Microalloying elements are added to a wide range of steels for improving microstructure, properties, processing or general performance. A survey is given on the various reasons for microalloying of engineering steels. In case hardening steels microalloying improves toughness and fatigue properties mainly due to a smaller and more homogeneous prior austenite grain size. In forging steels, microalloying controls the phase transformation during cooling after forging enabling shorter process routes. Furthermore, microallying contributes to the microstructural refinement improving the balance of fatigue, cyclic behavior and strength. Recently, microalloying supports the interface engineering in air hardening medium Mn steels and prevents embrittlement.

The evolution of austenite fraction and the associated solute partitioning during the intercritical annealing of medium-Mn steels are of great importance for austenite stabilization and the mechanical performance of this class of steels. In the present work, a 4.5Mn steel is subjected to a cyclic treatment and the evolution of the austenite fraction is measured with dilatometry. The evolution of austenite fraction and solute partitioning are simulated for a case where the starting time of the cyclic treatment is well before the equilibrium fractions have been established in the respective isothermal intercritical treatment. The evolution of austenite during thermal cycling in the intercritical range comprises of forward, inverse, and stagnant stages. The fraction of austenite formed decreases in each successive cycle while the kinetics of the evolution of austenite is controlled by the Mn diffusion in ferrite. Partitioning of Mn and C takes place from ferrite to austenite during the cyclic transformation. Due to the low diffusivity in austenite, wells form in the composition profiles in austenite of both Mn and C. These wells are the locus of the interfacial compositions of austenite, corresponding to the variation of the local equilibrium conditions during the thermal cycle.

Macroscopic failure of high-performance components such as the bearings in a gearbox of wind converters is usually caused by effects occurring on a microscopic scale, e.g., crack initiation at non-metallic inclusions. Failure of these components leads to high maintenance costs. This work focuses on modifying the chemical composition of the standard bearing steel 100Cr6 in order to increase its strain-hardening potential, and thus its damage tolerance while maintaining the degree of cleanliness. Al alloying is used to adjust a suitable combination of strengthening mechanisms. Dilatometric and metallographic analyses are performed to identify the phase transformation behavior and the microstructure constituents of the designed alloy. Special attention is paid to the formation and morphology of the k-phase, a Fe 3 AlC-phase which contributes to the strainhardening potential of the material. The mechanical properties are determined at several heat-treatment states and compared to the ones of conventional 100Cr6 steel. It is shown that the k-phase may be used for tailoring the strain-hardening potential during local plastic deformation, and thus reduce stress concentrations at inclusions.

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High performance components such as gear wheels shall be resistant to rolling-contactfatigue. This type of failure is usually caused by effects occurring on a microscopic scale, such ascrack initiation at non-metallic inclusions. Much effort has been invested so far in improving thesteel cleanliness. However, these high performance components often do not reach the desiredservice life. Preliminary failure within the guarantee terms still occurs which leads to high warrantycosts. Alternative to improving steel cleanliness, the damage tolerance of high performancecomponents could be increased by inducing the TRIP-effect around the crack tip. Due to high localstrain hardening, martensite transformation occurs. The high compressive stresses related to it coulddelay or stop crack propagation by reducing stress concentrations via plastic deformation. As aresult, rolling-contact fatigue resistance of carburized steels may be increased and preliminaryfailure may be avoided. Part I of this study focuses on modifying the chemical composition ofconventional 18CrNiMo7-6 steel with Al to develop a high-strength, yet ductile matrix with a highwork hardening potential. Dilatometric tests on laboratory melts analyze the possibility of adjustinga microstructure able to produce a TRIP-effect. Both isothermal annealing and Quenching andPartitioning (Q&P) are used to stabilize residual austenite and optimum process routes areidentified.