Abstract:
A rotary hearth sintering furnace composed of a debinding system, a part loading station, a rotary hearth furnace having multiple heating zones, an atmosphere system for maintaining certain atmospheres within different zones of the furnace, an unloader station and a cooling conveyor that are preferably controlled with a single programmable logic controller and operating station for sintering powder metal parts in a minimal amount of space.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present invention claims priority to United States Provisional Application Ser. NO. 60/654,223, filed Feb. 18, 2005. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to hearth furnaces, and more specifically to rotary hearth furnaces used for sintering stainless steel/high temperature powdered metal parts.  
         [0004]     2. Description of Prior Art  
         [0005]     Powder metal is one of four major methods of forming metal, the other three being casting, machining, and plastic forming of either hot or cold metal. Powder metal has many advantage over the other three processes. For instance, the labor associated with producing PM parts is generally lower than that required using other processes. In addition, close tolerances and unique material properties can be easily achieved using PM, as can production of intricately shaped parts, such as those requiring internal or external splines, gears, knurls, eccentric holes, hidden pockets, and the like. In addition, the material efficiency lowers part costs in that essentially 100% of the material used in the PM process is in the finished part, leaving virtually no scrap.  
         [0006]     One traditional drawback of powder metal is its lower density. Advances in metallurgy, however, have made high density powder metal parts having a unique binder possible when sintered at temperatures around 2500° Fahrenheit. The density that can be achieved, for instance, is now up to about 99.7% of the metal&#39;s theoretical density.  
         [0007]     Conventional process components are sintered at 2150° F. or below. This is the upper limit for conventional mesh belt sintering furnaces. Thus, the conventional mesh belt sintering furnaces will not be suitable for sintering operations of the powder metal parts that use a binder that produces higher density parts but requires sintering at temperatures around 2500° F.  
         [0008]     While ceramic belts may be substituted for the mesh belts to permit higher operating temperatures, the ceramic belts have lower loading capacity, typically around 6 lbs/sq. ft., which severely limits the throughput of this type of furnace.  
         [0009]     An alternative system is an elongated pusher style furnace wherein the parts are loaded in one end of the furnace, pushed through the furnace, and unloaded at the opposite end. This type of furnace can operate at the required temperatures and can process more pounds per hour than the belt system, but do have other drawbacks. For instance, the pusher system requires setter plates on which the parts will be placed and pushed through the furnace. These plates add significantly to the cost of operation, are prone to breakage from thermal cycling and handling, and can misalign and pile up while being pushed through the furnace. The plates also require a return system to bring them back to the load end of the furnace.  
       OBJECTS AND ADVANTAGES  
       [0010]     It is therefore a principal object and advantage of the present invention to provide a rotary hearth sintering furnace for sintering P/M parts.  
         [0011]     It is an additional object and advantage of the present invention to provide a P/M sintering furnace system that is can efficiently handle a large throughput of parts in a minimal amount of space.  
         [0012]     Other objects and advantages of the present invention will in part be obvious, and in part appear hereinafter,  
       SUMMARY OF THE INVENTION  
       [0013]     In accordance with the foregoing objects and advantages the present invention provides a rotary hearth sintering furnace essentially comprising a debinding system, a part loading station, a rotary hearth furnace having multiple heating zones, an atmosphere system for maintaining certain atmospheres within different zones of the furnace, an unloader station and a cooling conveyor. The system is preferably controlled with a single programmable logic controller and operating station. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The present invention will be more fully understood and appreciated by reading the following Detailed Description in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  is a top plan view of the sintering furnace system using plates to carry the parts;  
         [0016]      FIG. 2  is a top plan view of the sintering furnace system, wherein the parts are processed without plates;  
         [0017]      FIG. 3  is a cross-sectional view of the load chamber and furnace taken along section line  3 - 3  of  FIG. 1 ; and  
         [0018]      FIG. 4  is a cross-sectional view of the load chamber and furnace taken along section line  4 - 4  of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings, in which like reference numerals refer to like parts throughout, there is seen in  FIGS. 1 and 2  a powder metal (“P/M”) sintering system, designated generally by reference numeral  10 , and essentially comprising a rotary hearth sintering furnace  12 , a hearth loading chamber  14  in communication with furnace  12 , a hearth unloading chamber  16  in communication with furnace  12 , a debind conveyor  18  in communication with load chamber  14 , and a cooling conveyor  20  in communication with unload chamber  16 . P/M sintering system  10  is adapted to perform automated high temperature sintering of P/M parts  22  with high efficiency and high throughput. Furnace  12  is designed to operate at temperatures suitable for sintering P/M parts, for example, up to at least about 2500° F. It is contemplated, however, that the furnace be capable of operating at temperatures of up to about 3000° F. in order to be suitable for high temperature sintering applications other than for P/M parts.  
         [0020]     The sintering process begins at debind conveyor  18  where P/M parts are loaded at its front end. Conveyor  18  runs along a longitudinal axis A-A within a sealed compartment in which a first, controlled atmosphere will be maintained at a temperature of about 1100° F. to 1550° F. At this temperature, it is typically about a 20 minute process for the debinding to be complete.  
         [0021]     P/M parts typically contain an organic binder that holds the part together, and this binder is preferably burned off prior to entering the furnace for sintering. The first, controlled atmosphere for burning off the organic binder is typically 100% endothermic, Nitrogen-Hydrogen, Nitrogen bubbled through water, and a rich burning natural gas burner firing into the debind chamber. The precise type of atmosphere used, however, is dependant upon the type of binder being processed and the desired properties, the appropriate selection of which is known to one skilled in the art.  
         [0022]     It should be noted that debinding could be performed within the furnace if the furnace was large enough to accommodate the heat and atmosphere zone necessary to complete the debinding process. The debinding could also be done off-line relative to the sintering process, but this, of course, increases the amount of part handling necessary to complete the sintering process.  
         [0023]     Once coming off debind conveyor  18 , the parts either pass on to setter trays  24  positioned within loading chamber  14  or are moved directly onto a conveyor  26  positioned within chamber  14  that will convey the parts into furnace  12 . Typically, parts weighing over 2 pounds will go directly into furnace  12 , while parts weighing less than 2 ponds are more efficiently handled by loading onto tray  24  and then conveying the tray into furnace  12 . It is possible, however, to load all types of parts onto a setter tray or all parts directly into the furnace without trays. If trays are used, they remain in the furnace until they are loaded and unloaded. By keeping the trays at the furnace temperature, as opposed to room temperature, the efficiency of the sintering process is improved as there is no appreciable heat transfer between the trays and the parts that are placed thereon.  
         [0024]     Relative to the loading mechanisms and with reference to  FIGS. 3 and 4 , a two-axis loading system may be employed. When plates  24  are used, a plate loader  28  extends in a horizontal plane adjacent the bottom of chamber  14  and accepts setter plates  24  transported from the unload chamber  16 . PLC (programmable logic controllers) controlled hydraulic cylinders  30 , or equivalent systems such as servo-controlled systems, move setter plates  24  from the plate loader to the debind conveyor opening that is positioned in a horizontal plane vertically above the plane in which the plate loader extends, and where a part loader  32  moves parts from debind conveyor  18  onto setter plates  24 . Once setter plates  24  are loaded, the controller opens the furnace door and part loader  32  automatically moves the plates into furnace  12 . When plates are not used, the parts simply come in from the debind conveyor  18  and are raised from a part loader (the same as plate loader  28 ) onto the conveyor that will take them into furnace  12 .  
         [0025]     At the entry point of furnace  12 , an atmosphere pressure blower introduces the desired atmosphere. The atmosphere preferably consists of 75% Hydrogen/25% Nitrogen to 100% Hydrogen. Furnace  12  can be a conventional rotary hearth with upper and lower refractories  34 ,  36 , respectively, and a hearth  38  that rotates about the central axis of furnace  12 . Preferably, a servomotor drive system, oil lubrication and cooling systems, and a drive system comprising a large diameter thrust bearing with gear toothed outer race, a pinion gear, and a double reduction gear reducer, all controlled by the servomotor are employed, the arrangement of which would be known to one of ordinary skill in the art.  
         [0026]     The oil lubrication and cooling system circulates oil through the base  40  of furnace  12  to cool the base and from there migrates thorough the hearth and provides lubrication to the hearth bearing. A self contained pump unit cools and filters the oil in the system. The oil is gravity fed from furnace  12  to the pump unit where it is filtered, cooled, and pumped back into the base of the furnace.  
         [0027]     The setter plates  24  are designed to be preferably about 3 times as long as wide and be placed onto hearth  38  with their longitudinal axis aligned with the radial axis of furnace  12 . The rack system is designed to have plates  24  be keyed into hearth  38  and be stackable. The setter plates  24  and fixturing system that key them into hearth  38  will be made from refractory or ceramic material capable of withstanding the sintering temperatures and hydrogen atmosphere maintained within the furnace.  
         [0028]     After plates/parts are loaded onto hearth  38 , they begin their rotational travel around furnace  12 . The first 180 degrees of travel are in a series of ramped heat zones  40  that are maintained in a hydrogen atmosphere at up to about 2550° F. Depending on the P/M part, the number of heat zones can be adjusted to ramp up or down in temperature as quickly or as slowly as necessary. In addition, a series of atmosphere ports  42  are positioned at predetermined positions around the furnace  12  to provide a consistent, positive flow of the desired atmosphere, preferably hydrogen.  
         [0029]     After the parts have traveled at least 180 degrees of the way around furnace  12 , they enter a cool down zone  44  that gradually reduces the temperature to which the parts are directly exposed prior to exiting furnace  12 .  
         [0030]     After the parts have revolved around furnace  12  for about 324 degrees, they are unloaded from hearth  38  and into unload chamber  16 . A door separating furnace  12  from unload chamber  16  receives a signal from the controller that a plate  24  (or parts) are positioned for movement into chamber  16  and is opened and then closed as soon as the plate/parts are appropriately moved out of the furnace. Unload chamber  16  is virtually identical to load chamber  14 , containing all the same elements (designated with the same reference numerals except for the addition of a “′” sign on the drawings.) As opposed to the parts being introduced into load chamber  14 , however, the parts are passed from the unload chamber  16  onto an unload conveyor that takes them through a cooling chamber  50  for a predetermined distance. The cooling is preferably effected with a forced gas convection cooling system with the conveyor  20  riding on a water jacket. At the exit of cooling chamber  50 , the sintered P/M parts are taken for further processing.