Failure analysis of diesel engine flywheel ring-gears

This paper presents a failure analysis on three diesel engine flywheel ring-gears used in trucks. The ring-gears fractured when starting engines at low temperature (−25 °C). The fracture occurred from the root fillet region on the side of the load contact surface in all cases. Fractographic studies indicate that the fracture surfaces of three failed ring-gears have similar features, which exhibit cleavage fractures reflective of low toughness of the material. The detailed metallurgical analysis was conducted on the fractured gear teeth and compared with the ring-gear unfailed in service. The failed ring-gears present general metallurgical characteristics that network of grain boundary ferrite even Widmannstätten structure ferrite appeared in the induction hardened region and a continuous network of grain boundary ferrite appeared in the core and tooth groove regions. The appearance of the network ferrite makes the strength of the gear teeth decrease particularly in tooth groove region. The thinner induction hardened layer in the tooth groove region is also responsible for the decrease in strength of the ring-gears. It has been found that there are some forging cracks in the root fillet region. Additionally, the smaller root fillet radius makes the stress concentration in the fillet regions. The metallurgical defects with machining defect result in premature embrittlement fracture of the ring-gears. The unsuitable normalizing technology or forging technology should be responsible for the formation of continuous network of grain boundary ferrite.