The formation mechanism of fracture surface separations and their effects on the toughness for ferrite-pearlite steels

The formation mechanism of fracture surface separations for ferrite-pearlite steels is investigated by combining experimental testing and numerical simulation, and the effects of fracture surface separations on the toughness are discussed. The fracture surface of Charpy impact samples for both the as-rolled and the normalized ferritepearlite steel were compared. The fracture surface separations occur in the as-rolled plate above the ductilebrittle transition temperature instead of the normalized plate. The fracture surface separations are triggered by transverse tensile stress, which is supported by band-shaped pearlite with poor deformability. It is demonstrated that the formation of fracture surface separations is not only dependent on band-shaped pearlite and temperature but also on the stress state. The normal stress S11 in the X-direction is the largest, and the normal stress S33 distribution concentrates on the plate center, which can explain the location of the fracture surface separations. In contrast to the dispersed distribution of the normal stress S33 for the as-rolled plate, the large normal stress in the normalized plate is in a linear arrangement, resulting in a layered crack. The effects of fracture surface separations on the materials toughness are highly dependent on the number, length, depth and location of the separations.