Microstructure evolution and hard phase formation in boron-containing tool steels for additive manufacturing

• Cooling rate is the primary factor controlling hard phase formation in boron-containing tool steels. • Higher solidification rates favor eutectic carbo-borides, while slower rates favor borides. • Elemental segregation governs microstructure evolution in additive manufacturing. • Chromium content and hard phase type predominantly determine hardness. This study investigates the processability and microstructure formation of carbon-martensitic FeCB-based tool steels, designed for powder bed fusion-laser/metal (PBF-LB/M) with 6, 8, and 10 mass% chromium. The alloys follow the Low Transformation Temperature (LTT) concept, exploiting transformation-induced plasticity during martensitic transformation to reduce cold cracking. Since cracking is largely driven by thermal stresses from solidification (segregation effects, phase formation, and local austenite stability) and cooling below the MS-temperature, the focus is placed on microstructure evolution during solidification. Two PBF-LB/M parameter sets were used to vary cooling rates. CALPHAD-based predictions, phase-field simulations, synchrotron X-ray diffraction, and scanning electron microscopy were employed to characterize phase evolution. While CALPHAD captured the general trends in phase evolution, it showed limitations in describing the formation and composition of complex carbo-borides under rapid solidification conditions. Phase-field simulations reproduced dendritic growth and hard phase formation. High cooling rate favored M 23 (C,B) 6 formation, while slower cooling promoted Fe 2 B and Cr 2 B. Mo 3 B 2 formation was significantly suppressed under rapid solidification. Retained austenite ranged from 8 to 14 vol% and hard phases reached 20–24 vol%. A maximum hardness of ∼900 HV1 was achieved. These results demonstrate that understanding solidification-driven microstructure formation is essential for designing tool steels suited for PBF-LB/M processing.