Einfluss der Prozesskette auf die Feinkornbeständigkeit von mikrolegierten Einsatzstählen

Considering the processing of case hardening steel components, the machining as well as the heat treatment offer great capability for cost savings. By increasing the carburising temperature from today approximately 940°C up to e.g. 1050°C significant process time reductions are achievable due to the acceleration of diffusion and absorption processes. While increasing the carburising temperature, the appearance of prohibited coarse austenite grains has to be avoided. However the former austenite grain size is an attribute of the overall process chain from steel melting to case hardening. In this work, the influence of the manufacturing process on grain size stability of case hardening steels has been investigated. To simulate the hot forming process in laboratory scale compression tests (Rastegaev samples) have been performed to different standard steel grades (16MnCr5 / 20NiCrMo2) as well as two micro-alloyed (16MnCr5Nb / 20NiCrMo2Nb) steel grades. An increase in grain size stability during carburising simply depends on the size of precipitates. Unfortunately the use of a mean precipitate size is not significant, because the size distribution is modified by every single process step - even the case hardening itself. The results of this work show, that if deformation is carried out by warm deformation below a specific temperature T1 - which is a function of the chemical composition - grain size stability is assessed by the condition of precipitates upon delivery. The low temperature prevents a change in precipitate morphology by low diffusion. Increasing the deformation temperature the grain size stability during following case hardening depends on the deformation degree. The higher the deformation degree, the bigger is the risk of illegal grain coarsening. With a further increase of the deformation temperature - above a specific temperature T2 - no dependence of grain coarsening temperature and deformation degree is observed, assumed that the material contains an adequate content of micro-alloying elements Nb and Ti as well as Al and N.