Typology of cleavage fracture initiation sites of a low alloy steel after different heat treatments

Starting from a single nuclear heavy component steel, with a low inclusion content, 18 different heat treatments conditions were applied to vary its microstructure. An extensive fractographic study was then carried out on impact toughness and notched tensile specimens within the brittle fracture domain. A common typology of fracture initiation sites was identified over all treatment conditions; these initiation sites can be divided into three main categories. First, in most tests, brittle fracture was initiated by a carbide located at a grain boundary. Second, inclusions such as titanium nitrides or manganese sulfides triggered the brittle fracture for some cases. A few initiation sites were located on a grain boundary but did not exhibit any prominent microstructural constituent like an inclusion or a precipitate. For the studied specimens, the type of initiation site was neither correlated with the heat treatment parameters not with the testing conditions.