CA2152550A1

CA2152550A1 - Oxidation resistant cobalt-based alloy

Info

Publication number: CA2152550A1
Application number: CA 2152550
Authority: CA
Inventors: Narayana Sastry Cheruvu
Original assignee: Individual
Current assignee: CBS Corp
Priority date: 1994-06-27
Filing date: 1995-06-23
Publication date: 1995-12-28
Also published as: ITPD950126A1; JPH0849032A; ITPD950126A0; IT1281951B1

Abstract

A cobalt-based alloy is provided that exhibits superior oxidation resistance to high temperature, gas turbine operating conditions. The cobalt-based alloy is beneficially employed as the substrate for gas turbine blades, blade rings, and vanes. The cobalt-based alloy contains as major constituents the common elements of cobalt, chromium, nickel, and tungsten. Selected minor amounts of aluminum and hafnium are incorporated into the cobalt-based alloy to improve its oxidation resistance. Optionally, minor amounts of such elements as tantalum and silicon can also be employed in the formulation of the alloy composition to further improve oxidation resistance.

Description

21~2550 - 1 - 57,917 ~MPROVED O~IDATION RESISTANT COBALT-BASED ALLOY

FIELD OF THE INVENTION
The present invention is directed towards cobalt-based alloys useful as substrates for gas turbine components exposed to relatively high temperature operating conditions.
Specifically, the invention is directed towards cobalt-based alloys containing selected minor amounts of aluminum and hafnium to improve oxidation resistance at such operating conditions.
BACKGROUND OF THE INVENTION
Recent developments in the gas turbine power generation systems relate to the increased temperatures and pressures of operating the gas turbines which allow for increased power output. Firing temperatures have increased from about 980C (1800F) in about 1970 to about 1150C
(2100F)-1425C(2600F) today. These harsh operating conditions require that the first stage turbine components, such as the blade rings, blades, and vanes be made from a suitably durable alloy material. The ability to operate at such high temperatures is in part due to the recent advances in air cooling technology. However, although air cooling can be used to dissipate enough heat to sufficiently extend the service life of most superalloys commonly used for first section turbine components, its use of substantially decreases the efficiency of the turbine. Generally, blade rings can show signs of oxidation wear after about 100 hours of operation at the harsher operating conditions.

- 21525~

- 2 - S7,917 The first section combustion turbine blade and vane components are also exposed to thermal stresses during frequent heating and cooling cycles. These turbine components must be made of an alloy material having excellent oxidation s and hot-corrosion resistance (particularly in gaseous sulfur-containing conditions), high resistance to thermal fatigue, relatively good weldability for ease in manufacture and repair, sufficient creep strength, and good castability.
Various alloy compositions have been proposed or used for such blade, vane, and blade ring components. Such alloys are primarily cobalt-based, which are generally easier to weld and offer good corrosion resistance. Commonly known alloys are set forth in Table I.
TABLE I
Nominal Elemental Compositions Alloy Composition (wt.%) X-40 X-45 FSX-414 GTD-484 Cr 25 25 29 29 Ni 10 10 10 10 Co Bal Bal Bal Bal Fe Mo C 0.5 0.25 0.25 0.3 B 0.01 0.01 0.01 Ta - - - 0.5 Hf The FSX-414 and GTD-484 alloys contain higher levels of chromium for alleged oxidation/corrosion resistance.
However, it is believed that such high levels of chromium may - 2152~SO

- 3 - 57,917 actually decrease the oxidation resistance of the alloy composition at operating temperatures of above about 815C
(1500F).
The oxidation and corrosion resistance of first stage combustor components can be enhanced by applying a protective overlay coating to the base superalloy surface.
The overlay coating can therefore extend the service life of the component when exposed to higher temperature firing conditions. Overlay coatings are generally of either the MCrAlY type where M represents Ni, Co, Fe, and combinations thereof, or of the "aluminide" type, which are produced by an aluminizing technique forming cobalt and nickel aluminide on the component surface. These coatings form a barrier to oxygen diffusion to the base alloy component surface.
A need exists in the art to provide an alloy composition for use as a substrate for gas turbine blades, vanes, or blade rings that can withstand high temperature combustion firing. The alloy material should especially exhibit enhanced oxidation stability during operation.
SUMMARY OF THE INVENTION
A novel cobalt-based alloy is set forth that can be advantageously employed as the base alloy for various metal components within a gas turbine. The alloy is principally used to fabricate turbine blade rings, blades, or vanes, especially those which operate in relatively high temperature conditions. The cobalt-based alloy contains relatively small quantities of selected elemental compounds to improve the oxidation resistance of the turbine component made therefrom.
The alloy is referred to as a "cobalt-based" alloy in that cobalt is present in a greater ~uantity than any other element. Typically, the cobalt content is from about 38-67%
wt. of the alloy. The cobalt-based alloy additionally contains chromium, nickel, and tungsten as a major constituents. The chromium is present in an amount of from about 21-25% wt., the nickel is present in an amount of from about 8-12% wt., and the tungsten is present in an amount of from about 6-9% wt.

21S25~i0 - 4 - 57,917 The high temperature oxidation resistance of the cobalt-based alloy is improved by the addition of selective amounts of aluminum and hafnium. The aluminum is present in the cobalt-based alloy in an amount of from about 0.1-1.5% wt.
and the hafnium is present in an amount of from about 0.15-1.5% wt. Tantalum can also be incorporated into the cobalt-based alloy in an amount of from about 1-4~ wt. to improve the oxidation resistance. Also, silicon can optionally be added into the cobalt-based alloy in an amount of from about 0.15-1%
wt. to further improve oxidation resistance.
The cobalt-based alloy can additionally contain various other common metallurgical elements, such as incidental elements and impurities, that do not detract from the oxidation and other performance characteristics of the alloy and that are commonly found in commercially available cobalt-based alloys. Such elements include carbon, manganese, iron, titanium, zirconium, niobium, and sulfur.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is an isometric view of a gas turbine vane.
Fig. 2 is an isometric view of a gas turbine rotating blade.
DETAILED DESCRIPTION OF THE INVENTION
An improved alloy composition is set forth that exhibits superior oxidation resistance at high temperature, gas combustion turbine operating conditions. The alloy composition i8 advantageously employed as the substrate material for various metal components found in a gas turbine, especially first stage combustion blade rings, blades, and vanes, preferably blade rings. An example of a gas turbine vane 4 is shown in Fig. 1 and an example of a rotating blade 1 with outer shroud 2 and inner shroud 3 is shown in Fig. 2.
The alloy composition is a cobalt-based alloy in which the chromium content is not increased to impart improved oxidation resistance as in the FSX-414 and GTD-484 alloys that contain about 29% wt. chromium. The chromium content of the present alloy composition is generally from about 21-25% wt., and more preferably from about 22.5-24.25% wt. The 215255~

- 5 - 57,917 maintenance of the chromium content at levels below about 29%
wt. allows for enhanced properties such as improved oxidation resistance.
The improved oxidation performance of the cobalt-based alloy composition results from the incorporation ofselected amounts of aluminum and hafnium. The addition of such components improves the oxidation stability of the alloy composition, specially when it is exposed to temperatures of at least about 890C (1650F). The composition forms an oxide layer that acts as a protective coating on the surface of the component and that prevents further oxidation.
Aluminum is incorporated into the cobalt-based alloy in an amount such that the properties of the alloy are improved for use in the high temperature turbine environment.
Aluminum can be present in the alloy composition in an amount of from about 0.1-1.5% wt., preferably from about 0.25-1.25%
wt., and more preferably from about 0.5-1% wt. Aluminum is beneficial in that upon its oxidation a protective oxide scale is formed on the surface of the component. The aluminum content is preferably maintained below the recited upper levels to improve the weldability of the cobalt-based alloy.
Further, higher levels of aluminum can also lead to a rapid rate of oxide formation and subsequent spalling of that oxide.
Hafnium is incorporated into the cobalt-based alloy composition to aid in the adherence of the aluminum oxide to the surface of the component. Hafnium can be present in the alloy composition in an amount of from about 0.15-1.5% wt., preferably from about 0.25-1.% wt., and more preferably from about 0.25-0.7% wt. The addition of hafnium in excess of the recited upper limits can detract from the creep strength of the component and can also cause detrimental reactions within the die cavity mold upon formation of the component leading to various surface imperfections.
Tantalum can be present in the alloy composition to further increase the oxidation performance of the resulting component. Tantalum is generally present in an amount of from about 0.75-4% wt., preferably from about 1-3% wt., and more - -- 21525~

- 6 - 57,917 preferably from about 1-2% wt. Silicon can also be present in the alloy composition in an amount of from about 0.15-1%
wt., preferably from about 0.25-0.75% wt., and more preferably from about 0.25-0.5~ wt.
The alloy composition is said to be cobalt-based in that cobalt (Co) is present in a greater quantity than any other element and is generally referred to as constituting the "balance" of the alloy composition. The amount of cobalt in the alloy compositions is therefore generally about 38-67%
wt., preferably about 40-65% wt. The other elements that are present in a substantial amount are chromium (Cr), as noted above, nickel (Ni), and tungsten (W). The amount of nickel is generally from about 8-12% wt. and preferably from about 9-llt wt. of the alloy composition. The amount of tungsten is generally from about 6-9% wt. and preferably from about 6.5-7.5% wt. of the alloy composition. Cobalt-based alloy components, as compared to those made from nickel-based alloys, are preferred for use in the gas turbine since they are easier to repair weld, exhibit better thermal fatigue strength, and exhibit longer life under high temperature and low stress conditions.
The alloy composition can also contain other elements useful in enhancing the overall properties of the alloy for use in rigorous operating conditions found within the first stage turbine section. Such elements include carbon (C), manganese (Mn), iron (Fe), titanium (Ti), zirconium (Zr), columbium or niobium (Nb), and sulfur (S). Other elements, such as incidental and impurity elements, can be found within the alloy composition so long as they do not detract from the improved oxidation resistance characteristics of the alloy when used as the substrate for components exposed to high temperature environments such as those found in the first stage turbine section. Table II sets forth the elemental chemistry for the preferred alloy compositions of the present invention.

21525S~

- 7 - 57,917 TABLE II
Nominal Elemental Alloy Composition Composition (wt%) Broad Preferred C 0.2-O.S 0.2-0.3 Si 0.15-1 0.25-S
Cr 21-25 22.5-24.5 Mn 0.15 max 0.1 max Fe 2 max 1.5 max Ti 0.15-1 0.2-0.5 Al 0.1-1.5 0.5-1 Ni 8-12 9-11 Hf 0.15-1.5 0.25-0.7 W 6-9 6.5-7.5 Zr 0.75 max 0.05 max Nb 0.05 0.25-0.5 Ta 1-4 1-2 S 0.05 max 0.01 max Co balance balance The alloy composition can be advantageously employed as a substrate for gas combustion turbine vanes, blades, and blade rings. The alloy composition is especially designed to perform with superior oxidative resistance under extreme thermal conditions found most commonly in first stage turbine sections. The temperatures in these sections are generally at least about 900C (1650F), usually about 1150C (2100F)-1425C(2600F), and in some cases about 1250C (2300F)-1425C(2600F). The advanced cooling technology extends the operational life of turbine components that are made from the - 21~25~

- 8 - 57,917 present alloy compositions and exposed to such thermal operating conditions.
The turbine component life can be further extended by applying a protective coating to the surface of the turbine component. Various coatings have been developed to protect the surface of the substrate. One type of such coatings is referred to as an "overlay" coating. These coatings are generally denoted as MCrAlY coatings where the M represents such elements as Ni, Co, Fe, and combinations thereof. These coatings derive their protective capability from their ability to form a thin layer of alumina scale on the outer exposed surface. This alumina layer has been found to be quite beneficial in oxidation resistance. Additives such as yttrium, hafnium, and silicon have been incorporated into such overlay coatings to improve coating overall performance to improve the alumina adherence to the substrate, and to aid in the regeneration of the alumina scale. The overlay coatings are typically applied to the substrate surface through such processes as low pressure plasma spraying, physical vapor deposition, ion plating, and sputtering or slurry sintering.
Examples of such overlaying coatings are set forth in U.S.
Pat. Nos. 4,615,865; 4,585,481; 4,198,442; 4,101,715; and 3,754,903, which are incorporated by reference herein in their entireties. Generally, such overlay coatings contain from about 5-40% wt., preferably about 15-30% wt. chromium; about 8-35% wt., preferably about 10-20% wt. aluminum; 0.1-3% wt., preferably 0.1-2% wt. yttrium; the balance being a combination of cobalt and nickel. The overlay coating can also contain silicon in an amount of from about 0.1-7% wt., and hafnium in an amount of from about 0.1-2% wt.
Another class of coatings for the oxidation and corrosion protection of the substrate is the "aluminide"
coatings. These coatings are generated by an aluminizing technique such as pack diffusion or chemical vapor diffusion.
These coatings are formed by interactions between an aluminum source and the substrate surface. The aluminum forms cobalt and nickel aluminide at the surface of the cobalt and nickel - - 21525~i ~

_ 9 _ 57,917 based superalloy substrates, respectively. The coating characteristics are largely affected by the substrate chemistry and deposition process parameters. Examples of such coatings are shown in U.S. Pat. No. 5,000,782 which is incorporated herein by reference in its entirety.
Combinations of the two classes of coatings have also been used to formulate protective coatings by aluminizing an overlay coating as shown in U.S. Pat. Nos. 4,933,239;
4,910,092; and 4,897,315, which are incorporated herein by reference in their entireties.

Claims

1. A gas turbine comprising a turbine blade, vane, or blade ring made from a cobalt-based alloy comprising:

(a) from about 21-25% wt. chromium;

(b) from about 8-12% wt. nickel;

(c) from about 6-9% wt. tungsten;

(d) from about 0.1-1.5% wt. aluminum; and (e) from about 0.15-1.5% wt. hafnium.

2. The gas turbine of claim 1 wherein the aluminum content of the cobalt-based alloy is from about 0.5-1% wt.

3. The gas turbine of claim 1 wherein the hafnium content of the cobalt-based alloy is from about 0.25-0.7% wt.

4. The gas turbine of claim 1 wherein the cobalt-based alloy further comprises from about 1-4% wt. tantalum.

5. The gas turbine of claim 1 wherein the cobalt-based alloy further comprises from about 0.15-1% wt. silicon.

6. The gas turbine of claim 1 wherein the cobalt-based alloy consists essentially of:
(a) from about 21-25% wt. chromium;
(b) from about 8-12% wt. nickel;
(c) from about 6-9% wt. tungsten;
(d) from about 0.1-1.5% wt. aluminum;
(e) from about 0.15-1.5% wt. hafnium;
(f) from about 1-4% wt. tantalum;

(g) from about 0.2-0.5 carbon;
(h) iron, present in an amount up to about 2% wt.;
(i) from about 0.15-1% wt. titanium.

7. A gas turbine component selected from the group consisting of blade rings and blades, comprising a cobalt-based alloy comprising:
(a) from about 21-25% wt. chromium;
(b) from about 8-12% wt. nickel;
(c) from about 6-9% wt. tungsten;
(d) from about 0.1-1.5% wt. aluminum; and (e) from about 0.15-1.5% wt. hafnium.

8. The gas turbine component of claim 7 wherein the aluminum content of the cobalt-based alloy is from about 0.5-1% wt.

9. The gas turbine component of claim 7 wherein the hafnium content of the cobalt-based alloy is from about 0.25-0.7% wt.

10. The gas turbine component of claim 7 wherein the cobalt-based alloy further comprises from about 1-4% wt.
tantalum.

11. The gas turbine component of claim 7 wherein the cobalt-based alloy further comprises from about 0.15-1%
wt. silicon.

12. The gas turbine component of claim 7 wherein the cobalt-based alloy consists essentially of:
(a) from about 21-25% wt. chromium;
(b) from about 8-12% wt. nickel;
(c) from about 6-9% wt. tungsten;
(d) from about 0.1-1.5% wt. aluminum;
(e) from about 0.15-1.5% wt. hafnium;
(f) from about 1-4% wt. tantalum;

(g) from about 0.2-0.5 carbon;
(h) iron, present in an amount up to about 2% wt.;
(i) from about 0.15-1% wt. titanium.

13. The gas turbine component of claim 7 wherein said component comprises an exposed surface and further comprising a protective coating proximate to the exposed surface of the component, said protective coating comprising an aluminide coating or a MCrAlY coating wherein M is selected from the group consisting of cobalt, nickel, iron, and mixtures thereof.

14. A gas turbine vane comprising a cobalt-based alloy comprising:
(a) from about 21-25% wt. chromium;
(b) from about 8-12% wt. nickel;
(c) from about 6-9% wt. tungsten;
(d) from about 0.1-1.5% wt. aluminum; and (e) from about 0.15-1.5% wt. hafnium.

15. The gas turbine vane of claim 14 wherein the aluminum content of the cobalt-based alloy is from about 0.5-1% wt.

16. The gas turbine vane of claim 14 wherein the hafnium content of the cobalt-based alloy is from about 0.25-0.7% wt.

17. The gas turbine vane of claim 14 wherein the cobalt-based alloy further comprises from about 1-4% wt.
tantalum.

18. The gas turbine vane of claim 14 wherein the cobalt-based alloy further comprises from about 0.15-1% wt.
silicon.

19. The gas turbine vane of claim 14 wherein the cobalt-based alloy consists essentially of:
(a) from about 21-25% wt. chromium;
(b) from about 8-12% wt. nickel;
(c) from about 6-9% wt. tungsten;
(d) from about 0.1-1.5% wt. aluminum;
(e) from about 0.15-1.5% wt. hafnium;
(f) from about 1-4% wt. tantalum;
(g) from about 0.2-0.5 carbon;
(h) iron, present in an amount up to about 2% wt.;
(i) from about 0.15-1% wt. titanium.

20. The gas turbine vane of claim 14 wherein said vane comprises an exposed surface and further comprising a protective coating proximate to the exposed surface of the vane, said protective coating comprising an aluminide coating or a MCrAlY coating wherein M is selected from the group consisting of cobalt, nickel, iron, and mixtures thereof.