The fracture resistance of a ferritic-austenitic duplex steel

For a nitrogen alloyed ferritic-austenitic duplex steel X3CrMnNiMoN 25 6 4 the crack growth resistance is recorded in terms of the J-integral, J, and the energy dissipation rate, D. Due to the anisotropic microstructure of elongated ferrite and austenite domains, very different resistance curves appear for three different crack plane orientations. Metallographic and stereophotogrammetric studies lead to a model which is able to explain these curves. It is found that the geometrical arrangement of the (brittle) ferrite and the (ductile) austenite domains is the main influence factor on the crack growth resistance (and not the volumetric fraction of ferrite and austenite). Since no standard J(IC)-values can be determined, a procedure is proposed to evaluate an alternative measure of the fracture initiation toughness, i.e. ''quasi-initiation'' J-integral values. Copyright (C) 1996 Acta Metallurgica Inc.