High temperature metallic seal

Abstract

A metallic seal includes a cold formed substrate layer and one or more additional layers. At least one of the layers offers improved resistance to high temperature stress relaxation.

Description

BACKGROUND OF THE INVENTION

This invention relates to seals, and more particularly to metallic seals.

A variety of metallic seal configurations exist. Key metallic seals are commonly held under compression between two opposed flanges of the elements being sealed to each other. Such metallic seals may be used in a variety of industrial applications.

Key examples of such metallic seals are of an annular configuration, having a convoluted radial section which permits the seal to act as a spring and maintain engagement with the flanges despite changes or variations in the flange separation. Certain such seals have an S-like section while others have a section similar to the Greek letter Σ with diverging base and top portions. Other similar seals are formed with additional convolutions. One exemplary seal is sold by The Advanced Products Company, North Haven, Conn., as the E-RING seal. Such seals are commonly formed as a monolithic piece of stainless steel or superalloy. Such seals are commonly formed from sheet stock into a shape which is effective to provide the seal with a desired range of compressibility from a relaxed condition. These seals are installed in applications in a compressed state as shown in FIG. 1. The total compression (Δh_T) consists of an elastic component (ΔH_EL) and plastic component (Δh_PL) so thatΔh_T=Δh_EL+Δh_PL With continued exposure at elevated temperatures, the plastic component Δh_PL grows resulting from creep and the elastic component Δh_EL decreases with time. As a result, the sealing load or the capability of the seal to follow the flange movement also diminishes with time resulting from the reduced Δh_EL. This phenomenon is called stress relaxation.

BRIEF SUMMARY OF THE INVENTION

Therefore, long-term applications of current metallic seals are generally limited to about 1300° F. because the current cold formable nickel-based superalloys such as INCONEL 718 (Special Metals Corporation, Huntington, W.V.) and WASPALOY (Haynes International, Inc., Kokomo, Ind.), lose their strength at temperatures greater than 1300° F. and stress relax because of the dissolution of γ′ precipitates.

There are other cast metallic alloys, such as MAR M247 (a cast superalloy used in manufacture of turbine engine blades available from Cannon-Muskegon Corporation, Muskegon, Mich., as CM 247) which are used at ultra high temperatures (about 2000° F. or 1100° C.) for thick cross-section cast and wrought components. These alloys can not readily be rolled into thinner gauges and cold formed into static seal shapes.

Recently developed mechanically alloyed strips such as MA 754 of Special Metals Corporation and PM 1000 of Plansee AG, Reutte, Austria, with superior high temperature strength characteristics are also very difficult to fabricate into seal shapes.

Some of the refractory alloy strips such as molybdenum base (e.g., titanium-zirconium-molybdenum (TZM)) and niobium base alloys, although cold formable, have poor oxidation resistance above 1200° F. (649° C.). Therefore, it is believed that no current metallic alloy can readily be cold formed into seal and used at demanding elevated temperature applications requiring enhanced stress relaxation resistance.

One aspect of the present invention advantageously combines the cold formability of the current sheet alloys and stress relaxation resistance of other metallic alloys and composites which are not cold formable. Seal shapes are formed with cold formable alloys and a layer of creep/stress relaxation resistant alloys is deposited on the already formed substrate. The substrate can be either a fully formed or partially formed shape of the seal to achieve any thickness profile on the strip. Thickness can be preferentially built up in areas with high stress.

The deposition of the creep/stress relaxation resistant layer can be accomplished by processes such as:

• thermal spray of molten alloy droplets and powder;
• thermal spray of creep resistant alloys with micron (10⁻⁶ m) and submicron size ceramic particles such as zirconia, alumina and silicon carbide;
• vapor deposition such as electron beam physical vapor deposition (EB PVD);
• slurry coating of ceramics and curing at elevated temperatures; and/or
• electroforming of high temperature alloys with or without micron or submicron size ceramic particles.

The resultant metallic composite structure can advantageously be fabricated cost effectively to provide complex creep/relaxation-resistant structures for ultrahigh temperature applications. Other high temperature formed structures such as high temperature ducting, combustor liners and components for gas turbine engines can also be fabricated using this technology. Such structures may be advantageous substitutes for more expensive ceramic elements.

A second aspect involves providing an oxidation-resistant coating to a stress relaxation-resistant but oxidation-prone substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a radial sectional view of a metallic seal.

FIG. 2 is a radial sectional view of a first metallic seal according to principles of the invention.

FIG. 3 is a radial sectional view of a second metallic seal according to principles of the invention.

FIG. 4 is a radial sectional view of a third metallic seal according to principles of the invention.

FIG. 5 is an enlarged view of the seal of FIG. 4.

FIG. 6 is a graph of test data showing stress relaxation for various materials at 1600° F. (871° C.).

FIG. 7 is a graph of test data showing stress relaxation for various materials at 1800° F. (982° C.).

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 2 shows a seal 20 formed as an annulus having symmetry about a central longitudinal axis 500. In operation, the seal is held in compression between opposed parallel facing surfaces 502 and 503 of first and second flanges 504 and 505 to isolate an interior volume 506 from an exterior volume 507.

The seal is formed as a convoluted sleeve having first and second layers 22 and 24 and extending from a first end 26 to a second end 28. In the exemplary embodiment, the first layer 22 is generally interior of the second layer 24 and has first and second surfaces 30 and 32. In an exemplary manufacturing process, the first layer 22 is initially formed as a flat strip of cold formable material (e.g., it may be formed into a complex shape at a temperature which is less than half its Fahrenheit melting temperature and, preferably, at ambient conditions (room temperature)). The ends of the strip may be welded to form a sleeve, the two faces of the strip thereby becoming interior and exterior faces of the sleeve. The sleeve may be deformed into a convoluted shape such as that shown in FIG. 2, the interior and exterior sleeve faces becoming the surfaces 30 and 32, respectively, and the end rim surfaces of the sleeve in part defining the ends 26 and 28. After any optional additional further cleaning, machining, or surface treatment, the second layer 24 is deposited on the first layer 22. In the illustrated example, the layer 24 is gradually built up on the surface 32 with a substantially uniform thickness of a similar order of magnitude to the thickness of the layer 22. There may be additional optional machining, polishing, or surface treatment of the layer 24. Typically, however, there will be no additional machining or polishing involved. The result of this process is the production of an integrated seal in which the layers are held together not merely by macroscopic mechanical interfitting but adhesion at the microscopic level between the inner surface 40 of the layer 24 and the outer surface 32 of the layer 22. A major portion of the outer surface 42 of the layer 40 constitutes the external surface of the seal in contact with the volume 507. Portions 44 and 46 of the surface 42, slightly recessed from the ends 26 and 28, face longitudinally outward and provide bearing surfaces for contacting the flange surfaces 502 and 503 to seal therewith. Each layer makes a substantial contribution to the longitudinal compression strength and performance of the seal. Preferably in an anticipated range of operation, each contributes at least ten percent and, preferably, 30%.

Exemplary thermal operating conditions for the seal are in the range of 1600-2000° F. (871-1093° C.) or even more. A more narrow target is 1700-1900° F. (927-1038° C.). This does not necessarily mean that the seal can not be used under more conventional conditions. Under the target operating condition, the coating layer (e.g., the second layer 24) has a higher resistance to stress relaxation or creep than does the substrate layer (e.g., the first layer 22). Preferably the substrate layer is formed of a nickel- or cobalt-based superalloy. Particularly preferred materials are WASPALOY and HAYNES 230® (UNS No. N06230). Preferred coatings are cast γ′ hardened nickel-based superalloys. Particularly preferred coating materials are MAR M2000 and MAR M247. FIGS. 6 and 7 show stress relaxation according to the ASTM E-328 test for various candidate substrate and/or coating materials at low and mid target temperatures of 1600 and 1800° F. (871 and 982° C.) respectively.

FIG. 3 shows an alternate seal 120 having first and second layers 122 and 124. Potentially otherwise similar to the seal 20, the seal 120 has a more uneven thickness of the layer 124. In particular, the layer 124 is relatively thin near the contacting surface portions 144 and 146 near the seal ends 126 and 128.

FIG. 4 shows an alternate seal 220 of generally overall similar configuration to the seals of FIGS. 1-3. Structurally, the seal consists essentially of a single layer 222 of a cold formed refractory alloy strip (e.g., TZM). The entire exterior surface of the layer 222 is covered by a protective coating 224 which is not expected to substantially contribute to the strength of the seal. The coating is, however, effective to protect the underlying layer 222 from oxidation at elevated temperatures (e.g., at a target temperature in excess of 1200° F. (649° C.)). A preferred coating is molybdenum disilicide (MoSi₂) applied as a slurry coat followed by baking to produce a fused silicide coating. One specific silicide-based coating technology is available from Hitemco East, Old Bethpage, N.Y., and is formed by a process including the acts of slurry coating and baking the coating at high temperature.

Another preferred coating is nickel aluminide (Ni₃Al or NiAl) formed by first electroplating nickel to the substrate layer 222 and then slurry coating with aluminum and baking. Alternative coatings include at least one of gold, platinum, iridium, nickel, and nickel-tungsten. Such coatings may be applied by electroplating, vapor deposition, or thermal spray.

Other coatings include oxides of yttrium, zirconium, hafnium, boron, and their combinations and may be formed by a thermal or a slurry coating process. Yet other coatings may include combinations of the foregoing coatings and processes.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, the relaxation resistant material layer may be located in discrete locations along the length of the seal rather than continuously along the length. Such refractory material may be localized to portion of the seal where the greatest flexing occurs. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. A process for manufacturing a seal for sealing between interior and exterior volumes when held under compression, comprising: cold forming an annular first seal layer; and applying a second layer to a first surface of the first layer via a process selected from the group consisting of: thermal spray of molten alloy droplets and powder; thermal spray of creep resistant alloys with ceramic particles; vapor deposition; slurry coating of ceramics and curing at elevated temperatures; electroforming of high temperature alloys; and combinations thereof.

2. The process of claim 1 wherein: the first layer is formed having a bellows-like section prior to the application of the second layer; and after the application of the second layer, there is no machining step which removes a portion of the applied second layer material.

3. The process of claim 2 wherein the step of cold forming comprises one of: cutting an annulus from a tube and deforming the annulus to provide a radial section of enhanced compressibility; forming and welding a strip into an annulus and deforming the annulus to provide a radial section of enhanced compressibility; and deforming a flat strip to provide a section of enhanced compressibility and further deforming and welding the strip to provide an annulus having a radial section of enhanced compressibility.

4. A process for manufacturing a seal having a central longitudinal axis and forming a seal between interior and exterior volumes when held under compression between opposed first and second parallel faces of respective first and second flanges, comprising: cold forming an annular first seal layer from a refractory material; and applying an oxidation resistant coating to the first layer.

5. The process of claim 4 wherein the coating entirely covers the first seal layer.

6. The process of claim 4 wherein the coating comprises at least one of molybdenum disilicide and nickel aluminide and is formed by a process including the acts of slurry coating of at least one component of the coating and baking.

7. The process of claim 4 wherein the coating comprises at least a silicide is formed by a process including the acts of slurry coating of at least one component of the coating and baking.

8. The process of claim 4 wherein the coating comprises at least one of gold, platinum, and iridium is formed by a process including at least one of electroplating, vapor deposition or thermal spray.

9. The process of claim 4 wherein the coating comprises one or more oxides of yttrium, zirconium, hafnium, and boron.

10. A process for manufacturing an article, comprising: cold forming a substrate from a first nickel- or cobalt-based superalloy; and applying a coating of a γ′ hardened second nickel-based superalloy to the substrate.

11. The process of claim 10 wherein the application is via thermal spray process and provides the coating having a thickness of at least 10% of a thickness of the substrate.

12. The process of claim 10 wherein the article has a creep resistance at 982° C. greater than a cold formed article of like dimensions consisting essentially of the first superalloy.

13. The process of claim 10 wherein said substrate is formed from said first nickel-based superalloy.