Nickel-chromi Um-cobalt-molybdenum Alloys

Abstract

Directed to nickel-chromium-cobalt-molybdenum alloy characterized by good strength in short-time and long-time testing over a range of temperatures up to circa 2,000.degree.F., by excellent resistance to cyclic oxidation, by structural stability when subjected to long-time heating at intermediate temperatures and by excellent workability both hot and cold.

Description

Development of alloys for use at elevated temperatures has now been proceeding for a substantial time. As a result, many alloys are currently available which offer satisfactory properties under many conditions of stress and temperature. Most of the available alloys depend for strength at elevated temperatures upon the presence therein of precipitation hardening phases such as the well-known gamma prime phase. It is known that the gamma phase dissolves within the matrix of the alloy when the temperature of exposure is raised sufficiently high. This solution temperature is on the order of 1,900.degree. F. Accordingly, alloys which are characterized by compositions which produce the gamma prime phase are not ordinarily considered usable at temperatures at which the gamma prime phase dissolves to any substantial extent. Accordingly, service applications in which the precipitation hardening alloys are employed are generally limited to an upper temperature range of possibly 1,600.degree. F. Of course, many other criteria of usefulness must be applied to a particular alloy depending upon the service requirements. Thus, in certain applications, such as in gas turbine combustion cans where very high operating temperatures (e.g., 2,000.degree. F. and above) are encountered, oxidation resistance and long-time structural stability of the alloy become of major importance. In such service, temperatures are encountered which are sufficiently high to cause substantially complete solution of precipitation hardening phases such as gamma prime. At the very high temperatures, e.g., temperatures on the order of 2,000.degree. F., cyclic oxidation also becomes a problem since it is found that many alloys which may be satisfactory against oxidation at the high temperature involved have such a constitution that the protective oxide breaks down or becomes mechanically separated when cyclic oxidation conditions are encountered, leading to further oxidation of metal upon subsequent heating. Accordingly, under conditions of cyclic oxidation, alloys which might appear to be satisfactory during long-term exposure to oxidation at a very high temperature are found in fact to be unsatisfactory when cyclic oxidation is encountered. It is known that cyclic conditions are encountered in jet engine operation as in start-up and shut-down. Furthermore, parts which operate at very high temperatures are also subjected to heating and cooling through intermediate temperatures, e.g., 1,400.degree. F. Thus, stability in the intermediate temperature ranges of alloys intended for service at very high temperatures is also of interest to designers for many reasons, including, for example, repair weldability particularly of light-section parts.

Accordingly, a demand has arisen for an alloy which would combine substantial strength at very high temperatures, e.g., 2,000.degree. F., with high resistance to cyclic oxidation and long-time structural stability over a wide range of temperatures. It is to the solution of these problems that the present invention is directed.

It is an object of the present invention to provide an alloy which combines substantial strength at very high temperatures with long-time structural stability upon exposure to a wide range of temperatures and high resistance to the deleterious effects of cyclic oxidation.

Generally speaking, the present invention is directed to an alloy consisting essentially of, by weight, about 20% to about 24% chromium, about 0.8% to about 1.5% aluminum, about 9.5% or about 10% to about 15% or even about 20% cobalt, about 7% to about 12% molybdenum, not more than 0.15% carbon, up to about 0.6% titanium, up to about 0.006% boron, up to about 0.1% zirconium, up to about 0.05% magnesium, and the balance essentially nickel. The alloy should have a low content of incidental elements and impurities such as sulfur (0.015% max.), phosphorus (0.03% max.), and should not contain more than about 1% of copper. The iron content of the alloy should also be limited and for best stress-rupture properties, particularly at 2,000.degree. F., should not normally exceed about 5%, or even about 2%. Titanium, magnesium, boron and zirconium within the aforementioned ranges may be employed individually or in combination as deoxidizers in producing the alloy. In view of the low contents of aluminum and titanium in the alloy, essentially no age hardening occurs therein. The contents of cobalt and of molybdenum are extremely important in terms of conferring substantial elevated temperature strength to the alloy. Thus, the molybdenum content should be within the range of about 7% to about 12% and the cobalt content should be within the range of about 9.5% or about 10% to about 15%, or even about 20%, as it is found from experimental data that within these ranges of cobalt and molybdenum the best strength combinations are provided. Chromium and aluminum within the aforementioned ranges are also important in the alloy particularly from the standpoint of providing oxidation resistance, particularly cyclic oxidation resistance, thereto. It is found that tungsten additions are not particularly effective in relation to the effect of this element upon strength. It appears from the available experimental data that tungsten in amounts up to about 8% is not sufficiently useful to compensate for the increased cost and increased density resulting from the use of this element. It is found that small additions of rare earth elements such as cerium (as misch metal) and lanthanum appear to offer some improvement in cyclic oxidation resistance at 2,000.degree. F. Accordingly, up to about 0.15% of these elements may be employed in the alloy for improvement in oxidation resistance without encountering offsetting effects in terms of strength. As previously stated, the carbon content can be up to about 0.15%. Preferably, for best strength at the very high temperatures of 1,800.degree. F., and above, the carbon content is maintained in the range of about 0.04% to about 0.1%, e.g., about 0.06% to about 0.08%. Columbium adversely affects cyclic oxidation resistance of the alloy, and, accordingly, is not present in more than impurity amounts.

Preferred alloys produced in accordance with the invention contain about 0.06% to about 0.08% carbon, about 22.0% chromium, about 1.0% aluminum, about 0.35% titanium, about 12.5% cobalt, about 9.0% molybdenum, about 0.003% boron, and the balance essentially nickel. The alloy in wrought form, annealed at 2,150.degree. F. and air cooled, provides at room temperature a yield strength (0.2% offset) of about 42 thousands of pounds per square inch (ksi.), a tensile strength of about 106 ksi, an elongation of about 70% and a reduction in area of about 57%; and at 2,000.degree. F., provides a yield strength (0.2% offset) of about 7 ksi, a tensile strength of about 11 ksi, an elongation of about 90% and a reduction in area of about 77%. Preferred alloys also provide 100 hour rupture lives as follows: at 1,500.degree. F., about 20,000 psi; at 1,700.degree. F., about 8,800 psi; and at 1,900.degree. F., about 3,700 psi.

Alloys in accordance with the invention are preferably produced by vacuum melting as more consistent properties are thereby produced, although air melting may be employed. The alloys are readily workable both hot and cold. Furthermore, the alloys are highly resistant to the effects of cyclic oxidation for prolonged time at 2,000.degree. F., and do not develop embrittling phases after prolonged exposure to temperatures in the range of about 1,200.degree. F. to about 1,600.degree. F. In addition the alloys are readily weldable by common gas-shielded arc-welding processes, including the metal-inert gas (MIG) process, using filler metal of matching composition or other standard welding materials. The alloy provided in accordance with the invention is also useful itself as a MIG filler metal, e.g., in the form of wire, strip, etc., in welding other nickel-chromium alloys, and nickel-chromium-iron alloys, especially alloys which have demonstrated a tendency toward cracking when welded using the MIG process in conjunction with other filler materials. Welds produced using the alloy of the invention as filler material are as strong at room temperature and at temperatures up to about 2,000.degree. F. and as oxidation resistant in the weld metal as the wrought alloy of the invention. In general, the alloy can be used as a MIG filler metal in welding alloys containing about 19% to 30% chromium, up to about 2% aluminum, e.g., about 0.8% to about 1.7% aluminum, up to about 0.6% titanium, up to about 15% molybdenum, up to about 50% cobalt, up to about 16% tungsten, up to about 4% columbium, up to about 20% iron, up to about 0.015% boron, up to about 0.1% zirconium, up to about 3% tantalum, and the balance essentially nickel.

In order to provide those skilled in the art with a better understanding of the invention the following illustrative examples are provided:

EXAMPLE I

A series of 10 kilogram melts was produced having compositions as given in the following Table I. Alloys 1 through 7 were produced by air induction melting, while Alloys 8 through 10 were produced by vacuum induction melting. The resulting ingots were all successfully forged to 9/16 inches square bar stock for mechanical testing. In some cases cold rolled strip 0.125 inches thick was also produced from this material for cyclic oxidation testing.

TABLE I __________________________________________________________________________ Alloy No. %C %Fe %Si %Ni %Cr %Al %Ti %Co %Mo %B % Other __________________________________________________________________________ 1 0.04 0.13 0.03 Bal 21.82 0.89 0.29 14.14 9.41 0.005 2 0.04 0.10 0.03 Bal 21.96 0.91 0.30 10.24 9.05 0.005 0.038 Ce 3 0.04 0.12 0.03 Bal 21.92 0.97 0.31 10.16 8.85 0.005 0.03 La 4 0.04 0.17 0.08 Bal 21.70 0.92 0.40 9.98 9.10 0.005 5 0.05 3.13 0.03 Bal 21.89 1.02 0.38 10.16 9.24 0.0057 6 0.06 0.16 0.41 Bal 21.94 1.01 0.39 10.12 9.28 0.0046 7 0.04 0.10 0.05 Bal 21.43 0.90 0.30 19.80 9.07 0.005 8 0.007 1.16 0.03 Bal 21.62 0.89 0.35 9.92 9.01 0.0079 9 0.016 0.22 0.03 Bal 21.66 0.92 0.35 10.13 9.28 0.0069 10 0.06 0.16 0.02 Bal 21.69 0.93 0.35 10.04 9.23 0.0061 __________________________________________________________________________

Hot-forged square bar material from the heats as set forth in Table I were subjected to an anneal at 2,150.degree. F. for 1 hour followed by air cooling, and short-time tensile tests were performed thereon at room temperature and at 2,000.degree. F. with the results set forth in the following Table II.

TABLE II __________________________________________________________________________ Room Temperature 2000.degree.F. Alloy No. 2% Y.S. T.S. Elong. R.A. 2% Y.S. T.S. Elong. R.A. Ksi Ksi % % Ksi Ksi % % __________________________________________________________________________ 1 44.2 116.0 64.0 61.3 10.9 10.9 70.0 68.7 2 37.0 92.5 39.0 54.5 10.6 10.7 45.0 45.0 3 46.7 123.0 54.0 65.3 10.6 10.6 47.0 43.6 4 46.3 116.0 56.0 58.7 10.5 10.5 79.0 73.6 5 45.7 114.8 58.0 60.3 5.7 10.1 74.0 55.3 6 44.5 111.6 59.0 54.4 7.6 18.0 88.5 68.7 7 67.0 126.0 50.0 64.5 15.5 15.5 43.0 78.0 8 42.0 106.5 64.0 65.8 5.3 9.6 84.0 56.0 9 41.5 109.0 66.0 69.6 7.5 9.1 78.0 48.4 10 46.0 112.5 66.0 63.8 8.2 10.8 52.0 36.4 __________________________________________________________________________

Stress rupture tests were performed upon material annealed at 2,150.degree. F. for 1 hour from the heats at 1,500.degree. F. and 2,400 psi, and at 2,000.degree. F. and 3,000 psi with the results set forth in the following Table III.

TABLE III __________________________________________________________________________ 1500.degree.F./24,000 psi 2000.degree.F./3,000 psi Alloy No. Life Elong. R.A. Life Elong. R.A. (hours) % % (hours) % % __________________________________________________________________________ 1 26.3 86.0 65.5 57.4 26.5 25.5 2 23.2 116.0 72.0 19.2 17.5 22.5 3 16.9 97.0 73.5 17.9 21.5 21.5 4 24.7 117.0 73.0 50.0 70.0 56.5 5 22.1 105.0 70.0 36.1 61.5 44.0 6 32.8 97.0 70.6 36.1 50.0 46.0 7 46.0 105.0 67.0 38.1 46.7 43.9 8 26.4 115.0 79.5 18.5 128.0 68.0 9 30.4 129.0 79.5 19.5 66.5 47.0 10 39.9 100.0 70.0 73.5 43.5 32.5 __________________________________________________________________________

Creep tests performed upon the alloys of Table I indicated that at 2,000.degree. F. minimum creep rates of the order of 0.005% to about 0.006% per hour were observed at 2,000.degree. F. and a 600 psi stress whereas at a 1,000 psi stress at 2,000.degree. F. minimum creep rates in the range of about 0.006% to about 0.018% per hour were observed.

EXAMPLE II

This example illustrates the special utility of the alloy provided in accordance with the invention as a MIG welding filler metal.

Two 1-inch thick heat joints were made between hot rolled plate specimens of an oxidation-resistant alloy containing about 0.07% carbon, about 60% nickel, about 1.3% aluminum, about 23% chromium, about 0.33% titanium, about 0.16% silicon and the balance essentially iron under conditions of severe restraint and with a joint design of the V-groove butt type, using the MIG process with argon shielding gas and matching composition filler metal 0.062 inches in diameter. For one weld the voltage was 33 to 35 and for the other the voltage was 29 to 31. was 270 to 290 DCRP. Filler wire was fed at about 173 inches per minute. Both weldments revealed a heavy oxide on the deposited weld beads. Radiographic examination revealed oxide particles in the deposited weld metal. Side bend tests on transverse slices cut from the welds revealed numerous fissures in the weld deposits. In addition, hot cracking was observed by macroexamination of the welds. The welds were clearly unsatisfactory.

The foregoing experiment was repeated using filler metal of essentially Alloy No. 4 composition. The weld beads appeared to be relatively free of oxide and were of good quality. Radiographic examination of the welds revealed essentially no defects in the welds. Side bend tests on transverse slices cut from the welds revealed no fissures when bent about a pin 1.5 inches in diameter. No hot cracking was evident. A cyclic oxidation test at 2,000.degree. F. in air for 300 hours using a 15 minute heating and a 5 minute cooling cycle performed upon a specimen 1/8-inch thick machined from the welded plate and measuring 3 inches by 3/4 inches with the weld appearing transverse of the center of the specimen indicated no loss of oxidation resistance in the weld area.

EXAMPLE III

A five thousand pound heat was produced in a commercial vacuum induction furnace and was cast to form an ingot about 11 inches .times. 45 inches .times. 50 inches. Portions of the heat were converted to 2 inch plate, 3/4 inch diameter hot rolled bar and 0.062 inch cold rolled sheet. No difficulties were experienced in either hot or cold processing of the material. In cold finishing the sheet a cold reduction of 58.7% was employed. The heat (Alloy No. 11) contained about 0.02% carbon, about 0.04% manganese, about 0.23% iron, about 0.05% silicon, 57.5% nickel, about 21.85% chromium, about 1.07% aluminum, about 0.4% titanium, about 0.028% magnesium, about 9.94% cobalt, about 8.86% molybdenum, and about 0.0018% boron. Properties of the hot rolled rod product annealed at 2,150.degree. F. for 1 hour and air cooled, and the cold rolled sheet product annealed at 2,150.degree. for about 5 minutes and then air cooled, were determined by means of the short-time tensile test at various temperatures in the annealed condition with the results set forth in the following Table IV.

TABLE IV __________________________________________________________________________ Yield Strength Tensile Reduction Hd. Temp. Ksi Strength, Elong., in Area R.sub.b .degree.F. 0.2% offset Ksi % % __________________________________________________________________________ ROD STOCK Room 40.6 105.0 70.0 72.7 79 1000 28.4 78.7 71.0 60.3 73 1200 26.5 77.0 71.0 64.0 77 1400 25.8 62.0 83.0 67.5 72 1600 25.5 36.8 122.0 88.5 74 1800 23.6 23.6 109.0 90.0 66 2000 9.5 9.5 132.0 91.0 66 SHEET STOCK Room 35.1 89.3 68.5 61 2000 8.9 8.9 102.0 80 __________________________________________________________________________

The sheet in the cold rolled condition had no difficulty passing a severe bend test upon itself after annealing in the temperature range of 1,800.degree. F. to 2,150.degree. F.

Stress-rupture tests performed on the rod stock and on the sheet so produced demonstrated that, in the 2,150.degree. F., annealed condition, the material exhibited a 100 hour life at 1,500.degree. F. and a stress of 17,500 psi, at 1,700.degree. F. and a stress of 71000 psi, and at 2,000.degree. F. and a stress of 3100 psi.

EXAMPLE IV

A further 5,000 pound heat was produced in a commercial vacuum induction furnace and was flux cast in air to form an ingot about 11 inches .times. 45 inches .times. 50 inches. The heat (Alloy No. 12) contained about 0.07% carbon, about 0.13% iron, about 0.04% silicon, about 22.51% chromium, about 1.05% aluminum, about 0.41% titanium, about 0.029% magnesium, about 12.67% cobalt, about 8.91% molybdenum, about 0.0051% boron and the balance essentially nickel. The ingot was press forged to a slab about 9.5 inches .times. 42 inches in section and was then hot rolled to a slab 2 inches by 50 inches in section at 2,200.degree. F. with no difficulty. Three 2 inch .times. 2 inch .times. 50 inch billets were abrasive cut from the slab and were hot rolled to 3/4 inch diameter rod. The remainder of the slab was cut in half and hot rolled to hot bands which were about 0.32 inch .times. 52 inch in section. One of the hot bands was annealed in a continuous furnace at 1,950.degree. F. and was cold rolled to 0.190 inch gauge. At this point, the material was again annealed at 1,950.degree. F. and was cold rolled to 0.062 inch gauge. The 0.062 inch gauge material was continuously annealed at 2,150.degree. F. Tensile tests were conducted at various temperatures upon hot rolled rod and upon the sheet material annealed at 2,150.degree. F. for 1 hour with the results set forth in the following Table V.

TABLE V ______________________________________ Yield Strength Tensile Reduction Temp. Ksi Strength, Elong., in Area .degree.F. 0.2% offset Ksi % % ______________________________________ ROD STOCK Room 42.9 106.6 70.0 57.2 1000 28.3 84.1 69.0 57.6 1200 24.8 82.4 75.0 54.5 1400 25.6 64.8 84.0 64.6 1600 28.4 40.7 120.0 92.5 1800 21.0 21.4 124.0 94.1 2000 7.4 11.5 90.0 77.5 SHEET STOCK Room 47.0 111.0 54.0 1000 31.5 85.5 56.0 1200 28.5 84.0 62.0 1400 30.0 67.5 76.0 1600 30.5 36.0 92.0 1800 14.5 19.5 58.0 2000 7.5 10.5 58.0 ______________________________________

Annealing tests on the sheet demonstrated that material annealed at temperatures of 2,100.degree. F. and higher withstood the bend test upon itself without cracking.

Stress rupture tests conducted upon the hot rolled rod annealed at 2,150.degree. F. for 1 hour indicated a 100 hour life at 1,500.degree. F. and 20,000 psi, at 1,700.degree. F. and 9,400 psi and at 2,000.degree. F. and 4,500 psi.

EXAMPLE V

Hot rolled rod material from alloys 6, 8, 9, 10, 11 and 12 annealed at 2,150.degree. F. for 1 hour were subjected to long-time exposure at 1,200.degree., 1,300.degree., 1,400.degree., 1,500.degree. and 1,600.degree. F. and then subjected to impact testing with the results set forth in the following Table VI.

TABLE VI __________________________________________________________________________ Heat Treatment Alloy No. 6 Alloy No. 10 Alloy No. 11 Alloy No. 12 Temp. .degree.F. Time Impact CVN Hardness Impact CVN Hardness Impact CVN Hardness Impact CVN Hardness hrs. ft. lbs. R.sub.b ft. lbs. R.sub.b ft. lbs. R.sub.b ft. lbs. R.sub.b __________________________________________________________________________ 1200 50 -- -- -- -- 240 81 190.5 82 1200 1000 49.5 94 1300 50 60 97 128 90 122 92 79 93 1300 1000 50 94 1400 50 58 93 68 93 69 85 68 89 1400 1000 70.5 88 1500 50 46 93 64 93 39 86 73 90 1500 1000 78.5 87 1600 50 39 93 67 92 52 83 86 87 1600 1000 87 84 __________________________________________________________________________

X-ray examination of the material after long-time heating established that the only phase which appeared as a result of the heating was a carbide phase corresponding to the M.sub.23 C.sub.6 phase.

EXAMPLE VI

Sheet material from Alloys 2, 3, 4, 11 and 12 was subjected to a cyclic oxidation test in air employing a 15 minute heating to 2,000.degree. F. and a 5 minute cooling cycle for a total test time of 1,000 hours. At the conclusion of testing, the total depth of attack on the sheet specimens was about 0.0026 inches for each of Alloys 2 and 3, about 0.0042 inches for Alloy 4 and about 0.003 inches for Alloy 11 and about 0.003 inches for Alloy 12. Periodic weighing of the samples during the course of the test demonstrated that there was essentially no change of weight in any of the samples during the course of the testing thereby demonstrating that each of these alloys was highly resistant to the 2,000.degree. F. cyclic oxidation test.

The alloy of the invention may be melted in conventional melting equipment such as air or vacuum induction furnaces or electroslag furnaces. It is useful in applications such as gas turbine combustion liners, in ducting systems for aircraft, etc. It is particularly useful in any application in which cyclic oxidation at temperatures about 1,800.degree. F., e.g., 2,000.degree. F. and higher, are encountered.

Tests in aqueous solutions of common mineral acids demonstrated that the alloy has good resistance to corrosion therein. Thus, the alloy displayed good resistance to various concentrations of nitric acid, together with good resistance to sulfuric acid in concentrations up to 30% at 80.degree. C. and up to 10% at boiling temperatures. Moderate resistance was found to hydrochloric acid in concentrations to 30% or more at 80.degree. C. as well as an excellent resistance to all concentrations of phosphoric acid at 80.degree. C. even in the presence of up to 1% hydrofluoric acid. Accordingly, the alloy is useful in areas where resistance to acid corrosion is required.

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.

Claims

We claim:

1. A welding material in wire or strip form for use in inert gas shielded metal arc welding consisting essentially of by weight, about 20% to about 24% chromium, about 0.8% to 1.5% aluminum, about 9.5% to about 20% cobalt, about 7% to about 12% molybdenum, not more than 0.15% carbon, up to about 0.6% titanium, up to about 0.006% boron, up to about 0.1% zirconium, up to about 0.05% magnesium and the balance essentially nickel.

2. A (i) solid-solution, (ii) essentially non-age-hardenable, (iii) weldable nickel-base alloy characterized by not only (iv) good resistance to oxidation but also by (v) high resistance to cyclic oxidation at temperatures as high as 2,000.degree. F., the alloy also processing, (vi) structural stability after long-time heating and (vii) substantial strength at room temperature and at elevated temperatures, said alloy consisting essentially of, by weight, about 20% to about 24% chromium, about 0.8% to about 1.5% aluminum, about 9.5% to about 20% cobalt, about 7% to about 12% molybdenum, not more than 0.15% carbon, up to about 0.6% titanium, up to about 0.006% boron, up to about 0.1% zirconium, up to about 0.05% magnesium, up to about 0.15% of a metal from the group consisting of cerium and lanthanum, and the balance essentially nickel.

3. An alloy in accordance with claim 2 containing about 10% to about 15% cobalt.

4. An alloy in accordance with claim 2 containing about 0.06% to about 0.08% carbon, about 22% chromium, about 1% aluminum, about 0.4% titanium, about 12.5% cobalt, about 9% molybdenum, up to about 0.006% boron and the balance essentially nickel.