US2188399A - Electric resistance element - Google Patents

Description

'18 I in atmospheresi'which. changedsin character; such S practice Oman industrialvscaler,

Patented Jan. 30; 1940 UNITED STATES v Z,188,399 A NT-OFFICE 1 pa sam Q ELECTRIC RESISTANCE ELEMENT I Clarence George Bieber; Huntington; W; Va as signer to The lnternationaliNickel Company,

'. Inc., New York, any" Bio Drawing;

'10 ticularly forv electrics-resistance; elements intended to operate: at elevated -.temb ratures:; especially above, 180091; IQLQQIP;YI}1QY 1,OI xlreasonsginclud ing their suscep'tibili itoazembrittlement"orioxidation .at such: high temperatures; particularly as from oxidizing to reducing; 1 -Althgu gh; many proposals have been :made; none asfar: as 21am aware, has been; whollysuccessful and; satisfactory commercially,: especially when *{carried :into' a An-obiect tor the :inven on :is to; Davide's electric resistance element: comprising 2a: heat-- -.-treated' chromium "plated nickel or; nickel alloy wire which can be operatediin, oxidizingror ire ducing atmospheres at elevated temperatures in cluding 1800-2500 F. 21:

A further object of the invention is-to provide an electric resistance .elemefltutilizingthe electrical resistance properties; of nickel cr'ia nickel nickel duertoloiddationrma, 1. V It is also within. thelcontemplationofthe inventionto provide a processfor heat treating chromium ,plate'd nickel or nickel alloy:.-wire whereby an 'electric' resistance element can be produced in a suitable condition-for use in'electric resistance furnaces. V a 1 Other objects rand advantages oithe invention will becomeapparent from thefollowing description of a preferred embodiment of the and below '-'2600 ;F. iwithout. destructionipf. the

invention and a procedure for-carrying the invention into practice. 1

According to the present and inexpensive resistance heating element is providedby utilizing the desirable features of nickel and' nickel'alloys andavoiding the shortcomings thereof. 1 have discovered that'envelopment of nickel or nickel alloy with chromium, especially the electrodeposited type, and the subsequent formation, of a bond of a nickelchromium alloy provides an electric resistance element with adesirable temperature coefficient, an excellentresistance to oxidation and emperatures and changing atmospheres including oxidizing and reducing and with high melting point. I a v A method of producing the new electric resistance element is to electrolytically a deposit Application Serial N0 1. 109,307

alloy at elevated temperatures above 1800?- F.-

invention an emblembrittlement, especially at'super or elevated tern a relatively wmnmaw m Noe i; "(1936.

.thintla yers of chromium about; '.ooo2" to 'about 1 0.0Q5 'f:,-thick, upon nickel'or a nickel alloy wire :yorfgthe like.- Thesplated product issubiected to -aspecial-heat) treatment in a hydrogen Iat'mos pheret; It is .preferred to cefie'ct' the treatment by heating the wire rapidlyby the passage of an electric current threthrough; The heat i :i-treatmentEis such-as'to cause chromium to diffuse f a "-rilltofthe surface skin'or layer oi the nickel or nickel alloy so astoformran intermetallic alloy of chromium with thebase metalor alloy. The treated, wire: constitutes an improve ,electric resistance element which can be instal ed in any of electric resistance heating furnace and will function satisfactorily ataj much higher wyotherfbase metals-or, alloys known to the trade.

a abetter understanding: of the. invention, a

preferred embodimentand a preferred-procedure A of lmakingrthepresentilow,priced resistance eleg-ment :willibe' described; 1

-: .;;Nickel wire of :any suitable diameter is pickled ree =fr'om -oxide,'. etc. and 1 carefully cleaned with ipg'removez any grease;or,..dirt etc, It is preferred -'to:then:clean{thexwire electrolytically in a so-'v avlutio'n containing 10 per cent of sodium car edroxider gThe wire acts as a cathode with a ,cleaned'further as cathode ina solution of about 50 'sulfuricfacid wi'tha current density of 1about20 amperes per squareyfoot. The cleaned }wire isflnallyrrinsed with water. The wire" is coated with about 0.00021 to about 0l005" of chromium by the electrodepos'ition in a chromic i j [acid bath containing approximately 250 grams of the chromic acid and' a bout 2.5 gramssulfuric acid perliter'using a current density of temperature "of about 1159-120". F.

After this plating operation; the 1 chromium plated nickel wire is subjected toheat treatment 10G1-b01i tetrachloride or sodium tri-phosphat'e to approximately i; ampere per square inch and a- -For thepurpose of giving those skilled infthe bonate and about- 1 per cent off'sodiumhy- .ccurrent densityofabout m amperes per square" Q g .foot. Following this treatment," the wire is by passing a regulated electric current there- Y through sofas to maintain the wiretemperature 1 at about 1800 to about 2000 E. for about 1' to about? hours'in a hydrogen atmosphere which I have found to be best suited for the diilusion or alloying of the chromiumwith the nickel; The 7 product is, then an electric resistance v heating an electrical resistance furnace. Tests have demonstrated that it will operate satisfactorily without oxidation at a higher temperature than any base-metal resistor, is more convenient to use -than non-metallic resistors {of the 'carborunf dum type, and is cheaper to the public.

element capable of being used for any kind of l5 t,v.; temperature;. than Jprior: electric resistance "eleg mentsacomposed bf chromium-nickel. alloy or In carrying the invention into practice, the improved electric resistance element was subjected to severe tests to demonstrate its usefulness and to establish its unique properties. Some of these tests will now be described.

A rectangular furnace with a heating chamber 5" x 8" x 14" was provided with a hair-pin type of the improved electric resistance heating element made in accordance with the procedure outlined hereinabove by electro-depositingabout 0.001 inch of chromium upon a nickel wire about T g" in diameter and heat treating the same at about 1800 to about 2000 F. .in hydrogen for about 2 hours to alloy the chromium with the nickel. This furnace has been used.intermittently for 40 heats of 8 hours each at temperatures of 2000-2300 F. in an oxidizing atmosphere, and for 8 heats of Shows each at temperatures of 2000-2100 F. in a hydrogen atmosphere. practically as good a condition asat the time of installation.

A cylindrical furnace about 6" in diameter and 10" high was provided with a helical unit constituted of the present electric resistance element of diameter nickel wire plated with about 0.001 inch of chromium after heat treatment in hydrogen for about two hours at'about 1800 to about 2000 F.- This furnace has been used for 50 heats of about 4-8 hours each at temperatures of about 2150 to about 2350 F. in

a hydrogen atmosphere and for 12" heats of about 8 hours. each at temperatures of about The heating unit was found to be in (b) Reheated to 2000" for 8 hours and cooled over-night, r

(c) Reheated to 2100 for 8 hours and cooled over-night,

(d) Reheated to 2200 for .8 hours and cooled over-night,

(e) Reheated to 2300 for 8 hours and cooled over-night,

(f) Reheated to 2500 F. for and showed that the nickel interior had actually been in a molten condition inside the chromium shell, and that the wire had operated for a time as a molten resistor.

It'is to be observed that the presentapplication provides an improved electric resistance heating element which possesses new and unique properties and superior characteristics to prior electric resistance heating elements. Some of the new, improved and unique properties and 2000 to about 2100" F. in an oxidizing atmoscharacteristics may be appreciated from the following schedule:

Schedule l V V Prior alloy of 607 New chrome Prior silo of 80% Prior alloy of 85% 1' 257 Fe, 117 plated nickel Ni, 20 Cr Ni, 15% Gr Cn40% Mn 0 Resistance .Q l6 microhms per 100 microhms 85 microhms per 105 microhms per 1 cm per cm. cm cm Temperature resistance co-efiiclent 002 per F .00006 per "F 00008 per "F .0001 per l Maximum useful temperatures... 2600 F 2000 F 1900 F 1400" F. Resistant to atmospheres Excellent. Good Fair Poor.

. do ;.do Good Resistant to fluctuation Fair.

phere. The improved heating unit or' electric resistance element is in practically as good a condition as when it was originally installed.

Careful examination of the improved electric resistance elements in both furnaces described hereinabove showed practically no signs of deterioration, their electrical characteristics "have not changed and a practically constant electric current has continued to give the same furnace temperatures at all times. The remarkable 'efliciency of the novel electric resistance element can be judged further by comparison with the fact that a A" diameter conventional heating unit made from an alloy containing approximately per cent of chromium and about 80 per cent of nickel was used'in the cylindrical furnace and was found to have failed due principally to oxidation after three heats of about 4 hours each at about 2300'F. in a hydrogen atmosphere.

A further description of my new results will show the remarkable performance at elevated temperatures of my improved resistance element. Some 0.051" nickel wire with. a heat treated 0.001 inch chromium coating was wound on a refractory cylinder and assembled into a tube furnace. This furnace was subjected to the fol lowing heating cycles in air:

(a) Heated to 1900 F for 8 hours andcooled' over-night,

It is also to be observed that the new and improved electrical heating resistance element has a longer life at temperatures of 2000" F. and above. The life ofthe present improved electrical resistance heating element is distinctly superior to the life obtainable with prior electric resistance heating elements as may be clearly seen'from the following table:

'Table Life at Newyctc. Prior. etc. ggy

rnfiiiim infiifiia iti it,

2i00F V. T T Indefinite 1000 57 -l7 2200F 9e0 V 22 11 ,5

N0TE. Indeflnite means lonucr than 1080 hours or 10 weeks."

Furthermore the present invention has been applied to a nickel-chromium wire which was coated with a film of nickel having a thickness of about 0.00025 to about 0.0005" and then plated with afilm of chromium having a thickness of about 00002 to about 0.0005 This doubly plated wire had increased life at temperatures This doubly plated wire is very'useful for operating'at high temperatures. A satisfactory composition of the nickel-chromium alloy was found to be 50% to about 90% of nickel, 10% to about 25% of chromium, .l% to about 25% of iron and especially 80% of nickel, 19.8% of chromium and .2% of iron.

Other advantages of my improved resistance wire will become readily apparent to those familiar with the subject. For instance, I could chromium plate a chromium-nickel resistance wire and increase its operating range to higher temperatures, but I prefer to provide a less expensive article of manufacture by utilizing nickel, or some of its commercial alloys, as a base product. r

Although the present invention has been described in connection with preferred embodimentsit will be appreciated that variations and modifications may be resorted to without departing from the spirit and scope of the invention. For instance nickel alloys may be used. Thus nickel alloys having the following composition may be used:

I The size of the wire or element may vary depending upon specific conditions. For practical purposes the size may vary from 0.005" to about 1" in diameter or thickness.

ventional types, such as rounds, flats, strip, ribbon or T grid. Moreover tungsten or molybdenum may function like chromium but are notas commercial as chromium. All of these variations and modifications are to be understoodto be within the purview of the present specification and the scope of the appended claims.

What I claim is:

' 1. As a new article of manufacture, a composite electrical resistance heating unit comprising a core constituted of a nickel-chromium alloy, an outer layer of chromium enveloping said core, an intermediate layer of nickel, and diffusion zones between the outer layer and the nickel layer and between .the nickel layer and the core intimately bonding the layers with each other and with the corejsaid chromium layer being substantially more resistant to failure at temperatures in excess of 2200" F. than said core.

'2. As a new article of manufacture, a composite electrical resistance heating unit comprising a core containing a nickel-chromium-iron alloy, an intermediate layer of nickel surrounding said core, an outer layerof at least one metal of the group consisting of chromium, molybdenum and tungsten surrounding said nickel layer, and diffusion zonesbetween the outer layer and the nickel layer and between the nickel layer and the core intimately. bonding the layers with each other and with the core, said outer layer being substantially more resistant to failure at temperatures in excess of 2200 F. than said core.

CLARENCE GEORGE BIEBER.

The heating element may have various shapes including the con-