Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel

An investigation was conducted into the effect of carbide distribution on fracture toughness in the ductile-brittle transition temperature region of an SA 508 steel used for nuclear reactor pressure vessels. Tensile properties and elastic-plastic cleavage fracture toughness were measured in the transition temperature region, and the fracture toughness data were interpreted by using a simple fracture model containing carbide size distribution. This modeling study indicated that the critical nearest-neighbor distance between coarse carbides was an important microstructural factor affecting elastic-plastic fracture toughness, since it satisfied a linear relationship with the critical distance between a crack tip to a cleavage initiation site. These findings suggested that reducing the total number of carbides, particularly the number of M₃C carbides larger than the critical size, and homogeneously distributing fine M₂C carbides, were useful ways to improve fracture toughness in the transition temperature region.