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:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a divisional application of U.S. patent application Ser. No. 10/002,684 (abandoned) filed Oct. 24, 2001 and entitled “HIGH TEMPERATURE METALLIC SEAL” which claims benefit of U.S. Provisional Patent Application Ser. No. 60/242,759 filed Oct. 24, 2000 and also entitled “HIGH TEMPERATURE METALLIC SEAL.” The disclosures of Ser. Nos. 60/242,759 and 10/002,684 are incorporated by reference herein as if set forth at length. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates to seals, and more particularly to metallic seals. 
   (2) Description of the Related Art 
   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 that
 
 Δh   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, West Va.) 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 (MoSi 2 ) applied as a slurry coat followed by baking. Another preferred coating is nickel aluminide (Ni 3 Al or NiAl) formed by first electroplating nickel to the substrate layer  222  and then slurry coating with aluminum and baking. Alternative coatings include gold, nickel, and nickel-tungsten. 
   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.