1. Field of the Invention
The present invention relates generally to a high strength high chromium cast iron. The invention also relates to a valve rocker arm for an internal combustion engine of an automotive vehicle, which is made of the high strength high chromium cast iron.
2. Description of the Background Art
In an automotive internal combustion engine, a valve drive mechanism is provided for driving intake valves and exhaust valves in synchronism with engine revolution. The valve drive mechanism generally comprises a camshaft and a cam follower which convert rotation of the camshaft into a reciprocating motion for axially driving the intake and exhaust valves.
The cam follower comprises a valve rocker arms adapted to be driven by cams carried by the camshaft. The rocker arm is formed of aluminum alloy or high chromium cast iron. In case of aluminium alloy, the rocker arm is formed by die-casting. On the other hand, in case of high chromium cast iron, the rocker arm is formed by integral casting. According to advancing of automotive technologies for higher performance engine, requirement for compact and light weight engine with long life and maintenance free construction.
One example of high chromium cast iron rocker arm has been disclosed in Japanese Patent First (unexamined) Publication (Tokkai) Showa 56-84442. In this Japanese Patent First Publication, ferrochromium alloy used for high chromium cast, is composed of Cr, C, Si, Mn and so forth. In the disclosure, the ferrochromium alloy contains about 30 wt % of Cr with 9 to 13 of Cr/C composition ratio and with greater than or equal to 15 of Cr/C/S composition ratio. More specifically, the disclosed composition of the ferrochromium allow is as follow:
C: 2.4-3.2 Wt % PA0 Si: 0.5-1.0 Wt % PA0 Mn: less than 1.0 Wt % PA0 Cr: 25-35 Wt %. PA0 C: 2.5-3.7 Wt % PA0 Si: 1.0-2.0 Wt % PA0 Mn: 0.5-1.0 Wt % PA0 Cr: 15-20 Wt % PA0 Ni: 0.3-0.7 Wt % PA0 P: less than 3 Wt % PA0 S: less than 0.1 Wt % PA0 Fe: remainder and inevitable impurities.
The high chromium cast iron randomly forms needle structure carbide precipitated on the surface which contacts with cam of a camshaft which is made of chilled cast iron valve shaft of intake and exhaust valve, pivot and so forth. Furthermore, the high chromium cast iron contains martensite base matrix, in which residual sustenite or ferrite is distributed. Such structure of cast iron can cause substantial wearing of the associated components, such as cam, valve shaft, pivot and so forth. On the other hand, the valve rocker arm per se can cause severe scarfing wearing.