Understanding Precipitation Kinetics in an Additively Manufactured Hot Work Tool Steel

Additive manufacturing (AM) of tool steels often produces complex microstructures. Conventional alloys such as H13 present significant challenges in laser powder bed fusion (PBF-LB) due to their high carbon content, which leads to elevated residual stresses and an increased risk of cold cracking in the as-printed state. As a result, tool steels typically require modification or complete redesign to achieve optimal processability in PBF-LB. Osprey HWTS 50 is a newly developed hot-work tool steel alloy specifically engineered for AM, featuring a reduced carbon content compared to traditional grades like H13. This adjustment, along with tailored levels of chromium, molybdenum, and vanadium, not only improves printability but also enhances tempering resistance and reduces susceptibility to cold cracking. The alloy’s composition promotes the formation of temper-resistant vanadium- and molybdenum-rich carbides, rather than the chromium-rich carbides typically observed in H13, which is crucial for controlling the precipitation sequence and optimizing mechanical properties. Heat treatment remains essential for tailoring the mechanical response of AM tool steels. However, the unique microstructures and presence of nanoscale precipitates in these materials mean that conventional interpretations of precipitation behaviour may not apply. This study investigates the precipitation mechanisms and sequences that arise from the modified alloying strategy in Osprey HWTS 50. Both as-built and as-quenched samples underwent isothermal double tempering at 350, 575, and 650 °C. Atom Probe Tomography (APT) was used to analyse the size, morphology, and composition of the resulting precipitates. Directly tempered specimens exhibited higher concentrations of nitrogen than those subjected to quenching and tempering. In over-aged (650 °C) condition, vanadium-rich MC and molybdenum-rich M2C carbides could be distinguished. The evolution and chemical makeup of these precipitates allowed for the identification of phase transformations at various temperatures, defining the precipitation sequence in this alloy system.