Microstructural changes in high strength steels exposed to large deformation and high strain rates

Doctoral thesis, 2008

Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption and energy usage in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off,high-velocity forming and cold ring rolling are three such manufacturing methods that exhibit great potentials in terms of material waste reduction. However, all three processes involve large degrees of deformation that is not homogeneously distributed in the samples and the scientific knowledge regarding deformation mechanisms active in these processes is low. In order to allow for process optimisations a thorough understanding of associated deformation behaviour and microstructural changes is needed. Three steels have been used in this work: two bearing steels and one carbon steel. Through studies employing high-velocity parting-off incorporating impact velocities of 5-285 m/s, the fracture mechanisms active during material separation was shown to be a mix of ductile shear and ductile tensile fracture and to some extent failure by adiabatic shear banding. High-velocity forming tests were then conducted to evaluate parameters controlling the strain localisation and initiation of adiabatic shear bands (ASBs). Strain and strain rate were shown to be important for strain localisation. However, most important was shown to be the microstructure coupled to hardness where quenched and tempered samples developed ASBs while spheroidise annealed samples did not. By using electron microscopy the microstructure in the ASB regions generated by parting-off and forming were compared and shown to be composed of three types of structures: Within the ASB nanocrystalline equiaxed grains with a size of 50-150 nm were found, while adjacent to the ASB the microstructure consisted of a mixture of equiaxed grains and highly elongated subgrains. Outside this region only highly elongated subgrains were found. The elongated subgrains were shown to have a mutual orientation, adjacent subgrains having {110} type of planes parallel. This could indicate that formation of ASBs is a mechanically rate controlled process. In addition it was shown that smaller carbides were dissoluted while larger spheroid carbides remained and possibly also facilitated the refinement of microstructure in the formation of ASBs. Cold ring rolling tests were done to further investigate the effect of large deformations on microstructure and texture. The plastic deformation was shown to be most severe near the inner diameter of the rings decreasing towards the area of the outer diameter. By employing electron back scatter diffraction the ring rolling process was shown to result in a combined fibre texture where <110> was parallel to the rolling direction and <111> was parallel to the ring radial direction in the centre of the ring, indicating more or less plain strain in this region. Below the inner diameter the texture was of the {110} type indicating contribution of shear.