Micromechanics driven design of ferritic–austenitic duplex stainless steel microstructures for improved cleavage fracture toughness

Ferritic–austenitic duplex stainless steels are known to offer favorable combinations of good mechanical properties and corrosion resistance to be used for structural purposes. Lean duplex grades have already been introduced for consideration to replace standard 18–8 austenitic stainless steels in various industrial applications. Ferrite and austenite represent different deformation behaviors and contribute to the resulting fracture toughness characteristics. Micromechanical crystal plasticity based assessment of this cleavage fracture behavior is the subject area of current work. The objective is to bridge cleavage fracture models to full field crystal plasticity imaging based modeling of microstructures of ferritic–austenitic duplex stainless steels. The goal is to introduce means to computationally assess the effects of different multi-phase microstructural morphologies to cleavage fracture toughness and develop both the respective constitutive and cleavage fracture modeling capabilities. Such means can be used as an aid to develop better cleavage resistant, while in the current context lean, steel grades. Three different steels are investigated with differing austenite phase morphologies, and their behavior with respect to fracture mechanical response evaluated by micromechanical modeling. The effect of austenite fraction and morphology in terms of improving fracture toughness, a critical parameter concerning these steel grades and improvement of their cleavage fracture properties, is investigated. Deleterious features such as large ferrite grain size and microstructural property mismatching are identified and their implications to fracture toughness and development of applicable modeling capabilities for ferritic–austenitic duplex steels discussed. Simple design task of varying the austenite phase fraction is performed using synthetic microstructural modeling and the results evaluated with respect to their influence on the fracture toughness ductile-to