Cutting tools are commonly made of high-speed steel to simultaneously fulfill the requirements regarding hardness, strength, and toughness. The microstructure of high-speed steel alloys in as-cast condition is characterized by a microstructure consisting of a metal matrix and eutectic carbides. [1][2][3][4] The further manufacturing route of these materials comprises hot forming to break down the carbide network [1,2] and subsequent heat treatment. The behavior of the carbides during austenitization was examined in several publications. [3][4][5][6][7][8][9] Metastable carbides with the stoichiometry M 2 C (M¼ metal atoms, C ¼ carbon), which are present after casting, are transformed to carbides of the types MC and M 6 C with the transformation path of [3,[10][11][12] Dependent on the austenitization temperature and time and thus on the amount of dissolved carbides, different contents of retained austenite remain after quenching. [9] During typical subsequent tempering at temperatures where a secondary hardening effect occurs, retained austenite decomposes, and the precipitation of finely dispersed tempering carbides occurs. [13][14][15] Besides the conventional production route with casting and hot forming, alternative manufacturing processes like spray forming [16][17][18] or powder metallurgy (PM) with hot isostatic pressing are available. Several studies found that powder metallurgic high-speed steel exhibit higher toughness and better cutting performance compared to cast material. [19][20][21][22][23] In most cases, this is explained by the finer distribution, the isotropic properties, and the more regular shape of the eutectic carbides due to the rapid solidification of the powder particles during powder manufacturing. The carbides are affected in their type, [8] size, [5,24] and distribution [25] by the process variables during primary manufacturing and different heat treatments. Mishnaevsky et al. have shown that the fracture in tool steels typically starts in or along eutectic carbides. [26] It is, therefore, to be assumed that the morphology of the eutectic carbides must have an influence on the mechanical properties of high-speed steels.High-speed steel alloys typically exhibit a transformation gap in the isothermal time-temperature-transformation (TTT) diagram. The scientific consensus is that no transformation of the metastable austenite takes place in the temperature regime of the transformation gap. [27] Kešner et al. found that the size of carbides is affected by hardening in a salt bath under isothermal conditions at temperatures between 300 and 650 °C, which is in the temperature regime of the transformation gap of high-speed steels. [28] However, previous works have paid no attention to the effect of different heat treatment parameters on the shape of eutectic carbides. Our preliminary work shows that isothermal