US2400255A - Electric resistance elements and the like - Google Patents

Description

Patented May l4, 1946 ELECTRIC RESISTANCE ELEMENTS AND THE LIKE Leonard Bessemer Pfeil, Birmingham, England, assignor to The International Nickel Company, Inc., New York, N. Y.', a corporation of Delaware No Drawing. Application May 15, 1942, Serial No. 443,174. In Great Britain May 27, 1941 Claims.

The present invention relates to heat-resisting alloys and articles made therefrom and to a method of producing such alloys and articles, and more particularly to heat-resisting alloys and articles having an improved combination of prop erties.

It is well known to incorporate in heat-resisting alloys, such, for example, as those used for electrical resistance elements, small quantities of one or more elements such as the rare earth metals, particularly cerium, the alkaline earth metals, particularly calcium, thorium, etc., in addition to those of which the alloy is basically composed, for the purpose of improving the high temperature properties and in particular the service life of the alloys and articles made therefrom. As an example, the heat-resisting properties, and in particular the resistance to oxidation, of alloys consisting basically of 80% nickel and chromium are materially improved by the inclusion of small quantities of calcium, cerium, calcium and cerium or calcium and thorium. Many other elements may be used and, for convenience, in

view of their capacity for increasing the life of the alloy under high temperature conditions, they are referred to herein as long-life elements.

I have found that the long-life elements, when incorporated in the alloys to improve their life, may have an adverse effect on other properties of the alloys. In an increasing number of applications of heat-resisting alloys, resistance to creep at high temperatures is an essential requirement, and long-life additions to the alloys may seriously impair this resistance. Furthermore, the addition of the long-life elements to the molten alloys may be a matter of difliculty, as many of the long-life elements are extremely reactive. Again, the presence of long-life elements in the alloys may render them difficult to fabricate, and in some cases the alloys cannot be hot worked.

I have discovered that some of the elements will produce the desired effect of an increase in the service life of the basic alloys if they are applied in the form of films to the surfaces of the alloys.

It is an object of the present invention to provide heat-resisting alloys and articles having an improved combination of properties.

It is another object of the present invention to provide heat-resisting alloys and articles made therefrom having high service life combined with improved resistance to creep.

It is a further object of the present invention to provide a novel method of improving the life are extremely reactive.

of heat-resisting alloys and articles made therefrom.

It is also an object of the present invention to provide a method of improving the life of heatresisting alloys and articles made therefrom without seriously impairing the creep resistance of the alloy or article.

Other objects and advantages of the invention will become apparent to those skilled in the art from the following description.

The elements which I have found may be applied as films are thorium, calcium, zirconium and the rare earth elements. According to this invention the alloys or articles that are made therefrom and by their nature are necessarily subjected to high temperatures in service are provided on their surfaces with films of one or more of these long-life elements in the elementary form or in the form of oxides or hydroxides or in the form of salts that will decompose to oxides at high temperatures.

The service life of the alloys or articles provided with such surface films is not always so great as when the long-life element is included in the alloy, but compensating advantages are often obtained. For example, the addition of the elements to the molten alloys may be a matter of difiiculty, as many of the long-life elements Again, the presence of long-life elements in the alloys may render them diflicult to fabricate, and in some cases the alloys cannot be hot-worked. These difiiculties are avoided altogether by means of the invention. In an increasing number of applications of heatresisting alloys, resistance to creep at high temperatures is an essential requirement, and longlife additions to the alloys may seriously impair this resistance. Surface application of the longlife elements has the great advantage of allowing of the production of articles and machine parts for high temperature operations having high resistance to. oxidation and without any reduction in their normal resistance to creep.

It appears that the long-life elements produce the effect indicated by their influence on the' formation of scale on the surface of the alloy. Whether this is so or not, I find that what is necessary is to produce a fine dispersion of the long-life element or elements on the surface. This need not be continuous, although the particles that compose it should be uniformly distributed, and it must not be a thick coating such as would either completely exclude oxygen from the surface or would tend to flake ofi under rapid changes of temperature. With such a thick coating the desired efl'ect is not produced.

f the long-life elements defined above, the

best results are obtained with thorium and the rare earth elements. The latter may advantageously be used in the form of a so-called mischmetall containing cerium and other ele in, a solution of a salt of a long-life element, or.

to deposit an oxide or hydroxide of the element anodically. Inthe former case, the finished artivie should first be heated to give it a light oxide scale as this causes more satisfactory wetting of the surface and also makes the coating more adherent. The oxidised article may be dipped into the solution, e. g., into a solution of any ap-.

propriate concentration, e. g. about 10%, while 1 hot and withdrawn while still hot enough toevaporate oil the solvent, or it may be dipped told and then heated to remove the solvent and decompose the salt of the long-life element to oxide. Solutions of the nitrates of the long-life elements may be most conveniently used in most cases and slightly acid solutions containing about 10% by weight of the salt give a film of the deired thickness.

When anodic deposition is used, the article to he treated should have a clean metallic surface produced by bright annealing, pickling or sand blasting, and should be degreased before placing in the depositing bath The bath may contain any soluble salt of the long-life elements of any concentration from about 10% to saturation, and this should be made progressively more alkaline aluminium and the like.

until precipitation commences. Acid should then be added until the precipitate disappears and then a further slight excess. tions may be varied over a wide range, but successful deposition may be obtained by using a 10% solution, making the article to be plated the anode and increasing the applied voltage from about 2 volts until deposition is indicated by the evolution of gas round the article being treated. If the voltage is increased further the deposition becomes more rapid, but the gas evolution tends to strip off the film. The gelatinous film deposited should have a thickness of'a few thousandths of an inch. After coating the article is heated to dehydrate the film and to convert it to oxide.

Sometimes I may apply the film to the alloy or partly-fabricated article and thereafter fabricate, or complete the fabrication of, the article, and then I may use other methods of forming the film. For instance, the surface of the alloy or partly-fabricated article may be provided with an integral film by cementation, brought about by heating the alloy in a powdered long-life element in an inert atmosphere, or in a mixture of an oxide or other compound of the element with a reducing agent. Again, the alloy or partly-fabricated article may be passed through a bath of the molten element or this may be electrolytically deposited from a bath of a molten salt of it. Of course these methods may also be used to form films on finished articles, but for this purpose the two methods first described are preferred because of their simplicity.

Again, the condi-.

The following results have been obtained in tests on an 80/20 nickel-chromium alloy having a life of 33 hours at 1200 C.

Treatment Hours Alloy dipped lll solution of mlschmctalP' nitrate 59 Alloy coutod nnodically with mischmetall" oxide 65 Alloy dlppod in solution of thorium nitrate 54 Alloy coated anodlcally with thorln (thorium dioxide). 65 Alloy dippod lll calcium-thorium solution 60 nickel-chromium, nickel-cobalt-chromium, co-'- -balt-iron-chromium, iron-chromium, iron-chromium-aluminium, and complex alloys based on systems such as these alloys but containing in addition one or more of the elements molybdenum, tungsten, titanium, columbium, zirconium, These alloys include those containing about 10 to 35% chromium, and may optionally contain from a small amount, say about 0.01%. to about 20% of one or more metals from the group consisting of molybdenum, tungsten, titanium, columbium, zirconium, aluminium, silicon and manganese, and the balance substantially all metal of the iron group, i. e. metal of the group consisting of nickel, iron and cobalt. As will be apparent to those skilled in the art, these alloys may also contain small amounts of incidental elements and impurities, e. g. sulphur, phosphorus, etc. While the total of iron group metals will usually exceed about 50%, the invention contemplates not only those alloys in which one of the iron group metals exceeds about 50% but also those alloys in which no one particular iron group metal exceeds about 50% although the sum of all the iron group metals will exceed about 50%.

Although the present invention has been described in conjunction with preferred embodiments, it is understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such variations and modifications are considered to be within the purview and scope of the appended claims.

I claim:

1. A method of improving the high temperature service life of chromium-containing heatresisting alloys substantially free from long-life elements which comprises providing such alloys with an adhering surface film of at least one member selected from the group consisting of thorium, calcium, zirconium and the rare earth and salts thereof that will decompose to oxides at high temperatures.

3. Electrical resistance wires, tapes, ribbons and the like made of a chromium-containing heatresistlng alloy and having a surface film of at least one member selected from the group consisting of thorium, calcium, zirconiumandthe rare earth elements, oxides and hydroxides thereof .and salts thereof that will decompose to oxides at high temperatures,

4. An alloy composed essentially of chromium, nickel and iron and containing from 10 to 30% chromium, 20 to 90% nickel and to 50% iron, said alloy being provided on its surface with a film comprising at least one member selected from the group consisting of thorium, calcium, zirconium and the rare earth elements, oxides and hydroxides thereof and salts thereof that will decompose to oxides at high temperatures.

5. A heat-resisting chromium-containing aly provided on its surface with a film comprising a member selected from the group consisting of mischmetall" or compounds thereof.

6. A heat-resisting chromium-containing alloy provided on its surface with a film comprising a member selected from the group consisting of thorium and compounds thereof.

7. A method of increasing the service life of an article that is made from a heat-resisting chromium-containing alloy and that by its nature is necessaril subjected to high temperatures in service, comprising forming a film on the surface of said article by dipping said article in a solution of at least one memberof the group consisting of the salts of thorium, calcium, zirconium and the rare earth metals.

8. A method of increasing the service life of an article that is made from a heat-resisting chromium-containing alloy and that by its nature is necessarily subjected to high temperatures in service, comprising forming a film .on the surface of said article by anodic deposition thereon of at least one member of the group consisting of the oxides and hydroxides of thorium, calcium, zirconium and the rare earth metals.

9. A method of increasing the service life of an article that is made from a heat-resisting chromium-containing alloy and that by its nature is necessarily subjected to high temperatures in service, comprising applying to the allo a film of at least one member selected from the group consisting of thorium, calcium, zirconium and the rare earth elements, oxides and hydroxides thereof and salts thereof that will decompose to oxides at high temperatures, and thereafter fabricating the article from said alloy,

10. A method of increasing the service life of an article that is made from a heat-resisting chromium-containing alloy and that by it nature is necessarily subjected to high temperatures in service, comprising partly fabricating the article from said alloy, applying to the partly-fabricated alloy a film of at least one member selected from the group consisting of thorium, calcium, zirconium and the rare earth elements, oxides and hydroxides thereof and salts thereof that will decompose to oxides at high temperatures, and thereafter completing the fabrication of said article.

LEONARD BESSEMER PFEIL.