Brittle Cracking of Flanks of a Big Gear

Several teeth of a big helical gear were found cracked following coarse-grinding teeth after the gear was carburized. Visual observation shows that cracks are present only on the flanks at one side having an acute angle to the gear end. The cracks propagated from the flanks toward the gear end. SEM observation on the fracture surfaces reveals that intergranular and quasi-cleavage brittle fracture is the cracking mechanism of the flanks. The cracked flanks at the ??acute angle?? side have a weak ability to support the grinding stress induced by grinding tooth operation, which is mainly responsible for cracking of the flanks. The thick case-harden depth and weak inter-crystalline strength nature of the carburized layer facilitate the initiation and propagation of crack. Unsuitable grinding operations probably aggravate cracking of the flanks.