Influence of annealing time on the microstructure and properties of additively manufactured X2CrNiMoN25–7–4 duplex stainless steel: Experiment and simulation

Abstract

Duplex stainless steels (DSS) manufactured additively using the powder bed fusion – laser beam/metal (PBF-LB/M) process exhibit a predominantly ferritic microstructure due to the process inherent ultrahigh cooling rates. In this microstructural constitution, the high potential of a balanced two-phase ferritic-austenitic microstructure typical for DSS is not exploited, which is why these steels are subsequently heat-treated. The present study focuses precisely on this heat treatment considering PBF-LB/M-processed X2CrNiMoN25–7–4 DSS solution annealed at different times and aims to explain the influence of the as-built microstructure on the phase formation during solution annealing as a function of heat-treatment time and how different annealing times affect the mechanical and corrosive properties. The as-built microstructure was investigated using electron backscattered diffraction (EBSD), X-ray diffraction (XRD), and atom probe tomography (APT) and the subsequently solution annealed ones using EBSD, XRD, secondary electron microscopy (SEM), energy dispersive (EDS) and wavelength dispersive X-ray spectrometry (WDS). Additionally, the results were compared to DICTRA® simulations. The corrosive properties were examined by potentiodynamic measurements in 0.5 molar sulfuric acid, whereas the mechanical properties were characterized by tensile testing. The ferritic microstructure in the as-built condition is characterized by a supersaturation with nitrogen, accompanied by the formation of nanoscale Cr2N. During heat treatment, these precipitates serve as nucleation sites for austenite formation, so that a balanced, ferritic-austenitic microstructure is already achieved after 3 min. Compared to longer annealing times (5 and 60 min), the highest strength and corrosion resistance is obtained with an annealing time of 3 min, as longer times result in grain coarsening.