Thermal ageing on the deformation and fracture mechanisms of a duplex stainless steel by quasi in-situ tensile test under OM and SEM

To clarify the effects of the long-term thermal ageing embrittleness on the deformation and fracture mechanism of casting duplex stainless steels (CDSSs), quasi in-situ tensile tests under OM and SEM are conducted for Z3CN20-09M CDSSs thermally aged up to 10,000 h. Regardless of thermal ageing, the single sliding, multi-gliding and cross-slipping are the main deformation mechanisms in the austenite and ferrite under tensile load. The long-term thermal ageing mainly affects the initiation and the propagation of the cracks and then influences the fracture mechanisms. For the as-received specimen, the cracks initiate in the locations of the impurity particles both within the austenite and ferrite grains and between the δ/γ phase boundaries. For the spinodal decomposition, the precipitation of the G-phase and the coarsening of the spinodally decomposed structure in ferrite, the cracks mainly originate in the δ/γ phase boundary and the slip steps in ferrite. The fracture mode changes from the microvoid coalescence ductile fracture of the as-received specimen, to the quasi-cleavage fracture in ageing time of 3000 h and to the cleavage fracture in ferrite and the quasi-cleavage fracture in austenite in ageing time of 10,000 h.