Influence of carbide particles on crack initiation and propagation with competing ductile-brittle transition in ferritic steel

Carbide particles play an important role for the fracture toughness of ferritic steels in the ductile-brittle transition (DBT) region as cracks mainly originate from a broken or debonded carbide. It is well-known that size, volume fraction, strength and distribution of carbides are relevant for the competition of ductile and brittle mechanisms of failure. In the present study, the influence of the carbide strength and carbide-ferrite interaction on the fracture toughness of ferritic steels is investigated in the DBT regime by a micromechanical, deterministic FEM model. The carbide particles are resolved discretely in the fracture process zone at the crack tip. Cleavage of ferrite, of carbides and debonding of the carbide-ferrite interface are modeled by cohesive zones. The simulated fracture toughness in the DBT region do compare well with experimental data from literature. Thus, the capability of the model is demonstrated to capture the competition of the various microscopic mechanism.