Based on all the previous analysis, the fracture process of nodular cast iron crankshaft is as follows: in the induction quenching process, the effective coil width of the inductor is too large, resulting in too long heating size, and the rolling groove in Zone IV is quenched; Subsequently, in the rolling process, the structure of the rolling groove and edge is extruded, slipped or even crushed. While the graphite in Zone IV is deformed, small cracks are generated around the deformed graphite.
When the full speed and full load bench test is carried out, due to the influence of force and the sharp angle effect of flake graphite, the cracks around graphite expand and extend, and part of the metal on the journal surface falls off; On the one hand, the surface of the journal and bearing bush is strained by the falling metal, on the other hand, the lubricating oil film between the bearing bush and the journal is damaged, and the bearing bush of nodular cast iron crankshaft is short of oil. Poor lubrication leads to direct contact friction between the bearing bush and the journal. The high temperature generated by friction leads to re austenitization of the journal and secondary quenching during bearing cooling, High temperature also eventually leads to sintering and locking of bearing bush and journal.
At the same time, due to inertia rotation, the nodular cast iron crankshaft is subjected to excessive bending and torsional stress near the journal fillet, and the stress is concentrated at the loose defect position at the edge of the rolling groove. When the stress exceeds the fatigue limit at the loose position, it promotes the formation and rapid expansion of fatigue cracks, and then the nodular cast iron crankshaft breaks instantaneously.