US3343950A - Nickel-chromium alloys useful in the production of wrought articles for high temperature application - Google Patents

Description

United States The present invention relates to age-hardenable alloys particuarly suitable for use at elevated temperatures and, more particularly, to age-hardenable, nickel-base alloys having an advantageous combination of mechanical characteristics at elevated temperatures.

Heretofore, the art has endeavored to provide alloys suitable for high temperature applications and particularly alloys to be used as materials for rotor discs in gas turbines. The special requirements imposed on alloys for this purpose have been discussed in US. Patent No. 3,151,981 of R. A. Smith and I. Heslop. The alloys described and claimed therein contain from 0.02 to 0.09% carbon, from 14 to 22% chromium, from 10 to 20% cobalt, from 3 to 10% molybdenum, from 2 to 3.5% titanium, from to 0.8% aluminum, the sum of the titanium and aluminum contents being greater than 2.5%, from 2 to 5.25% columbium, from 0 to 25% iron, from 0.001 to 0.01% boron, and from 0.01 to 0.1% zirconium, the balance, apart from impurities and residual deoxidants, being nickel. Molybdenum may be replaced by an equal atomic percentage of tungsten up to a maximum tungsten content of by weight, and columbium by an equal Weight of tantalum up to a maximum tantalum content of 3%. In order to avoid the formation of embrittling phases in the above-mentioned alloys, the contents of iron and columbium in these alloys have to be specially correlated within the ranges set forth, the highest columbium contents being associated with the lowest iron contents and vice versa. Increasing the iron content, while making the alloys cheaper, also reduces their tensile strength and proof stress at high temperatures, and for the highest strength the iron content should not exceed 5%.

While the tensile and creep properties of alloys of the type set forth in the aforementioned patent are excellent up to temperatures of about 600 0, these properties fall off at higher temperatures, and the continued development of gas turbine engines has given rise to a need for alloys having greater strength at temperatures above 600 C. Although some improvement in tensile strength results from increasing thetotal content of the hardening elements titanium and aluminum, this has been found to lead to a drastic reduction in tensile ductility, and to make the alloys almost unforgeable. Although many attempts were made to overcome the foregoing difliculties and other disadvantages, none, as far as we are aware, was entirely successful when carried into practice commercially on an industrial scale.

It has now been discovered that age-hardenable, nickelbase alloys, having excellent mechanical properties at temperatures above 600 C., can be produced by carefully controlling and restricting the ranges of specific alloying '7 atent C 3,343,950 Patented Sept. 26, 1967 ice elements contained therein, such as chromium, titanium, aluminum, columbium and vanadium. By way of further explanation, it has been found that if, at the same time as the total content of aluminum and titanium is increased, the ratio of aluminum to titanium is increased and the content of columbium is decreased, the tensile strength of the alloys at high temperatures is improved while adequate ductility is still retained. Titanium can indeed be entirely absent. This discovery is doubly surprising, since in the alloys of our previous specification, in which titanium predominated, on the one hand aluminum contents above 0.8% greatly reduced the tensile ductility, and on the other hand too low a columbium content was associated with a low tensile strength and proof stress. When the composition of the alloys is carefully controlled in this way the chromium content must also be controlled in order to avoid embrittlement of the alloys on prolonged exposure to high temperatures. Furthermore, to ensure that the alloys are not notch-sensitive, that is to say that their stress-rupture life is not reduced by the presence of notches, the total content of the hardener elements titanium, aluminum, columbium and vanadium must be restricted.

It is an object of the present invention to provide an age-hardenable, nickel-base alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C.

Another object of the invention is to provide an agehardenable, nickel-base alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C. and which is particularly resistant to high temperature embrittlement and is notch-insensitive.

The invention also contemplates providing a rotor disc formed from an age-hardenable, nickel-base alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C. and which is particularly resistant to high temperature embrittlement and is notch-insensitive.

Other objects and advantages Will become apparent from the following description.

Generally speaking and in accordance with the present invention, alloys contemplated herein advantageously contain, in percentages by weight, from about 11% to about 16% cobalt, from about 10.1% to about 17% chromium, from about 0.02% to about 0.1% carbon, from about 5% to about 9% molybdenum, from about 2.53% to about 5.2% aluminum, up to about 1.73% titanium, about 1% to about 2.5% columbium, up to about 1.75% vanadium, about 0.001% to about 0.01% boron, about 0.01% to about 0.1% Zirconium, the balance being essentially nickel. The term balance essentially nickel,'as commonly understood by those skilled in the art, does not exclude small amounts of other incidental elements, e.g., deoxidizers and impurities, which are commonly present in nickel alloys and do not adversely affect the novel charateristics of the present invention. In this regard, small amounts of manganese and silicon, e.g., up to about 1% of each, are tolerable in the present alloys. Iron is an undesirable impurity in the present alloys since it increases their tendency to embrittle on prolonged exposure to high temperatures, and its content should therefore not exceed 1%, though greater amounts, up to 5%, may be tolerated J at chromium contents within the lower part of the range. Preferably, the total content of all impurities and residual deoxidants does not exceed 2%.

Within the limits set out above, the amounts of aluminum and titanium must be such that the ratio of aluminum to titanium (Al:T i) is at least 2:1 by weight and the total content of aluminum and titanium (AH-Ti) is from 3.8% to 5.2%. At lower ratios of aluminum to titanium the high temperature tensile strength of the alloys falls off very sharply. A total aluminum-l-titanium content of at least 3.8% is needed to ensure that the alloys have adequate strength at high temperatures, but if this total exceeds 5.2% the tensile ductility of the alloys at high temperature is reduced and hot working becomes very difiicult or even impossible. Columbium and vanadium also contribute to the strength of the alloys, and to obtain adequate high temperature strength these elements must be present in a total amount (Cb-t-V) of at least 2%.

The substitution of vanadium for an equal weight of columbium in the alloys improves their tensile ductility and hot workability, and preferably the alloys contain at least 0.5% vanadium. On the other hand, vanadium impairs the high temperature tensile and creep strengths of the alloys and reduces their resistance to oxidation. For this reason the vanadium content must not exceed 2% and the ratio of vanadium to columbium (VzCb) must not exceed 1.5:1.

Commercially available sources of columbium are usually contaminated by tantalum and small amounts of tantalum will, therefore, usually be present in the alloys of the invention. If desired, tantalum may also be deliberately added to the alloys so that up to one half or even the whole of the columbium content is replaced atom for atom by tantalum.

Small additions of boron and zirconium have markedly beneficial etfects on tensile ductility at temperatures of 600 C. and above, and both of these elements must be present. However, as the content is increased, the melting point of the alloys falls, and if more than 0.1% boron or 0.1% Zirconium is present, serious deterioration in the hot working properties of the alloys results.

In addition to the need to restrict the contents of the main hardening elements in the manner just described, it is also important that the contents of each of the other constituents of the alloy should be within the limits set forth.

At chromium contents less than about of the resistance of the alloys to oxidation, and particularly to attack by the products of combustion of gas turbine fuels, decreases, and preferably the chromium content is at least 12%. If the chromium content exceeds 17% the alloys become brittle on prolonged exposure to high temperatures, and preferably the chromium content does not exceed 16%.

The cobalt content has a very marked elfect on the strength of the alloys. Their high temperature tensile strength is reduced when the cobalt content is decreased below 11% or increased above 16%, and preferably the cobalt content is from 13% to 15 Molybdenum has a beneficial effect on both tensile strength and ductility at high temperatures, and the presence of at least 5% molybdenum is very desirable. On the other hand, excessive additions of molybdenum carry the penalties of increased density, decreased machinability and increased ditficulty in hot working, and the content should, therefore, not exceed 9%.

In the alloys of this invention tungsten is not equivalent to molybdenum and its presence has many disadvantages. It reduces the tensile ductility of the alloys, increases their density and thus the stress in rotating components made from them, and impairs the workability of the alloys. Nevertheless small amounts of tungsten, up to a maximum of 2%, can be tolerated as a replacement for half its weight of molybdenum. Preferably, however, the alloys are entirely free from tungsten.

In addition, the total amount of the hardening elements aluminum, titanium, columbium and vanadium is also important. To ensure adequate high temperature strength this total must be at least 6.3%. Increasing the total hardener content increases the high temperature strength, but at the same time it tends to make alloys notchsensitive, reduces their tensile ductility at high temperatures, increases their tendency to embrittle, and makes them harder to forge. For these reasons the total hardener content must not exceed 8.5% or even 8%, and prefera'bly it is not more than 7.5%

In carrying the invention into practice, the alloys can be air melted, but to ensure the best creep properties and workability they are preferably melted and cast under vacuum. If they are melted in air they are preferably deoxidized by means of calcium or magnesium and refined by holding under vacuum in the molten state for some time before casting. The pressure during this refining treatment should not be more than 0.1 mm. Hg and is preferably lower, e.g. 5 microns or less; the temperature is suitably from 1400 C. to 1600 C.; and the holding time is at least 5 minutes and preferably is at least 10 minutes. The cast ingots may be processed to bar or rotor disc form by conventional extrusion, forging or pressing techniques and may be further processed to sheet by extrusion, forging, hot rolling and col-d rolling.

The alloys of the present invention are of the age hardenable type and require suitable heat treatment in order to develop the critical combination of properties required. Both solution heating and aging treatments are required. The former is most important since it largely decides the values of creep strength and proof strength that can be achieved in a given alloy. Very high solution heating temperatures give the highest possible creep re-. sistance, while on the other hand lower solution heating temperatures favor increased proof strength. A suitable heat treatment for parts made from the alloys comprises solution treatment for from V2 to 8 hours at 900 C. to 1150 C., followed by cooling at any practical rate (e.g., by air cooling or oil quenching or, in the case of sheet, water quenching) and then aging. Aging treatments can be carried out at temperatures in the range of 650 C. to 850 C. or 900 C. and may involve one or more stages of heating for from 4 to 40 hours at successively lower temperatures in this range. A preferred heat treatment to give the highest level of proof strength, together with reasonable creep strength, consists in solution heating for from 1 to 8 hours at 1050 C. to 1150 followed by aging for from 4 to 20 hours at 750 C. to 850 C.

For the purpose of giving those skilled in the art a better understanding of the invention and/ or a better appreciation of the advantages of the invention, the fol lowing illustrative examples are given:

Example I A series of tensile tests was carried out at 750 C. and impact tests at room temperature on a number of alloys that contained, besides the elements set forth in Table I, 0.05% carbon, 14% cobalt, 7% molybdenum, 0.003% boron, 0.05 zirconium, balance nickel. The test pieces were machined from forged bar of the alloys, and the tensile test pieces were given the following heat treatment before testing; solution heated for 1 hour at 1130 C., air cooled, aged for 4 hours at 820 C., air cooled, again aged for 16 hours at 700 C. and air cooled. Elongations were measured on a gauge length of one inch.

The impact tests were performed on Charpy V-notch test pieces that had been solution-heated for 1 hour at 1130 C., air cooled, and then heated at 750 C. for 1000 hours to simulate long-time exposure in service. The results of these tests are set forth in Table 1:

No. 10, with 20% chromium, contained sigma phase at all four of the temperatures used.

TABLE I Tensile Properties at 750 0. Alloy Impact No. Cr Ti Al Nb V Strength Y.S. U.T.S. Elong. (ft-lb.) (tons/in?) (tons/in!) (percent) 0. 5 4.0 1. 5 1. 5 64 74 5 10 0. 5 4.0 l. 5 l. 5 61 74 12 6. 5 0. 5 3. 5 l. 5 l. 5 58 63 ll 12 0.5 4.0 1.0 1.0 58 70 10 9.4 0.5 4.0 2. 2.0 66 76 7 3. 6 0.5 4.0 1. 1. 5 64 70 2.1 3. 5 1.0 l. 5 1. 5 51 66 3 5. 8 2. 5 2.0 1.5 l. 5 56 67 10 2. 5 0. 5 4. 75 49.6 64 7 1 Molybdenum content 5 0.

Tensile Strength '1.S.I.=Long tons (2,240 lbs.) per square inch.

Alloys Nos. 1 to 5 are in accordance with the present invention, while Nos. 6 to 9 are not. Alloys 7 and 8 (as does Alloy 9) deviate from the invention in having an aluminum to titanium ratio (Al:Ti) of less than 2:1, while Alloy 6 has too high a chromium content. It is readily apparent that alloys within the invention exhibit high temperature mechanical properties and resistance to embrittlement superior to those outside the invention, which include No. 9, one of the best disc alloys hitherto available and which is in accordance with our US. Patent No. 3,151,981. This is true despite the fact that Alloy No. 9 was melted and cast under vacuum, whereas all the others in the above table were melted and cast 1n air.

Comparison of the results for Alloy No. 2 with those 5 of No. 7 shows the much poorer tensile properties that result from the use of AlzTi ratios less than 2: 1.

Alloy No. 6, with 18% chromium, has very poor impact properties, and comparison of the impact properties of Alloys Nos. 4, 2 and 5 show how the impact strength decreases as the total content of niobium and vanadium is increased. Comparison of Alloy No. 2 with No. 3 shows that decrease in the aluminum content somewhat improves the impact strength at the expense of some loss in strength. The best all-round combination of properties is shown by Alloy No. 1.

Example II A series of Charpy V-notch tests were carried out to further illustrate the particularly good resistance to notch sensitivity and to high temperature embrittlement of an alloy containing chromium as compared with alloys having higher chromium contents. The alloys tested contained, besides the amount of chromium set forth in Table II, 0.05% carbon, 14% cobalt, 7% molybdenum, 0.5% titanium, 4.0% aluminum, 1.5% columbium, 1.5% vanadium, 0.003% boron, 0.05% zirconium, balance nickel. Each alloy was tested at room temperature after heating for a prolonged period of 1000 hours at one or X-ray examination of the specimens of Alloy No. 2 (15% chromium) heated at 750 C. and 900 C. did not reveal any of the embrittling sigma phase, but the Alloy Example III Stress-rupture tests were carried out at a stress of 33.5 t.s.i. and a temperature of 732 C. on un-notched and notched samples of Alloy No. 2, after a heat treatment consisting of solution heating at 1130 C. for 1 hour, aircooling, aging at 850 C. for 5 hours and again air-cooling showed that this alloy is notch-strengthened. The unnotched specimen broke after 308 hours with an elongation of 4.3%, while the notched specimen was unbroken after 600 hours.

A most advantageous range of alloy composition contemplated herein for alloys having an advantageous combination of properties, including high temperature strength and ductility combined with resistance to notch sensitivity and high temperature embrittlement, is as follows: about 0.04% to about 0.06% carbon, about 12% to about 16% chromium, about 13% to about 15% cobalt, about 6% to about 8% molybdenum, up to about 0.6% titanium, about 3.7% to about 4.8% aluminum, the total content of aluminum and titanium (Al+Ti) being about 4% to about 5%, about 1.4% to about 1.7% columbium, about 1.2% to about 1.7% vanadium, the total content of aluminum, titanium, columbium and vanadian being about 6.6% to about 7.4%, about 0.001% to about 0.01% boron, about 0.01% to about 0.1% zirconium, balance essentially nickel. A particularly advantageous composition consists essentially of about 0.05% carbon, about 15 chromium, about 14% cobalt, about 7% molybdenum, about 0.5% titanium, about 4.0% aluminum, about 1.5% columbium, about 1.5% vanadium, about 0.003% boron, about 0.05% zirconium, balance essentially nickel.

While the alloys of the invention are particularly suitable for making wrought rotor discs for gas-turbine engines, their advantageous combination of properties also makes them useful as material for large bolts for service at elevated temperatures, for example, for securing the casings of steam turbines. The threads of such bolts constitute notches, so it is most important that the alloy used be not notch-weakened.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. An age-hardenable alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C. and which is further characterized by notch insensitivity and by resistance to high temperature embrittlement, said alloy consisting essentially of about 0.02% to about 0.1% carbon, about 10% to about 17% chromium, about 11% to about 16% cobalt, about to about 9% molybdenum, about 2.53% to about 5.2% aluminum, up to about 1.73% titanium, the total content of aluminum and titanium being about 3.8% to about 5.2% and the ratio of aluminum to titanium being at least 2:1, about 1% to about 2.5% columbium, up to about 2% vanadium, the total content of columbium and vanadium being about 2% to about 4% and the ratio of vanadium to columbium not exceeding 1.5 :1 about 0.001% to 0.01% boron, about 0.01% to about 0.1% zirconium, and the balance essentially nickel, the total content of aluminum, titanium, columbium and vanadium being about 6.3 to about 8.5%.

2. An alloy as set forth in claim 1 in which the vanadium content is at least 0.5%.

3. An alloy as set forth in claim 1 wherein columbium is at least partially replaced by an equal atomic percentage of tantalum.

4. An alloy as set forth in claim 1 wherein molybdenum is partially replaced by an equal atomic percentage of tungsten up to a maximum. of 2% by weight of tungsten.

5. As a new article of manufacture, a turbine rotor disc formed from the alloy set forth in claim 1.

6. An age-harden-able alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C. and which is further characterized by notch insensitivity and by resistance to high temperature embrittlement, said alloy consisting essentially of about 0.04% to about 0.06% carbon, about 12% to about 16% chromium, about 13% to about 15% cobalt, about 6% to about 8% molybdenum, about 3.7% to about 4.8% aluminum, up to about 0.6% titanium, the total content of aluminum and titanium being about 4% to about 5%, about 1.4% to about 1.7% columbium, about 1.2% to about 1.7% vanadium, about 0.001% to about 0.01% boron, about 0.01% to about 0.1% zirconium, and the balance essentially nickel, the total content of aluminum, titanium, columbium and vanadium being about 6.6% to about 7.4%.

7. As a new article of manufacture, a turbine rotor disc formed of the alloy set forth in claim 6.

8. An age-hardenable alloy having an advantageous combination of mechanical characteristics at temperatures above about 600 C. and which is further characterized by notch insensitivity and resistance to high temperature embrittlement, said alloy consisting essentially of about 0.05% carbon, about 15% chromium, about 14% cobalt, about 7% molybdenum, about 4% aluminum, about 0.5% titanium, about 1.5% columbium, about 1.5% vanadium, about 0.003% boron, about 0.05% zirconium, and the balance essentially nickel.

References Cited UNITED STATES PATENTS 3,061,426 10/1962 Bie'ber -171 3,151,981 10/1964 Smith @1211 75-171 3,155,501 11/1964 Kaufman et a1. 75171 3,166,412 1/1965 Bieber 75 17 1 HYLAND BIZOT, Primary Examiner.

R. O. DEAN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 5,545,950 September 26, 1967 Edward Gordon Richards et a1 It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

in the heading to the printed specification, lines 6 and 7 for "International Nickel Company" read The International Nickel Company, Inc column 2, line 45, for "10.1%" read 10% lines 56 and S7, for "charateristlcs" read -character1st1cs column 5, line 59, for "0H 1% boron" read 0.01% boron line 47, for "10% of the" read 10% the column 7, line 11, after "1.5:1" insert a comma.

Signed and sealed this 8th day of October 1968 (SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD BRENNER Attesting Officer Commissioner of Patents

Claims (1)

1. AN AGE-HARDENABLE ALLOY HAVING AN ADVANTAGEOUS COMBINATION OF MECHANICAL CHARACTERISTICS AT TEMPERATURES ABOVE ABOUT 600*C. AND WHICH IS FURTHER CHARACTERIZED BY NOTCH INSENSITIVITY AND BY RESISTANCE TO HIGH TEMPERATURE EMBRITTLEMENT, SAID ALLOY CONSISTING ESSENTIALLY OF ABOUT 0.02% TO ABOUT 0.1% CARBON, ABOUT 10% TO ABOUT 17% CHROMIUM, ABOUT 11% TO ABOUT 16% COBALT, ABOUT 5% TO ABOUT 9% MOLYBDENU, ABOUT 2.53% TO ABOUT 5.2% ALUMINUM, UP TO ABOUT 1.73% TITANIUM, THE TOTAL CONTENT OF ALUMINUM AND TITANIUM BEING ABOUT 3.8% TO ABOUT 5.2% AND THE RATIO OF ALUMINUM TO TITANIUM BEING AT LEAST 2:1, ABOUT 1% TO ABOUT 2.5% COLUMBIUM, UP TO ABOUT 2% VANADIUM, THE TOTAL CONTENT OFCOLUMBIUM AND VANADIUM BEING ABOUT 2% TO ABOUT 4% AND THE RATIO OF VANADIUM TO COLUMBIUM NOT EXCEEDING 1.5:1 ABOUT 0.001% TO 0.01% BORON, ABOUT 0.01% TO ABOUT 0.1% ZIRCONIUM, AND THE BALANCE ESSENTIALLY NICKEL, THE TOTAL CONTENT OF ALUMINUM, TITANIUM, COLUMBIUM AND VANADIUM BEING ABOUT 6.3% TO ABOUT 8.5%.