Insights into Microstructure Evolution of Additively Manufactured High-speed Steels

Compared to conventional means, additive manufacturing (AM) has demonstrated its reliability in producing refined microstructure and improved performance in various high-speed steels. Due to the complex thermal histories encountered during the AM process, the high-speed steel with high contents of carbon and alloying elements undergoes intricate phase transformations, leading to complex microstructures with multiple precipitates that are crucial in strengthening steels. This PhD study focuses on the phase evolution during the rapid solidification process of M50 steel and carbide precipitation during post-heat treatment of S390 steel. Both steel grades fall into the category of high-speed steel family, with the former containing about 0.8 wt.% and the latter of 1.6 wt.% in carbon concentration. An improved understanding of the processing-microstructure-property relationship for AM high-speed steels has been achieved through a combination of post-mortem characterisation on precipitates and in-situ phase tracking during rapid solidification and post-heat treatment. Quantitative characterisation of primary carbides using scanning electron microscopy and focused ion beam together with nanoprecipitates by atom probe tomography and small-angle neutron scattering reveals the role of secondary phases. Fine prior-austenite grains, impeded by primary carbides, and nanoprecipitates appearing during the secondary hardening, contribute to the overall strength enhancement of around 610 MPa. Phase evolution during tempering was investigated further through ex-situ neutron scattering and in-situ synchrotron X-ray diffraction, revealing the most reduction of retained austenite and precipitation of nanoparticles in the initial 120 mins at 560 ℃. The primary carbides grow by 60 nm within 2 mins and nanoparticles precipitate with a size of 1.4 nm after 60-minute tempering accompanied by an exceedingly high hardness of 921 HV. Additionally, the microstructure evolution of AM M50 steel was investigated by operando synchrotron X-ray diffraction, unveiling cooling rates in the order of 105 K/s during liquid-solid transformation. The step-wise martensite formation was observed during solid-solid transformation. The rise in the carbon content of martensite from 0.58 wt.% to 0.68 wt.% was derived from the increased martensite tetragonality during the cooling process. The insights gained in this study serve as a valuable guide for designing future steel groups and developing heat treatment procedures tailored for the AM process.