Failure analysis of a gear tooth facture of a rolling mill decelerator

Metallographic examination revealed that the grain size of the material are too coarse and the tempered structure retained a thick needle martensite orientation, thus reducing the steel toughness and raising the gap sensitivity. Heat treatment experiment showed that the coarse grains mostly resulted from the ultra high-temperature quenching. The impact toughness of the gear material was too low whereas the yield ratio of the gear material was too high, with a tested value up to 0.89. The hardness of the gear material was also too high, reaching a value of HRC37. The uniformity of the gear material, such as the dispersion of sulfur-rich inclusions, is poor. Fractographic analysis indicates that a brittle multi-source fatigue fracture caused by the microstructural defects of the material.