Patent Publication Number: US-2006017204-A1

Title: Steel-shelled ceramic spacer block

Description:
FIELD OF THE INVENTION  
      The present invention relates to spacer blocks positioned between aluminum ingots in preheat furnaces and, more particularly, to an improved spacer block that is more robust and has a longer useful life.  
     BACKGROUND OF THE INVENTION  
      Heating of aluminum ingots is a well-established practice for achieving desired properties in the ingot and to render the ingot sufficiently malleable for reduction in thermo-mechanical processes. During a preheating step, aluminum ingots are heated to temperatures below the melting point of the aluminum alloy. Preheating serves to control the metallurgical properties of the ingot, reduce cracking, and reduce the forces needed to further process the ingot. Up to six ingots can typically be vertically stacked in a preheat furnace at one time. Spacer blocks are typically positioned between the stacked ingots to maintain a gap between the ingots and prevent them from sticking to one another, allow hot gases to circulate between the ingots for faster heat-up, and provide uniform exposure to the furnace atmosphere.  
      Conventional blocks are solid blocks of an aluminum alloy, which may be the same as or different from the alloy of the ingot supported thereby and have a size of about 1 to 4 inches×2 to 6 inches×6 to 24 inches. Each of these spacer blocks weighs over ten pounds. A single operator may handle 400 to 500 spacer blocks per shift.  
      Additional drawbacks to conventional spacer blocks relate to their composition. When heated in a furnace, the metal of the ingot as well as the metal of the spacer blocks soften. When aluminum alloy spacer blocks are subjected to the weight of a conventional ingot load in a preheat furnace and temperatures of about 600° F. (316° C.) and higher, the strength of the spacer block begins to decrease. When subjected to higher preheat furnace temperature conditions of about 800° F. (427° C.) and higher, aluminum alloy spacer blocks exhibit a diminished strength capacity that is typically unsatisfactory for providing adequate ingot support.  
      In addition, oxide layers grow and volatile metals, such as magnesium and lithium, migrate to the surfaces of the spacer blocks and the ingots. The migrated metals cause the spacer blocks and the ingots to adhere to one another. Deformation and adhesion of the spacer blocks to the ingots is particularly problematic for the ingots at the bottom of the stack where the load is the greatest. When the preheat cycle is complete, a crane is used to remove an ingot from the stack and position the ingot at the beginning of a hot line rolling mill, reversing mill, or the like. An operator must remove any spacer blocks stuck to the ingot prior to any ingot processing. Occasionally, the spacer block can be removed from the ingot by simple hand pressure. However, often the spacer block is so tightly adhered to the ingot that it must be knocked off with a large hammer or an axe. Occasionally, a forklift or the like must be used to loosen the adhered spacer block from the surface of the ingot.  
      An additional problem associated with sticking of conventional spacer blocks to the ingot is the formation of marks, which are typically left on an ingot upon removal of the spacer block. Spacer blocks often produce defects in the surface of the ingot. When an ingot having such a defect is subsequently rolled, the defect becomes a surface imperfection in the rolled product. For many applications of rolled product, such defects are unacceptable in the marketplace.  
      Another drawback to the aluminum spacer blocks is the tendency of various aluminum alloys used for conventional spacer blocks to creep at high temperatures. At temperatures of about 900-1140° F. (482-616° C.), conventional spacer blocks having initial dimensions of 3 inch×3 inch&#39;12 inch can become deformed into dimensions of about 2.5 inch×3.5 inch×12.5 inch. Not all spacer blocks in a stack of ingots are always deformed similarly. Hence, in a set of spacer blocks used with a stack of ingots, the individual spacer blocks may have differing dimensions. Variable dimensions in the spacer blocks can aggravate sticking of the spacer blocks to the ingots. For example, when six spacer blocks are used for an ingot and two of the spacer blocks do not touch the ingot because they have been deformed, only four of the spacer blocks contact the ingot, thereby supporting the entire load. In this situation, the load per unit area borne by the four spacer blocks contacting the ingot increases by about 33%. At such higher loads, the adhesion between the spacer blocks and the ingots is aggravated.  
      High temperature creep of aluminum spacer blocks is also a problem in preheat furnaces operated at higher temperatures, e.g., at or above about 1120° F. (604° C.). It has become common practice in those circumstances to position the spacer blocks between the ingots so that a portion of the spacer block extends out between the ingots. During the preheat cycle, the portion of the spacer block which is sandwiched between the ingots becomes flattened to a thickness of about ½ inch while the remaining portion of the spacer block which did not support the ingot retains its original width and height of 3 inch×3 inch. In order to reuse spacer blocks that have been partially flattened, operators turn the spacer blocks between ingots. This often results in the entire spacer block being flattened into a thickness of about ½. When the spacer block between the ingots is greatly reduced to about ½ inch, airflow between the ingots is greatly reduced which results in uneven heating, extended cycle times, and insufficient exposure of the ingot surfaces to the furnace atmosphere.  
      Accordingly, a need exists for a spacer block for use in aluminum ingot preheat furnaces which is lightweight, does not stick to the ingot surfaces, and retains its shape when subjected to high temperature furnace conditions.  
     SUMMARY OF THE INVENTION  
      This need is met by the spacer member of the present invention, which may be used for supporting a metallic alloy product subject to heat treatment. The spacer block comprises a tubular housing with a core of a ceramic material. The tubular housing, which encloses the ceramic core, comprises a wall portion, two end portions, and a bore. The wall portion has at least two substantially flat surfaces in parallel with each other, with the flat surfaces having corner edges that have a radius of at least  3 / 8  inch and ends that are tapered at least ¼ inch inward toward the bore of the tubular housing. In addition, the flat surfaces also have a coating that reduces the sticking of a metallic alloy product. The end portions each have at least one aperture to allow the internal portions of the block to adjust to ambient atmospheric pressure.  
      The spacer member of the present invention may be produced by providing a tubular housing comprising a bore and a wall portion having at least two substantially flat surfaces in parallel with each other wherein the flat surfaces have corner edges and ends, tapering the ends at least ¼ inch inward toward the bore of the tubular housing, attaching an end portion having at least one aperture to an end of the tubular housing, filling the tubular housing with a ceramic material, and attaching another end portion having at least one aperture to the other end of the tubular housing. The flat surfaces may then be coated with a non-stick coating for preventing sticking of a heat-treated metallic alloy product to the spacer member.  
      A complete understanding of the invention will be obtained from the following description when taken in connection with the accompanying drawing figures wherein like reference characters identify like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1   a  is a perspective view of a tubular housing of a spacer member of the present invention.  
       FIG. 1   b  is a cross-sectional view of a housing of the spacer member of the present invention filled with a ceramic material.  
       FIG. 2  is a graph showing the average cold crushing strength for various preferred castable ceramic core materials. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      As shown in  FIGS. 1   a  and  1   b , the spacer member of the present invention includes a housing  20  with core  30  of a ceramic material. The housing  20  is preferably in the form of a tube having a wall portion with at least two substantially flat surfaces  40  in parallel with each other and a bore  90  defined by the wall portion that is structured to receive the ceramic core. As shown in  FIG. 1   a , housing  20  can have any functional dimensions, however, typical spacer members have a width (W) of from about 1 to about 4 inches, a height (H) of from about 2 to 6 inches, and a length (L) of from about 6 to 24 inches.  
      The thickness of the flat surfaces  40  of the housing  20  can be from about 1/64 inch to about ½ inch, preferably from about 1/16 inch to about ⅛ inch. While thicker walls can be employed, relatively thin walls are typically desirable due to the considerable weight savings. Thinner walls allow for a lower weight for the spacer member, however, if the walls are too thin (e.g., less than about 1/64 inch), the spacer member may be prone to crushing and tearing under the ingot load.  
      The exterior of the flat surfaces  40  of the housing  20  are preferably smooth to minimize any mechanical interlocking with ingot surface during a heat treatment. A suitable maximum roughness is an Ra of about 10 to about 10,000 microinches. The smoothness of the flat surface exterior  40  may be controlled by the extrusion process or rolling process used to manufacture the housing  20 . In one embodiment of the present invention, the surfaces  40  may be machined or polished as needed.  
      The exterior of the flat surfaces  40  of the housing  20  also may be coated with a material to further prevent ingot sticking in the preheat furnace. The preferred material used is the metal oxide coating nickel aluminide, but other metal oxide coatings such as nickel oxide, nickel aluminide, cobalt oxide, chromium oxide, molybdenum oxide, zirconium oxide, aluminum oxide, and magnesium oxide could also be used.  
      In one embodiment of the present invention, the thermal oxide of nickel formed on the exterior of the flat surfaces  40  of the housing  20  has a thickness of from about 5 nm to about 50 microns. In another embodiment, the thermal oxide of nickel formed on the exterior of the flat surfaces  40  has a thickness of from about 10 nm to about 2 microns.  
      While the exterior of only two opposing flat surfaces  40  need to be smoothed and/or coated as described above when used to support ingots in a preheat furnace, it is preferred that the exterior of all of the flat surfaces  40  are similarly treated. In this manner, a user need not be concerned which of the exterior surfaces  40  contact an ingot in a preheat furnace.  
      The flat surfaces  40  have corner edges  50  that have an outside radius of curvature R 0  of at least ⅜ inch and ends  80  that are tapered at least ¼ inch inward toward a bore  90  defined by the wall portion that is structured to receive the ceramic core. This reduces high stress risers that contribute to premature failure at the corner edges  50  and reduces the potential for sticking of the ingot to the spacer member. The rounded corner edges  50  may be any suitable shape having some degree of curvature, such as circular, elliptical, or ovular. In the preferred embodiment, the rounded corner edge  50  extends longitudinally along the length of the spacer member. By rounding the interior of the corner edges  50 , the load of the ingots applied to the housing  20  is partially shifted away from the edges  50  to reduce stress at the edges  50 . Furthermore, when spacer blocks have pointed or sharply angled corner edges  50 , sharp divots or deep deformations can form in the ingot at the point of contact between the spacer block corner edge  50  and the soft ingot. The sharper and/or deeper the resulting ingot deformation is, the more remedial processing is required to remove the defect from the ingot for subsequent use. Additional remedial processing contributes greatly to the expense of the resulting product. Accordingly, by rounding the corner edges  50  of the spacer members, the subsequent remedial ingot processing is reduced and the lifespan of the spacer member is prolonged.  
      The tubular housing  20  comprises a metal selected from the group consisting of high temperature steel, high temperature steel alloy, or cold rolled steel. High temperature steel or steel alloys are preferred because the solidus temperature of steel is significantly higher than the temperature of the preheat furnace conditions. Steel and steel alloys also exhibit tensile compressive yield strengths that are sufficient to support the weight of ingot loads at the preheat furnace temperatures. Preferred high temperature steel or steel alloys are 1018 and 1020. High temperature steel and steel alloys are particularly well suited for use in relatively high temperature furnaces employing temperatures of from about 800° F. to about 1,200° F. (427° C.-649° C.).  
      In one embodiment, the spacer member has a thickness of from about 0.5 to about 4 inches. Spacer members less than about 0.5 inch thick do not typically allow for adequate circulation of the furnace atmosphere between ingots, and spacer members sized larger than about 4 inches thick result in an ingot stack that is too tall for conventional preheat furnaces and may destabilize the ingot stack. In one embodiment of the present invention, housing  20  has a square cross-sectional configuration and dimensions of about 3 inch×3 inch×12 inch. In another embodiment, the housing  20  has a rectangular cross-sectional configuration and dimensions of about 2 inches×5 inches×16 inches. Each of these preferred embodiments are sized and configured to conform with the conventional spacer blocks presently used in the ingot processing industry, however, other cross-sectional configurations of the housing  20  are encompassed by the present invention.  
      Housing  20  is designed to enclose at least a part of the ceramic core  30 . The core  30  is preferably manufactured from a curable ceramic material. Ceramic materials typically have a relatively low density (compared to aluminum) and high strength. However, most ceramic materials are brittle and tend to crumble under impact loads, therefore spacer member includes housing  20  to retain the ceramic core  30 . The housing  20  also serves to prevent the ceramic material from contacting and damaging ingots during use. Accordingly, the ends  80  of the housing  20  should be substantially closed off to prevent escape of the ceramic core  30  during use as shown in  FIGS. 1   a  and  1   b.    
      The ceramic core  30  may comprise a castable material, such as calcium aluminates. The ceramic material preferably has a cold crushing strength of from about 500 psi to about 20,000 psi. Cold crushing strength is a measure of the static load the spacer member can withstand until failure occurs. The density of the ceramic material preferably is less than the density of conventional solid aluminum spacer blocks (about 173 lbs/ft 3  or 2.8 g/cc) to achieve significant weight savings for the spacer member of the present invention. Typically, the density of the ceramic material is not greater than about 150 lbs/ft 3  or 2.4 g/cc. Preferably, the density of the ceramic material is not greater than about 125 lbs/ft 3  or 2.0 g/cc. The properties of the ceramic material of cold crushing strength and density are balanced to obtain a suitable material for the core  30 .  
      Particularly preferred castable materials include Greenlite Express 24, CW108 Castable, HPV Castable, Reno Cast FSLC/A1, and Metroflo SR. These preferred castable materials are available from RHI Refractories (Greenlite &amp; CW108), Chicago Fire Brick Division (HPV), Renofractories, Inc. (Reno Cast), and Matrix Refractories, Inc. (Metroflo). These ceramic materials were evaluated for suitability for use in the core  30  of the spacer member of the present invention.  FIG. 2  illustrates the average cold crushing strength of each of these preferred castable materials. From the figure, Reno Cast FSLC/A1 and Metroflo SR have the greatest cold crushing strength and therefore would be the most preferred castable material.  
      At least one end portion  60  of the housing  20  comprises at least one aperture  70  having a diameter of from about 1/64 inch to about 1/16 inch sized to allow the inside of the spacer member to adjust to the ambient atmospheric pressure of the furnace while substantially retaining the ceramic core  30 . In another embodiment, a plurality of end portions  60  of housing  20  contains a plurality of apertures  70 . The end portions  60  are attached to the ends  80  of the housing  20 .  
      The method of making a spacer member includes: providing a tubular housing  20  comprising a wall portion having at least two substantially flat surfaces  40  in parallel with each other and a bore  90  wherein the flat surfaces  40  have corner edges  50  and ends  80 ; tapering the ends  80  at least ¼ inch inward toward the bore  90  of the tubular housing  20 ; attaching an end portion  60  having at least one aperture  70  to an end  80  of the tubular housing  20 ; filling the tubular housing  20  with a ceramic material; attaching an end portion  60  having at least one aperture  70  to the other end  80  of the tubular housing  20 ; and applying to the flat surfaces  40  of the wall portion a non-stick coating for preventing sticking of a heat treated metallic alloy product to the spacer member.  
      The housing  20  may be formed by extruding the steel into a tube of the desired shape or by providing a sheet of steel, shaping the sheet of steel into the desired configuration, and welding the edges of the sheet together to form a tube. In a preferred embodiment of the present invention, a low cost housing  20  is made from cold rolled steel and robotically welded. In another embodiment, the housing  20  is roll form welded in the same flow path.  
      After the housing  20  is formed, one end of the housing  20  may be closed off by attaching an end portion  60  comprising at least one aperture  70  having a diameter of from about 1/64 inch to about 1/16 inch sized to allow the inside of the spacer member to adjust to the ambient atmospheric pressure of the furnace while substantially retaining the ceramic core  30 . After an end  80  of the housing  20  is closed off, the uncured ceramic material is then poured into the housing  20  and allowed to cure. The other end  80  of the housing  20  may then be closed off. In this manner, the housing  20  acts as a shell surrounding the ceramic core  30 .  
      In a preferred embodiment, tapering would occur by first cutting the ends  80  along the corner edges  50  and then tapering the ends  80  inward toward the bore  90  of the tubular housing  20  via the use of a tapering means. However, one skilled in the art would know that tapering the ends could occur via the use of other methods. The length of the cut along the corner edges  50  can be from ¼ inch to 1 inch. A preferable means for cutting the corner edges  50  is milling, but could include sawing, shearing, or grinding. Tapering means includes anything that would be strong enough to bend the metal including a break or a radius forming jig. An end portion  60  is then attached to the housing  20  preferably by welding the end portion  60  to the ends  80  of the housing  20 . However, the end portion  60  could also be attached via the use of fasteners or any means that would properly attached the end portion  60  to the ends  80  of the housing  20 .  
      A coating comprising a thermal oxide of nickel, specifically nickel aluminide, is then formed on the exterior of the flat surfaces  40  of the housing  20 . Nickel or nickel alloys can be applied to the flat surfaces  40  of the housing  20  prior to forming the housing  20  or after the housing  20  is manufactured via conventional coating techniques, such as brushing, plasma spraying, thermal spraying, cold spraying, electroplating, electroless plating, cladding, plasma vapor deposition, sputtering, and electron beam evaporation.  
      After applying a nickel aluminide coating to the flat surfaces  40  of the wall portion, the housing  20  is preferably subjected to an oxidizing step. The oxidizing step comprises subjecting the housing  20  to a heating period in an oxidizing atmosphere, in which the housing  20  is held to an elevated temperature of from about 800° F. to about 1200° F. (427°-649° C.) for greater than 2 hours. The high temperature heating step is beneficial in forming a thick non-reactive oxide on the surface of the coating and to form a diffusion layer between the coating and the housing  20 . In another embodiment of the present invention, housing  20  can be subjected to a standard plasma spray process. In yet another embodiment of the present invention, housing  20  can be subjected to ozone or another oxidizing atmosphere for a period of time sufficient to allow a nickel oxide to form on housing  20 .  
      It will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed in the forgoing description. Such modifications are to be considered as included within the following claims unless the claims, by their language, expressly state otherwise. Accordingly, the particular embodiments described in detail herein are illustrative only and are not limiting to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.