US 3366478 A
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Jan. 30, 1968 H. 1.. WHEATON COBALT-BASE SHEET ALLOY Filed July 21, 1965 5 Sheets-Sheet 1 2 s 8 o c g ooco o 2 g g rn m QQQMQD 1.0 m. N
INVENTOR. HOLD LWHEATON K5 BY at t 2' ATTORNEY Jan. 30, 1968 H. L. WHEATON 3,366,478
COBALT-BASE SHEET ALLOY Filed July 21, 1965 I 5 Sheets-Sheet 3 05 O: I LIIJ Z o O o o 2 z lg -N|v91 'gg s I INVENTOR.
HAROLD L. WHEATON BY WW ATTORNEY United States Patent 3,366,478 COBALT-BASE SHEET ALLOY Harold L. Wheaten, Rolling Meadows, Ill., assignor to Martin-Marietta Corporation, New York, N. a corporation of Maryland Filed July 21, 1965, Ser. No. 473,662 9 Claims. (Ci. 75171) The present invention relates to a forgeable alloy useful in the construction of structures having excellent resistance to the combined effects of stress and oxidation at elevated temperatures and, more particularly, to a cobalt-base alloy as so used.
In the construction of gas turbine engines, especially those designed for use in aircraft, it is often necessary to fabricate components such as burner cans, after-burner liners, etc. from sheet metal. When installed in the completed gas turbine and placed in service, such components are subject to extremes of temperature and oxidizing conditions while at the same time being subject to moderate or excessive amounts of stress. Normally, in order to fabricate the desired components, it is first necessary to be able to produce metal sheet. Thereafter the metal sheet must be formable and must be weldable so as to facilitate the fabrication thereof. Alloys suitable for such usage must therefore exhibit a unique combination of characteristics. Heretofore two alloys have often been employed for the aforedescribed components. These prior art alloys are identified herein as alloys A and B. Alloy A, a cobaltbase alloy, has poor resistance to oxidation at temperatures in excess of about 1800 F. Alloy B, a nickel-base alloy, is relatively weak over the operative range of about 1500 F, to 2000 F. Thus neither alloy A nor alloy B exhibit the combination of characteristics which are deemed essential by the designers of the more modern gas-turbine engines. It is unfortunately the fact that new aircraft proposals call for gas turbine engines with greater thrust to weight ratios. In turn, it is the general rule that when higher performance engines are required, higher turbine inlet temperatures are involved. Thus alloys which heretofore could form the basis for a compromise between design requirements and practical utility can no longer be relied upon to satisfy the aircraft industrys need for turbine structures formed from sheet. As far as I am aware, the aforementioned problems have not heretofore been solved on a practical basis by the provision of an alloy having the optimum combination of characteristics required for the specified usage.
It has now been discovered that by the use of a specially controlled cobalt-base alloy sheet-metal structures can be provided having an adequate resistance to stress and oxidation at elevated temperatures in the range of about 1500 F. to about 2000" F.
It is an object of the present invention to provide a novel cobalt-base alloy.
Another object of the invention is to provide sheet made of a novel cobalt-base alloy.
An additional object of the present invention is to provide novel gas turbine structures made from novel cobaltbase alloy sheet.
A still further object of the present invention is to provide a novel process for the production of sheetformed gas turbine structures.
Other objects and advantages will become apparent from the following description taken in conjunction with the drawing in which:
FIGURE 1 is a graphical representation of the stress in pounds per square inch (p.s.i.) as related to life-to-rupture of an alloy of the present invention at temperatures from 1400 F. to 2000 F.;
FIGURE 2 is a graphical representation of the relationship between stress for life-to-rupture in hours and temperature for an alloy of the present invention, alloy A and alloy B; and
FIGURE 3 is a graphical representation of the gain in weight in milligrams per square centimeter (mg/cm?) over a period of about 100 hours at 2100 F. for an alloy of the present invention and alloys A and B.
Generally speaking the present invention contemplates a cobalt-base alloy containing, in addition to cobalt (in Weight percent), about 0.03% .to about 0.20% carbon, about 15% to about 30% chromium, about 10% to about 30% nickel, about 2% to about 12% tantalum and about 0.01% to about 0.5% zirconium. Optionally, the alloy can contain (in weight percent) up to about 3% tungsten, up to about 3% molybdenum, up to about 2% columbium (niobium), up to about 10% iron, up to about 0.2% titanium, up to about 0.2% hafnium and up to about 0.2% boron. Elements such as oxygen, nitrogen, hydrogen, tin, lead, arsenic, bismuth and the like are undesirable impurities and should be kept to an absolute minimum commensurate with commercial melting practice. Incidental elements and amounts of same such as up to 1.5% silicon, up to 1.5 manganese, up to 0.5% aluminum, up to 0.2% rare earth metals and the like can be tolerated. While such incidental elements can at times be useful, for example for deoxidation purposes, it is advantageous to maintain them at low levels of concentration.
In producing the alloys of the present invention, it has been found advantageous to melt under vacuum in an induction furnace. Briefly one can melt down the required cobalt, chromium, nickel and carbon under high vacuum and hold same in the molten condition until deoxidation is complete. Subsequent to the completion of the deoxidation, tantalum is added followed by the zirconium. \Vhen alloying is complete, the melt is cast under vacuum into ingots. Alternatively melting can be accomplished under protective covers or atmospheres other than vacuum.
The cast ingots are cropped to remove any portions containing pipe and are forged at temperatures in excess of about 1800 F. and up to about 2200 F. to reduce the section thereof and produce a shape suitable for rolling. The forged bars are finally hot rolled at temperatures equivalent to the forging temperatures to sheet. A typical sheet produced in such fashion, after annealing for 4 hours at 2175 F., had a thickness of 0.055 inch and an average grain diameter of about 0.090 millimeter.
In carrying the invention into practice it has been found advantageous to maintain the composition of the alloy such that the zirconium content is substantially complete- 1y chemically associated with the carbon content and with any residual oxygen and nitrogen which might be present in the alloy. For all around good characteristics one should maintain chromium within the range of about 18% to 22% (by weight) and nickel in the range of about 15% to 25% (by weight). Advantageously, tantalum is maintained at about to by weight, e.g. 7.5%. When the weight percent of carbon is about 0.03% to 0.10% the weight percent of zirconium is about 0.05% to 0.20%, the higher amounts of zirconium being present only when the higher amounts of carbon are present. Cobalt is essentially the balance of the alloy and is present in an amount from about 35% to about 60% (by weight) together with impurities and incidental elements associated with the alloy ingredients.
The alloy of the present invention is essentially a solidsolution hardened alloy. The elements chromium and tantalum contribute appreciably not only to oxidation resistance of the alloy but also to solid solution strengthening and should together total at least about 20% by weight. It is essential that the amount of tantalum not exceed about 12% by weight. Amounts of tantalum in excess of 12% not only reduce fabricability but also change the essential metallurgical character of the alloy to one which is subject to the formation of intermetallic compounds at temperatures in the ranges of 1300 F. to about 1600 F. Chromium contents in excess of 30% by weight embrittle the alloy while amounts of chromium less than 15 by weight tend to reduce the resistance of the alloy to oxidation. The sum total of the weight percentages of chromium plus tantalum plus tungsten plus molybdenum should not exceed about 35%. The nickel content of the alloy tends to improve its workability and fabricability and further tends to stabilize the high temperature facecentered-cubic form of cobalt. The small amount of zirconium in the alloy appears to be effective in increasing to a marked extent the forgeability of the alloy.
The advantageous results achieved by means of the present invention are depicted in the drawing. The data set forth in the drawing have been derived by testing samples of an alloy containing in weight percent about 0.05% carbon, about chromium, about 20% nickel, about 7.5% tantalum, about 0.10% zirconium with the balance being cobalt. This alloy composition was melted under vacuum, cast, forged and rolled as detailed hereinbefore. After annealing for four hours at 2175 F. stressrupture specimens fabricated, in accordance with American Society for Testing Materials Procedure No. E8-61T, exhibited the characteristics as depicted in FIGURE 1 of the drawing. Referring now thereto, it is to be observed that alloys of the invention exhibit highly useful lives-torupture under stress at temperatures from about 1400 F. to about 2000 F. This fact has importance in that a sheet-formed turbine structure does not operate at a single temperature but rather is subjected to a wide range of temperatures (and consequent thermal stress) depending upon the specific ambient conditions in the engine during its operating cycle. Thus a throttle-back from full power occurring during a flight can change the temperature of ducting, burner cans and after-burners by the order of 500 degrees in Fahrenheit units.
The following tabulation (Table I) shows that alloys of the invention exhibit an advantageous range of ductility as measured by percentage elongation when stressed for rupture in 100 hours at the indicated temperatures.
The nominal compositions in percent by weight of prior art alloys A and B mentioned hereinbefore are set forth in Table II.
FIGURE 2 of the drawing shows that the alloys of the invention exhibit substantially equal if not better stressrupture strength than alloy A. Over the temperature range of about 1400 F. to 1900 F. the stress for hours rupture of alloys of the present invention is substantially twice the stress for 100 hours rupture of alloy B. FIG- URE 3 of the drawing shows that at 2100 F., alloy A lacks any substantial utility in view of its rapid rate of oxidation.
Alloys of the present invention are weldable by heliarc welding. This factor, as pointed out hereinbefore, is important to the practical fabricability of structures made of sheet metal (sheet metal structures). Likewise these alloys exhibit advantageous room temperature tensile characteristics, e.g. an ultimate tensile strength (U.T.S.) of 130,000 p.s.i., a 0.2% offset yield (0.2% Y.S.) of 56,000 p.s.i. and an elongation (EL) of 48%. These tensile characteristics coupled with excellent bend ductility make alloys of the present invention readily formable into desired shapes by conventional tooling.
The alloys of the present invention can be fashioned into sheet and further formed and fabricated into turbine structures mentioned hereinbefore. In view of its novel and useful combination of engineering characteristics alloys of the present invention can also be used in the manufacture of furnace components such as muflles, troughs, plates, tools and the like for handling molten glass, thermocouple protection tubes, etc. Shapes other than sheet can also be fashioned. For example, the alloy of the present invention can also be made into strip, bar, plate, foil, tubing and wire.
While the present invention has been described in conjunction with advantageous embodiments, it is to be understood that modifications and variations may be resorted to wtihout departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations may be considered to be within the purview and scope of the invention and appended claims.
1. A workable alloy having excellent resistance to the combined efiects of stress and oxidation comprising in weight percent about 0.03% to about 0.20% carbon, about 15 to about 30% chromium, about 10% to about 30% nickel, about 2% to about 12% tantalum, about 0.01%
to about 0.5% zirconium, up to about 3% tungsten, up
to about 3% molybdenum, up to about 2% columbium, up to about 10% iron, up to about 0.2% titanium, up to about 0.2% hafnium, up to about 0.2% boron, up to about 1.5% silicon, up to about 1.5 manganese, up to about 0.5% aluminum, up to about 0.2% rare earth metals, with the balance of the alloy being essentially cobalt.
2. An alloy as in claim 1 wherein the carbon content is about 0.03% to 0.1% and the zirconium is about 0.05 to about 0.20%.
3. An alloy as in claim 1 wherein the perecntage of chromium plus the percentage of tantalum is at least about 20% and the percentage of chromium plus the percentage of tantalum plus the percentage of tungsten plus the percentage of molybdenum is not greater than about 35%.
4. An alloy as in claim 1 wherein the chromium content is about 18% to about 22%.
5. An alloy as in claim 1 wherein the nickel content is about 15% to about 25%.
6. An alloy as in claim 1 wherein the tantalum content is about to about 7. A Wrought alloy having excellent resistance to the combined effects of stress and oxidation containing in weight percent about 18% to about 22% chromium, about to about 25% nickel, about 5% to about 10% tantalum, about 0.03% to about 0.20% carbon, about 0.01% to about 0.5% zirconium With the balance being essentially cobalt together with impurities and incidental elements normally associated therewith.
References Cited UNITED STATES PATENTS 2,763,547 9/1956 Dyrkacz et al -171 2,974,036 3/1961 Thielemann 75-171 3,085,005 4/1963 Michael et al. 75171 3,314,784 4/1967 McQuillan et al. 7517l DAVID L. RECK, Primary Examiner. RICHARD O. DEAN, Assistant Examiner.