Investigation of cleavage fracture initiation in notched specimens of a C–Mn steel with carbides and inclusions

Cleavage initiation in notched specimens of a C–Mn steel with carbides and inclusions is investigated at −196 and −130 °C. The mechanical tests, microscopic observations and measurements, and FEM calculations are carried out. Two types of cleavage initiation sites are found. One is related to approximately spherical inclusions ahead of notch root (IC initiation), the other is related to spherical inclusions situated ahead of the elongated larger string inclusions (SIC initiation). In both IC and SIC initiation mechanism, the crack nucleation is induced by inclusions and the final fracture is controlled by propagation of a ferrite grain-sized crack into matrix grain. In the case of SIC initiation, a plate-like defect caused by the early debonding of the elongated larger string inclusions promotes the cleavage fracture, and the cleavage initiates in front of the plate-like defect. Temperature has pronounced effect on the type of the cleavage initiation site. At −196 °C the fractures are mainly the IC initiation mechanism, and at −130 °C all the fractures are the SIC initiation mechanism. The reason for this has been analyzed. The notch toughness of the materials tested is mainly determined by the cleavage initiation occurring in the weak constituents composed of inclusions and matrix grains, and is independent on the sizes and numbers of carbide particles.