Through the material composition test of gear failure fracture

scanning electron microscopy analysis

metallographic analysis

impact toughness test

and the analyses of the fracture limit

thermal stress concentration

the original defects of cast steel

cold-brittle character and other aspects are carried out

the conclusion is educed: In the cold scrip state and under the constraints of the tooth surface hardened layer

the destructive fracture is caused by the rapid expansion of the initial crack

which is induced by the concentration of large thermal stress due to the local sudden heating.Therefore

the improving measure of using the method of overall heating of the tank is proposed in order to avoid the occurrence of similar incidents.