A continuous heat treatment of steels reduces the process times from several hours to a few minutes. The resulting cost savings as well as lower decarburisation and distortion make this kind of 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 heat treatable and high speed steels this study compares current industrial continuous and isothermal heat treatments of the steels 42CrMo4 and HS 6-5-2. In this doctoral thesis a deeper insight in the structure-property relationship of continuously heat treated steels could be gained, whereby the results of this comprehensive investigation can be used to improve the continuous heat treatment to make it even more applicable for industrial use. With the aid of advanced characterisation methods as electron back scatter diffraction and atom probe tomography it could be shown that the crucial heat treatment step in case of the heat treatable steel 42CrMo4 is the austenitisation. Due to the shorter holding times at austenitisation temperature a homogenous distribution of the carbon concentration could not be achieved, which results in a larger martensitic block size. This, furthermore, deteriorates the mechanical properties as tensile strength and hardness. Nevertheless, those disadvantages can be compensated by applying a continuous tempering. The higher heating rates and the shorter holding times during continuous tempering lead to a finer distribution of precipitates, which subsequently improves the impact toughness of the steel. The microstructural evolution of the high speed steel HS 6-5-2 during an industrial continuous heat treatment is also investigated in this study. Here, after continuous hardening a higher amount of undissolved primary carbides retains embedded in the matrix, which reduces the concentration of alloying elements available for precipitation of secondary hardening carbides. The detail analysis of tempered specimens by means of atom probe tomography revealed that secondary hardening carbides in continuously heat treated steels contain less molybdenum and tungsten compared to isothermally heat treated steels. Nevertheless, although the mentioned difference in chemical composition of secondary hardening carbides is present in continuously hardened and tempered steels a degradation of the performance characteristics could be excluded with the aid of red hardness measurements.
|Translated title of the contribution||Charakterisierung der mikrostrukturellen und mechanischen Eigenschaften von kontinuierlich wärmebehandelten Stählen|
|Publication status||Published - 2016|
Bibliographical noteembargoed until null
- continuous heat treatment
- atom probe tomography
- electron back scatter diffraction