On the evolution of secondary hardening carbides during continuous versus isothermal heat treatment of high speed steel HS 6-5-2

The continuous heat treatment of high speed steels reduces the process times from several hours to a few minutes. The resulting cost savings as well as lower decarburisation and distortion make the continuous heat treatment favourable over an isothermal heat treatment. However, the microstructure-property relationship during continuous heat treatments is far from being well understood. In order to identify the key microstructural features for the future optimisation of continuous heat treatments of high speed steels this study compares a current industrial continuous and an isothermal heat treatment of steel HS 6-5-2 by means of light optical microscopy, scanning electron microscopy, X-ray diffraction, atom probe tomography, and red hardness. After continuous hardening the content of primary carbides is higher and the amount of retained austenite is lower compared to isothermal hardening. Due to the reduced time for dissolution of primary carbides a lower content of alloying elements is present in the martensitic matrix for subsequent tempering. Therefore, the chemical composition of the secondary hardening carbides after tempering is different for a continuous heat treatment. Although the difference in chemistry is quite pronounced, the deterioration of the hardness at elevated temperatures, which strongly influences the performance characteristics of the finished parts, is not altered.