Steel grades, currently used for automotive shafts and gears within the electrified powertrain, reach their load limits. As a consequence of the increased transmission of torque in the electrified compared to the combustion powertrain, innovative automobile manufacturers are striving to develop new steel designs and manufacturing routes for transmission components with improved mechanical properties. The mechanical properties are ultimately determined by the alloy-related effects and heat treatments, which are decisive regarding diffusion behaviour and precipitation kinetics.
Therefore, this thesis investigates the development of martensitic steel grades focusing on two reference steel grades and one new alloy design with different copper contents. The different copper contents were added to a chromium, aluminium and copper alloyed steel grade with low vanadium, molybdenum and niobium contents. Simultaneously, the Duplex heat treatment, consisting of carburising, ageing at elevated temperatures and plasma nitriding, is studied. In effect it means that this thesis aims to investigate the Duplex heat treatment and the influence and interaction of the three precipitation hardening mechanisms consisting of copper-particles-phase, carbides/carbo-nitrides and nitrides to increase the hardenability of steel. On a laboratory scale, the microstructures and hardenability of the steel grades were investigated by various characterisation methods, involving high-resolution atom probe tomography.
In the case-hardened samples with ageing at elevated temperatures, an increased copper content led to an increase in hardness. Although, coarse chromium rich carbides were found at the prior austenite grain boundaries. Within the copper precipitates, chromium was found to be enriched. This implies that chromium rich carbides use copper as a nucleation site.
The improved hardenability for Duplex heat-treated samples was confirmed for neglecting ageing before nitriding and reduced copper contents of ≤ 0.8 wt% copper. Ageing before nitriding increased the chances of unwanted phases at the grain boundaries and led to general coarsening of the precipitates, reducing the case hardness and eventually resulting in impaired mechanical properties during service. The effect of general coarsening of the precipitates was also found in the samples with increased copper contents after Duplex heat treatment. Coarser nitrides were preferably precipitated at coarser copper precipitates while finer ones precipitated homogeneously within the matrix or heterogeneously at finer copper precipitates. In this research, the coarser nitrides were found to be aluminium rich, while the fine homogeneous/heterogenous nitrides were chromium rich. Further, it was found that aluminium and chromium were enriched by on average 0.4 at% aluminium and 0.5 at% chromium within the copper precipitate, indicating that aluminium- and chromium rich nitrides preferably use copper as a nucleation site.
Although a high case hardness of > 1000 HV 1 was achieved after Duplex heat treatment, a diffusion front could not be prevented due to a chromium content > 4.5 wt% chromium. To improve the alloy design, these investigations were accompanied by thermodynamic equilibrium calculations. These suggested that a decrease in copper, chromium and nickel, while increasing vanadium and considering increasing molybdenum may lead to an improved precipitation behaviour. Further, a reduction to 3 wt% chromium supposedly reduces the build-up of a diffusion front while maintaining chromium rich nitride precipitation.
Thus, the combination of the proposed new alloy design and the Duplex heat treatment offers the potential to significantly improve mechanical properties compared to currently used case-hardened gear steels in automotive. In addition, it is shown that the influence and especially the interaction of Cu, carbide and nitride precipitates can improve the hardenability of steels.