Brittle fracture micromechanisms in bainitic and martensitic microstructures in a C-Mn-B steel

In the present work, the brittle fracture of a C-Mn-B steel conditioned for both bainitic and martensitic microstructures has been studied, after performing four point bending tests. Four point bend testing in single notched specimens has proved to be appropriate for studying the micromechanisms of brittle fracture, as opposed to fatigue precracked notch tests. These latter specimens exhibit very sharp variations of stress and strain distributions at microstructurally significant distances, causing difficulties in the identification of controlling fracture micromechanisms. Attention must be given to the fact that in both cases, brittle fracture has been triggered from the cracking of coarse brittle TiN particles, which are present in the microstructure as a result of the solidification process. This constancy of the defect size, whilst changing only the matrix strength, has permitted the rationalization of the higher value of [sigma][sub F] for martensite in terms of nucleation control process.