Evolution of microstructural heterogeneities in additively manufactured low-alloy steel

Understanding the complex phase transformations during additive manufacturing (AM) of low-alloy multi-phase steels is a necessary task to discover the mechanisms that lead to formation of AM-specific, heterogeneous microstructures. In the present study, investigations were carried out to gain fundamental insights on microstructure evolution of low-alloy steels during AM and upon post-AM heat-treatments. To this end, a low-alloy steel, with a composition similar to DP600 dual-phase (DP) steel, was processed by laser powder bed fusion (L-PBF). Subsequently, two post-L-PBF heat-treatments were applied to obtain ferritic-martensitic DP microstructures. The first heat-treatment consisted of austenitization followed by isothermal holding in ferrite (α)/austenite (γ) region (AIH), whereas the second comprised of inter-critical annealing (IC) in α/γ region. The as-built state exhibited a tempered martensitic microstructure with a weak (almost random) crystallographic texture in combination with compositional and morphological heterogeneities. Combination of multiphase-field simulation and multi-scale microstructure characterization revealed that the formation of compositional and morphological heterogeneities in as-built state was governed by consecutive liquid-solid (delta-ferrite (δ) → γ) and solid-solid (γ ↔ martensite (α΄)) phase-transformations. For both post-L-PBF heat-treatments, Mn-segregation bands that formed during L-PBF led to heterogeneous α΄ distribution after annealing. Electron microprobe analysis (EPMA) measurements revealed that the local Mn and C distributions were closely related to the spatial distribution of α and α΄. The AIH heat-treatment resulted in annihilation of morphological heterogeneities, namely coarse- and fine-grained clusters. The absence of austenitization in IC heat-treatment resulted in distribution of ferritic-martensitic DP microstructure in coarse- and fine-grained clusters that inherited the L-PBF specific grain morphology distribution. Lastly, the IC state showed overall best mechanical properties due to the conservation of the heterogenous clustered microstructure, which potentially aided to simultaneously obtain high tensile strength (890.9 MPa) and relatively high ductility (20.5%).