Powder Bed Fusion - Laser Beam of Low-Alloy and Tool Steels: Role of Processing and Heat Treatment

Licentiate thesis, 2025

Powder Bed Fusion–Laser Beam (PBF-LB) enables the production of complex, high-performance steel components with tailored microstructures. However, the rapid solidification and cyclic reheating that is typical of this process create unique microstructural features and thermal histories that differ significantly from those of conventionally processed steels. These characteristics influence phase transformations, precipitation behavior, and mechanical performance, highlighting the need to understand how processing and post-treatment parameters affect material properties.

This thesis investigates the influence of processing parameters and heat treatment on the microstructural evolution and mechanical response of low-alloy and tool steels fabricated by Powder Bed Fusion–Laser Beam (PBF-LB). The first part examines how variations in process parameters affect densification, microstructure, and mechanical performance in a low-alloy engineering steel. Fully dense and crack-free components (≥ 99.9%) were obtained across a wide processing window, exhibiting a refined lath martensitic microstructure. A distinct change in prior austenite grain morphology was observed in specimens processed with a 90 µm layer thickness, where columnar grains transitioned to more equiaxed structures. Increasing layer thickness led to a reduction in impact toughness, highlighting the trade-off between build productivity and mechanical performance. The second part focuses on the tempering behavior of an additively manufactured lean hot-work tool steel. Detailed microstructural characterization revealed sequential precipitation of V(C,N), M2C, and MC-type carbides with increasing tempering temperature. The directly tempered condition exhibited finer and more uniformly distributed precipitates, higher dislocation density, and improved thermal stability compared to the quenched-and-tempered condition.

Overall, these insights contribute to the development of robust processing and heat treatment strategies that support the broader industrial implementation of additively manufactured low-alloy and tool steels.