US2172023A - Heat-resistant alloy - Google Patents

Description

Patented Sept. 5, 1939 I UNITED-STATES PATENT OFFICE JohnD. Gat, Pittsburgh, Pa.

No Drawing. Application August 30, 1937,

Serial No. 161,680 1 2 Claims. (Cl. 75-124) This invention relates to heat resistant alloys ence of an element or elements whichv removes and, more particularly, to a heat resistant alloy highly soluble carbides from between the grain containing aluminum. boundaries by forming insoluble carbides which One object of this invention is to provide an are so finely and uniformly distributed through- 5 improved heat resistant alloy which is substanout the iron-aluminum mass that they have no '5 tially free from carbides at its grain boundaries. deleterious effect on ductility or corrosion re- Another object of this invention is to provide sistance of the metal. Such carbides have alow a heat resistant alloy having hot-working charsolubility at elevated temperatures and prevent acteristics comparable to the more expensive any substantial amount of carbides at grain alloys of the stainless type. boundaries, even after repeated heating and cool- 10 A further object is' to provide an inexpensive ing, which is common in the use of such heat alloy having improvedelectrical resistance propresistant articles. erties at elevated temperatures. The elements to accomplish this purpose in- The value of aluminum additions to steel to elude primarily titanium; although molybdenum,

prevent oxidation at elevated temperatures has tantalum, columbium or metals of similar char 15 long been recognized and discussed, but alumiacteristics may be utilized. Such an element mlm ad s cause excessive grain growth as titanium, for example, is added to the metal Which Prevents its being satisfactorily hot in quantities defined by the carbon content so cold-worked. This is attributed to the fact that, a t produce unifarnfly di t i t fine up n co lin h r n pr en i i nbides of low solubility throughout the mass. 20

minllm y segregates at the grain boundaries Under this invention, the carbides are substand forms a brittle, hard fi f complex artially eliminated from the grain boundaries of bides which surround the iron-aluminum grains. th a11 y,

This fil Causes brittleness and quick failure, I have found that the amount of titanium, for

due to rapid o ationof suc carbides example, should be about six (6) times the 25 creasing'amounts of carbon and aluminum natunt of carbon present. Thus, if the carbon rally tend to aggravate brittleness and large is approximately 10 per t th titanium grain structureaddition should be about six (6) times that Thus, carbon, even to the extent of only a few amount or approximately 050 per cent Thus,

I t unt o auminum, ianium an car on its being practically hot or cold reduced, as is 32 2 1 2 3 lowscarbon alloys so often desired. With the carbon content un- (A) High ductility at hot working tempera M giilarnugregnllugsgyt 1 3 3; i e gegg z gi tures and relatively low physical propeFrtiles at 3 the ductility becomes less while the ability to g gxi g g (1600 degres ren roll the metal becomes almost impossible. The total elimination of carbon might eliminate some (B) Hlgh duftmty a wa zi of the above difficulties but such elimination tures but relatwely hlgh yslca Ies w places it outside the capabilities of present industhe same elevated te.mpera' ures as 0 o trial furnaces. It is practically impossible to Table I reduce, in present furnaces, the carbon content to within limits which would make it-permissible A B to make iron-aluminum alloys free from carbide precipitations at the grain boundaries. Aluminum 2, 2% 8,

The present invention overcomes the above Titanium ess anl ver disadvantages in the manufacture of heat resist- $333 016% "'l"" Over ant alloys and, produces an iron-aluminum alloy gi ighi i g l residuals not exceeding 2% inherently more stable at elevated temperatures g;, than the usual iron-aluminum alloys. Such an Iron Remamder- 5 improved alloy is also ductile, machinable and Weldable, due to the fine distribution and elimination of the deleterious carbides from the grain boundaries.

This is primarily accomplished by the pres- While I have shown the amounts of titanium that should preferably be added in order to distribute the carbides uniformly so that the grain boundaries of the Fe-Al solid solution are substantially free therefrom, it is to be understood that I may use other elements for the same purpose. Thus, columbium may be used in amounts suiiicient to distribute the carbides which, as some tests indicate, will approximate about eight (8) to nine (9) times the amount of carbon.

In addition to the above mentioned advantages of such an alloy, wide distribution of these insoluble carbides throughout and within the grains leads to more nuclei of crystallization which promotes an inherent fine grain structure, and increases stiffness proportional to the amount of carbides present which exhibits itself primarily as strength at elevated temperatures, where strength is needed. Stiffness produced by fine distribution of carbides, does not affect either ductility or workability of the metal either in its hot or cold state.

Such an alloy can be worked into thin sections providing inexpensive heating elements having improved electrical properties over the more expensive and more diflicult to work chromimum and chromium-nickel alloys commonly used for this purpose. This improvement manifests itself in a material increase of ohmic resistance expressed in microhms per cubic centimeter. Thus, for example, an alloy of my invention containing about 8 per cent. aluminum and 0.30 per cent. titanium has a resistance of 160 microhms per cubic centimeter, as compared to 132 microhms per cubic centimeter at the same temperature for so-called Nichrome metal utilized especially for electric heating units.

While I have described one embodiment of the present invention, it is to be expressly understood that various modifications may be made in' forming the heat resistant alloy in accordance with the scope of the appended claims.

I claim:

1. A ductile, heat-resistant iron-aluminum alloy including a carbon content not greater than 0.16 per cent., not less than 6 per cent. aluminum and suflicient titanium to prevent any substantial presence of carbides at the grain boundaries of the alloy, said titanium being approximately six 6) times the amount of carbon and at no time exceeding 1 per cent.; manganese, sulphur, phosphorus -and silicon in combined amounts not to exceed 2 per cent., and the remainder being iron.

2. A ductile, heat-resistant alloy including a carbon content not greater than 0.16 per cent., not less than 6 per cent. of aluminum and with a sufficient member of the group consisting of titanium, molybdenum, tantalum and columbium to prevent any substantial presence of carbides at the grain boundaries of the alloy, said member being not greater than six (6) times the amount of carbon and at no time exceeding 1 per cent., manganese, sulphur, phosphorus and silicon in combined amounts not to exceed 2 per cent., and

the remainder being iron.

JOHN D. GAT.