Investigation On Defective Structure Of Large Cracked Gear And Its Failure Analysis

Two large carburized gears cracked when grinding teeth and on a statically placed state. Inspection in site show that for the two gears the circumferential cracking occurr-ed on the spoke plate between the two holes of gear. The macrostructure and micro-structure of gear material were analyzed by SEM and OPM in detail. Chemical compo-sition of core and micro-composition were examined by OES and EDX. Macro and micro fractures were investigated by visual and SEM to determine the cracking mechanism and the failure causes of the two gears. The main results can be concluded as follows:(1) Severe macro point-like segregation is present in the gear material. Content of Si, Mn, Cr, especially P, S in the segregation zones is much higher than in the non-segregation zones. The excessive alloying elements in the segregation zone result in the microstructure difference between the segregation and non-segregation zones.(2) The cracks initiated from the larger size segregation zones in the core of spoke plate of gear and propagated toward the two hole surfaces on the spoke plate and the upper and low ends of gear. Failure in intergranular mode and the intergranular facets associated with ductile tearing was found. Evidences indicate that the failure of the gea-rs is due to hydrogen embrittlement. The possible source for hydrogen was introduced during melting and casting operating.(3) The large segregation zones in the core of spoke plate of gear supply the favor-able metallurgical condition for hydrogen-induced fracture:presence of excessive impurities of S, P and microstructure of bainite susceptible to hydrogen embrittlement, which became the trap of hydrogen. As the hydrogen content in the segregation zone increased, the strength of interfaces decreased to lead to cracking of the hydrogen-enriched zone.