Phase transformations in hypoeutectoid carbon steel during laser powder bed fusion

Journal article, 2025

This study employed high-speed in situ X-ray diffraction, combined with modeling and simulation, to investigate the phase transformations in Fe-0.45 C steel during laser powder bed fusion (L-PBF). The observed rapid cooling rates (∼8.4*10⁴ K/s to ∼7.7*10⁵ K/s) induced two phase transformation shifts. First, the primary solidification mode shifted from δ-ferrite to austenite, aligning with predictions from the Kurz-Giovanola-Trivedi solidification model. Second, austenite transformed into martensite as the cooling rates exceeded the threshold for fully martensitic microstructure (∼500 K/s). Ex situ analyses further confirmed a martensitic microstructure, as the specimens were characterized by a high hardness (∼750–780 HV), fine lath morphology, unetched appearance, and a body centered tetragonal lattice structure. Martensite tempering during the intrinsic heat treatment (IHT) occurred primarily during the initial three cycles, with the most pronounced effect occurring after the first cycle. This was attributed to the efficient heat dissipation of the thin-wall specimens, which minimized heat accumulation and resulted in tempering being driven by reheating within the heat-affected zone. Further phase analysis and modeling identified cementite as the dominant carbide within tempered martensite, even after the first IHT cycle. This was linked to the high dislocation density, small parent grain size, and fine sub-grain structure of martensite, which facilitated transition carbide dissolution while accelerating cementite nucleation and growth. These findings provide an important understanding of microstructure formation during L-PBF of hypo-eutectoid carbon steels that can serve as a baseline for the development of alloy compositions that are tailored for the process.