Application of micromechanical models for predicting fracture toughness of sulphide controlled Fe 510 steels

As cleavage fracture follows a tensile stress criterion, it is possible to predict fracture toughness K Ic as a function of temperature when small scale yielding stress distribution in combination with the Ritchie, Knott and Rice criterion is used. With increasing temperature and thus pronounced plasticity the crack tip blunts and the experimental values are underestimated. The Schmidtmann and Nierhoff proposal for modifying the cleavage stress criterion and introducing the effect of crack tip blunting is compared with the RKR-model and with experimentally determined K Ic - values for four different qualities of Fe 510 steels. The steels mainly differ in sulphur content and sulphur shape control. Thus the effect of sulphur on cleavage fracture will be discussed.