US3174852A - High temperature chromium-tungstenmolybdenum alloy - Google Patents

Description

United States Patent Delaware No Drawing. Filed Apr. 14, 1959, Ser. No. 806,384 4 Claims. (Cl. 75-176) This invention relates to a chromium-tungsten-molybdenum alloy having outstanding oxidation resistance at elevated temperatures. It pertains particularly to a refractory metal alloy of this type which is designed for buckets and guide vanes of gas turbine engines in which metal temperatures reach 2000 F.

The nickel base alloy and cobalt base alloy blades commonly used today in gas turbine engines for aircraft normally have maximum service temperatures of approximately 1800 F. to 1900 F. This limitation necessarily restricts the performance and efficiency of these engines. Refractory metals, such as niobium, tungsten, molybdenum and chromium, have satisfactory high melting temperatures and sufficient potential availability to warrant investigation as high temperature turbine blade materials. However, each of these metals exhibits poor oxidation resistance at temperatures of 2000 F. or above. Therefore, such metals are unsatisfactory for use in turbine blades which necessarily are exposed to extremely hot oxidizing gases. During recent years attempts have been made to correct this deficiency by adding small amounts of Various alloying elements to these refractory base metals. However, these attempts have been unsuccessful since the resultant products still did not possess adequate oxidation resistance at the very high temperatures under consideration.

Accordingly, a principal object of the present invention is to provide a refractory alloy which can be employed as a turbine blade material at temperatures up to 2000 F. because of its outstanding oxidation resistance at such temperatures, coupled with good hot strength and other necessary physical properties. It is considered desirable that an alloy to be used for gas turbine service in air at 2000 F. have a 100-hour stress-rupture life at that temperature with at least a 15,000 psi. load. Such an alloy also should possess adequate fabricability and a melting point of at least 3000 F. It is preferable that turbine blades formed of this alloy have an oxide scale thickness of not more than 0.005 inch after 100 hours exposure in air at a temperature of 2000 F.

In accordance with the present invention, we have found that a refractory alloy comprising about 37% to 70% chromium, to 50% tungsten and 10% to 30% molybdenum satisfies the foregoing requirements to an outstanding degree. A chromium content of 45% to 65% appears to provide most advantageous results, particularly with respect to the combination of oxidation resistance and hot strength. The preferred ranges for the tungsten and molybdenum contents are to 30% and 15% to 25%, respectively. An alloy composed of 60% chromium, tungsten and 20% molybdenum, for example, has an oxide scale of less than 0.001 inch in thickness after 100 hours cyclic exposure in air at a temperature of 2000 F.

Small but effective amounts of readily oxidizable me- -F. at 16 hours exposure at temperature.

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tals, such as calcium, arsenic, misch metal, thorium, bismuth, silicon, zirconium, tantalum, aluminum and boron, may be added to the ternary alloy to further improve its extreme high temperature properties. In general, we have found these addition agents beneficial with respect to optimum physical properties in the following approximate maximum amounts: 0.3% calcium, 0.08% arsenic, 0.15% misch metal, 0.2% thorium, 0.3% silicon, 0.05% zirconium, 0.1% tantalum, 0.5% aluminum, 0.05% bismuth, and 0.5% boron. Small quantities of various other elments, usually less than 1%, can be tolerated in the chromium-tungsten-molybdenum alloy without affecting its physical properties.

Photomicrographs at 500 magnifications of our new chromium-tungsten-molybdenum alloy show a small amount of needle-like phase after exposure in air at a temperature of 2400 F. This needle-like phase has been tentatively identified as a nitrogen-rich phase which is not encountered if the metal is exposed to the same temperature in a purified argon atmosphere. Alloys containing 60% chromium, with calcium and misch metal added, appear less susceptible to this form of contamination than the lower chromium materials. In fact, alloys of this type did not show any metallographic evidence of contamination after 100 hours at a temperature of 2000 F., although a small amount of this needle-like phase was found when heated in air for 16 hours at a temperature of 225 0 F. It was further found that pre-oxidation of the chromium-tungsten-molybdenum alloy containing 0.02% calcium and 0.15 misch metal in pure oxygen for 30 minutes at 1975 F. increased the minimum temperature at which this phase was detected from 2250 F. to 2400 Evidently the oxidizing nitrogen-free atmosphere prevented initial N contamination and thus inhibited diffusion of nitrogen through the oxide layer.

When non-metallic inclusions are present in the grain boundaries of the alloy of this invention, de-oxidizing additions may be beneficially made. These integranular impurities appear to be reduced to the greatest extent by the addition of 0.05 titanium and 0.1% to 0.3% calcium.

Our tests have shown that excellent results are produced when 0.001% to 0.02% calcium and 0.03% to 0.15% misch metal are added in combination to a 60% chromium-20% tungsten-20% molybdenum alloy. The average thickness of the oxide scale on these alloys containing addition agents was not greater than approximately 0.001 inch after 100 hours exposure in air at a temperature of 2000 -F. Similar results were also obtained when the above-described small additions were made to various alloys comprising 37% to chromium, 10% to 50% tungsten and 10% to 30% molybdenum.

The following table lists the approximate chemical compositions of specific examples of the chromium-tungstenmolybdenum alloy of this invention and shows the thick ness of the total oxide scale formed by heating in air for hours at 2000 F. In each instance the balance of the base alloy composition is substantially all chromium.

Each of the alloys listed in the table has a melting point materially in excess of the 3000 F. desired minimum heretofore mentioned. These alloys were prepared by non-consumable arc-melting in an inert atmosphere of argon plus helium. Raw materials of maximum available purity were used. The constituents of the alloys may be added either simultaneously or successively.

Base Alloy in Percent Elements Added (Percent) Total (Balance Oxide Chromium Thickness in Inches Mo W Ca As MM Th Bi Si Zr Ta Al B o 1 Miseh metal.

Sound extrus1ons may be obtained w1th the chrom1umcons1st1ng essent1ally of about to 65% chromlum,

tungsten-molybdenum alloy of this invention by first hot pressing it'to approximately 10% reduction in thickness at a temperature of 3 15 0 F. Initial hot working produces a wrought recrystallized structure Which enables the alloy to be further hot Worked, such as by extrusion. Previously, it had been widely believed that a high molybdenum content alloy containing more than about 25 chromium lacked suitable high temperature oxidation resistance and could not be Worked Without cracking. As indicated above, however, the alloy described herein has excellent oxidation resistance and can be suitably hot Worked.

Stress-rupture tests were conductod on as-cast chromium-tungsten-molybdenum alloy test bars in an argon atmosphere at a temperature of 2000 F. under progressively increasing loads from 15,000 p.s.i. to 35,000 p.s.i. For example, an alloy consisting of about 60% chromium, 20% tungsten and 20% molybdenum with 0.02% calcium plus 0.15% misch metal added had a hot strength estimated in excess of 25,000 p.s.i. for 100 hours life at 2000 F. The results of this specific test were as follows.

Stress-rupture test at 2000 F. in argon Hours previous to Stress, p.s.i.: next higher stress 15,000 154.6

1 Failed.

While our invention has been described by means of certain specific examples, it is to be understood that the scope of our invention is not to be limited thereby except as defined in the following claims.

We claim:

1. An alloy having an average surface oxide scale thickness not in excess of about 0.001 inch after 100 hours exposure to air at a temperature of 2000 F. and having a stress-rupture life of more than 100 hours under a 15,000 p.s.i. load at a temperature of 2000 F., said alloy 15% to 30% tungsten, 15% to 25% molybdenum, and a small amount eifective to materially increase the hightemperature oxidation resistance of said alloy of at least one readily oxidizable metal selected from the group consisting of calcium, arsenic, misch metal, thorium, bismuth, silicon, zirconium, tantalum, aluminum and boron.

2. A gas turbine blade having an average surface oxide scale thickness not in excess of about 0.001 inch after hours exposure to air at a temperature of 2000 F. and having a stress-rupture life of more than 100 hours under a 15,000 p.s.i. load at a temperature of 2000 F., said blade being formed of an alloy consisting essentially of about 45% to 65% chromium, 15% to 30% tungsten, 15 to 25 molybdenum, and a small amount effective to materially increase the high-temperature oxidation resistance of said alloy of at least one readily oxidizable metal selected from the group consisting of calcium not in excess of about 0.3%, arsenic not in excess of about 0.08%, misch metal not in excess of about 0.15%, thorium not in excess of about 0.2%, bismuth not in excess of about 0.05%, silicon not in excess of about 0.3%, zirconium not in excess of about 0.05%, tantalum not in excess of about 0.1%, aluminum not in excess of about 0.5% and boron not in excess of about 0.5%.

3. A highly oxidation-resistant alloy having an average surface oxide scale thickness not in excess of about 0.001 inch after 100 hours exposure to air at a temperature of 2000 F. and having a stress-rupture life of more than 100 hours under a 15,000 p.s.i. loadat a temperature of 2000 F., said alloy consisting essentially of 45 to 65% chromium, 15 to 30% tungsten, 15 to 25 molybdenum, calcium not in excess of 0.3% and misch metal not in excess of about 0.15%.

4. An alloy having an average surface oxide scale thickness not in excess of about 0.001 inch after 100 hours exposure to air at a temperature of 2000 F. and having a stress-rupture life of more than 100 hours under a 15,000 p.s.i. load at a temperature of 2000 F., said alloy consisting essentially of about 15% to 30% tungsten, 15% to 25% molybdenum and the balance substantially all chromium.

(References on following page} References Cited by the Examiner UNITED STATES PATENTS Honda 75-176 Kurtz 75-176 Wainer 75176 5 Wlodek et a1. 75-176 Buck 75176 NAHAM MARMELSTEIN, RAY K. WINDHAM,

Examiners.

Claims (1)

1. AN ALLOY HAVING AN AVERAGE SURFACE OSICE SCALE THICKNESS NOT IN EXCESS OF ABOUT 0.001 INCH AFTER 100 HOUSR EXPOSURE TO AIR AT A TEMPERATURE OF 2000*F. AND HAVING A STRESS-RUPTURE LIFE OF MORE THAN 100 HOURS UNDER A 15,000 P.S.I. LOAD AT A TEMPERATURE OF 2000*F., SAID ALLOY CONSISTING ESSENTAILLY OF ABOUT 45% TO 65% CHROMIUM, 15% TO 30% TUNGSTEN, 15% TO 25% MOLYBDENUM, AND A SMALL AMOUNT EFFECTIVE TO MATERAILLY INCREASE THE HIGHTEMPERATURE OXIDATION RESISTANCE OF SAID ALLOY OF AT LEAST ONE READILY OXIDIZABLE METAL SELECTED FROM THE GROUP CONSISTING OF CALCIUM, ARSENIC, MISCH METAL, THORIUM, BISMUTH, SILICON, ZIRCONIUM, TANTALUM, ALUMINUM AND BORON.