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Improved mechanical properties by control of bainite transformation
The term bainite describes a variety of different microstructures that can be generated by continuous cooling or isothermal heat treatments. Based on systematic microstructural investigations a bainite characterisation scheme has been developed that distinguishes between different basic structures and substructures, the latter being described by location, type and form. According to this detailed description, microstructures have been quantitatively analysed using LOM, SEM, and EBSD-techniques. Some of the microstructural features can be further characterised by nanohardness measurements. The transformation kinetics of different bainite forms have been determined by dilatometer studies. The kinetics differs significantly depending on the amount of diffusion involved during the phase transformation. Several numerical approaches have been tested and critically evaluated in order to describe the kinetics. Furthermore, for various steel groups microstructure-property relations have been investigated with a focus on the development of cracks. By this, recommendations for optimised bainitic steels with improved balance of strength and ductility as wells as improved toughness can be deduced.