A new approach for processing defect-free hot-work tool steels: Quasi-isothermal laser powder bed fusion

• As-hardened properties after PBF-LB/M processing in the transformation gap. • No microstructural height-dependencies after quasi-isothermal processing. • Enhanced secondary hardening towards higher hardness and tempering temperatures. • Simulation of partial carbon equilibria by a novel computational approach. • Innovative cross-linking of empirical and simulated results. Powder-Bed-Fusion Laser-Beam/Metal (PBF-LB/M) enables the production of complex geometries with high design freedom. However, processing martensitic tool steels by Additive Manufacturing (AM) is challenging due to the risk of cold cracking during martensitic transformation. Traditionally, preheating above 200 °C is used to mitigate cracking, but this leads to complex, heterogeneous microstructures and requires additional thermal post-processing. The direct production of as-quenched parts by PBF-LB/M, eliminating the need for subsequent hardening, remains unexplored. This study investigates the quasi-isothermal processing of AISI H11 (X37CrMoV5-1) tool steel at elevated temperatures, specifically within the transformation gap between approximately 400 C and 620 C. The research focuses on understanding the time–temperature-transformation behavior and microstructural evolution during isothermal PBF-LB/M. Dilatometry and a novel computational approach combining thermodynamic and empirical calculations were employed to analyze microstructure formation and retained austenite stability. Results show that processing at 425 C produces a homogeneous, defect- and crack-free martensitic microstructure with about 15 vol% stable retained austenite, eliminating the need for subsequent hardening. Additionally, this microstructure enables more efficient secondary hardening compared to conventional methods, offering a promising, cost-effective route for PBF-LB/M processing of C-martensitic steels.