scope:

The brittle-to-ductile transition for cleavage fracture of steels has been examined with integrated considerations of micromechanical and macromechanical features. The evolution of transition temperature approaches to fracture-safe design has been based on concepts that metal ductility factors should override mechanical constraint factors, in the higher temperature range of the transition. The transition temperature range, determined by dynamic fracture tests, has provided the necessary guidance for the development of improved steels. Fracture mechanics concepts emphasize that macroscopic fracture toughness is controlled by mechanical constraint and flaw severity factors. While true, within limits, there has been an unwarranted extension of these principles to signify that the transition temperature does not have a basic significance to fracture processes. Contrary to popular beliefs, these concepts are not in opposition-metallurgical factors determine the intrinsic metal ductility, and mechanical parameters serve to describe the response of the metal to specific stress states. Recent investigations of the effects of large section size have demonstrated that increased mechanical constraint results in shifts of the transition temperature, as predicted by fracture mechanics theory. However, these shifts are of relatively small magnitude and, more importantly, do not eliminate transition temperature characteristics.