.0 percent chromium, 1.3 percent molybdenum, 0.5 percent vanadium, the remainder iron, are hardened by a quench from a high temperature (1,8001,900F for VASCOJET 1000) and later tempered to slightly decrease the hardness and increase the ductility by exposure to intermediate temperatures such as those utilized in the bakeout of vacuum arc devices in accord with the present invention.

I am aware that ferrous materials of largely unspecified characteristics have been used in vacuum arc devices previously. Thus, US Pat. No. 1,582,330 issued Apr. 27, 1926 on an application filed May 19, 1919 by L.S.Brach, wherein the use of nickel steel of unspecified hardness, ductility and other characteristics other than that the nickel steel does not, it is stated, emit gases, has been utilized in a lightning arrester, as gap electrode therefor. The teaching of the Brach patent does not bear a substantial relation to the use of the specific hard. ductile highly pure and gas-free arc electrodes-disclosed and claimed in accord with'the present invention since the state-of the-artin l919,-insofar as vacuum is concerned, was very crude and in no way ad,-

vanced to the point. where vacuum are devices as conistics of voltage recovery strength and rateof voltage recovery strength, which are essential in-accord with the present invention. I am' further aware that U.S. Pat. 3,038,980 issued June 12,1962 on an application filed Dec. 17 1959 toT. H. Lee makes the generalized statement that the property of ametal that appears to be most predominant in determining the voltage at which a breakdown between electrodes of such metal will occur is its modulus of elasticity in tension, i,e'., Youngs Modulus. While superficially it mayseem 'that Youngs Modulus of Elasticity is. related to'the hardness and/or ductility of devices in'accord with the present invention, it should be pointed .out that Youngs Modulus-of Elasticity is an indication of the tensile strengthof the material within the elastic range and. is generally achieved by computingthe ratio of stress to strain during elastic deformation. Ductility, on the other 'hand,is related to plastic deformation, which characteristics are entirely unrelated. Youngs Modulus is not an indication of the hardness of a material. Furthermore, Youngs Modulus of Elasticity is essentially the same forall irons and steels, whereas, as may be found by reference KING. 3 which will be discussed hereinafter, the voltage recoveryv strength and rate of voltage recovery strength which are critical-to the selection of ferrous arc electrode materials in accord with the present invention, vary substantially among ferrousdielectric strength. Accordingly, it is well within the contemplation of those skilled in the metallurgical arts to apply the teachings of the present invention to the use of many unenume'rated'ferrous alloys for the provision of improved vacuum are devices.

As used herein, and in the appended claims, the terms hardness, hard, and the like, are intended to connote a tensile strength in excess of 100,000 psi and preferably in excess of 150,000 psi and a Rockwell C value of at least approximately 30 and preferably at least approximately'40. As used herein, the terms high ductility", ductile and the like are meant'to connote a ductility as evidenced by a percentageelongation of a standard 2 inches long sample of standard (ASTM) cross section of at least 5 percent and preferably 10 percent. All percentages of compositions are expressed in weight percent.

FIG. 2 of the drawing illustrates a vacuum switch constructed in accord with the present invention. In FIG. 2, like. elements to those of FIG. 1 are identifiedv with like numerals. In FIG. 2, as in FIG. 1, envelope ll of vacuum switch '50 comprises upper and

lower endplates

13 and 14, respectively, separated by cylindrical insulating endwall member 12. A pair of

arc electrode members

17 and 18 defining an arcing gap 16 therebetween are supported respectively from endwall members l3 and 14 by are electrode-

support assemblies

19 and 20 respectively.

Arc support assemblies

19 and 20 are both comprised of a cuprous

inner core

21 and 23 respectively which may be surrounded by a

ferrous cladding member

22 and 24 respectively. The cuprous materials which meet the criteria of hardness-and ductility. and gas freedom, as set forth herein. 1

"The importance of ductility in. arc-electrodes of vac uum are devices in accord with the invention cannot be stressed too heavily. The enormous shocks caused by the electromagnetic forces of kilo-ampere arc can deform strong arc electrode structures. If the structures aremerely made hard, they may be brittle, and when loys such as maraging, precipitation-hardened, TRIP, and carbon steels which are capable of having their crystal structure grain size and other metallurgical characteristics altered by'treatment at temperatures which maycorrespond with acceptable temperatures for the bakeout of vacuum are devices in accord with the present invention, that this invention may be practiced with numerous such steels, many of which may provide the increased strength, hardness and ductility which are desirable and consistent with the attainment of more stablevacuum arc structures forthe-attaim ment of higher currentcargy-ipgQapabilities and-greater invention. r

Arc electrode support member 20 is reciprocally

core members

21 and 23 are electrically and mechanically affixed with lowresistance connection to are electrodes -17 and 18 respectively. A copper-bismuth or copper-beryllium alloy, for example, arcing button 20 is inserted into arc electrode 18 primarily to prevent welding between like material are

electrodes

17 and 18. The presence of a-low vapor pressurebutton in arc electrode 18 does not detractfrom the dielectric recovery strength and breakdown'strength characteristics obtained due to the use of ferrous electrodes in devices inaccord with'the invention. Thus, the small sizeof button-30 and its central location, in'view of the known electromagnetic tendency of an arc between two electrodes to propagatev transversely" and to seek the edges or; comers thereof rapidly removes the arc fromthe central button sothat its presence has essentially no effect on the voltage recovery strengthand rate of recovery thereof of devices in accord with the movable so as to move from a first circuit closed position in which it is in contact with fixed arc electrode 17 to an open circuit position in which it is separated from means forseparating

arc electrodes

17 and 18 while at the same time maintaining v.thenecessary high vacuum withi-n'switch envelope 11 An equipotential high-current conducting strap ofa-flexible braided conductor,

preferably copper or a cuprous'alloy, provides fo-r'good I I 1 electrical conduction between reciprocable assembly and .endwall 14 and is appropriately affixed thereto.

in operation, vacuum switch 20 is connected in circuit with a'circuit element to be protected as, for example, a transformer or motor, and is normally connected in line with a current conduction path by means of connector 44 on are electrode support member assembly 19 and a similar connection (not-shown) on support assembly 20 or, alternatively, one affixed to lower endwall member 14. Steady-state conduction of electric current is essentially through the

cuprous cores

21 and 23 of

arc support members

19 and 20, with a minimal length of conduction path through ferrous material of

arc electrodes

17 and 18, thus achieving the desirable electrical conductive characteristics of copper or cuprous materials together with the advantageous dielecbreaker actuating mechanism (not shown) to cause arc electrode support assembly 20 to be reciprocably movable downward, removing arc electrode 18 from contact with fixed arc electrode 17 and-causing the initiating of a high current are to be struck therebetween. Thereafter, conduction of current through the high current are and extinction thereof are essentially the same as in accord with the mode of operation of the triggerable vacuum arc device of FIG. 1 of the drawing.

The device of FIG. 2 may-be modified to form a triggerable switch by adding a trigger electrode assembly to one primary arc-electrode structure as is done in FIG. 1. Such a device is in a normally open position and is triggered to strike an arc.

As is stated hereinbefore, yet another unexpected advantage of devices in accord with the present invention is the discovery of a unique dielectric recovery strength characteristic of ferrous arc electrodes of the ductile,

hardenable steels utilized in devices of the invention. FIG. 3 of thedrawing illustrates atypical plot of recovery strength in volts plotted as a function of time after arcing of gas-free arc electrodes of a specific configura- -tion following a 250'ampere shaped current pulse utilized in vacuum are devices, as in thepresent invention,

fabricated from different electrode materials tested funder identical test conditions. In each instance, the

voltage in kilovolts between the electrodes starts from a zero value and rises to a breakdown value, the average of which is generally denominated as the recovery strength. Thus, two important characteristics are pres-v ent. First, the rate of recovery in kilovolts per microsecond and the recovery strength in kilovolts (KV). In FIG. 3, curve A represents the recovery strength curve forcopper electrodes. Curve B represents the recovery strength curve for beryllium electrodes. Curve C represents the recovery strength curve for electrodes made of two different steels typical of the hardened ductile steels in accord with the present invention, both of which have essentially the same recovery strength characteristics. These steels are VASCOMAX 200 and VASCOJET 1,000. Curve D represents the recovery strengthcharacteristic of 304-type stainless steel, a readilyavailable, commonly used, stainless steel, often found as shield materials and other instrumentalities in circuit breakers and such devices.

As is set forth hereinbefore, all ferrous materials are not suitable for vacuum arc devices in accord with the present invention. Thus, Curve D, representative of the recovery strength characteristic of type 304 stainless steel, exhibits a recovery rate of approximately 8 to 10 kilovolts per microsecond as compared with the recovery rate of approximately 15 KV per microsecond shown by Curve C, and saturates at a value of approximately kilovolts. Copper, on the other hand, shows both a slower recovery strength than that of the steels in accord with the present invention and saturates at a very low value of voltage, namely, 37 kilovolts. While this is not consistent with the curves shown inmy above-identified copending application wherein it was stated thatcopper electrodes show essentially the same recovery rate of hard, ductile steels, later, more precise test data indicates the herein described added advantage in recovery rate of hard, ductile steel electrodes to be a more representative value. Beryllium, although exhibiting a very rapid rate of recovery strength, is inferior to the steels in accord with the present invention in that it saturates at approximately 60 kilovolts.

Thus, from a consideration of curves of FIG. 3, it may be seen that the electrodes fabricated from hard, ductile, gas-free steels in accord with the present invention make devices fabricated in accord with the present invention quite superior to those utilizing copper in that the recovery rate is higher and the saturation voltage holdoff strength is almost three times as great. Ordinary type 304 stainless steel shows both a lower recovery rate and a saturation value which is approximately 60 percent that of steel ferrous materials in accord with the present invention. It is, therefore, apparent that great utility maybe found in the use of the hardened, ductile, gas-free, ferrous materials in accord'with this invention in the fabrication of vacuum arc devices.

Thus, it is readily apparent that the unique advantages found in electrodes made of hard, ductile, gasfree, ferrous materials, as set forth herein, are not characteristic of electrodes of ferrous materials, per se, particularly alloy steels such as nickel steels, noting for example the inferior performance of electrodes made of 304. stainless steel which contains approximately 18 percent chromium, 8 percent nickel and the balance iron. All of the electrodes, the characteristics of which are illustrated graphically in FIG. 3, were similarly gasfree so that although the devices in accord-with the present invention must contain electrodes which satisfy the gas-free criteria set forth hereinbefore, the subnor- .mal performance of type 304 stainless steel electrodes,

as is evidenced by the graph of FIG. 3, is not to be attributed to superior or inferior gas-freedom. Rather, as is set forth hereinbefore, the unique advantages of electrodes in accord with the invention, and devices in accord with the invention using such electrodes, is related to the unique combination of hardness and ductility found in hardenable steels, such as maraging steels, TRIP steels, carbon alloy steels, and precipitationhardened steels. Similarly, all of the steels used in electrodes-in accord with the present invention are found to have significant proportion of impurities therein which make it possible for the grain structure thereof to be susceptible of the appropriate hardening under the particular characteristic and manipulative processes whereby the hardening of the unique steels utilized in electrodes in. accord with the present invention is accomplished. I

In general, in: addition to the steels set forth herein, other vacuum-refined steels containing nickel, chromium, and often titanium, and other such similar transition metals, are sufficiently hard, ductile, and may be readily rendered sufficiently gas-free as to be uniquely adapted to possess the highly desirablehardening characteristics which optimize devices utilizing such ferrous arc electrode assemblies in accord with thepresent invention.

.Devices constructedinaccord with thepresent invention have been operated repeatedly to hold off 100,000 volts in open circuit position. Switch devices in accord with the invention have, in closed circuit position, routinely successfully carried 33,000 amperes peak current. This current has been interrupted successfully by causing an arc to be'struck which exhibits approximately a 50 volt drop and which has been exting'uished at a first occurring current zero with no noticeable deleterious effects to the arc electrodes, and such are has been extinguished and the applied voltage has failed to restrike on the next voltage half cycle.

While certain characteristics of the materials utilized in devices in accord with the invention are disclosed and claimed in my aforementioned co-pending application, it has, since the filing thereof, been discovered that certain other characteristics thereof are important andthat my invention is broader in scope than the specitic configuration of arc-electrodes illustrated therein.

Accordingly, the appended claims sl et'forth in true depth and scope the invention disclosed herei n. It is,

therefore, my intention to cover by the appdned claims all modifications and changes of 'the herein-disclosed embodiments as fall within the true spirit and scope of the foregoing disclosure. i

What I claim'as new'and desire to secure by Letters Patent of the, United States is:

1. A vacuum arc device adapted to carry high curv rents in one operative-state and to hold off high voltrial having a hardness as evidenced by a Rockwell C hardness of at least 30, a yield strength of at least 100,000 psi, and a-ductility as evidenced by at least a 5 percent'elongation of a standard 2 inches long sample of standard ductility test cross-section and having a substantial freedom of sorbe'd gases and gas-forming impurities therein-so as to withstand arcing current densities of approximately 200 amperes/cm for ahalf-cycle of power-alternating 4 voltage without the emission of any substantial quantity of gaseous material inconsistent with cone tinued maintenance of said low pressure after having been arced thereby.

.d. shield means surrounding said are electrode members to confine arcing specie to the interior thereof; and

e. means connecting said are electrode assemblies in circuit with an electric load.

2. The device of claim 1 wherein each of said ferrous electrode assemblies is a vacuum-melted steel.

3. The deviceof claim 2 wherein said ferrous electrode assemblies are fabricated from hardened steel having aRockwell C hardness in excess of 40, a yield strength in excess of approximately.150,000'psi, and a ductility corresponding to an elongation of at least approximately '10. percent of a 2 inches long standard cross section test sample.

4. The device of-claim 2 wherein said ferrous assemblies are fabricated of age-hardenedsteel which increases in hardness and yield strength during vacuum bakeout cycles used to fabricate vacuum arc devices.

5. The device of claim 2 'wherein said ferrous metal is precipitation-hardened.

6. The device of claim 4 wherein said bakeout temperatures are of the order of 900 l ,200F and are utilized for periods of from 5-24 hours.

7.The device of claim 2 wherein said ferrous material is a transformation induced plasticity steel.

8. The device of claim. 2 wherein said ferrous material is a temperature-hardened marag-ing steel.

9. The device of claim 3 wherein said ferrous material is temperature-hardened carbon alloy steel having from 0.2 to 0.6 weight percent carbon therein.

10. The device of claim 2 wherein the device further includes means for establishing an electric arc between said are electrode assemblies by establishing an electron-ion plasma therebetween.

11. The device of claim 10wherein said device is a triggerable vacuum gap device and the means for sup-- plying an electron-ion plasma therein is a trigger electrode assembly.

l2. The'device of claim 10 wherein the device is a vacuum switch and said arc electrodes are each'supported upon a highly electrically conductive support member which is secured in'low resistive contact with said ferrous arc-electrode.

-.13. The device of claim 12 wherein said electrode support members are surrounded by a sheath of hard, ductile, ferrous material.

14. The device of claim 3 wherein said ferrous assemblies are fabricated of a heattr'eatable steel which increases in ductility during vacuum bakeout cycles used to fabricate vacuum are devices of the order of 5004600C.

Claims

1. A vacuum arc device adapted to carry high currents in one operative state and to hold off high voltages in another operative state and comprising: a. an hermetically-sealed envelope evacuated to a pressure of 10 5 Torr or less; b. a pair of arc electrode assemblies disposed within said envelope and adapted to define therebetween an interelectrode gap and to sustain therebetween a high current arc which is sustained by conduction specie derived from said arcelectrodes; c. said electrode assemblies comprising ferrous material having a hardness as evidenced by a Rockwell C hardness of at least 30, a yield strength of at least 100,000 psi, and a ductility as evidenced by at least a 5 percent elongation of a standard 2 inches long sample of standard ductility test cross-section and having a substantial freedom of sorbed gases and gas-forming impurities therein so as to withstand arcing current densities of approximately 200 amperes/cm2 for a half-cycle of poweralternating voltage without the emission of any substantial quantity of gaseous material inconsistent with continued maintenance of said low pressure after having been arced thereby. d. shield means surrounding said arc electrode members to confine arcing specie to the interior thereof; and e. means connecting said arc electrode assemblies in circuit with an electric load.

3. The device of claim 2 wherein said ferrous electrode assemblies are fabricated from hardened steel having a Rockwell C hardness in excess of 40, a yield strength in excess of approximately 150,000 psi, and a ductility corresponding to an elongation of at least approximately 10 percent of a 2 inches long standard cross section test sample.

4. The device of claim 2 wherein said ferrous assemblies are fabricated of age-hardened steel which increases in hardness and yield strength during vacuum bakeout cycles used to fabricate vacuum arc devices.

5. The device of claim 2 wherein said ferrous metal is precipitation-hardened.

6. The device of claim 4 wherein said bakeout temperatures are of the order of 900*-1,200*F and are utilized for periods of from 5-24 hours.

7. The device of claim 2 wherein said ferrous material is a transformation induced plasticity steel.

8. The device of claim 2 wherein said ferrous material is a temperature-hardened maraging steel.

10. The device of claim 2 wherein the device further includes means for establishing an electric arc between said arc electrode assemblies by establishing an electron-ion plasma therebetween.

11. The device of claim 10 wherein said device is a triggerable vacuum gap device and the means for supplying an electron-ion plasma therein is a trigger electrode assembly.

12. The device of claim 10 wherein the device is a vacuum switch and said arc-electrodes are each supported upon a highly electrically conductive support member which is secured in low resistive contact with said ferrous arc-electrode.

13. The device of claim 12 wherein said electrode support members are surrounded by a sheath of hard, ductile, ferrous material.

14. The device of claim 3 wherein said ferrous assemblies are fabricated of a heat treatable steel which increases in ductility during vacuum bakeout cycles used to fabricate vacuum arc devices of the order of 500*-600*C.