In situ characterization of phase and microstructural evolution during multi-step heat treatment of an additively manufactured tool steel

Post-printing heat treatment of additively manufactured (AM) tool steels is often essential for optimizing mechanical properties, usually involving complex, multi-step heat treatment procedures. This study investigates the microstructural evolution and its impact on mechanical properties of an AM tool steel through successive heat treatment steps, including the as-built, spheroidized, quenched, sub-zero treated, and three repeated tempered conditions. For the first time, the dynamic mechanisms of phase transformation and carbide evolution during heat treatment of an AM tool steel are systematically revealed through the combined application of in situ synchrotron X-ray diffraction and multi-scale characterization techniques. (Cr,Mo,Mn,V)23C6 carbide precipitation was revealed alongside (retained) austenite, martensite and δ-ferrite, with the phase contents varying across the individual heat treatment steps. A strong correlation between (retained) austenite, (tempered) martensite and Vickers hardness was observed, with a final Vickers hardness of 577 ± 5 HV10 in the fully heat-treated condition. Furthermore, the as-built microstructure strongly influenced the subsequent thermal processing behavior as indicated by the stability of δ-ferrite throughout heat treatment, originating from austenite by-passing during LB-PBF, and the early formation of M23C6 carbides during spheroidizing, driven by Cr segregation resulting from the LB-PBF process. Therefore, this study highlights the influence of AM microstructures on heat treatment responses and offers new insights into carbide formation and phase transformations of AM tool steels. The findings emphasize the critical role of post-printing heat treatments in tailoring the microstructural and mechanical properties of tool steels, thus advancing the understanding of specific heat treatment strategies for AM components.