Evolution of microstructures in laser additive manufactured HT-9 ferritic martensitic steel

A comparative study to understand the influence of processing conditions on the microstructural and phase evolution in HT9 ferritic/martensitic (F/M) steels, fabricated using laser powder bed fusion and laser directed energy deposition methods, was undertaken. The microstructural and phase evolutions during the laser-based additive manufacturing processes were compared with the HT9 steel fabricated using conventional vacuum arc melting and explained through the thermokinetic conditions associated with these manufacturing processes. Electron back scattered diffraction microstructures of the cross-sections reveal that the L-PBF processed HT9 steel microstructures comprised δ-ferrite, martensite (α’) and retained austenite (γ), whereas the L-DED microstructure consisted of α’ and retained γ, and no evidence of retained δ-ferrite was found. The arc melted steel showed predominantly martensite and a small amount of retained δ-ferrite. The L-PBF and L-DED microstructures comprise 15–21 % and 3–5 % of retained γ along the build direction, respectively. A multiscale multiphysics thermal model approach was adopted to deduce the correlation between the experimentally observed phase fractions and process-induced thermo-kinetics. The nanoindentation based hardness and yield stress obtained through spherical indentation technique, correlates well with the fraction of retained austenite. The experimental findings coupled with modelling aspects offer valuable insights into the intricate interplay between processing routes, phase evolution, and mechanical attributes in HT9 steel.