抄録
The relation between micro-cracks and transition temperature was examined on low carbon steels containing lamellar pearlites. Specimens were pulled to fracture at different speeds at various testing temperatures below room temperature. Fractured specimens were observed under a microscope.
Four distinct temperature ranges were observable from the initiation mechanisms of microcracks. Below the transition temperature TC, at which the fracture stress decreased abruptly in the temperature range of nil-ductility, micro-crack were initiated in ferrite by cracking of boundary cementites and lamellar cementites. In the temperature range from TC to TB (nil-ductility transition temperature), micro-cracks were initiated at the intersection of deformation twins and at the barriers such as grain boundaries and pearlite colonies. In the temperature range from TB to TD (ductility transition temperature), different types of the initiation of micro-cracks were observed. In the vicinity of TB, micro-cracks were initiated at the ferrite boundaries and the boundaries between pearlite colonies and ferrites, but at the temperature near TD, lamellae of cementite parallel to the tensile axis were fractured and the cementite cracking grew into void to fracture. Above the temperature TD, fracture was advance by voids and the voids tended to become larger with increasing temperature.
