Brittle-to-ductile transitions and its relation to the deformability of cementite in fully pearlitic steels

Temperature dependence of the absorbed impact energy in fully pearlitic steels was investigated. Two-step brittle-to-ductile transitions were observed. The value of the activation energy associated with the first transition is comparable to that of low carbon ferritic steels, indicating that the first transition is the brittle-to-ductile transition of ferrite phase in pearlitic steels. It suggests that the first transition is controlled by the dislocation glide in ferrite. The value of the activation energy associated with the second transition is higher than that of the first transition. Micrographs of fracture surfaces and side surfaces after fracture tests suggest that the activation energy associated with the second transition should relate to the deformability of cementite in pearlite, that is, the second transition is controlled by the dislocation activity in cementite of pearlite.