Micromechanisms and the Charpy transition curve

Abstract Fracture mechanisms which play a role in the ductile brittle transition temperature (DBTT) as determined by Charpy tests are reviewed. Cleavage is triggered by heterogeneous plastic deformation which initiates cracks in carbides and in inclusions. It is then propagation controlled, in most cases at the interface of initiation sites or else at grain boundaries. The role of inclusions is often to raise the stress locally, thus allowing initiation in nearby small particles. It is usually considered that the DBTT, as the cleavage stress and the yield strength do, follows a relation with the inverse of the square root of the grain size; this is questioned. Fast cooling rates lower the DBTT owing to finer microstructures in martensite as well as in bainite; they produce also smaller carbides after tempering. The presence of delta ferrite raises the DBTT. The shape of inclusions has an important influence on the initiation as on the further propagation of cleavage cracks. Solutes atoms modify the yield strength as well as the fracture energy; nickel has a solution softening effect and raises the fracture energy; silicon lowers it. Intergranular fracture due to temper embrittlement is another mechanism which has an effect on the DBTT. It competes with cleavage mechanisms explaining the associated role of carbides.