Non-warping heat shield
United States Patent De Luca et al.  
 NON-WARPING HEAT SHIELD Inventors: Norman C. De Luca, Glenshaw&#39;.  
 Eugene A. Mizikar, Clairton, both of Pa.  
 Jones &amp; Laughlin Steel Corporation, Pittsburgh, Pa.  
 Filed: Apr. 18, 1974 Appl. No.: 461,814  
 Assignee:  
 US. Cl. 432/249; 432/10; 432/256; 432/260 Int. Cl. F23m 9/00 Field of Search 432/l0, 249, 256, 253, 432/260 References Cited UNITED STATES PATENTS Dailcy, Jr 432/260 ll/l949 Apr. 22, 1975 3.056.594 10/1962 Blackman et al 432/260 3.386.72! 6/1968 Jones et al. 3.661.371 5/1972 Skelton et al 432/260 Primary E \&#39;uminer.lohn J. Camby Attorney. Agent, or Firm-Gerald K. White; T. A. Zalenski [57] ABSTRACT The degree of localized overheating and temperature non-uniformity of coils encountered during batch heat-treatment processes is minimized through use of an insulated heat shield assembly having a central opening. The dimensionally stable heat shield is constructed so as to be resistant to warpage during the heat-treating process.  
 6 Claims, 6 Drawing Figures Fig.5.  
 PATENTEUA RZZIQB sum 3 9 {1 TIME, HOURS NON-WARPING HEAT SHIELD Our invention generally relates to an insulated heat shield which is useful in obtaining improved temperature uniformity between the various stacked coils during batch coil heat-treatment. The heat shield serves to suppress radiant heat energy and to limit convected heat energy transmitted to the top surface of the top coil in batch heabtreating furnaces of conventional construction. In order to accomplish the above stated result. the heat shield is placed on the top coil during the process so as to prevent the top surface of the top coil from locally overheating as a result of inherent furnace conditions.  
  Non-uniform heating of the coil stack. i.e.. overheating of the top coil. is believed to be related, in part, to the inherent tendency of heated furnace atmosphere gases to rise to the top of the furnace. the initially relatively cold and massive furnace base. and nonuniformities in the circulation of the furnace atmo sphere. However. the major cause of localized overheating is believed to be related to the fact that the top coil has a relatively greater surface area exposed to radiant and convected heat than other portions of the coil stack. This condition is especially noticeable when annealing covers are employed to cover the coil stack because the cover serves as a source of radiant energy.  
  Coil stack temperature uniformity is a desirable, and oftentimes essential, consideration in the successful attainment of uniform mechanical properties during the heattreatment of metals. This is especially true when the heat-treating temperature of a metal must be maintained within a narrow range due to metallurgical considcrations. Cold rolled high strength low alloy steels are typical of such steels. Uniformity is also generally desirable in the annealing of both plain carbon and alloy steels in which unduly broad temperature variations may lead to the formation of undesirable phase transformations. precipitated secondary phases, undesirable surface carbides. variations in hardness and other mechanical properties, etc. Our inventive heat shield is effective in reducing or eliminating problems of the above nature through the expedient of improved control of thermal uniformity.  
  The prior art is generally aware that temperature non-uniformity problems may be reduced through the expedient of providing insulating coverings at potentially overheatable regions of a furnace charge. For example, US. Pat. No. 3,370,993 discloses the use of asbestos to cover pipe ends during their heat-treatment.  
  Various types of cover plates are also known in the art of heat-treating metallic coils. Typical cover plates are illustrated in U.S. Pat. Nos. 2.495.561, 3,211,590. 3,302,939. and 2.678.815. Such plates are of various designs and are adapted for the particular purposes of the respective patents.  
  The heat shield assembly has utility in a wide variety of commercial batch heat treating systems. For example, it is immaterial as to whether or not the system employs an annealing cover, forced atmosphere circulation, or whether the furnace is direct fired or is radiant tube heated. The nature of the furnace atmosphere is likewise immaterial to the practice of the invention. In general, the heat shield of the invention is suitable for use in the heat-treatment of any product in which it is desirable to prevent localized overheating ofa portion of the charge. Furnaces in which either single or multiple stacks of coils are heat-treated may be used in combination with the heat shield.  
  Our invention comprises a heat shield which will reduce the extent of localized overheating of the charge and which will also be resistant to warpage due to thermal expansion and contraction during use in the heattreating process. Despite substantial thermal gradients across the shield thickness, shield warpage is minimized due to its particular structural configuration. Warpage prevention is not only an important factor relating to insulating efficiency during a use of the heat shield, but is also of commercial significance when it is considered that the heat shield is intended to be reused numerous times.  
  It is, thus, an object of our invention to provide a heat shield assembly that is capable of functioning to prevent localized overheating of furnace charges during batch heat-treating processes.  
  It is also our objective to provide a heat shield assembly which is capable of functioning to minimize temperature non-uniformity of the charge during heattreatment.  
  lt is a further objective to provide a heat shield assembly which is resistant to thermal warpage and is dimensionally stable during its use in a heat-treatment process and is further suitable for prolonged reuse in subsequent heat-treating processes.  
  These and other objectives and advantages will occur to those skilled in the art from the following description of the invention.  
  FIG. 1 is a top view of a quadrant of the heat shield assembly.  
  FIG. 2 is a side sectional view of the heat shield taken along Section AA of FIG. 1. This figure also illustrates that the heat shield is positioned over the top of a coil during its use in a heat treating process.  
  FIG. 3 illustrates an unshielded stack of coils in which coil temperatures were measured at the various indicated positions.  
  FIG. 4 illustrates a shielded stack of coils in which coil temperatures and heat shield temperatures were measured at the various indicated positions.  
  FIG. 5 is a graphical depiction of the thermal history of various portions of a stack of coils subjected to heattreatment without use of a heat shield.  
  HO. 6 is a graphical depiction of the thermal history of various portions of a stack of coils subjected to heattreatment with use of a heat shield.  
  The shield assembly comprises two attached substantially circular plates having central openings and containing multiple radial slots. Thermal warpage is minimized through use of the radial slots because the slotted portions of the respective plates can expand and contract independently of unslotted plate portions. The plates are fastened to each other in a loose or non-rigid fashion at locations proximate to their outer peripheries and also to their central openings. This type of connection enables the plates to move independently of each other and, hence. prevents potential warpage due to the transmission of temperature differential induced bending moments between the plates. An insulating material is interposed between the respective plates in order to provide the requisite insulating capability of the heat shield. In addition, spacers are provided between the respective plates in order to maintain dimen sional stability of the assembly and to prevent the crushing of the insulating material.  
  A preferred embodiment of the heat shield assembly is depicted in FIGS. 1 and 2. FIG. 1 is a top view of a quadrant of the heat shield while FIG. 2 is a side view of the heat shield taken along Section A-A of FIG. I.  
  As shown in FIG. 2, the heat shield assembly comprises upper plate 1 and lower plate 2. each plate having a central opening. Insulating material 3 is interposed between plates 1 and 2 so as to provide for the requisite heat insulating properties. A typical insulating material suitable for practice of the invention is FlBER- FAX ceramic fiber manufactured by The Carborundum Company. Niagara Falls. NY. This insulating material comprises a lightweight flexible batting of interlocked ceramic fibers. Of course, numerous other com mercially available insulating materials in loose form as well as in sheet or blanket form are also suitable.  
  The respective plates are fastened or held together at a location proximate to the respective central openings by U-shaped channel 4. The slightly oversized channel holds plates 1 and 2 together through slidable contact of its legs and the plate surfaces. Such arrangement permits plates 1 and 2 to thermally expand and contract independently upon heating and cooling. This feature precludes significant warpage of the assembly during use. U-shaped channel 4 also serves the additional function of limiting the passage of atmospheric gases between plates 1 and 2, thus, improving the insulating efficiency of the heat shield. Channel 4 also serves to prevent the loss of insulating material 3 from the assembly. It is understood that other suitable fastening means could be employed to hold plates 1 and 2 together at the central opening. For example. a loose bolting arrangement or the like would be also suitable. if such fastening arrangement were utilized, one would also then need to provide a ring or other member to avoid the loss of insulating material. For this reason the U-shaped channel is a preferred fastening means. Bolting arrangement 5 is used to attach plates 1 and 2 at an outer radial position. As is apparent from the drawing. the bolting arrangement provides a loose means of attachment so as to permit plates 1 and 2 to expand and contract independently from each other upon heating and cooling. This arrangement also precludes significant warpage of the assembly. Ring member 6 is placed between plates 1 and 2 near the outside perphery of the assembly for the purpose of preventing the loss of insulating material. There is no need to affix ring 6 to either plate 1 or 2 as the ring will remain in place inherently due to its structural configuration. Fastening means similar to that of U-channel 4 may be substituted for bolting arrangement 5 and ring member 6. However. such substitution involves significantly more weight. Thus, the illustrated bolting arrangement is a preferred fastening means.  
  Centering means 7 is attached to the lower leg of U- shaped channel 4 for the purpose of facilitiating the placement of the heat-shield on top of coil 9 prior to the start of the heat-treatment. Centering means 7 fits into the inside diameter of coil 9 this assuring uniform positioning of the heat shield. When it is considered that commercial practice of the invention on a typical industrial scale involves the positioning of a rather heavy object on the top of a rather high stack of coils with a crane or similar lifting device, it will be readily appreciated that such centering means is of substantial assistance. This design also facilitiates the stacking of shield when they are not in use. Centering means 7 may consist of a series of radial spokes or a solid shell-like member in the form of a truncated cone. The use of solid shell-like centering means is preferred when the invention is practiced in a furnace assembly in which the annealing atmosphere is controllably circulated with the use of a centrally located base fan because a solid shell-like member will serve to prevent atmosphere gas from passing over the top coil instead of passing through convector plates which separate the coils in the bottom portion of the stack. The open bottom of the solid shell-like truncated cone, of course, fixes the effective central opening of the heat shield through which circulating atmosphere gas passes. in the event that it is desired to reduce or even to close the area of this already existing central opening it is possible to insert a ring or disc into the truncated cone to effect such area adjustment. One could also merely place a ring or disc on top of the central opening of plate 1 to accomplish an equivalent areal adjustment.  
  FIG. 1 represents a top view of the heat shield. This view illustrates two additional features of the invention. Radial slots 10 serve to prevent warpage of the two plates. The multiple radial slots permit the outer peripheries of the respective plates to expand and contract independently of the central portion of the plates. thus. minimizing potential plate warpage. Spacing members 11 are placed between plates 1 and 2 to pre vent curshing of insulating material 3 and to maintain dimensional stability of the assembly. Only one plate should be attached to spacing members 11 so as to avoid potential warpage of the assembly. By such method of attachment, the spacer is free to move in sliding contact with the unattached plate during thermal expansion and contraction. Tack welding has been found to be a suitable attachment technique. Hairpin shaped solid bars are a perferred type of spacing means due to assembly fabrication considerations. However, discrete spaced vertical bars or pipes attached to one plate only would also constitute equivalent spacing means.  
  The improvement in coil temperature uniformity obtained with the use of the heat shield is readily apparent from a consideration of the trial results discussed below.  
  A three-high stack of low carbon steel coils was annealed in a direct fired multiple stack portable type annealing furnace. The same th ee coils were heated according to the same annealing cycle with and without use of the heat shield of the invention. FIBERFAX ceramic fiber in blanket form was used in the heat shield as the insulating material. The batch annealing furnace contained an HNX atmosphere which was circulated downwardly through the respective coil inside diameters by a base fan. HNX atmospheres typically comprise about 7% H 93% N less than 01% CO, and less than 0.l% CO and have a dew point range of about -4()F to 60F. The intent of the annealing cycle was to obtain a temperature range of from l,280F. to l,330F in the annealing stack. This treatment is conventionally utilized to anneal cold rolled low carbon steel coils. An upper limit of l,3 30F. is selected so as to avoid the formation of austenite while the lower limit of l,280F is needed to assure that desired machanical properties are obtained.  
  FIG. 3 illustrates the coil locations in which temperature measurements were taken for the unshielded coil stack. Coil temperature measurements were also taken at the same locations for the heat shielded coil stack. These locations are illustrated in FIG. 4. Also. denoted in FIG. 4 are locations A and B which correspond to the temperatures measured throughout the cycle at the top and bottom of the heat shieldv All temperature measurements were taken with mineral insulated sheath thermocouples which were wrapped in the coil at the indicated locations.  
  The coil sizes, heat-treating furnace, and furnace conditions were selected to be representative of actual industrial conditions. Starting from the lowermost coil. the coil weights were 26.36 tons. 12.29 tons. and l4. tons, respectively; the coil heights were 65 in.. 30 in., and 37 in., respectively; and the outside coil diameters were 64.8 in.. 65.3 in., and 63.2 in, respectively.  
  FIG. 5 represents a plot of temperature measurements throughout the annealing cycle taken at the locations shown in FIG. 3. This data is typical of the thermal history of batch annealed coils which have not been protected by a heat shield. As can be seen from the plot. a substantial portion of the top coil was overheated. i.e., heated above the maximum aim temperature of 1.330F. Also of interest is the relatively short time of exposure any coil to the aim range of l,280F. to 1.330F.  
  On the other hand. marked improvement in temperature uniformity and prevention oflocalized overheating of the top coil resulted for the same coils when the heat shield of the invention was placed upon the top coil. The above conclusion is amply demonstrated by reference to FIG. 6. As may be observed, overheating of the top coil and residence time in the aim annealing temperature range are significantly improved with use of the heat shield. Also, of significance is the reduction in temperature spread between the three coils.  
  Curves A and B of FIG. 6 indicate the substantial thermal gradient which is created across the heat shield thickness during the annealing cycle. In addition to contributing to the above discussed improvements in temperature uniformity, the thermal graident also leads to certain advantages of substantial operating significance. Use of the heat shield permits the furnace operator to utilize higher furnace temperatures during the heat-up stage of the cycle. Higher initial furnace temperatures then lead to shorter heat-up times, thus, resulting in shorter overall cycle times. In the event that one did not utilize a heat shield. lower furnace temperatures would be required upon heat-up due to the tendency of the topmost coil to overheat. It is also apparent that shorter soaking times may be utilized with heat shielded coils due to increased temperature uniformity and the consequent ability to more readily maintain the entire charge within a desired temperature range.  
 We claim:  
  I. A heat shield assembly having a central opening for the prevention of localized overheating during batch heat-treatment of coils, comprising:  
 a. a first substantially circular plate having a central opening and containing multiple radial slots whereby thermal warpage of said first plate would be minimized upon heating and cooling during said heat-treatment;  
 b. a second substantially circular plate having a central opening and containing multiple radial slots whereby thermal warpage of said second plate would be minimized upon heating and cooling during said heat-treatment;  
 c. a first fastening means in contact with said first and second plates at a position proximate to said respective central openings of said first and second plates for loosely fastening said first and second plates so as to permit said first and second plates to expand and contract independently upon heating and cooling;  
 cl. a second fastening means in contact with said first and second plates at an outer radial position of said first and second plates for loosely fastening said first and second plates so as to permit said first and second plates to expand and contract independently upon heating and cooling;  
 e. insulating material located between said first and second plates; and  
 f. spacing means located between and in contact with said first and second plates so as to prevent the crushing of said insulating material and to maintain dimensional stability of said heat shield assembly.  
  2. A heat shield assembly having a central opening for the prevention of localized overheating during batch heat-treatment of coils as recited in claim I, wherein:  
 said first fastening means comprise a U-shaped channel member having legs in slidable contact with said first and second plates so as to permit said first and second plates to expand and contract independently upon heating and cooling.  
  3. A heat shield assembly having a control opening for the prevention of localized overheating during batch heat-treatment of coils as recited in claim 2, wherein:  
 said second fastening means comprise both members attached loosely to said first and second plates so as to permit said first and second plates to expand and contract independently upon heating and cooling.  
  4. A heat shield assembly having a central opening for the prevention of localized overheating during batch heat-treatment of coils as recited in claim I. wherein:  
 said spacing means comprise hairpin shaped bars.  
  5. A heat shield assembly having a central opening for the prevention of localized overheating during batch heat-treatment of coils as recited in claim 1, which further includes:  
 positioning means connected to said heat shield assembly proximate to said central opening of said first and second plates for assisting in the placement of said heat shield assembly on a top surface of a top coil.  
  6. A heat shield assembly having a central opening for the prevention of localized overheating during batch heat-treatment of coils as recited in claim 5, wherein:  
 said positioning means comprise a solid shell in the form of a truncated cone.