US3433631A - High temperature alloy - Google Patents

Description

United States Patent Oflice 3,433,631 HIGH TEMPERATURE ALLOY Robert G. Frank and John W. Semmel, Jr., Cincinnati,

Ohio, assignors to General Electric Company, a corporation of New York No Drawing. Filed Apr. 12, 1967, Ser. No. 630,223 US. Cl. 75-171 6 Claims Int. Cl. 'C22c 19/04, 19/00 ABSTRACT OF THE DISCLOSURE Cobalt-based alloys, which have good high temperature strength and oxidation-resistance and which are not subject to embrittlement upon high temperature aging comprise, in addition to cobalt, 15-20% chromium, 8-12% nickel, 3-7% molybdenum, less than 7% tungsten, less than 0.07% carbon, and small amounts of iron, manganese, and silicon, with a trace of sulfur. The preferred composition is 20% chromium, nickel, 6.5% molybdenum, less than 0.07% carbon, and balance cobalt.

BACKGROUND OF THE INVENTION The invention relates to cobalt-based alloys which exhibit high strength at temperatures in excess of 1,200" Fahrenheit and which are relatively ductile at room temperature after prolonged exposure to temperatures in excess of 1000 Fahrenheit.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

Electrical power generators, aerospace propulsion devices, and similar applications, because of repeated operation at high temperatures over long periods of time, often require as structural materials alloys which are initially strong, weldable, and formable into a variety of mill products, such as tubing, sheets, etc. These alloys also must be oxidation-resistant at high temperature and must be structurally stable, i.e. able to retain their mechanical properties after high temperature exposure. Generally, cobalt-based alloys including 18-25 percent chromium, 2-15 percent tungsten, 10-20 percent nickel, 1-2 percent tantalum and/or columbium, 0.050.3 percent car bon, and small amounts of iron, silicon, and manganese have been used in applications of these types. Heretofore these alloys have bad one major deficiency. This has been their susceptibility to embrittlement or loss of ductility upon prolonged exposure to high temperatures. As structural failure must be avoided, this problem establishes an environmental limitation on the use of prior art alloys. Thus there has been a need for an alloy of the type described above which can withstand prolonged exposure to high temperatures without loss of ductility.

One object of the present invention, therefore, is to provide a wrought metal alloy which retains its strength at high temperature and is not subject to deterioration of its mechanical properties upon prolonged exposure to high temperature.

It is also an object of the present invetnion to provide an alloy which is capable of being formed into various mill products such as tubing and sheet, is capable of being welded, has good high temperature strength, and is not subject to embrittlernent produced by [high temperature aglng.

BRIEF SUMMARY OF THE INVENTION These and other objects are met in accordance with the present invention by the provision of a cobalt based alloy comprising -20 percent chromium, 8-12 percent nickel,

Patented Mar. 18, 1969 at least 3 percent molybdenum, less than 7 percent tungsten, and less than 0.07 percent carbon. Small amounts of iron, manganese, and silicon may also be included. In the preferred form, the alloy comprises about 20 percent chromium, 10 percent nickel, 6.5 percent molybdenum, and less than 0.07 percent carbon.

DETAILED DESCRIPTION OF THE INVENTION The present invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. It may be better understood, however, together with the further objects and advantages thereof, by reference to the following detailed description.

In the course of the present invention it has been found that the high temperature aging of high strength cobaltbased alloys is associated with the precipitation of certain metallic carbides and intermetallic compounds of cobalt and tungsten. These metallic carbides are generally represented as M C and M 0 while the intermetallic compounds whioh appear to be most detrimental are Co W types. Alloys in which part or all of the tungsten is replaced by molybdenum, and the tungsten and carbon contents are limited to less than 7 and less than 0.07 percent, respectively, match closely the characteristics of prior art high strength cobalt-based alloys but are notable for the absence of any tendency to embrittle on heat aging above 1,000 Fahrenheit. Generally, the alloy compositions of the present invention are in the range 15-20 percent chromium, 8-12 percent nickel, 3-7 percent molybdenum, less than 7 percent tungsten, less than 0.07 percent carbon, small amounts of iron, manganese, and silicon and a trace amount of sulfur. Although manganese and silicon may be deliberately included as deoxidizers, iron is generally present only as a residual element from one of the charge materials. Car-bon, up to the maximum of 0.07 percent, may be added as a strengthening agent. The preferred alloy comprises in addition to cobalt, 20 percent chromium, 10 percent nickel, 6.5 percent molybdenum, and less than 0.07 percent carbon. Manganese, silicon, and carbon may also be included in the preferred alloy depending on the need for deoxidation and strengthening. These factors may in turn depend upon melt procedure, intended use of the alloy, etc.

Although the alloys of the present invention need not be made by any particular process, the following are examples of a vacuum process in which several heats of alloy comprising the preferred composition of the present invention were made. (Alternatively, air melting or other vacuum processes may also be employed.)

EXAMPLE 1 Cobalt, chromium, nickel, molybdenum, iron, and manganese were charged to an aluminum oxide crucible in a furnace which was then evacuated to 1 micron mercury pressure. After the charge was melted and heated to 2,750 Fahrenheit, it was held at this temperature until it was refined, i.e., the boiling had subsided. The charge was then cooled to the freeze point and, after a small quantity of aluminum had been added, reheated to 2,750 Fahrenheit. Then argon, at one half atmosphere pressure, was admitted to the furnace and a small quantity of nickel-magnesium was added to the charge to lower the silicon content. Following this, the charge was tapped and poured into ingots. The composition of the alloy produced in this example is listed as Heat No. 1 in the table below.

EXAMPLE 2 An alloy, listed as Heat No. 2 in the table, was produced in a process differing from that in Example 1- only by the inclusion of high carbon chromium in the original charge and the addition of graphite along with aluminum.

3 EXAMPLE 3 An alloy, listed as Heat No. 3 in the table, was produced in a process differing from that of Example 1 only by the inclusion of silicon in the original charge, the addition of calcium-silicon along with aluminum upon the intermediate cooling of the charge and the omission of the nickel magnesium addition just before pouring.

EXAMPLE 4 An alloy, listed as Heat No. 4 in the table, was produced as in Example 3 except that high carbon chromium llgeat Weight percent Cr N1 M0 w 0 Fe Mn si s on 1 20.20 10.12 6.72 0.1 0.01 1.85 0.10 0. 01 0. 003 13:11. 2 20.20 10. 00 0. 98 01 0. 07 1.35 0.16 0.02 0.008 Hal. 3 20. 27 10.00 6.92 01 0. 01 1.90 0.09 0.28 0.004 Bal. 4 20.29 10. 24. 6.66 01 0. 04 1.88 0. 05 0.12 0.011 Bal.

was included in the original charge, graphite was added What we claim as new and desire to secure by Letters along with calcium-silicon and aluminum upon intermedi- Patent of the United States is: ate cooling of the charge. 1. A high temperature, high strength alloy, free of the Ingots from each of the above heats were annealed tendency to become embrittled upon prolonged exposure and rolled into sheet form by conventional methods. Porto high temperatures consisting essentially of 15-20 pertions of the annealed sheets were then aged at 1,600 cent chromium, 8-12 percent nickel, 3-7 percent molyb- Fahrenheit for 100 hours; other portions of these andenum, less than 0.07 percent carbon, and balance cobalt. nealed sheets were aged at 1,600 Fahrenheit for 1,000 2. An alloy, such as that recited in claim 1, further inhours. eluding up to 0.2 percent manganese.

After the heat treatment and aging the various samples 3. An alloy, such as that recited in claim 1, further were bent around a very small radius and angles of 180 including up to 0.3 percent silicon. were produced in each sample without failure. These re- 4. An alloy, such as that recited in claim 1, comprising sults were in contrast to the embrittlement and failure 20 percent chromium, 10 percent nickel, 6.5 percent observable in similar tests using prior art high tempermolybdenum, and less than 0.07 percent carbon. ature alloys. The embrittlement or lack thereof as indi- 5. An alloy, such as that recited in claim 4, further incated by the bend tests of alloys, the compositions of eluding less than 0.2 percent manganese, and less than 0.3 which are listed in the table, was corroborated by photopercent silicon. niicrographs and X-ray diffraction results. In addition, 6. An alloy, such as that recited in claim 1, further inthese alloys were characterized by hot strength, weldeluding up to 7 percent tungsten. ability, oxidation-resistance, and formability characteristics comparable with the prior art high temperature References Cited x f ih c l f th bl't f th 11 f th UNITED STATES PATENTS sur erevlenceo ea11yo eaoyso e present invention to retain their ductility after high tem- 2746860 5/1956 Bmder et 75 171 perature aging, tensile tests were made on several alloys 2996379 8/1961 Faulkner *171 of the compositions represented in the table, with respect 3177O74 4/1965 Luce et a] to ductility, percent elongation of these sam les is thou ht to be partially pertinent. It was found that a iter hoi irs RICHARD DEAN y Exanmcrof aging a l,600 Fahrenheit, the alloys of the present 50 US. Cl. X.R.

invention exhibited a percent elongation generally above 30 percent. The superior ductility retention of the alloys of the present invention became more clearly evident after aging for 1,000 hours at 1,600 Fahrenheit when it was