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 characterization 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 analyzed using light optical microscope (LOM), scanning electron microscope (SEM), and electron beam backscatter diffraction (EBSD)-techniques. Some of the microstructural features can be further characterized by nano-hardness measurements. The transformation kinetics of different bainite forms has 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 optimized bainitic steels with improved balance of strength and ductility as wells as improved toughness can be deduced.