Failure Analysis And Processing Technology Improvement Of The Bicycle Flywheel Parts And Its Molding Die

For the bicycle multi-flywheels core's cold extruding die and the bicycle flywheel bit of fourteen teeth, the fracture failure reason was analyzed by the optical microscope, scanning electron microscope and the microhardness tester. The improved measures of the die's processing technology and the flywheel bit's producing technology were proposed in the paper.For the bicycle multi-flywheels core's cold extruding die, it was found that a lot of non-uniform carbide belts are obvious with the observation of the microscopic structure. The results of observing fracture appearance and testing of carbon component nearby the fracture surface indicate that the carbide concentration region is the region of generating the fracture crack. The existence of non-uniform carbide belts is the inner reason of fracture failure, and the accumulation of the stress produced in the process of cold extruding is external reason of fracture failure. The suggestions are presented as follow: (1) the incoming raw material worked the die should be forged firstly. (2) quenching and tempering temperature are adjusted to 1210℃and 560℃, respectively. (3) after extruding parts about 3500, the punch is anealed at 280℃for 2h to relieve the thermal stress .The characteristics of the carburized layer and distribution of the carbon content are analyzed for the bicycle's flywheel bit of fourteen teeth. Compared with Shimano's product, the results show that the depth of the carburized layer of the teeth from the top to root is heterogeneous due to the different deformation in variant regions of the flywheel blank formed by cold-extruding. There is the over carburizing area, which consists of brittle carbide mainly, in the superficial coat of the teeth because of high carbon potential in the carburizing processing. Especially, over carburizing area of the root of the teeth, which has the larger size and highest microhardness value, is mainly the region of generating fracture. The suggestions are presented as follow: (1) the cold extruding blank should be on an annealing process ,temperature at 650℃for 10min before carburizing heat treatment (2) In the beginning and end of the carburizing process the carbon potential are adjusted to 0.85 and 0.80 (C %), respectively.