Rocker arm and method of casting

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.

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).sub.7 C.sub.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.

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 
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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) 
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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.