Method of forming tappet of the kind having ceramic seat plate
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to a tappet for use between a push rod and a cam in an internal combustion engine and more particularly to a method of forming a tappet of the kind having a ceramic seat plate for contact with a cam. 
     2. Description of the Prior Art 
     With a view to improving the wear resistance of a tappet and thereby meeting the demand for a higher output of an engine, there has been proposed a tappet consisting of a tappet main body made of a single metallic material and a ceramic disk or seat plate brazed to an end of the tappet main body. The tappet main body is made of such steel that can be transformed to martensite by air cooling, such as SNCM 630 (nickel-chromium-molybdenum steel according to Japanese Industrial Standards) and SKC 24 (hollow drill steel according to Japanese Industrial Standards), so that it can be hardened by making use of the heat for brazing. 
     By the combination of metal and ceramics, the prior art tappet aims at attaining an improved durability with a reduced cost, i.e., aims at improving the durability by utilizing an excellent wear resistance of ceramics while lowering the cost by utilizing the low machining cost and low material cost of the metal. 
     However, the metal employed for the prior art tappet needs to have such a property that it can be easily brazed to the ceramic seat plate and be hardened by making use of the heat for brazing, and is therefore limited to an expensive steel such as SNCM 630, etc. Such a steel however is poor in machinability. Thus, when the entire of the tappet main body is made of such an expensive steel, a high material cost and high machining cost inevitably result. Accordingly, the prior art tappet is disadvantageous from the cost point of view. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a method of producing a tappet which comprises a first step of forming a first part of a tappet main body from low carbon steel and carburizing the first part, a second step of forming a second part of the tappet main body from metal and welding the first part and the second part together to constitute the tappet main body, and a third step of brazing a ceramic seat plate to an end of the second part. 
     The low carbon steel forming the first part has such a property that when carburized it can be hardened by making use of heat for the brazing of the third step. The metal forming the second part has such a property that it can be hardened by making use of heat for the brazing of the third step. 
     This method is effective for overcoming the above noted disadvantages or shortcomings inherent in the prior art tappet. 
     It is accordingly an object of the present invention to provide a novel method of forming a tappet for an internal combustion engine, which can reduce the manufacturing cost without deteriorating the durability of the tappet. 
     It is a further object of the present invention to provide a novel method of the above described character which can attain an excellent durability by utilizing a comparatively cheap material. 
     It is a further object of the present invention to provide a novel method of the above described character which can reduce a necessary amount of metal which is comparatively expensive, poor in machinability but suited for being brazed to a ceramic plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded sectional view of a tappet according to an embodiment of the present invention; and 
     FIG. 2 is a sectional view of the tappet of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, a completed or finished tappet 1 consists of a tappet main body 2 made of metal and and a ceramic seat plate 3 brazed to a lower end of the tappet main body 2. The tappet main body 2 is made up of an upper part 2a and a lower part 2b which are welded together to constitute an integral unit. 
     The upper part 2a is generally hollow cylindrical and has a bottomed end portion having at an inner side a semispherical recess 4 for contact with a push rod 5. The upper part 2a is formed from low carbon steel which is comparatively cheap and has a good forgeability, such as SNC 815 (nickel-chromium steel according to Japanese Industrial Standards), SNCM 616 (nickel-chromium-molybdenum steel according to Japanese Industrial Standards), SNCM 420 (nickel-chromium-molybdenum steel according to Japanese Industrial Standards), etc. In the meantime, the steel can be other than the above described kinds so long as it has a good machinability before carburizing and, when carburized, it can be hardened by low-speed cooling such as air cooling. The upper part 2a is for installation in a hole of a cylinder block 6 of an internal combustion engine for up-and-down movement therein. 
     On the other hand, the lower part 2b is formed from steel such as SNCM 630, SKC 24, etc. In the meantime, the steel forming the lower part 2b can be other than the above described kinds so long as it has a property of being easily brazed to the ceramic plate 3 and being hardened by making use of the heat for brazing. 
     The above described ceramic plate 3 is for example a circular plate formed from a silicon nitride material which is sintered at atmospheric pressure and contains 90 wt % of Si 3  N 4 . The ceramic plate 3 is thick enough to endure an impact applied thereto from a cam 7 which drives a valve (not shown) by way of the tappet 1 and the push rod 5. 
     The method of producing the tappet 1 will now be described. The upper part 2a, lower part 2b and ceramic plate 3 are prepared so as to be independent or separated constituent parts prior to assembly. 
     In the first step, the low carbon steel forming the upper part 2a is carburized. The carburizing may be gas carburizing, solid carburizing, liquid carburizing, etc. While a desired carburized depth differs according to the size of the tappet 1, the material from which the upper part 2a is formed and the degree of severeness with which the tappet 1 is used, it was revealed by the test samples which will be described hereinlater that in the case of the tappet 1 sized to be 50 mm long and 20 to 30 mm in outer diameter and made of SNC 815, SNCM 616, SNCM 420, etc. a sufficient effect was attained when the carburized depth was 0.6 mm. 
     In the second step, the upper part 2a and the tappet main body 2. For ease of welding the carburized layer at the lower end of the upper part 2a is removed by cutting prior to the welding. The welding may be any welding such as electron beam welding, laser beam welding, friction welding, etc. 
     In the last third step, the ceramic plate 3 is brazed to the lower end or bottom surface of the tappet main body 2, i.e., the lower end of the lower part 2b. The brazing is performed by preferably employing a silver brazing material containing Ti (titanium). A desired brazing metal is an Ag-Cu-Ti alloy, Ag-Cu-In-Ti alloy, Ag-Ti alloy, Cu-Ti alloy or Ag-Cu-Ni-Ti alloy. The brazing surface of the ceramic plate 3, i.e., the surface of the ceramic plate 3 to be brazed to the tappet main body 2, may be metallized by vapor deposition prior to the brazing. When the brazing surface is metallized, a desired brazing metal is an Ag-Cu alloy or Ag-Cu-In alloy. 
     A brazing atmosphere is desired to be a nonoxide atmosphere such as an atmosphere of vacuum, Ar, H 2  or N 2 . Particularly, in the case where the brazing metal contains Ti, atmosphere of vacuum or Ar is desired. A brazing temperature is set so as to be causative of hardening the carburized upper part 2a and the lower part 2b. 
     Then, three test samples (a), (b) and (c) and one comparative sample which have such a cross section shown in FIG. 2, whose upper part 2a is sized to be 21 mm in maximum outer diameter and 35 mm long and whose lower part 2b is sized to be 35 mm in maximum outer diameter and 13 mm long, were produced under the following conditions and subjected to the following durability test. The test result is shown in Table 1. 
     Production of test samples (a), (b) and (c): 
     (1) Upper part 2a 
     The upper parts 2a for the test samples (a), (b) and (c) were cold forged from SNC 815, SNCM 616 and SNCM 420, respectively and processed by gas carburizing at 900° C. and for 2 hours so that a carburized layer of 0.6 mm in thickness was formed. In the meantime, &#34;carburized depth&#34; herein used means &#34;effective depth&#34; according to Japanese industrial Standards 0557-1965. Thereafter, the carburized layer at the welding surface to be joined with the lower part 2b is removed by cutting. 
     (2) Lower part 2b 
     The lower parts 2b were formed by cutting from a round bar of SNCM 630. 
     (3) Ceramic seat plate 3 
     The ceramic seat plates 3 were formed from a powdered material containing 90 wt % of Si 3  N 4 , sintering aides of AL 2  O 3  and Y 2  O 3  and into a circular plate by means of a pressing and sintered at ordinary pressure in atmosphere of N 2  and their opposite flat sides were processed by grinding. 
     (4) The upper parts 2a were welded to the respective lower parts 2b by electron beam welding in the case of the test samples (a) and (b) and by laser beam welding in the case of the test sample (c). 
     (5) The brazing were performed by interposing a brazing metal of Ag-Cu-In-Ti alloy between the lower part 2b of each tappet main body 2 and each ceramic seat plate 3 and holding them in a vacuum at 790° C. for 30 min. in the case of the test sample (a) and at 795° C. for 30 min. in the case of the test samples (b) and (c) and in N 2  gas for substitution. Thereafter, the brazed integral unit of the tappet main body 2 and the ceramic seat plate 3 was cooled in a furnace. 
     Production of comparative sample: 
     (1) Upper part 
     The upper part 2a was formed from a round bar of S45C by cutting, heated up to 900° C. and oil hardened. 
     (2) The lower part 2b and ceramic seat plate 3 were the same with those of the test samples (a) to (c). 
     (3) The lower part 2b and ceramic plate 3 were first brazed together under the same condition to that of the test sample (b) and thereafter the upper part 2a was joined to the lower part 2b by laser beam welding. 
     Durability Test: 
     The test samples (a), (b) and (c) and comparative sample of the tappet 1 were subjected to a durability test by being installed on a 15-liter diesel engine and setting the valve clearance to be three times of a standard value (0.4 mm) while operating the engine at 170% of rated speed (about 2,500 rpm). 
     By such a severe durability test, the comparative sample was broken at the welded portion 15 hours after the beginning of the test. In contrast to this, no damage was found in the test samples (a), (b) and (c) which were produced according to the method of this invention even 200 hours after the beginning of the test, and therefore it was confirmed that the test samples (a), (b) and (c) were excellent in durability. 
     From the foregoing, it will be understood that since an upper part and lower part are first welded together to constitute a tappet main body and thereafter a ceramic seat plate is brazed to the tappet main body, the upper part and lower part can be hardened by making use of the heat for brazing while a welding distortion at the welded portion between the upper part and lower part can be removed by making use of the heat for the brazing, thus making it possible to attain a high strength at the welded portion and therefore making it possible to attain a tappet having an improved durability. 
     It will be further understood that the hardening of the upper and lower parts and the removal of welding distortion from the welded portion can be attained at one common process, thus making it possible to reduce the number of processes necessitated and therefore the manufacturing cost. 
     It will be further understood that since the tappet main body is adapted to be made up of separate upper and lower parts, the necessary amount of expensive metal suited for welding to the ceramic seat plate can be smaller and furthermore the simply shaped lower part can be easy in machining even though it is made of metal of a poor machinability, thus making it possible to reduce both the machining cost and the material cost. 
     It will be further understood that by partially cutting off the carburized layer from the portion of the upper part to be welded to the lower part, the weldability between the upper part and lowerpart can be improved, thus making it possible to further increase the strength at the welded portion. 
     
                                           TABLE 1                                 
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        Material                                                          
              Material                                                    
                    Kind            Hardness                              
                                          Hardness                        
        of    of    of     Brazing  of    of    Test                      
        upper part                                                        
              lower part                                                  
                    welding                                               
                           condition                                      
                                    upper part                            
                                          lower part                      
                                                result                    
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Test (a)                                                                  
        SNC815                                                            
              SNCM 630                                                    
                    Electron                                              
                           790° C. × 30 min.                 
                                    Hv 390                                
                                          Hv 390                          
                                                No damage                 
sample              beam welding                was                       
of   (b)                                                                  
        SNCM616                                                           
              SNCM 630                                                    
                    Electron                                              
                           795° C. × 30 min.                 
                                    Hv 400                                
                                          Hv 400                          
                                                found                     
this                beam welding                after                     
invention                                                                 
     (c)                                                                  
        SNCM420                                                           
              SNCM 630                                                    
                    Laser  795° C. × 30 min.                 
                                    Hv 390                                
                                          Hv 390                          
                                                200 hr.                   
                    beam welding                                          
Comp.   S45C  SNCM 630                                                    
                    Laser  795° C. × 30 min.                 
                                    Hv 500                                
                                          Hv 395                          
                                                Broken                    
sample  hardened    beam welding                after 15                  
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                                                hr.