Abstract:
In a process of casting a rocker arm as one piece from high chrome cast iron, a Ca-Si base addition agent is added to the molten metal in an amount of 0.3 to 0.6% by weight. The agent is effective as a globulizer to round high-hardness carbide precipitates directing toward the casting surface.

Description:
This application is a continuation of application Ser. No. 07/569,440 filed on Aug. 20, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of making a rocker arm utilizable as an element of a valve mechanism in an automotive internal combustion engine. 
     2. Description of the Prior Art 
     Anti-wear rocker arms, as described in JP A Sho 63-303030, are producible as one piece from high chrome cast iron by casting. The anti-wear property results from high-hardness carbides such as (Fe, Cr) 7  C 3  in the matrix. 
     High-hardness Fe-Cr-composite carbides tend to form a directional solidification in which acicular precipitates direct to the casting surface. When a rocker arm makes a sliding engagement with a cam member, the acicular carbides abrade the sliding surface of the associated cam member. It is known that acicular carbides can be rounded or globulized when tungsten is added in an amount of more than 10% by weight to the high chrome cast iron. However, it can not be a solution to the problem, because the addition of more than 10% of tungsten is costly. What is worse, tungsten produces tungsten carbides that are highly hard and abrasive to the cam member. 
     SUMMARY OF THE INVENTION 
     The present invention is intended to solve the problem as described above and has as an object to provide a method of manufacturing a rocker arm that is less abrasive to the associated cam member. 
     The present invention is achieved as a result of extensive research made from a finding that an addition agent for treating molten high chrome cast iron is effective to globulize Fe-Cr-composite carbides. 
     In accordance with the present invention, a Ca-Si base addition agent is added in an amount of 0.3 to 0.6% by weight to a molten metal in a process of casting a rocker arm as one piece from high chrome cast iron. 
     The Ca-Si base addition agent contains about 30% of Ca, about 60% of Si, and about 10% of impurities. One or more of Ba, Sr, Mg, Zr, Ce, Mn, and Al may be added in an amount of 0.5 to 5% by weight to the agent. 
     The high chrome cast iron contains more than 10% of Cr and preferably has the composition consisting, by weight, of C: 2.5-3.7%, Si: 1.0-2.0%, Mn: 0.5-1.0%, Cr: 15-20%, Ni: 0.3-0.7%, P: no more than 0.3%, S: no more than 1.0%, and Fe and unavoidable impurities: the balance. The cast iron may additionally contain one or more of W, Mo, V, Nb, Ta, Ti, and B in an amount of 3 to 10%, by weight, if necessary. 
     When the Ca-Si base addition agent is added to the molten metal not as a deoxidizer but as a globulizer, it should be added in an amount of no less than 0.3% by weight. The amount of less than 0.3% of the agent is insufficient to round carbide precipitates or achieve the object of the present invention. On the other hand, the amount of more than 0.6% increases cost without more improvement of the roundness. Therefore, the addition agent should be added to the molten metal in an amount of 0.3 to 0.6% by weight. The Ca-Si base addition agent is moderate in reaction, operable with ease, and contributable to a high yield. 
     When such elements as Ba, Sr, Mg, Zr, Ce, Mn, and Al are added to the Ca-Si base addition agent, the rounding of Fe-Cr carbides is accelerated. The reason for this is that these elements have not only the action to retard the fading of the inoculation-effect but also a high deoxidation due to a strong affinity to oxygen. However, when these elements are added in an amount of less than 0.5% by weight, the desired effect is not obtained, but in an amount of more than 5% by weight, no more improvement is expected. Therefore, the elements should be added in amount of 0.5 to 5% by weight to the agent. 
     The inventive method of casting a rocker arm as one piece from high chrome cast iron is characterized in that a Ca-Si base addition agent is added to the molten metal in an amount of 0.3 to 0.6% by weight. The Ca-Si base addition agent is effective not only to deoxidize the molten metal but also to globulize precipitates of Fe-Cr carbide. As compared with the conventional method in which the addition agent is given as a single deoxidizer to allow Fe-Cr carbides to form acicular precipitates, the method of the invention can produce a rocker arm of the type having globular precipitates of Fe-Cr carbide. This results is the advantages that the rocker arm is the same in abrasion resistance as the conventional but less aggressive against the other member and thus the cam member is less worn when coupled with the inventive rocker arm than when coupled with the conventional one. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
     FIGS. 1 to 7 are photomicrographs showing the metal structure of Examples according to the present invention; and 
     FIGS. 8 and 9 are photomicrographs showing the metal structure of Comparisons. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is described in detail with the sample rocker arms made from high chrome cast iron in which the molten metal has the following composition by weight: 
     TC: 3.50% 
     Si: 1.50% 
     Mn: 0.70% 
     Ni: 0.50% 
     Cr: 18.00% 
     Mo: 1.00% 
     W : 5.00% 
     Fe: Balance 
     In the process of casting, the molten metal of each sample was treated with the individual agent of which the content, amount and elements added are given in Table 1. Every sample was hardened and tempered in the same way after casting. The samples were classified to 11 Examples and 2 Comparisons on the basis of the composition of the addition agent. Examples and Comparisons except Examples 3, 7, 9, 11 were etched and photomicrographed as illustrated in FIGS. 1 to 9. The photomicrographs show white carbide grains distributed in the black pearlitic matrix. Each photomicrograph was analyzed by an image analyzer by which both the real and circumcircular areas of each carbide grain were measured for calculation of the ratio of the former to the latter. The average value of the ratios indicates the roundness or globularity of each of Examples and Comparisons. The values of globularity are also given in Table 1. 
     
                       TABLE 1______________________________________SAMPLES                 Globularity______________________________________Example 1 shown in FIG. 1                   43%Agent: Ca--Si: 0.4%(Elements added: none)Example 2 shown in FIG. 2                   45%Agent: Ca--Si: 0.3%(Elements added: Ba: 2%)Example 3 not shown     48%Agent: Ca--Si: 0.5%(Elements added: Sr: 3%)Example 4 shown in FIG. 3                   50%Agent: Ca--Si: 0.6%(Elements added: Mg: 2%)Example 5 shown in FIG. 4                   42%Agent: Ca--Si: 0.4%(Elements added: Zr: 3%)Example 6 shown in FIG. 5                   47%Agent: Ca--Si: 0.4%(Elements added: Ba: 1%, Zr: 1%)Example 7 not shown     52%Agent: Ca--Si: 0.5%(Elements added: Sr: 1%, Mg: 1%)Example 8 shown in FIG. 6                   45%Agent: Ca--Si: 0.5%(Elements added: Mn: 3%)Example 9 not shown     43%Agent: Ca--Si: 0.4%(Elements added: Ce: 3%)Example 10 shown in FIG. 7                   55%Agent: Ca--Si: 0.5%(Elements added: Ba: 2%, Mg: 1%, Ce: 0.5%)Example 11 not shown    48%Agent: Ca--Si: 0.4%(Elements added: Zr: 2%, Sr: 1%, Mn: 1%)Comparison 1 shown in FIG. 8                   28%Agent: Al: 0.4%(Elements added: none)Comparison 2 shown in FIG. 9                   32%Agent: Ca-- Si: 0.2%(Elements added: none)______________________________________ 
    
     As seen in FIGS. 1 to 7, Examples have carbides in the shape of globular grains. In contrast, FIGS. 8 and 9 show that Comparisons have carbides in the shape of sharp or acicular grains. This difference in shape of carbides is also understood from the values of globularity in Table 1. Examples have the globularities of more than 42% while Comparisons have the globularities of 28% and 32%. It is known that, when the rocker arm has the globularity of more than 40%, it is unaggressive against the associated cam member with the result that the cam member is improved in weariness. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.