Transformation of nanoscale inclusions in 316L stainless steel processed by laser beam powder bed fusion during isothermal heating

Transformation mechanisms and reaction kinetics for formation of nanoscale inclusions in as-built 316L stainless steel produced by laser beam powder bed fusion (LB-PBF) during subsequent isothermal heating process were investigated and clarified experimentally and theoretically. The resulting changes in morphologies, size distributions, number densities, and chemical compositions of the inclusions were measured and discussed, along with microstructure and texture of the steel. The results showed that with increasing isothermal heating time and temperature, the columnar grains in the as-built LB-PBF 316L stainless steel transformed into equiaxed grains, which grew gradually and exhibited a large number of twins in the FCC structure. During isothermal heating, the reaction of Si in the steel with MnO–Cr₂O₃ in the nanoscale inclusion resulted in a transformation from the homogeneous oxide MnO–SiO₂–Cr₂O₃ to an inclusion with an obvious core–shell structure, and the core part was eventually rich in Si and the shell part was predominantly rich in Mn and Si, depending on the heating temperature and time. An Ostwald ripening model used for predicting the growth of nanoscale inclusions during isothermal heating verified that the observed effects of isothermal heating time and temperature were predicted for Si diffusion control.