Development of Fracture Splitting Method for Case Hardened Connecting Rods

The fracture splitting (FS) method for case hardened connecting rods has been developed to improve engine performance while decreasing production costs. The FS method is widely used for automotive connecting rods because it effectively improves their productivity. Normalized forging steels, microalloyed forging steels and powder metals have generally been used as the material in the FS method as they are easily split due to their brittleness. On the other hand, the materials to be used for high performance motorcycles are case hardened low carbon steels because they allow the connecting rods to be lightweight due to their high fatigue strengths. These materials, which have a hardened area of approx. 0.5mm in depth from the surface, have a ductile texture inside. This texture obstructs the crack propagation and makes the split force too high to split without deforming the bearing area. In order to solve these issues and make FS a successful method for making motorcycle connecting rods, the experimental evaluations have been completed. It was found that the temperature and the strain ratio affected the brittleness of the fracture surface, and the ductile-brittle transition line was selected. It was also found that modifying the bearing design is effective to avoid the harmful tiny fractions occurring at the junction of another crack propagation through a FEM method. This newly developed method has shown an improvement in the circularity of the bearing area while decreasing the production cost by 30%. The connecting rods manufactured using this new method have been applied for our mass-produced high performance motorcycles since 2003.