Microstructure Evolution and Precipitation Strengthening Behaviour of Additively Manufactured High-speed Steels

Additively manufactured (AM) high-speed steels were investigated, focusing specifically on the microstructure evolution during post-treatment in S390 steel and the rapid solidification process in M50 steel. An improved understanding of the processing-microstructure-property relationship for AM high-speed steel is achieved through a combination of post-mortem microstructure characterisation on precipitates and in-situ tracking of phase evolution. Quantitative characterisation of primary carbides and nanoprecipitates highlights the strengthening through nanoprecipitates that contribute to the exceedingly high hardness of 921 HV. Phase evolution during tempering was examined through in-situ synchrotron diffraction and ex-situ small-angle neutron scattering, revealing primary carbide growth by 60 nm within 2 minutes and nanoparticle precipitation with a size of 1.4 nm after 60-minute tempering. Additionally, the microstructure evolution of AM M50 steel was investigated by operando synchrotron diffraction, unveiling cooling rates in the order of 10⁵ K/s during liquid-solid transformation. After printing, the carbon content of 0.47 wt.% in the matrix was derived from the martensite tetragonality. The insights gained serve as a valuable guide for designing future steel groups and developing heat treatment procedures tailored for the AM process.