We discuss the correlation between segregation of carbide forming elements (vanadium) and void initiation and propagation in low-alloyed Cr-Mo steels. The internal defects are created during blooms straightening in radial type of casting machine due to strain deformation field in the temperature range characteristic for vanadium nitride, carbide or carbonitride precipitation. Based on the statistical analysis (1097 continuously cast blooms) of three low-alloyed Cr-Mo steel variants with different vanadium content, we conclude that the void occurence is strongly associated with the level of vanadium content. The experiments performed by means of microstructural, fractographic investigations and microchemical point analysis proved that preffered areas for void initiation are interdendritic segregations in the core of bloom. We observed vanadium carbide precipitates that are situated close to the cracks, whereas the content of vanadium was 7,5 times more then in an average smelt composition. The further development of defects is connected with fast heating up to the austenization temperature in soaking pit. Our results confirm that marked decrease of defects can be achieved by optimization of production process, i. e. optimized casting speed, steel overheat above the liquidus temperature and/or application of M-EMS.
Continuously cast round blooms from vanadium microalloyed steel denoted as 25CrMo4 type are very sensitive to internal defect creation during casting. Further propagation of internal cracks is affected by suboptimal heating preceding the hot rolling process. FEM offers powerful tool for simulation of the temperature gradients and stress-strain behavior. In this paper it is demonstrated that proper selection of FEM model is essential to get trustworthy results corresponding with practical observations. It is possible to find optimum between two contradictory requirementsacceptable computation time severity and trustworthy results, if the simple model is compared with experimental data and/or more sophisticated models.
Identification of the mechanical properties of high-strength steel using digital image correlation. In this paper an experimental procedure to identify the plastic behaviour of sheet metals up to large strains using full field measurement is presented. The tests were conducted on notched specimens. This geometry generates a heterogeneous strain field which was measured during the test using a digital image correlation system. The advantage of using a heterogeneous strain field in the identification procedure is that a complex state of stress-strain can be analyzed at the same time and much more information can be obtained in a single test. On the other hand, the stress field cannot be directly computed from the test and a suitable identification procedure must be developed. Here, the virtual fields method (VFM) adapted for large strains and plasticity was used to identify the hardening behaviour and the anisotropy of the material. The values obtained with the VFM were compared with the results from a standard identification made using uniaxial tensile tests.
This paper is dedicated to the development and optimization of the porous titanium materials suitable for biomedical usage in traumatology. Main aim of the presented research activities is focused on preparation of biocompatible titanium based materials with controlled porosity. It was found that titanium specimens with total porosity approximately 40 % revealed mechanical properties very similar to those of human cortical bone. Two-layer specimens with controlled porosity were prepared and tested by electron microscopy for post-sintering cracks. All tested specimens with controlled porosity were cracks free. Future works will include preparation of geometrically more complicated shapes, machining andin vitrocells proliferation testing.
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