Origin of fissures on fracture surfaces of impact samples of HSLA steels with ferrite/pearlite microstructures

Abstract

A microstructural examination was carried out investigating the early stages of fissure formation. It was found, in certain steels particularly control rolled, that the tendency to start a fissure, i.e. initiate a crack, which could then propagate parallel to the surface plane in impact tested specimens, was very common. However, whether these grew sufficiently to form deep cracks and relieve the triaxial stress system associated with a notch depended on the degree of grain elongation. Cracks in the early stages of fissure formation were always intergranular, initiated at inclusions or carbide particles that were situated at the boundaries. The further development of the cracks to form a fissure occurred by the decohesion of the grain surfaces until the crack length was sufficient for normal brittle behaviour to take place or the crack had widened to such a degree that the leverage was great enough to make unpeeling of the grain surfaces very easy. Heavily control rolled steels in which the grains had become markedly elongated making crack propagation along the boundaries relatively easy, were therefore, most prone to forming deep fissures. For normalised steels which had essentially an equiaxed grain structure, although cracks initiated at the grain boundaries, they were not able to spread to adjacent grains and growth was stopped abruptly.