The transition behaviours of fracture have been examined on lamellar pearlite steel containing proeutectoid cementite. The transition temperature was determined from the tensile properties-temperature curve. Micro-cracks of the fractured specimen were measured on the longitudinal section, and then the relation between the initiation and propagation of micro-cracks and testing temperature was discussed.
In the temperature range below the ductile transition temperature TD, three types of micro-cracks were observed (types a, b and c). Type a was the micro-crack which was initiated by cracking of proeutectoid cementite and propagated by subsequent separation of pearlite. Type b was the micro-crack grown from the cracking initiated at the pearlite colony boundary. Type c was the micro-crack initiated by the cracking of lamellar cementite which was connected to proeutectoid cementite. Above TD, dotted micro-cracks in lamellar cementite (type d) and transgranular cracks of pearlite colony (type e) were observed in addition to micro-cracks of types a, b and c.
