抄録
The CGF (Continuous Grain Flow) forging method is widely applied to crankshafts of high-speed marine diesel engines in recent years. CGF-forged crankshafts are considered to have an advantage in fatigue strength compared with free-formed forged crankshafts because of two factors: 1) continuous grain flow along the axial direction of the shaft, and 2) small segregation of impurities in the surface layer of the shaft. In general, however, the forged grain flow of CGF-forged crankshafts has a local discontinuation at the fillet between the crank-pin and the crank-arm or between the crank journal and the crank-arm, which originates in the shaft machining process. In this study, effects of the local discontinuation of the forged grain flow on the ultra-high-cycle fatigue strength were investigated by fatigue tests and by FEM calculations. Based on the investigation, it is clear that the effects of continuous grain flow on the fatigue strength are invalid in the event that a local discontinuation of the forged grain flow at the fillet exists.
