Intergranular Fracture of Spoke Plate of a Large Gear

Abstract

A crack occurred on the spoke plate of a large marine gear apparently when the gear was being moved at the fabrication site. Fractographic analysis of the fracture surfaces in this study revealed that the crack initiated from the core of spoke plate, and propagated circumferentially toward the surfaces of two holes on the spoke plate and simultaneously to the upper and low ends of the gear. Intergranular fracture was found in crack origin and in regions where the fracture developed slowly. The intergranular facets were associated with ductile shearing between the facets. Transgranular cleavage fracture was apparent in the fast crack propagation region. Metallographic observations also revealed that a type of macrocasting defect, general spot segregation, was generally present on the gear material. A large size macro-spot segregation zone was present near the crack origin and presented a favorable metallurgical condition for hydrogen-induced fracture. The conditions conductive to hydrogen embrittlement include the presence of excessive amounts of S and P, and the appearance of bainite. Therefore, it was concluded that the crack of the spoke plate of the gear was hydrogen induced. The probable source for hydrogen was introduction during the melting and casting process.