Impact tensile tests of tufftrided low carbon steel were carried out at various temperatures by using a drop-weight type impact testing machine designed in our laboratory. The initiation of crack during the fracture process was observed by using a stroboscope. The fractography and the observation of microstructure near the crack tip were performed by optical and electron microscopes.
The transition temperature rose by tufftriding, and this rise became more remarkable with increasing treating time. This can be explained by the increase of yield stress and the decrease of brittle fracture stress. At the high temperature side of transition region in tufftrided steel, brittle cracks initiated near the surface of hardened layer and propagated to the core. The fracture mode changed from brittle fracture (cleavage) to ductile fracture (dimple) at the boundry between the case layer and the core.
By considering these characteristics, a fracture model of tufftrided steel was proposed and the fracture mechanism was discussed.
