US 2703661 A
Description (OCR text may contain errors)
I 2,703,6fil Patented Mar. 8, 1955 United States Patent Office TELEVISION TUBE Charles R. Taylor, Trenton, Ohio, asslgnor to Armco SteelmCorporatlon, Mlddletown, Ohio, a corporation ofO o v No Drawing. Application 1m 14, 1950, Serial No. 168,147
2 Claims. (Cl- 220-23) My invention relates to the provision of a ferrous alloy suitable for the manufacture of electrical devices in which metal must be sealed in or against glass. While the utility of the invention is not so limited, a particular field of utility for such an alloy today lies in the provision of television tubes in which the body is made of a suitable alloy and a glass screen is fused to an open .end of the metallic body. It will be understood that the bodies of such devices are generally conical or funnel-like in shape, the larger open end of the body being usually either circular in cross-section, or rectangular.
It will be understood that one requirement for a suitable body alloy in such devices embraces those physical characteristics of formability and workability, hardness, strength and the like as are necessary for the physical formation of the body as such, and it is an object of my invention to provide alloys having such characteristics.
For the successful sealing of metal against glass, it is necessary that the metal have substantially the same coeflicient of expansion as the glass and that it have the proper oxidizing characteristics. These, however, are
not the only or even the most important requirements.
Since considerable degrees of heat must be employed in the sealing, especially in structures of the class to which reference has been made, it is necessary that the metal on heating and cooling undergo no sharp dimensional discontinuities. It is an object of the invention to provide an alloy meeting all of these requirements.
Dimensional discontinuities upon heating and cooling are caused by the metal undergoing phase changes. Hence, it is an object of my invention to provide an alloy which will not undergo phase changes over a very wide range of temperature, say, from -40 C. to 1200 C. Still more specifically, it is an object of my invention to provide an alloy which will remain in the ferritic form throughout such a range of temperature.
It is finally an important object of my invention to provide an alloy which is very substantially less expensive than alloys heretofore proposed for like purposes.
These and other objects of my invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, I accomplish in that series of alloys which I shall hereinafter more fully describe.
Alloys hitherto proposed for the purpose have taken generally two forms. One form is a ferrous alloy containing about 28% of chromium, which is expensive. Another form is one containing about 17% of chromium but having also a content of columbium, titanium, molybdenum or tungsten which, again, makes the alloy expensive. In the practice of my invention, I have succeeded in producing satisfactory alloys for the purpose which do not contain excessive quantities of chromium and which also do not contain expensive alloying ingredients of the character mentioned in this paragraph.
The nature of my invention will be more clearly understood in the light of a discussion of the factors involved in the avoidance of dimensional discontinuities upon temperature change by keeping the metal in the ferritic form throughout a wide range of temperature. If one plots a phase diagram of iron-chromium alloys, charting percentages of chromium on the abscissa and temperatures along the ordinate, there will be a loop extending to the right from the left-hand vertical axis. Within the area bounded by this loop, as determined by the temperature and the chromium content, the alloy will exist in the gamma-phase of iron, known as austenite. It will be clear on such a chart that if a high enough percentage of chromium is chosen, it will be possible to heat the alloy to a very high temperature without passing through the area of the loop, i. e. without changing the alloy from the ferritic to the austenitic form, the ferritic or alpha-phase of iron being represented by the area outside the loo This will explain the eflicacy of the first type o? alloy hitherto proposed, since in it the quantity of chromium is so large that a wide range of temperature variation will lie entirely outside the austenitic area.
Other elements than chromium, however, have an important bearing on the position of the loop in the phase diagram mentioned, movingthe end of it to the right or left depending upon the nature of the elements in question. This means that an alloy composition lying near the right end of the loop may or may not pass through the loop formers.
In the second previously suggested type of alloy, as mentioned above, elements such as columbium or titanium have the effect, I believe, of combining with carbon and nitrogen present so that these elements cannot exert their etfect in extending the austenitic range. As a consequence, a smaller quantity of chromium may be employed to maintain the ferritic condition throughout a wide range of temperature. Alloys hitherto suggested they; contained carbon in the general range of .05% to 0- I have now found that excellent results may be obtained in alloys meeting the requirements of this invention having very much smaller chromium contents than have hitherto been possible in the absence of other alloying ingredients such as columbium, titanium, molybdenum or tungsten, providing the carbon is kept to an unusually low value and providing nitrogen is also kept low or is immobilized or rendered insoluble by an element such as aluminum or zirconium which can combine with the nitrogen, forming nitrides. Such elements, and in particular aluminum, are much cheaper than the alloying ingredients formerly employed. Aluminum does not react with carbon so that the carbon content must be kept low, as already indicated. The carbon content of commercial heats cannot economically be made substantially lower than about .015%. In my new alloys, other elements which act as austenite formers, principally nickel and manganese, should be kept as low as practicable.
In my new alloys the chro ium content is relatively low and may vary generally om 14% to 23% with a preferred range of 16% to 2 so that the cost of the alloy is not unduly increased by the requirement of a high chromium content. It is essential in my invention that the carbon be kept low and I indicate 0.4% as a substantial maximum.
The aluminum, where employed, can vary from .02% to 1%, with about .15% preferred. These figures refer to the total aluminum content of the alloy, including that present as aluminum nitride and aluminumv oxide. Zirconium can replace some or all of the aluminum, in amounts ranging from substantially .02% to substantially 1%.
The alloy can be formed from metal with any normal nitrogen content, chromium-bearing steels ranging usually from .0l0% to .060% of soluble nitrogen. Roughly, the quantity of aluminum should vary directly with the quan tity of nitrogen within the ranges for both set forth above. It may be pointed out that while there should be present enough aluminum to render a substantial portion of the nitrogen insoluble if over about .030% nitrogen is present, an excess of aluminum, within the range set forth over that quantity required for the nitrogen, does no harm.
of .15% to 2%. It has some advantage in the physical characteristics of the metal.
As already indicated, other austenite-forming elements, such as manganese and nickel, should be kept as low as is practicable, and I indicate substantially 30% manganese and .35 nickel as preferred maxim'a for these elements.
Greater quantities of either or both elements may be present, but if so, more chromium will be required to prevent the formation of austenite. It will be understood that with amounts of nickel or manganese approaching, say, 1%, so much chromium will be required that much of the economic advantage of my alloy will be lost.
I attain the objects of my invention essentially by avoidance of the gamma loop through a proper balance between austenite and ferrite formers in the alloy. My alloys are inexpensive because they do not contain expensive alloying ingredients other than chromium and at the same time require comparatively low chromium values which are not difiicult to attain in the manufacture of chromium steels in the electric furnace. By reason of their tolerance for austenite formers, my alloys may be made from inexpensive scrap materials, as is conventional in processes of producing chromium-bearing steels, and they may also be made by those processes in which chromium-bearing steels are made in the electric furnace from chromium ores as raw material. It is, of course, clear in the light of the teachings above that my alloys could be produced by melting together the desired pure ingredients; but this is expensive, whereas a very great field of utility of my invention lies in inexpensive manufacture from inexpensive raw materials.
Further, there are no expensive steps involved in the production of my alloys. When they are made from scrap, the desired compositional ranges may be attained by the selection of scrap of known compositions, as well as by the addition of alloying ingredients in ways known in the art. The unusually low carbon contents of my materials may be attained by known procedures for lowering carbon during the melting and refining of a heat in the electric arc furnace, as taught in Patent No. 2,455,073 issued November 30, 1948, in the name of Loveless. Again, a low-carbon melt of the alloy may be made by producing a low-carbon base and alloying it with lowcarbon ferro-chrome, as in Patent No. 1,665,146, issued April 3, 1928, in the name of Reinartz. Also it is possible to decarburize a heat in the electric arc furnace by blowing with oxygen prior tov the adjustment of the chromium content.
Considering the proper balance of austenite and ferrite formers, it may be pointed out that within the compositional ranges given, where the austenite formers tend to be high, it is advisable also to keep the ferrite formers near the high end of their ranges. The reverse is also true, so that if the alloys are low in austenite formers, the ferrite formers need not be so high.
As an example of a particular alloy which has been found very successful in attaining the objects of this invention, I give the following:
Per cent Chromium 17 Carbon Aluminum .15 Manganese -25 Nickel .30 Silicon 30 Balance substantially all iron with the impurities normal to straight chromium steels.
I have further found that the lower the carbon in my product, the less will be the deleterious efiect of a given quantity of nitrogen since both are austenite formers. Thus in an alloy which is low both in carbon and in nitrogen, the quantity of aluminum may be diminished or aluminum may be omitted entirely. In the absence of aluminum. the chromium content may be varied to compensate for the effect of carbon and nitrogen provided both the carbon and nitrogen are low. For example, successful alloys may be made in following my invention without aluminum providing the carbon content is .03% or lower, providing the nitrogen content is .03% or lower, by somewhat increasing the chromium content if that should be found necessary. With .03%
carbon and .03% nitrogen, successful aluminum-free alloys can be made with a chromium content of 18% or slightly higher. A mtrogen content as low as .03% is frequently attained in chromium-bearing steels made by the chromium ore process as exemplified by Patent 1,154,400, issued April 10, 1934, to Arness.
It will be understood by those skilled in the art that as the chromium content of an alloy increases above, say, 17% or 18%, the expense of producing it increases very substantially. As a consequence, from the standpoint of cost the lower chromium contents are of primary interest. It is thus less expensive to produce an alloy containing, say, 17% of chromium, which alloy must also contain aluminum, than it is to produce an alloy containing, say, 18% or 19% of chromium but free of alurmnum. However, where the chromium-bearing steel can be produced with a low nitrogen content, it is within the scope of my invention to carry the carbon to a low value and omit the addition of aluminum.
In describing my new alloy as ferritic," it is to be understood that it is substantially free from the austenitic phase and approaches pure ferrite. In its preferred form, no austemte can be detected metallographically. It is essential that the alloy be sufficiently free from austenite that no abrupt changes or reversals in direction occur in the dilatometnc cooling curve, although within this limitation, the term ferritic is not inten ed to exclude some detectable austenite;
Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention in certain exemplary embodiments, what I claim as new and desire to secure by Letters Patent is:
1. A television tube comprisin a glass face plate fused to a hollow metal body, said y being of an alloy consisting substantially of the following ingredients in the following proportions:
the balance being substantially all iron with impurities characteristic of straight chromium stainless steels.
2. A composite article comprising a metal body which is a fern'tic ferrous alloy consisting of chromium from 14% to 23%, carbon not greater than substantially .04%, the said alloy containing not more than substantially .03% soluble nitrogen, and containing also from .02% to 1% of a metal chosen from a class consisting of aluminum and zirconium, the balance of said alloy being iron taken with the normal impurities characteristic of straight chromium stainless steels, and a glass body, said glass body being fused to said metal body.
References Cited in the file of this patent UNITED STATES PATENTS 1,627,780 Jonas May 10, 1927 1,850,953 Armstrong Mar. 22, 1932 1,947,417 Holst Feb. 13, 1934 2,402,031 Ffleld June 11, 1946 FOREIGN PATENTS 337,767 Great Britain Nov. 5, 1930 483,679 Great Britain Apr. 23, 1938 888.948 France Dec. 27, 1943 OTHER REFERENCES Transactions, American Institute of Mining and Metallurgical Engineers, vol. 113, pages 128, 129, 138 and 139, pub. in 1934 by the A. I. M. M. E., N. Y.
Transactions, American Society for Metals, vol. 23, pages 14, 21, 22, 23, 42 and 43, pub. in 1935 by the Amer. Soc. for Metals, Cleveland, Ohio.
Nitrogen in Chromium Alloy Steels, page 10, copyright 1941, by the Electro Metallurgical Co., New York.
Academic Des Sciences, Comptes Renclus, vol. 226, June 1948, pages 2150 and 2151.
Metal Progress, July 1, 1947, page 94.