US3432294A

US3432294A - Cobalt-base alloy

Info

Publication number: US3432294A
Application number: US449825A
Authority: US
Inventors: Harold L Wheaton
Original assignee: Martin Marietta Corp
Current assignee: Martin Marietta Corp
Priority date: 1965-04-21
Filing date: 1965-04-21
Publication date: 1969-03-11
Anticipated expiration: 1986-03-11
Also published as: GB1143214A

Description

March 11, 1969 H. L. WHEATON COBALT-BASE ALLOY l of 4 Sheet Filed April 21, 1965 FIG. 2

INVENTOR Y m m T JR A 0 E H n A I- w m M A w B March 969 H. L. WHEATON 3,432,294

COBALT-BASE ALLOY I Filed April 21, 1965 Sheet 3 of 4 INVENTOR.

HAROLD .WHEATON BY 06 M ATTORNEY.

RUPTURE TIME HRS.

March 11, 1969 H. L... WHEATON 3,432,294

COBALT-BA S E ALLOY Filed April 21, 1965 Sheet 3 of 4 g 92 Z .2 2 CE 0 u j :12 g on:

I v 2 g E m i 5 o q a 3 3 a lSd -3:.H'l HI'IOH OOOI 80d SSBELLS INVENTOR AT TOBJXEY March 11, 1969 H. 1.. WHEATON 3,432,294

COBALT-BASE ALLOY' v I Filed April 21, 1965 Sheet 4 of 4 05 g 2 if F SHHQNI (13333 INVENTOR.

HAROLD L. WHEATON BY J ATTORNEY United States Patent 8 Claims ABSTRACT OF THE DISCLOSURE An alloy suitable for use under load at high temperatures and having an excellent combination of engineering characteristics. The alloy is a carbide-hardened cobaltbase material containing critical, related amounts of carbon, chromium, nickel, tungsten, tantalum, titanium and zirconium.

The present invention relates to alloys and, more particularly, to cobalt-base, carbide-hardened alloys especially adapted for use at elevated temperatures at high stress.

For a number of years during the development of the gas turbine engine for aircraft and stationary usage, it has been the goal to produce stronger and stronger cobaltbase alloys. Now that the race for brute strength of cobaltbase alloys is being won by achievement of useful strengths at temperatures of the order of 80% of the absolute melting temperature of the alloys in question, more and more attention is being directed to the need for the development of other highly useful and necessary characteristics in useful high-strength alloys.

One of the problems which has plagued developers of cobalt-base, carbide-hardened alloys is the problem of producing a strong alloy which not only is relatively metallurgically stable at temperatures at the extremely high ranges of 1800 F. and higher, but also is resistant to drastic embrittlement resulting from exposure to lower temperatures, e.g. temperatures about 1000 F. to about 1600 F. It is well recognized in general that exposure to temperatures above the recrystallization range will alter the metallurgical characteristics of an alloy, such alteration being due, at least in part, to grain growth. At lower temperatures other changes can occur. In cobalt-base, carbide-hardened alloys of the kind in question, hardness increases are experienced when the alloys are subjected for prolonged periods of time at temperatures of the order of 1300 F. to 1600 F. For example, a prior art, precision-cast, carbide-hardened cobalt-base alloy might exhibit an as-cast hardness of 40 R (Rockwell C units) which hardness could increase to 50 R upon exposure for about 48 hours at 1500 F. Should such an increase in hardness accrue during engine operation, for example during operation of a gas turbine engine incorporating water injection, an increase in hardness along with an inherent decrease in ductility and increase in brittleness could prove to be a fatal deficiency for the engine part in question. It is, therefore, very important that a cobaltbase carbide-hardened alloy should exhibit as low an ascast hardness as is compatible with achieving useful levels of strength at elevated temperatures.

Mechanical characteristics, other than brute strength at extremely high temperatures, which are necessary for a successful gas turbine alloy include good high temperature creep strength and rupture ductility, good high temperature oxidation resistance, good thermal conductivity and acceptable room-temperature strength and ductility. All in all, a successful gas-turbine alloy of the kind in question must meet and pass many tests in order to be practically and commercially useful. As far as I am aware,

iCe

no one has heretofore provided the art with an alloy having such an advantageously high combination of engineering characteristics, as is exhibited by the alloy of the present invention, wherein a substantial balance of characteristics is attained without substantial sacrifice of any particular feature.

It has now been discovered that by means of a special control of alloying elements within specially controlled and, advantageously, interrelated ranges a novel carbidehardened, cobalt-base casting alloy can be produced having an advantageous balance of engineering characteristics and possessing highly enhanced practical utility.

It is an object of the present invention to provide a novel cobalt-base alloy.

It is another object of the present invention to provide a novel cobalt-base, carbide-hardened alloy.

A still further object of the present invention is to provide precision-cast engineering structures made of a novel cobalt-base, carbide-hardened alloy.

Another object of the present invention is to provide novel, precision-cast gas-turbine structures made of a novel cobalt-base, carbide-hardened alloy.

An additional object of the present invention is to provide a process for the production of a novel alloy and for the production of engineering structures, e.g. gas-turbine structures comprised thereof.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawing in which:

FIGURE 1 is a sketch depicting the as-cast microstructure of an alloy in accordance with the present invention, as viewed at about 250 power magnification;

FIGURE 2 is a sketch depicting the as-cast microstructure of a useful prior art alloy, as viewed at about 250 power magnification;

FIGURE 3 is a graph showing life-to-rupture in hours for an alloy of the present invention under various loads in pounds per square inch (p.s.i.) at various temperatures between and including 1400 F. and 20,00 F.;

FIGURE 4 is a graph showing the interrelation between stress for hour life (in p.s.i.) and temperature (in degrees Fahrenheit) for an alloy of the present invention and some prior art alloys; and

FIGURE 5 is a graph illustrating the creep behavior of any alloy in accordance with the present invention, as compared to the creep behavior of some prior art alloys.

Generally speaking, the present invention is directed to a cobalt-base, carbide-hardened alloy containing in weight percent about 0.6% carbon, about 18% to about 24% chromium, about 7% to about 15% nickel, about 6% to about 9% tungsten, about 2% to about 5% tantalum, about 0.1% to about 0.5% titanium, about 0.1% to about 1% zirconium with the balance being essentially cobalt together with impurities and incidental elements normally associated with cobalt-base alloys. The alloy of the present invention is particularly adapted to be formed by precision casting into engineering structures useful at elevated temperatures such as encountered in gas turbines.

Among the incidental elements which may be present in the alloy of the present invention are up to about 3% iron, up to about 0.5 each of manganese and silicon, up to about 0.1% boron, up to about 0.2% mischmetal (a mixture of rare earth metals), up to about 2% of columbium and up to about 2% of molybdenum. It is advantageous to maintain the impurity elements sulfur, hydrogen, lead, tin and others of like activity at as low levels as possible in order to avoid the deleterious effects caused by interstitial infiltration of the crystal lattice by hydrogen and the like and the formation of low melting phases by lead, tin, sulfur, and the like.

When producing the alloy of the present invention, it is highly advantageous to employ vacuum mtlting wherein the alloying elements cobalt, chromium, nickel, tungsten and carbon are melted together under a high vacuum and held in the molten condition thereunder until deoxidation has been completed. Subsequent to completion of said destress is concentrated in areas where hot spots can develop. Further, massive carbide structures, especially in semicontinuous or continuous form, provide brittle areas through which stress-induced cracks can propagate rapidly.

oxidation tantalum is added to the molten metal followed 5 It is to be noted that among themselves, the carbideby titanium and zirconium. When alloying is completed, forming elements are advantageously interrelated such the melt is cast either into bars suitable for remelting or that the amount of tungsten in weight percent is about directly into finished shapes by means of suitably shaped twice the amount of tantalum in weight percent, the weight molds, e.g. molds produced by investing a fugative model percent of zirconium is about 2.5 times the weight percent in a ceramic matrix. Advantageously casting as well as of titanium and the sum of the weight percent zirconium melting is carried out under a high vacuum. Alternatively, plus twice the weight percent titanium is less than about one can melt and cast under an inert atmosphere such as 1.5%. While these interrelations are somewhat empirical argon. However, when such is done, it is necessary to in nature and can, in practice, be varied somewhat within employ deoxidizing agents, such as silicon, aluminum the alloy or range limits set forth hereinbefore, experiand rare earth elements in order to kill the melt prior mental experience has shown that one cannot ignore these to the addition of tantalum, titanium and zirconium. interrelations and still achieve a commercially advan- When melting under vacuum conditions, carbon acts tageous alloy. Generally, as indicated by the mean of the not only as an alloying ingredient but also as a deoxidizer. alloy ranges set forth hereinbefore, it is advantageous that Thus when melting in the usual manner as employed unratio of the atom percent of carbon to the total atom perder vacuum, i.e. in a high frequency induction furnace, it cent of carbide-forming elements, to wit, tungsten, is necessary to carefully adjust the carbon content of the tantalum, zirconium and titanium be maintained at about original charge to produce a final carbon content within 0.72 in order to attain hig h creep rupture characteristics. the range of about 0.4%, to about 0.7%, advantageously The aforementioned advantageous ratios of tungsten to about 05% to about 0 7% I o de t id lo of tantalum and zirconium to titanium serve the same purstrength and oxidation resistance, one should avoid pose. On the other hand, the aforementioned maximum chromium contents below about 19%. On the other hand, summation of percentages of zirconium plus titanium is use of chromium contents above about 23% can in certain intain d for the purpose of assuring good' oxidation instances lead to brittleness in the alloy. It is advantageous resistance at elevated temperatures. for all round good characteristics (especially low as-cast For purposes of the present invention it is to be noted hardness and enhanced resistance to oxidation) to employ that molybdenum is not the equivalent of tungsten and about 10% nickel in the alloy. However, one can use plus columbium as a substitute for a part of the required or minus 2% as a practical, efiicacious and advantageous tantalum. While the alloys of the present invention can range for this element. Nickel contents in excess of 15% n ain up to about 2% by weight of molybdenum as an drastically lo tu t th, impurity or incidental element, this element is not a sub- The carbide formers, tungsten, tantalum, titanium nd stitute for the required amount of tungsten. Where oxidazirconium, are advantageously employed in amounts of tioh resistance is 1101 a Prime factor, one can employ about 7% tungsten, about 3.5% tantalum, about 0.2% columbium as Substrate for a P of the required titanium and about 0.5% zirconium in conjunction with tantalum on a /2 to 1 by weight basis. However, in the 0ut 0.6% carbon. When used in these amounts, the usual case this is not recommended. Usually, columbium carbide-forming elements, in conjunction with the carbon in amounts P to 05% y Weight can Preseht in the tend to produce a bl di d bid phase i h alloy as an impurity or incidental element associated with cobalt-chromium-nickel matrix which stable, dispersed tantalum In like ner small amounts of hafnium can phase does not form a continuous network throughout the he Present in the alloy of the Present invention as an matrix. The importance of thi type of bl structure, element introduced in association with zirconium. which is depicted in FIGURE lot the drawing, is the fact For the P p Of giving those Skilled in h a a that a high basic thermal conductivity of the alloy due better understanding of the invention and/01 a ett r to a continuous metal matrix is achieved upon casting appreciation of the advantages of the invention, the and remains substantially unimpaired even after prolhwingihhstl'ative examples are given! longed exposure to temperatures of about 1400 F. to abtput 1900 F. Thus the alloy of the present invention, Example w en cast into objects such as guide vanes for gas turbine A11 3 h i f engines, permits the efficient transfer of heat from locally ing 0.(i carbon, 2l i ga ilir rri iil if l tig 1ii cle l illqlfefilsigfaltledli02203?!cgbtggggerilporgglnsaglglgrglgngl g tungsten, 3.5% tantalum, 0.2 .0% titanium, 0.5% zirconsomewhat higher carbon contentspi fi 131 l 8 1um wlth the balance essentially cobalt is melted under t 11 Super Y h" vacuum In a hlgh-frequency lnduction furnace and poured ggrfiiggistggnia sgm-rgogllrlllllglsbgcaartbgihlghsiglsi at 2800 F. under vacuum into molds to produce tensile mal conductivity of magsive semi c-ontinuous carbfi test bars, stress-rupture tesbbars, oxrdatlon test specimens phases is melatively 10w com ared t the the 1 l e and a1rfo1l-sect1ontest spec mens for thermal shock tests. ductivity of a cobalt ano P 0 h COII- Addlt onal alloy compositions comprismg alloys 2 and y matr x and because heat trans- 3 (wlthln the lnvention) and alloys A to H (outside the fer is slowed at phase boundaries, ob ects made of pnor invention) and having compositions in weight percent, as art alloys having a mlcrostructure such as depicted in set forth in Table I, were made in the same manner as [FIGURE 2 are likely to fail at hot spots especially when the alloy of the example.

TABLE I Alloy No Pertgnt Peasant Peisltint Pergnt Peggent PerTcent Perlt zient Perzcpnt Ohgr,

0. 52 20.8 Bal. 7. s 0.51 21.7 Bal. 5.1 0. 53 21. 4 Bal. 7. 1 0. 51 21.7 Bal. 6. 2 0. 55 21. 3 Bal. 8. 4 0.30 20.9 Bal. 5. 6 0.60 25 Bal. 7.2 0.85 21.5 Bal. 10 0. 45 21 Bal. 11 0.50 25 Bal. 7.5

The data set forth in Table II which pertains to the alloys of the example and Table I shows that the lives to rupture at 2000 F. and 8000 pounds per square inch (p.s.i.) of alloys of the present invention are at least substantially equivalent to the lives to rupture of alloys outside the present invention. Further, the stress (in p.s.i) for 100 hour life versus temperature curves for the alloy of the example and alloys Nos. F, G, and H are depicted in FIGURE 4 of the drawing. These curves show that the alloy of the present invention exhibits unexpectedly superior strength at temperatures in excess of about 1550 F. when compared to alloys (i.e. alloys F, G, and H) previously suggested for use in gas turbines and other structures subjected in use to high temperatures.

TABLE II Elongation, Alloy No. Temperature Stress Life percent F.) (p.s.i.) (Hours) (1 gauge length 1 2, 000 8,000 11.1 7 2 2,000 8, 000 79.0 11 3- 2,000 8,000 77 11

A

2, 000 8, 000 109. 1 10

B

2, 000 8, 000 135. 4 18 2,000 8, 000 51. 5 7

D

2, 000 8, 000 20. 2 28

E

2, 000 8,000 43. 2 20 F 2,000 8, 000 30. 5 13

G

2, 000 8, 000 17. 8 13

H

2, 000 8, 000 5. 0

1 N at available.

Alloy C, which contains no titanium and only a trace of zirconium, and Alloy E, which contains 1.8% columbium substituted for 3.6% tantalum, are considerably weaker than the alloys of this invention (i.e., l, 2, and 3) as are the prior art alloys, F, G, and H.

Alloys substantially of the composition of alloy No. 1 (that is, alloys made to conform to the chemistry of alloy No. 1 but exhibiting the normal variations in chemistry occurring in foundry practice) exhibit lives to rupture at various temperatures and under differing stresses as indicated by the linear plots on the graph depicted in FIGURE 3 of the drawing. A comparison of FIGURES 3 and 4 of the drawing shows, for example, that at 1800 F., the life-to-rupture of alloy No. 1 can be expected to be about 4 times as great as the life-to-rupture of alloy No. F when both are tested under a load of about 14,500 psi For the sake of completeness, it is to be noted that the test data set forth in Table II and in FIGURE 4 of the drawing was obtained by employing the standard test for life-to-rupture, as described in ASTM E8-61T and by using as-cast specimens 0.25 inch diameter and 1.4

inches in length exclusive of that length required for gripping. An additional pertinent point is, that at temperatures in excess of about 1600" F., the creep behavior of alloys of the present invention differs substantially from that of prior cobalt-base alloys suggested for use at 1800" F. and higher. FIGURE 5 of the drawing illustrates this point showing the creep behavior of an alloy in accordance with the invention when tested at 2000 F. under a load of 8000 p.s.i. as compared to the creep behavior of comparably tested Alloys Nos. F and G. FIG- URE 5 shows that under the test conditions specified, the alloy of the present invention exhibits a well-defined, low second stage creep rate as compared to the ill-defined, high second stage creep rate of prior alloys suggested for use at temperatures in excess of about 1600 F.

In addition to rupture strength, a basic requirement for any high temperature structural material is oxidation resistance. Materials possessing inadequate oxidation resistance must be coated which adds to the cost and lowers reliability. The oxidation behavior of the alloys listed in Table I is shown in Table III. Alloys with weight gains under 5 mg./cm. and having adherent scales can be considered oxidation resistant while those having weight gains over 5 mg./cm. or scales that spall do not have suflicient oxidation resistance to be used in the uncoated condition at 2000 F.

A comparison of alloy A with alloy 1 indicates the beneficial effect of nickel on oxidation resistance While a comparison of alloy B and alloy 1 shows the deleterious effect of high zirconium plus titanium. Alloy C has fairly good oxidation resistance but relatively low creep rupture strength because of the absence of zirconium and titanium. Alloy E, which contains the atomic equivalent of columbium substituted for tantalum, has poor oxidation resistance even with a chromium content of 25% andthe addition of 10% nickel. The oxidation data for prior art alloys F and G are included for comparative purposes although their rupture strengths are such that oxidation data are somewhat academic.

Another novel aspect of the alloys of the present invention, when compared with some other cobalt-base, carbide-hardened alloys, is shown by the as-cast and aged hardness data in Rockwell C units (R set forth in Table IV.

Hardness alter Alloy N0. hardness (Re) 48 hours at 1,500 F. (Re) A comparison of alloy A and alloy 1 shows the necessity of using nickel to control the as-cast and aged hardnesses in order to avoid undesirable embrittlement. Alloy D, having a low carbon content, illustrates the fact that relatively low hardnesses can be attained by lowering the carbon content. However, as shown in Table II, the low carbon content results in a low life-to-rupture at 2000 F. Alloy F, on the other hand, illustrates the fact that high hardnesses in the as-cast and aged conditions do not necessarily result in a high life-to-rupture. Table II shows that alloy F exhibits a life-to-rupture which is not significantly different from the life-to-rupture of alloy D even though there is a marked difference in both the carbon contents and the hardnesses of these alloys. Since, as is shown, cobalt-base, carbide-hardened alloys increase in hardness by an increment of about 8 to 10 Rockwell C units by virtue of being subjected to temperatures of the order of 1500 F. for about 48 hours, the lower as-cast hardness of alloys in accordance with the invention indicates highly enhanced utility especially when viewed in light of the data set forth in Tables II and III. A composite view of the data of Tables II, III and IV shows that alloys of the present invention exhibit a unique combination of strength, oxidation resistance, and resistance to embrittlement not present in alloys outside the invention or alloys of the prior art.

A further measure of the unique utility of the alloys of the present invention as compared to other cobalt- Table V Alloy No.: Mils deflection after 500 cycles 1 31 F 96 G 26 The data in Table V shows that alloys of the present invention exhibit unique utility when subjected to thermally induced stress. Thermally induced stress and fatigues resulting therefrom is a day-to-day hazard which must be faced by any alloy used for any part in the hot-zone of an aircraft gas turbine engine. Basically the data in Table V shows that the thermal conductivity as measured by resistance-to-defiection of the alloys of the present invention is significantly better than the thermal conductivity of the v. prior art, cobalt-base, carbide-hardened alloy F and essentially equivalent to prior art alloy G which, as mentioned previously, has poor oxidation resistance and low creep-rupture strength. This large difference is primarily due to the metallographic factors discussed hereinbefore which, in turn, result from the metallurgical and elemental makeup of the novel alloys.

The alloys of the present invention also exhibit excellent room temperature (77 F.) and high temperature tensile characteristics are demonstrated by the data set forth in Table VI relating to alloy No. 1.

TABLE VI Ultimate Yield Percent Temperature tensile strength, Percent reduction strength percent elongation in area (K s.i.) offset (K si.)

1 R.T.=Room temperature.

Alloys of the present invention (the compositions of Which are expressed herein in terms of percent by weight) are highly useful for the manufacture of precision cast objects including nozzle guide vanes, turbine blades, and after burner components. Where necessary, the alloys of the present invention can be machined in the manner common to all carbide-hardened, cobalt-base alloys, i.e. by grinding.

Although the present invention has been described in conjunction with advantageous 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 inven tion and appended claims.

I claim:

1. A carbide-hardened alloy consisting essentially of, in weight percent, about 0.4% to about 0.7% carbon, about 18% to about 24% chromium, about 7% to about 15% nickel, about 6% to about 9% tungsten, about 2% to about 5% tantalum, about 0.1% to about 0.5% titanium, about 0.1% to about 1% zirconium with the balance being essentially cobalt together with impurities and incidental elements normally associated with cobalt-base alloys, said alloy being characterized by a high combination of engineering characteristics necessary for structures used under stress at elevated temperatures in oxidizing atmospheres.

2. A precision cast alloy object made of the alloy of claim 1.

3. A precision cast, gas turbine structure made of the alloy of claim 1.

4. An alloy as in claim 1, wherein the carbon content is at least about 0.5%, the chromium content is at about 19% and the nickel content is about 10%.

S. An alloy as in claim 1 wherein the amounts of the elements: carbon, tungsten, tantalum, titanium and zirconium are related such that the weight percent of tungsten is about twice the weight percent of tantalum, the weight percent of zirconium is about 2.5 times the weight percent of titanium, and the ratio of the atomic percent of carbon to the total of the atomic percents of tungsten, tantalum, zirconium and titanium is about 0.72.

6. A cobalt-base, carbide-hardened alloy consisting essentially of cobalt and, in weight percent, about 19% to about 23% chromium, about 8 to about 12% nickel, about 0.5% to about 0.7% carbon, about 7% tungsten, about 3.5% tantalum, about 0.2% titanium and about 0.5% zirconium.

7. A precision cast, vacuum-melted, carbide-hardened alloy consisting, in weight percent, of about 0.6% carbon, about 21.5% chromium, about 10% nickel, about 7% tungsten, about 3.5% tantalum, about 0.2% titanium, about 0.5% zirconium with the balance being essentially cobalt.

8. A precision cast, carbide-hardened alloy consisting of about 0.5% to 0.7% carbon, about 19% to 23% chromium, about 8% to 12% nickel, about 6% to 9% tungsten, about 2% to 5% tantalum, zirconium and titanium in amounts such that the minimum percentage of titanium is about 0.1%, the maximum percentage of zirconium is 1% and the percentage of zirconium is about 2.5 times the percentage of titanium, up to about 3% iron, up to about 0.5% manganese, up to about 0.5% silicon, up to 0.1% boron, up to 0.2% mischmetal, up to 2% columbium, up to 2% molybdenum and the balance being essentially cobalt.

References Cited UNITED STATES PATENTS 2,763,547 9/1956 Dyrkacz et al. 171 2,920,956 1/1960 Nisbet et al 75-171 2,974,036 3/1961 Thielemann 75171 3,026,199 3/1962 Thielemann 75171 RICHARD O. DEAN, Primary Examiner.

US. Cl. X.R. 14832 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,432,294 March 11, 1969 Harold L. Wheaton It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 56, "an", first occurrence, should read the Colu; 2, line 46, "any should read an Column 3, line 1 "mtlting" should read melting line 6, after "oxidation" insert a comma. Column 4, lines 32 and 33, "as a substitute for a part of the required tantalum shoull read (niobium) is not the full equivalent of tantalum line 38, "substrate" should read substitute line 53 "0.0% carbon" should read 0.60% carbon Column 5, TABLE II, fourth: column, line 1 thereof, '11. 1" should read 102. l same table II, in the heading to the fifth column thereof, "Elongation, percent (1 gauge length" should read Elongation percent (l gauge length) Column .6, .TABLE I in the heading to the second column thereof, "Temperature T. should read Temperature (F. Column 7, line 19, fatigues" should read fatigue line 26, after "the" cancel "v. same column .7, TABLE VI, in :the heading to the second and third column thereof, line 4, (Ks. i.) each occurrence, should read K.S.I. Column 8, line 19, before "about", second occurrence, insert least signed and sealed this 7th day of April 1970.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents