US 3459545 A
Description (OCR text may contain errors)
United States Patent 3,459,545 CAST NICKEL-BASE ALLOY Clarence George Bieber, Suifern, and John J. Galka,
Tuxedo, N.Y., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 468,154, June 29, 1965. This application Feb. 20, 1967, Ser. No. 617,071
Int. Cl. C22c 27/00 US. Cl. 75--171 6 Claims ABSTRACT OF THE DISCLOSURE Directed to a nickel-base alloy particularly useful in the form of castings such as in the form of turbine blades The present invention is a continuation-in-part of our copending US. application Ser. No. 468,154, filed June 29, 1965, now abandoned.
The present invention is directed to a nickel-base casting alloy which develops high strength at elevated temperatures and is characterized by improved corrosion resistance and good castability, and, more particularly, to a nickel-base casting alloy especially useful for the production of vacuum melted and vacuum cast aircraft gas turbine blades for use in engines wherein corrosive attack due to oxidation and sulfidation is encountered.
The gas turbine industry has now become firmly established and is capable of producing gas turbine engines for use in aircraft, automotive, marine and stationary applications which are characterized by long life and reliability. The requirements imposed upon gas turbine engines by actual and prospective users has placed greater and greater demands upon the builders of gas turbines. In turn, the requirements for parts to be used in gas turbine engines has continually been based upon higher and higher performance standards. A particularly sensitive part in the gas turbine mechanical structure is the turbine blading. It has been found that the temperature requirements imposed upon turbine blading materials is always being raised to a higher level. Furthermore, as performance standards imposed upon the gas turbine engine and the constituent parts thereof are raised to higher levels, it is found that other problems are encountered which must be solved. Corrosion of the blading materials due to oxidation, sulfidation and other effects due to the ambient atmosphere encountered in engine service is one of these. Accordingly, the engine builders have come to expect that the blading materials will not only possess very high strength at temperatures on the order of 1800 F. and higher but will be resistant to the corrosive effects encountered in service, particularly during the long exposure times in service now being achieved.
One alloy which has been widely employed with good "ice to oxidation and thermal fatigue, and by good structural stability when exposed to the effects of temperature and stress for extended periods of time. The alloy has been used to manufacture cast turbine blades which have performed satisfactorily in many hundreds of gas turbine engines. An alloy described in US. application Ser. No. 336,458, now Patent No. 3,301,670, and nominally containing about 10% chromium, about 4% molybdenum, about 1% columbium, about 2% tungsten, about 2% tantalum, about 6.5% aluminum, about 1% titanium, about 0.12% carbon, about 0.02% boron, about 0.1% zirconium, and the balance essentially nickel, offers materially improved strength properties at temperatures of about 1800 F. than the alloy sold under specification AMS 5391. It is found, however, that the aforementioned alloys have not met more stringent requirements in regard to corrosion resistance such as sulfidation resistance as more recently imposed by the aircraft gas turbine manufacturers. It has long been postulated that chromium is an element which could usefully be employed in increased amounts for the purpose of improving corrosion resistance, e.g., sulfidation resistance, of nickelbase high temperature alloys. However, it is found that when an attempt is made to improve the sulfidation resistance of alloys such as those described hereinbefore merely by increasing the chromium content thereof that other wholly undesirable effects are encountered. These effects include materially reduced elevated temperature strength, increased susceptibility to the formation of undesirable phases during long time exposure to elevated temperature, reduced ductility, etc. For example, when the chromium content of an alloy as described in US. application Ser. No. 336,458, now Patent No. 3,301,670, and having the nominal composition set forth hereinbefore is increased from about 10% to about 17%, the life to rupture at 1800 F. and 22,000 pounds per square inch (p.s.i.) is reduced from about 200 hours to about 23 hours. Such a result is wholly unsatisfactory and is not acceptable, particularly when it is borne in mind that the AMS 5391 alloy described hereinbefore is required to have a rupture life of 30 hours at 1800 F. and 22,000 p.s.i. and regularly provides a rupture life of about 50 hours at 1800 F. and 22,000 p.s.i. Accordingly, the art has been presented with the problem of providing a nickel-base casting alloy which would have materially improved corrosion resistance, e.g., sulfidation resistance, as compared to the AMS 5391 alloy but which would still retain essentially the rupture properties developed in the AMS 5391 alloy.
We have now discovered a nickel-base casting alloy having substantial rupture strength at temperatures of the order of up to about 1 800 F. and having improved corrosion resistance, e.g., sulfidation resistance, as compared to prior nickel-base alloys employed for the production of cast gas turbine blades.
It is an object of the present invention to provide a nickel-base casting alloy having improved corrosion resistance, e.g., sulfidation resistance.
It is a further object of the invention to provide a nickel base casting alloy characterized by highly useful stressrupture properties at temperatures of the order of up to about 1800 F. and having improved corrosion resistance, e.g., sulfidation resistance, as compared to prior nickelbase casting alloys when employed for gas turbine blading purposes.
Other objects and advantages of the invention will become apparent from the following description.
Broadly stated, the present invention comprises a nickelbase alloy having high stress-rupture properties and having improved corrosion resistance, e.g., sulfidation resistance, at elevated temperatures, when produced as castings in the vacuum melted and vacuum cast form which contains about 15%, e.g., 15.5% or 16%, to about 18% chromium, about 0.5% to about 2.5% molybdenum, about 0.5% to about 2% columbium, about 1% to about 3% tungsten, about 1% to about 3% tantalum, about 6% or 6.5 to about 7.5% or about 8% of aluminum plus titanium, up to about 10% or cobalt, about 0.025% to about 0.25% carbon, about 0.01% to about 0.04% or 0.05 boron, about 0.01% to about 0.2%; zirconium, and the balance essentially nickel.
An essentially cobalt-free alloy contains, by weight, about 16% or 16.5% to about 17.5% chromium, about 1% to about 2% molybdenum, about 0.75% to about 1.25% columbium, about 1.5% to about 2.5% tungsten, about 1.5% to about 2.5% tantalum, about 5.5% or 6% to about 6.5% aluminum, up to about 0.5% titanium, about 0.03% to about 0.07% or 0.08% carbon, about 0.015% to about 0.025% boron, about 0.05% to about 0.15% zirconium, up to about 1% cobalt, and the balance essentially nickel. Such alloys are characterized by a rupture life at 1800 F. and 22,000 p.s.i. of at least about 30 hours or more in combination with improved corrosion resistance, e.g., sulfidation resistance, at elevated temperatures.
It is important that the alloys provided in accordance with the invention be produced using the purest materials commensurate with reasonable cost. Thus, the contents of subversive impurities such as lead, bismuth, tellurium, sulfur, selenium, phosphorus, oxygen, nitrogen, hydrogen, arsenic, antimony, tin and thallium should be as low as possible. Cobalt may be present in the alloy in amounts up to about 10% or about 15%, e.g., about 4% to 12%, as this element appears to contribute to the sulfidation resistance of the alloy, although cobalt increases cost. Iron may be present in impurity amounts, e.g., up to about 1%, as iron decreases the ability of the alloy to resist microstructural changes after long exposure to elevated temperature. Silicon and manganese are detrimental elements and should not be present in amounts exceeding about 0.3% or, more advantageously, about 0.2% or about 0.1%, of each.
It is found that cobalt-containing alloys, i.e., alloys containing about 4% to about 12% cobalt, display improved properties from the standpoints of strength and corrosion resistance. These alloys more advantageously contain reduced proportions of aluminum, e.g., 3% or 3.5% or more of aluminum, increased proportions of carbon, e.g., up to about 0.25% carbon, and increased proportions of titanium such that the titanium content is at least about six times the carbon content and is up to about 4% or 4.5%. In such alloys, the chromium content may be reduced to as low as about 15% with concomitant increase in high temperature properties. Such alloys are strong at 1800 F., have improved stress-rupture strength at lower temperatures, e.g., 1350 F. to 1500" F., and have good room temperature ductility, even when cast using currently common fine-grain casting techniques which involve the use of inoculated molds. In addition, the alloys retain the long-time structural stability which characterizes the aforementioned essentially cobalt-free alloys. These alloys accordingly contain about 15% to about 18% chromium, about 4% to about 12% cobalt, about 0.75% to about 2.2% molybdenum, about 1% to about 3% tungsten, about 0.5% to about 2% columbium, about 1% to about 3% tantalum, about 3% to about 6% or 7% aluminum, titanium in amounts at least six times the carbon content up to about 4%, about 0.1% to about 0.2% carbon, about 0.01%} to about 0.04% or 0.05% boron, about 0.01% to about 0.2% zirconium, and the balance essentially nickel. Usually, the total content of aluminum and titanium in the alloys does not exceed about 7% or 7.5 For alloys containing about 10% cobalt, a preferred range comprises about 15.5 to about 17% chromium, about 8% to about 11% cobalt, about 0.75% to about 2.2% molybdenum, about 1.8% to about 3% tungsten, about 0.75% to about 1.25% columbium, about 1% to abo t a a m,
about 3% to about 4% aluminum, about 3% to about 4% titanium, about 0.14% to about 0.2% carbon, about 0.01% to about 0.05% boron, about 0.05% to about 0.15 zirconium, and the balance essentially nickel. For alloys containing about 5% cobalt, a preferred range comprises about 15.5% to about 17% chromium, about 4% to about 6% cobalt, about 0.75 to about 2.2% molybdenum, about 1.8% to about 3% tungsten, about 0.75% to about 1.25% columbium, about 1% to about 2% tantalum, about 3% to about 4% aluminum, about 3% to about 4% titanium, about 0.14%; to about 0.2% carbon, about 0.01 to about 0.03% boron, about 0.05 to about 0.15% zirconium, and the balance essentially nickel.
The aforementioned cobalt-containing alloys, when properly heat treated, will have a stress-rupture life of at least about 30 hours at 1800 F. and 22,000 p.s.i. stress and of at least about hours at 1350 F. and 90,000 p.s.i. stress. More advantageous alloys containing nominally 4% aluminum and 3% titanium will have stressrupture lives of at least about 50 hours and at least about 200 hours under the respective test conditions.
In the alloy provided in accordance with the invention, chromium and molybdenum are carefully controlled in amount to enable obtaining improved corrosion resistance at temperature and satisfactory stress-rupture properties as contemplated in accordance With the invention. Thus, chromium is desirably about 16% to about 18% and molybdenum is about 1% but does not exceed about 2.5 In some instances, chromium may be increased up to about 19% or 20% with improvement in corrosion resistance, e.g., sulfidation resistance, in instances wherein microstructural stability during long periods of exposure to elevated temperatures are of reduced importance. It is important from the standpoint of stress-rupture life that as chromium is increased from 15% to about 20%, the molybdenum be reduced from about 2.5 to about 1% or about 0.5%. The elements columbium, tantalum, tungsten and aluminum all contribute in the controlled amounts employed to obtaining the high strength developed in the alloy provided in accordance with the invention. When any of these elements is employed in amounts either great or lesser than those given hereinbefore, less satisfactory results from the standpoints of strength or ductility or both are obtained. Boron and zirconium are beneficial elements having reference to the stress-rupture properties. It is found that carbon is an important element in regard to the development of satisfactory stress-rupture properties in the alloys. For example, in one instance, an essentially cobalt-free alloy otherwise in accordance with the invention but containing ony 0.01% carbon developed an unacceptable stress-rupture life of only 10 hours at 1800 F. and 22,000 p.s.i. whereas a similar essentially cobalt-free alloy containing 0.03% carbon developed a satisfactory stress-rupture life of 40 hours at 1800 F. and 22,000 p.s.i. On the other hand, when carbon exceeds about 0.08%, the stress-rupture properties of the alloys are again detrimentally affected, unless the appropriate adjustments in composition, particularly with regard to cobalt, aluminum and titanium noted hereinbefore, are made. Titanium may be employed in amounts not exceeding about 0.75 or, more advantageously, not more than 0.5% or even 0.25%, in the special essentially cobaltfree alloys which contain not more than about 0.08% carbon described hereinbefore. However, the aforementioned cobalt-containing alloys also having increased contents of titanium, e.g., up to about 3.5% or 4% titanium, with reduced aluminum contents such that the total content of aluminum plus titanium is about 6.5 to about 7.5 advantageously contain substantially increased carbon contents, e.g., about 0.14% to about 0.18% or about 0.2%. Such alloys also advantageously contain about 2% molybdenum for increased strength, particularly at 1350 F., while maintaining freedom from the formation of brittle phases, e.g., sigma phase, on long-time heating.
h lo g a e 1 i c udes the composition of In addition, the tensile tests conducted upon cast-to-size test bars at room temperature have indicated a yield strength (0.2% offset) of about 120,000 p.s.i. along with TABLE I Percent Mo Cb eighteen alloys produced in accordance with the invention by vacuum melting and vacuum casting. The balance of the alloy in each case is essentially nickel.
elongations of the order of 3% or 5% or more for alloys within the invention.
In order to demonstrate the improved corrosion resistance achieved in accordance with the invention, a number of corrosion tests were conducted wherein a specimen of the test alloy was heated in contact with a molten mixture of 90% sodium sulfate and sodium chloride in an air atmosphere to a temperature of about 1700 F. for
a period of about four hours. It was found that the specimens of AMS S391 alloy were destroyed in the course of the test whereas specimens made of the alloy of the present invention were not attacked. The aforementioned cobalttaining alloys withstand the severe molten salt corrosion test when subjected thereto for eight hours or even sixteen hours. Alloy No. 16 withstood this severe test for a period of hours without attack but started to show slight evidence of corrosion after 200 hours. However,
40 Alloy No. 18 still showed no evidence of corrosion after 200 hours in the test. This alloy was strong in all conditions of testing as shown in Table III and was microstructurally stable after long-time elevated temperature exposure, i.e., the alloy did not develop a brittle phase such as sigma phase. In another test conducted at 1450 F. in an oxidizing gas atmosphere containing sulfur dioxide using specimens coated with a mixture in equal parts of sodium sulfate and magnesium sulfate, it was found that the corrosion resistance of the alloys provided in accordance with the invention was on the order of ten to fifty times better than that of the AMS 5391 alloy. The alloys also exhibit improved elevated temperature sulfidation resistance under alternating reducing and oxidizing conditions.
Castings produced in accordance with the invention may be employed in the as-cast condition with good results. Heat treatment of the castings may be employed for the purpose of improving certain properties. For example, when it is desired to improve stress-rupture life of the aforementioned cobalt-free alloys, the castings may be subjected to a solution heat treatment comprising a heating at about 2100 F. to about 2200 F., e.g., 2150 F., for a time of about one to about ten hours, e.g., about two hours. The solution heat treatment may be followed by an aging treatment at about 1600 F. to about 1700 F., e.g., about 1650 F., for about ten hours to about fifty hours, e.g., twenty-four hours. The aforementioned cobalt-containing alloys are heat treated by solution heating in the temperature range of about 1925 F. to about 2075 F., e.g., about 1975 F. to about 2050 F., for a time of about one to about ten hours, e.g., about two to about six hours. The solution treatment may be followed by an aging treatment at about 1500 F. or 1550 F. to about 1650 F. for about 24 to about 16 hours. An advantageous heat treatment comprises two beatings within 75 the temperature range of about 1925 F. to about 2075 fiMMMMHfiMHEHUBBBBBB 0 0 0 0 0 O 0 0 0 U 0 0 0 0 0 0 0 0 n ea m n m l23 8333m334 W l 0 .L n w C 11 1 a 0 u u mu m Q m m w 7 ,S m S t 1t 24 0382241313102727535874 8347764437463 4386 6 6 5 5 I0 e 5555 S HF. 1111112 .11 .1111 1 55 w W 0 w m m 6445 6657 9 2 4. 42234452455 0 0 H 0000 0 t w um m hhhh h mam 922225120555220552 S .55 O 4446 6 n L 2. 2 .22 222 w M D 222 1 O n L L LL LLL t Pwm mmmm m m U mm mn .1 m e no .4 u :l llllllllmw ovnmllwlll 0% e W C w 31 m S u 3 1 0 0. m n es mama m n w hO h H 6551 6 0 0 0 MW f. mm HWHM H wm W 2492145236 24 27 589766 M. 6436634464H m 297 5 S .1 a a a 3 0.36am m fiumfimmfi a an mm 2 that... a. m a mmaaam ma t a 636 e h d l 2 e h D. mhmmm mmamm w a m. a, 0 C C n as y. as I 622111121 1121 222 m m C m m H H W F F T L .1 L of S 0000 00 e o s 1 0 m mm O MHRM E hhhh hh E K m m m ab S 5 t 1 L 22 2 6 L L 0 0 2 eo B rrrrm r. B 1 1 1 .mmm15mm559m5m 7 A mtmamrm Mm A 0 m am a a r i T T m n mm uwee eos w U ln O W n mmn-ow m nummmmm mmmmmmwm m P a m mmwmaem m 0 MW at P eIm m Manama o ABBCAAAABDEFGAEEEEEEEEEE DEFGEEEEEEEEEE m m m m .adddddnd II n n n N 0 C t eeee SW PH nmmh w m mfl m mh m n N n u n n u 1 t r. seeeeete mm 4 53 T HHHH H m m m m m H M S m T n 00 S a S b 1t n a n n n n u u .a .Hd H. .1 S .w N u u u E ;m S t 4L 5 u Tn. u h m n. 0 6 f t u v. mm h m t m s m u u .1 "m m T.m a m C ABCDEF G A L 2 5 6 6 79111111111 1 Specimen unbroken.
7 F., e.g., about 1975 F. to about 2050 F. for about two to about six hours, with the first heating being at a higher temperature, followed by an aging treatment. For example, a four hour heating at 2050 F., followed by a four hour heating at 1975 F., followed by an aging treatment has been used with good results.
Castings produced from the alloys provided in accordance with the invention may be employed not only in cast aircraft, industrial, marine and automotive gas turbine blades but also in cast stationary gas turbine components such as guide vanes, nozzle partitions and other cast gas turbine components which are subjected to corrosive environments at elevated temperatures. Sulfur compounds are normally present in fuels used for gas turbines. It is found that when gas turbine engines also ingest salt, as is the case in marine service of various types, that attack upon the hot surfaces of the engines is vastly accelerated. The invention is particularly useful for the production of parts which must operate in the presence of salt, e.g., sodium chloride, as encountered in gas turbines operated at sea, including marine and aircraft gas turbines.
Castings may be produced from the alloys provided in accordance with the invention using commercial vacuum melting and vacuum casting equipment and employing investment molds, static casting, etc. The alloys should be prepared by vacuum melting. It is also desirable to vacuum cast the alloys. In commercial practice, it is permissible to prepare remelt stock by vacuum melting and then remelt and cast under an argon atmosphere.
1. An alloy having an improved combination of elevated temperature stress-rupture strength and corrosion resistance consisting essentially of about 15% to about 18% chromium, about 8% to about 11% cobalt, about 0.75% to about 2.2% molybdenum, about 1.8% to about 3% tungsten, about 0.5% to about 2% columbium, about 1% to about 3% tantalum, about 3% to about 4% aluminum, about 0.1% to about 0.2% carbon, about 3% to about 4% titanium with the total content of aluminum and titanium not exceeding about 7.5%, about 0.01% to about 0.05% boron, about 0.01% to about 0.2% zirconium, and the balance essentially nickel.
2. An alloy in accordance with claim 1 wherein the total content of aluminum and titanium is at least about 6.5%.
3. An alloy according to claim 1 wherein the total content of aluminum and titanium does not exceed about 4. An alloy according to claim 1 wherein the carbon content is about 0.14% to about 0.2%.
5. A corrosion resistant alloy having high elevated temperature rupture strength consisting essentially of about 16% chromium, about 10% cobalt, about 2% molybdenum, about 2.5% tungsten, about 1% columbium, about 1.25% tantalum, about 4% aluminum, about 3% titanium, about 0.18% carbon, about 0.02% boron, about 01% zirconium, and the balance essentially nickel.
6. A corrosion resistant alloy having high elevated temperature rupture strength consisting essentially of about 16% chromium, about 10% cobalt, about 2% molybdenum, about 2.5% tungsten, about 1% columbium, about 1.25% tantalum, about 3% aluminum,
' about 4% titanium, about 0.18% carbon, about 0.02%
boron, about 0.1% zirconium, and the balance essentially nickel.
References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary Examiner