WO1997005297A1

WO1997005297A1 - Cobalt alloy

Info

Publication number: WO1997005297A1
Application number: PCT/US1996/009765
Authority: WO
Inventors: Narayana S. Cheruvu
Original assignee: Westinghouse Electric Corp
Current assignee: Westinghouse Electric Corp
Priority date: 1995-07-28
Filing date: 1996-06-10
Publication date: 1997-02-13
Anticipated expiration: 1998-01-28

Abstract

There is provided by the invention a cobalt-based alloy comprising the following elements in weight percent: Carbon 0.35 to 0.55; Tantalum 3.00 to 4.50; Columbium 0.00 to 0.75; Chromium 22.0 to 25.0; Nickel 9.0 to 11.0; Titanium 0.15 to 0.50; Tungsten 6.5 to 7.5; Aluminum 0.10 to 0.25; Zirconium 0.00 to 0.05; and Cobalt balance. The cobalt-based alloy of the invention can extend component life in high temperature applications with exposure to frequent heating and cooling cycles, such as combustion turbine first stage vanes.

Description

COBALT ALLOY

FIELD OF THE INVENTION This invention relates to the field of cobalt-based alloys. More particularly, the invention relates to a cobalt- based alloy that extends component life in high temperature applications involving exposure to frequent heating and cooling cycles, such as combustion turbine first stage vanes.

BACKGROUND OF THE INVENTION Service life of high temperature components, such as gas turbine blades and vanes, depends upon the material's creep strength and upon its low cycle fatigue strength. Low cycle fatigue strength, in turn, depends upon ductility. However, strength and ductility are inversely related properties.

In some applications, such as the combustion turbine first stage, vanes are exposed to thermal stresses during frequent heating and cooling cycles, making material with high ductility essential. Insufficient ductility results in cracking (from low cycle fatigue) and reduced service life. That is, the components are not sufficiently ductile to withstand repeated thermal shocks from alternate heating and cooling as the turbines are cycled on and off. With higher gas turbine inlet temperatures expected in the future, thermal stresses will significantly increase, thus further reducing service life of high^* temperature components. Hence, for improved reliability and availablity, the engines of the future will demand a material with high ductility.

U.S. Patent 3,432,294, which patent is incorporated by reference as if fully set forth herein, discloses a carbide-hardened cobalt based alloy (hereinafter referred to as the "MAR-M 509 alloy"). The MAR-M 509 alloys as taught by U.S. Patent 3,432,294 consists 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 and relatively high levels of zirconium at about 0.1% to about 1% with the balance being essentially cobalt together with impurities and incidental elements normally associated with cobalt-based alloys. Among the incidental elements disclosed to be present in U.S. 3,432,294 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% of columbium and up to about 2% of molybdenum.

With the MAR-M alloy, there is has been reported difficulty in effectively applying protective coatings due to the mold metal reaction.

U.S. Patent 4,082,548, which patent is incorporated by reference as if fully set forth herein, teaches a cobalt- based alloy (hereinafter referred to as the "ECY-768 alloy") having a minimum practicable of zirconium so that detrimental inter-dendritic carbide oxidation is suppressed and to alleviate the surface integrity and mold metal reaction problems of the prior art alloy MAR-M 509. The ECY-768 alloys as taught by U.S. Patent 4,082,548 consist essentially of, in weight percent, 0.55% to 0.65% carbon, 22.5% to 24.25% chromium, 9% to 11% nickel, 6.5% to 7.5% tungsten, 3% to 4% tantalum, 0.1% to 0.5% titanium and relatively low levels of zirconium at 0.0% to 0.05% with the balance being essentially cobalt together with 0.10% to 0.20% aluminum, 0.0% to 1.5% iron, 0.0% to 0.01% boron, 0.0% to 0.4% silicon and 0.0% to 0.1% manganese. While the strength of is MAR-M 509 and ECY-768 are increased by thermal exposures during coating treatments or by exposure to service, the ductility of existing vanes made of these alloys is lowered. Accordingly, there remains a need for improved cobalt-based alloys which extend the service life of high temperature components, such as combustion turbine first stage vanes, which are exposed to frequent heating and cooling cycles. SUMMARY OF THE INVENTION

The invention provides a novel cobalt-based alloy having optimized levels of carbon, while maintaining a minimum practicable zirconium level, which stabilizes the microstructure of components made of such alloys thereby reducing ductility loss caused by thermal treatment or long term service. The alloy of the invention, inter alia , extends component life, and improves reliability of combustion turbine vanes.

There is provided by the invention a novel cobalt- based alloy comprising the following elements in weight percent: Carbon 0.35 to 0.55; Tantalum 3.00 to 4.50; Columbium 0.00 to 0.75; Chromium 22.0 to 25.0; Nickel 9.0 to 11.0; Titanium 0.15 to 0.50; Tungsten 6.5 to 7.5; Aluminum 0.10 to 0.25; Zirconium 0.00 to 0.05; and Cobalt balance. Further provided by the invention are novel cast components, such as gas turbine vanes, cast from the alloy of the invention.

Other objects and advantages will become apparent from the following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. IA and IB are photomicrographs (1000X) of an ECY-768 alloy component from Westinghouse Electric Corporation, Pittsburgh, PA; as cast, before thermal treatment (A) ; and after thermal treatment for 30 hours at 1900^βF, or about 1038°C (B) . The arrows in Fig. IB denote carbon precipitates formed after thermal treatment. Fig. 2A and 2B are photomicrographs (4OOX and 1000X respectively) of an alloy of the invention (M4) ; as cast, before thermal treatment (A) ; and after thermal treatment at

1900T, or about 1038°C (B) . No significant precipitation of carbides was seen after thermal treatment.

Fig. 3 is a graph comparing ductility of two alloys of the invention (M3 and M4) with an ECY-768 alloy after exposure to a thermal cycle (- 30 hours) at 1900"F (about 1038^βC) . Fig. 4 is a graph comparing tensile strength of two alloys of the invention (M3 and M4) with an ECY-768 alloy after exposure to a thermal cycle (- 30 hours) at 1900 (about 1038°C) .

DETAILED DESCRIPTION OF THE INVENTION Generally speaking, the present invention is directed to a cobalt-based alloy comprising the following elements in weight percent: Carbon 0.35 to 0.55; Tantalum 3.00 to 4.50; Columbium 0.00 to 0.75; Chromium 22.0 to 25.0; Nickel 9.0 to 11.0; Titanium 0.15 to 0.50; Tungsten 6.5 to 7.5; Aluminum 0.10 to 0.25; Zirconium 0.00 to 0.05; and the balance being substantially Cobalt. Of course the alloy of the invention may further comprise impurities and incidental elements generally associated with cobalt-based alloys. Among the incidental elements which may be present in the alloy of the invention are, for example, Iron 0.0 to 1.5; Boron 0.00 to 0.01; Silicon 0.00 to 0.40; and Manganese 0.00 to 0.10. All element amounts disclosed herein are in weight percent unless otherwise indicated.

The disclosed alloy of the invention is an improvement over the ECY-768 alloy now in use. This disclosed chemical modification includes removing the excess carbon believed to be the agent of lowered ductility, while maintaining minimal practicable levels of zirconium, thereby stabilizing the vane microstructure during the coating thermal treatment or long-term service. Total carbon content was lowered from a nominal 0.6% to between 0.375% and 0.55%. Thus, the original excess 0.1% carbon is substantially no longer available in solution in the matrix for precipitation during the coating thermal cycles and/or service. The alloy of the invention additionally comprises the element columbium in the range from 0.0 to 0.75 and a range for tantalum of 3 to 4.5. The amounts of columbium and tantalum can be varied in the alloy of the invention depending on the amount of carbon employed as one function of both the columbium and tantalum is to help minimize carbide formation by tying up carbon as tantalum or columbium carbide. Thus, the amount of these elements are readily selected by those skilled in the art to reduce carbide formation- during thermal exposure and extended service.

Material analysis reveals that ductility of existing ECY vanes (made of the Westinghouse ECY-768 alloy) is lowered by thermal exposure during coating treatments and/or by exposure to service. Coating processes generally include two thermal cycles: 1800-1900^βF (about 982-1038°C) for six hours maximum (cycle 1); and 1825-1900°F (about 996-1038^βC) for thirty hours maximum (cycle 2) . Studies of a MAR-M 509, an alloy of the Martin

Marietta Co., New York, NY, and studies of ECY-768 an alloy of the Westinghouse Electric Co., Pittsburgh, PA have shown that the coating thermal cycles or service exposure increases strength and lowers ductility. Table I tabulates the effects of coating thermal treatment on the strength and ductility of an alloy ECY-768 (ductility as measured by percentage elongation and by reduction of area) . "Before Treatment" tensile properties (i.e., before coating thermal cycles, or "as cast") are compared with "After Treatment" tensile properties (i.e. , after coating thermal cycles) . For example, in carrot or test bar #1, YS (yield strength) increased slightly from 51.2 before treatment to 59.3 after treatment while RA (% reduction of area) drops dramatically from 12.0 before treatment to 2.9 after treatment and EL (% elongation) drops from 9 to 2.5. Overall, the average ductility loss is «75%.

The effects of coating thermal cycles on tensile strength and ductility, as measured by ASTM-E8, in ECY-768 are shown in Table I. TABLE I

TENSILE PROPERTIES OF ECY-768 BEFORE AND AFTER COATING THERMAL TREATMENT

STRENGTH* DUCTILITY⁸ STRENGTH DUCTILITY

CARROT BEFORE TREATMENT AFTER TREATMENT YS UTS EL RA YS UTS EL RA ksi ksi % % ksi ksi % %

1 51.2 83.5 9.0 12.0 59.3 98.3 2.5 2.9*

2 47.9 92.3 4.5 3.5 65.8 81.5 2.9 4.0

3 39.4 82.6 10.7 12.5 60.8 97.0 3.9 3.4

4 51.8 92.0 4.6 6.7 48.4 90.6 4.2 4.7

.5 52.4 92.9 5.3 7.4 52.8 89.1 4.9 3.3

6 52.8 94.8 5.0 6.8 50.7 89.7 3.5 4.4

7 53.5 94.8 4.0 4.9 50.8 94.0 3.3 5.2

8 39.2 93.5 15.1 16.0 57.6 93.9 2.7 3.2*

9 48.9 67.6 6.4 8.3 65.9 80.6 2.3 2.8*

10 44.9 69.3 6.8 9.9 60.6 81.7 3.3 3.5

11 40.6 66.2 8.2 6.8 61.8 82.3 2.1 2.5*

12 41.7 67.8 7.2 6.2 59.1 80.6 2.6 2.5*

13 40.4 67.5 6.6 8.7 62.9 83.2 2.0 2.4*

14 37.7 67.0 8.2 8.5 65.5 83.9 2.0 2.7*

15 41.0 69.8 8.9 13.0 63.6 82.3 2.1 2.6*

16 42.3 69.0 6.0 7.5 60.8 81.9 2.1 2.4

17 39.6 66.9 7.3 9.0 61.8 82.3 2.2 3.7

18 39.9 68.7 10.3 11.9 60.7 82.1 2.2 2.3

19 38.6 67.7 8.4 8.4 49.8 70.3 2.0 1.4

20 43.6 71.5 8.5 8.1 62.3 77.5 1.9 1.9

21 41.8 71.7 8.2 13.2 64.5 83.5 2.1 2.5

22 42.9 70.2 8.2 9.3 62.8 80.3 2.1 2.1

23 42.5 70.9 7.9 9.4 62.1 82.7 2.6 1.8

^♦Average of two tests Carrot bars were taken at random. ^A STRENGTH: YS = yield strength UTS = ultimate tensile strength ^B DUCTILITY:

EL = elongation

RA = reduction of area The microstructural change responsible for lowering ductility is believed to be the precipitation of carbides from the matrix during the coating thermal cycles. Figures IA and IB are optical micrographs (power 1000X) of the ECY-768 microstructure that compare a sample "Before Treatment" with a sample "After Treatment". A precipitation of carbides has taken place during the thermal cycles and is clearly shown in Fig. IB. It is this additional carbide precipitation during the coating thermal treatment that is believed responsible for the increased tensile and yield strengths and decreased ductility.

The ECY-768 matrix, and the carbides present, were further analyzed before and after thermal exposure. Results showed that in the "as cast" structure, most of the carbon precipitated as primary MC carbides (Tantalum Carbide and Tungsten Carbide) and as a smaller amount of M₂₃C₆. Where total carbon content was nominally 0.6%, these carbides together accounted for 0.5%. Thus, 0.1% carbon remained in solution in the matrix to precipitate during the coating treatment. Carbon remaining in solution in the "as cast" matrix (0.1%) was determined to be the carbon that later precipitated as carbides (M₂₃C₆) during the coating thermal cycles. Further, the 75% decrease in ductility (shown in Table I) is believed to be primarily caused by the precipitation of this excess 0.1% carbon during heating.

Figures 2A, 2B, 3 and 4 show results of tests on sample bars made of alloys of the invention (M3 and M4) : Figure 2A and 2B show optical micrographs of an alloy of the invention (M4) that compare the microstructure "as cast" or "Before Treatment" (power 400X) with the microstructure "After Treatment" (after coating thermal cycles) (power 1000X) . Note that no significant additional carbides precipitated from the M4 matrix during the coating thermal cycles.

Figures 3 and 4 compare ductility and tensile strength of alloys of the invention (M3 an M4) with ECY-768 after exposure to thermal cycles. Ductility (EL, RA) increased significantly in the M3, M4 alloys: by » +300 to 400%. Strength (YS, UTS) is surprisingly maintained in the M3, M4 alloys: at « -10%. In addition, creep rupture test results (data not shown) showed M3 and M4 to be comparable to ECY-768. (That is, under creep loading, service life is not reduced with M3 and M4.)

Thus, alloys of the invention, such as the alloy referred to herein as "WES-100", chemistry as shown below, has increased ductility without adversely affecting the tensile and/or creep strength. The disclosed chemistry for WES-100, described as a range including M3 and M4 is as follows in weight percent:

ELEMENT M3 - M4(wt%) ND = not determined

C 0.39 - 0.50 BAL = balance

Si 0.06 - ND (% remaining)

Mn <0.01 - ND

Cr 22.99 - 22.01

Fe 0.08 - ND

Ti 0.19 - 0.19

Al 0.18 - 0.21

Co BAL - BAL

Ni 9.91 - 10.03

W 6.84 - 6.83

Zr <.01 - ND

Cb <.02 - 0.42

Ta 3.11 - 3.84

S 0.0013- ND

The alloy of the invention is produced by methods known to those in the art and the alloy is particularly suitable to be formed by casting into components useful at elevated temperatures such as are encountered by gas turbine vanes.

Benefits of the alloy of this invention include extended component life by increasing material ductility without compromising tensile strength. Additionally, the alloy of the invention is believed to improve reliability of combustion turbine vanes currently cast of the ECY-768 alloy. The alloy of the invention thus provides a benefit both in gas turbine vanes and in other applications such as aircraft turbines.

Claims

CLAIMS:

1. A cobalt-based alloy comprising the following elements in weight percent:

Carbon 0.35 to 0.55; Tantalum 3.00 to 4.50; Columbium 0.00 to 0.75; Chromium 22.0 to 25.0; Nickel 9.0 to 11.0; Titanium 0.15 to 0.50; Tungsten 6.5 to 7.5; Aluminum 0.10 to 0.25;

Zirconium 0.00 to 0.05; and Cobalt balance.

2. The alloy of claim 1 further comprising the following elements in weight percent: Iron 0.0 to 1.5;

Boron 0.00 to 0.01;

Silicon 0.00 to 0.40; and

Manganese 0.00 to 0.10.

3. A cast alloy component made of the alloy according to claim 1.

4. The cast alloy component of claim 3 wherein said component is a gas turbine vane. 5. A gas turbine having a plurality of vanes, at least some of said vanes being made from an alloy comprising the following elements in weight percent:

Carbon 0.35 to 0.55; Tantalum 3.00 to 4.50;

Columbium 0.00 to 0.75;

Chromium 22.0 to 25.0;

Nickel 9.0 to 11.0;

Titanium 0.15 to 0.50; Tungsten 6.5 to 7.

5;

Aluminum 0.10 to 0.25;

Zirconium 0.00 to 0.05; and

Coba1t ba1ance.

6. The gas turbine of claim 5 wherein said at least some of said vanes further comprises the following elements in weight percent: Iron 0.0 to 1.5; Boron 0.00 to 0.01; Silicon 0.00 to 0.40; Manganese 0.00 to 0.10.