Metal working – Method of mechanical manufacture – Prime mover or fluid pump making
Reexamination Certificate
2000-01-24
2003-12-02
Vidovich, Gregory (Department: 3726)
Metal working
Method of mechanical manufacture
Prime mover or fluid pump making
C029S557000, C072S340000, C148S211000, C451S038000
Reexamination Certificate
active
06655026
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a production process for a connecting rod for internal combustion engines, and specifically relates to a strengthening method for inner surfaces of a bearing into which a piston pin and a crankpin fit in sliding and relative rotating connection.
Generally, a connecting rod such as the above comprises a small end formed at an end of a rod-like portion as a main element and a big end formed at the other end of the rod-like portion. A piston pin of a piston is fitted into the small end and a crankpin of a crankshaft is fitted into the big end in sliding and relative rotating connection. The connecting rod converts the reciprocation of the piston to the rotation of the crankshaft. Such a connecting rod repeatedly receives inertial force from the piston and explosive pressure in the combustion chamber, so that fluctuation of the load, such as tension and compression, repeatedly affects each end of the rod (hereinafter collectively referred to as the “bearing portion”). Generally, a bearing metal is fixed in the inner surface of the bearing in order to reduce sliding resistance and noise. In this case, there is a concern that relative sliding between the inner surface of the bearing portion and the bearing metal will occur by the fluctuating load which impinges on the bearing portion, and that fretting corrosion will occur. The fretting corrosion causes remarkable decrease of fatigue strength. Therefore, the thickness of the bearing portion must be additionally designed in consideration of the decrease in the fatigue strength, so that a design to reduce the weight of the apparatus is difficult. A structure in which the bearing metal is omitted for the sake of a light-weight design is known. In this structure, the piston pin and the crankpin are directly supported by the inner surface of the bearing portion, and a lubricating oil is supplied through oil paths, which are formed in the connecting rod, to the sliding surface. In this structure, the sliding portion tends to be galling and seize, and there was a problem that it was difficult to ensure sufficient fatigue strength.
Carburizing and quenching processing has been employed to ensure or improve fatigue strength. The carburizing and quenching processing is a heat processing method in which a product formed of steel with a relatively low carbon content is heated in a carburizing gas comprising CO gas and CH
4
gas, etc, as a main component so as to disperse and permeate carbon into the surface layer of the product, and then the product is quenched so as to harden only the surface layer. However, oxygen-containing components such as H
2
O, CO
2
, and CO are also included in the carburizing gas, so that oxygen permeates into the surface layer during carburization. The oxygen combines with elements such as Cr, Mo, Si, etc., which are originally added for improving quenching properties, and oxides precipitate at the grain boundaries in the steel. As a result, a grain boundary oxidized layer, which is relatively soft, is formed to a depth of about 50 to 10 &mgr;m, and the quenching properties of the surface layer after the carburization is deteriorated, and desired fatigue strength cannot be obtained.
Japanese Patent Application Laid Open (Kokai) No. 62-203766 discloses a process for improving fatigue strength by the carburizing and quenching processing. According to the process, the grain boundary oxidized layer (referred to as the “abnormal carburized layer” in the publication) is removed by chemical dissolution after the carburizing and quenching processing, and then shot-peening is performed on the treated surface. Japanese Patent Application Laid Open No. 6-71520 discloses a process in which the grain boundary oxidized layer is removed by CBN (cubic boron nitride) grinding, and then shot-peening is performed on the ground surface. In both processes, the grain boundary oxidized layer must be removed, and the portion in which the compressive residual stress has been formed by the quenching is exposed on the surface, and then the shot-peening is performed thereon, so that new compressive residual stress is imparted.
The quenching depth according to the carburizing and quenching processing, that is, the depth to which the compressive residual stress is formed, varies according to the condition of the carburizing and quenching processing (temperature, carbon concentration, processing time, etc.). It will be the best for the surface hardening processing if the hardest portion, in which the compressive residual stress exhibits the maximum value, can be exposed as a surface. However, in the conventional processes, the processing of fresh surface, from which the grain boundary oxidized layer is removed or the surface on which the shot-peening was performed, is not always the portion which exhibits the highest compressive residual stress. According to the research of the inventors, the depth to which the compressive residual stress exhibits the maximum value is approximately 100 to 200 &mgr;m, and is deeper than the usual grain boundary oxidized layer (approximately a depth of 10 to 50 &mgr;m, as mentioned above). Therefore, it has not been possible for the surface to exhibit the maximum value of the compressive residual stress even if the grain boundary oxidized layer is removed and shot-peening is performed thereon.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a production process for a connecting rod for internal combustion engines, in which the fatigue strength is remarkably improved by an optimum surface hardening processing and the problem of the sliding properties, such as galling and seize, can also be solved.
The present invention provides a production process for a connecting rod for internal combustion engines, the process comprising a carburizing and quenching processing for forming a carburized layer in a surface layer of a material of a connecting rod for internal combustion engines, a grinding processing for grinding the carburized layer formed in a bearing portion of the material until the depth at which compressive residual stress exhibits the maximum value, and a shot-peening processing of the surface ground by the grinding processing.
In the invention, first the carburizing and quenching processing is performed on the material of the connecting rod which is formed of steel with low carbon content. By the carburizing and quenching processing, a carburized layer, which is provided with compressive residual stress and is hardened, is formed at the surface layer of the material. Carburizing and quenching processing in the invention refers to a series of processes in which the material is retained in the carburizing gas at 900° C. or more, for example, and then the material is rapidly cooled and tempered. A grain boundary oxidized layer exists in the outermost surface of the formed carburized layer.
FIG. 1
shows the characteristics of the surface layer after the carburizing and quenching processing is performed on low-carbon steel, the hardness, the proportion of retained austenite, and the compressive residual stress. These characteristics are approximately as shown in
FIG. 1
, even though some differences will be observed according to differences in processing conditions. In
FIG. 1
, the portion indicated by the oblique lines on the left shows the grain boundary oxidized layer. According to
FIG. 1
, the hardness is constant over some interval from the surface, and it becomes lower below a certain depth. The proportion of the retained austenite is high in the vicinity of the surface, and decreases as the depth increases. The compressive residual stress is small in the vicinity of the surface, increases as the depth increases, and then decreases after it exhibits the maximum value at a certain depth. After the carburizing and quenching processing in which such characteristics are exhibited, the grinding processing for grinding the carburized layer formed in the bearing portion of the material until the depth at which compressiv
Iida Zenji
Koike Seiichi
Muroga Shigeki
Takashima Mitsuo
Arent Fox Kintner & Plotkin & Kahn, PLLC
Compton Eric
Honda Giken Kogyo Kabushiki Kaisha
Vidovich Gregory
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