To predict the solidification and the product properties of complex alloyed tool steels, a deep understanding of the transformation behaviour during and after solidification, depending on the chemical composition, is a key competence. The knowledge of primary carbide formation plays an essential role in higher-alloy tool steels, where many different carbide types (e.g. M7C3, M23C6, M6C und MC) can form. Especially with higher alloyed complex carbidic steels, thermodynamic calculations often show significant differences. In order to be able to make precise thermodynamic statements about new alloys, reliable investigation methods are essential.
In the framework of a comprehensive method development, the quaternary Fe-C system with 10 w.t.-% Cr and 3% W (typical subsystem of cold work steels, with M7C3 and M23C6 carbides) and the Fe-C system with 6% W and 5% Mo (simplified high speed steel without vanadium, with M6C and MC carbides) were selected. Very homogeneous and pure model alloys were produced by induction melting and subsequent centrifugal spin casting in a copper mould. The well-established DSC method (Differential Scanning Calorimetry) was used to investigate all high-temperature phase transition temperatures.
As regular DSC measurements of as-cast materials do often not show the equilibrium transformation temperatures of carbides, a special time-temperature DSC setup was developed. Regular diffusion annealing of as-cast carbidic steels is very time-consuming, but by in-situ heat treatment during the DSC measurement in the semi-solid-zone (30% liquid phase fraction), a perfect equilibration can be achieved within minutes due to the high diffusion. To prove the potential of the in-situ high-temperature equilibration in the DSC, addition “equilibration and quenching” experiments were performed at selected temperatures in a Tammann-type furnace and were investigated by SEM-EDS and XRD analysis. By combining these methods, carbide types and the transformation temperatures can be verified to evaluate and construct complete phase diagrams.