Fracture behaviour and related microstructures of tool steels parted-off at high rates of strain

Licentiate thesis, 2007

Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off, or adiabatic cutting, has shown to result in very low material waste as well as narrow dimensional tolerances of parted-off samples. However, for optimisation of the process improved knowledge is required regarding fracture characteristics and deformation mechanisms associated with the process. For evaluation of samples cut by commercially available technology, incorporating impact velocities of 5-10 m/s, a hydraulic high-velocity pressing machine with a parting-off tool was used. In addition, for evaluation of the possible influence of a higher impact velocity on the shear localisation, a non-conventional method was developed. In that case velocities up to 285 m/s could be employed. By measurement of velocities an estimation of fracture energies could be made. Microstructure evolution during deformation and fracture was characterised using optical microscopy combined with scanning and transmission electron microscopy. It has been concluded that for impact velocities of 5-285 m/s, the parting-off is, within the full range of velocities, initiated through shearing resulting in ductile shear fracture. However, depending on sample size and velocity the fracture mechanisms active in some parts of the fracture are ductile tensile indicating a triaxial stress state. Microstructural studies showed that the depth of the severely deformed region below the fracture surface is more dependent on the size of the sample than on the microstructure, resulting in much smaller deformed regions for smaller samples. The severe deformation results in cracks: For small samples only microcracks initiated on precipitates such as carbides and MnS inclusions, but for larger samples inter- and intragranular cracking also occurred. Electron microscopy of the severely deformed region below the fracture surface has shown three different types of structures: Right below the fracture surface a white-etching band (appears white in optical microscopy) consisting of nanocrystalline equiaxed grains was occasionally found. The grain size within this region was between 50 and 150 nm. The region below the white-etching band consisted of a mixture of equiaxed grains and highly elongated subgrains. The third region, located below region two, is composed of highly elongated subgrains. In the case of speroidise-annealed 100Cr6 parted-off at an impact velocity of 225 m/s the elongated subgrains were shown to have a mutual orientation, adjacent subgrains having (110) type of planes parallel. This could be an indication of formation of white etching bands being a mechanically rate controlled process involving dynamic recovery. In the case of parting-off pearlitic 100CrMn6 steel it has been shown that the lamellar spacing of the cementite decreases and cementite lamellae aligned in a direction perpendicular to the generated fracture surface are severely distorted during the extreme deformation, resulting in breaking up of the lamellar structure. The white-etching band consists of small finely distributed carbides in a ferrite matrix.