COMAT 2014 - Recent Trends in Structural Materials

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Authors: Martin Rund, Josef Volák, Miroslava Šindelářová
Abstract: The evaluation of actual mechanical properties of the in-service structures after some time of operation or determination of local properties for detailed FEM simulation yields the necessity to obtain relevant material data with high accuracy from small volume of the experimental material. Therefore, non-destructive or semi-destructive techniques using small size samples are being developed. The use of small-scale samples also enables the evaluation of material properties in various locations of tested component; for example, the mechanical properties of the individual regions of welds, local properties determination for properties anisotropy assessment and properties determination in cases when small volume of the experimental material is available e.g. residual service life assessment of in service components, bulk nanostructured materials… There are shown results of small size tensile tests (M-TT) and small sized fatigue tests (SFT). In the case of small size specimens testing a machining becomes more pronounced that in the case of standard sized specimens. The current study brings information on the machining influence on the expected results obtained by small size specimens in the case of quasi-static tensile tests and fatigue test.
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Authors: Bohuslav Mašek, Ivan Vorel, Hana Jirková, Petr Kurka
Abstract: All sectors of industry experience high demand for shaped products with as good mechanical properties as possible at low costs. Automotive industry, in addition, requires that the parts are of lightweight construction. Consequently, new types of materials and processes have to be combined to design new production chains capable to meet this demand. For instance, there are high-strength low-alloyed steels, whose final properties are attained by advanced heat treating techniques. One of such techniques is the Q&P process which can deliver excellent ultimate strengths exceeding 2000 MPa at a sufficient elongation level of 10 %. When combined with an unconventional forming method, it allows complex-shaped parts with outstanding mechanical properties to be made. One example of such combined procedure is the sequence of internal high pressure forming, hot stamping and Q&P processing. In the present study, thin-walled hollow stock was processed using such a combined procedure. After stepwise optimization of processing parameters, products with martensitic structure and a small amount of bainite were obtained. In all locations of the product which were tested, the ultimate strength exceeded 1950 MPa and elongation reached 15 %
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Authors: Daniela Hauserova, Jaromir Dlouhy, Zbysek Novy
Abstract: Accelerated carbide spheroidisation enables significant shortening of time necessary for formation of steel microstructure consisting of ferritic matrix and globular carbides. Conventional way to obtain such structure is long duration soft annealing after semiproduct hot forming. Research of accelerated carbide spheroidisation showed that it is possible to spheroidise lamellar pearlite in minutes. Accelerated carbide spheroidisation produces very similar microstructure like conventional soft annealing in morphological point of view, but carbide particles and grain size of the matrix is significantly smaller. Finer microstructure results in higher hardness in comparison with conventionally annealed steel. Finer microstructure also ensures more homogeneous and finer structure after final heat treatment, after hardening. This fact indicates, that properties of final product differ according to previous structure after soft annealing. Finer carbides in structure enhance hardness and facilitates carbide dissolution during austenitisation. This effect enables quenching temperature lowering. Steel 100CrMnSi6-4 microstructure and properties are compared in view of various kind of spheroidisation and quenching temperature.
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Authors: Eva Rykalová, Zdeněk Peřina, Radek Fabian, Petr Jonšta
Abstract: Infrared radiometric long-waved systems are widely used in various industries as well as in research and development. This long-waved system is the perfect tool for quick diagnosis of the state of the equipment, easy defect detection, which is reflected by higher temperatures due to increased friction and wear. Infrared radiometric long-waved systems are used especially in civil engineering, electrical engineering, metallurgy and many other industries. They are also used to optimize and improve of the production processes. The series of measurements using the infrared radiometric long-waved system in steel plant were carried out due to prepared research project. Images of temperature fields of ladles with liquid steel, cooled exhaust knee of electric arc furnace and ingot mould were obtained during solidification of steel ingot. Information about the state of wear of the refractory lining of the ladle, exhaust knee can be gained from obtained images. The obtained results can be used for more accurate numerical simulations of the process of casting and solidification of steel ingots.
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Authors: Eva Peterková, Radko Samek
Abstract: The article deals with more precise method of bending moment determination for rigid plastic work hardening material. Unlike earlier methods, a new methodology takes into account the real shape of the deformed tube cross-section after bending. The real centroid position of the tube cross-section, the change of wall thickness and the irregularly deformed tube cross-section are considered. The paper presents a possible way of determining the real bending moment, reflecting the geometrical changes of the “flattened” cross-section. This approach is based on the subsequent integration of elementary bending moments, which reflects successive changes of the bending stress and a material model as well.
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Authors: Hana Jirková, Kateřina Opatová, Štěpán Jeníček, Bohuslav Mašek
Abstract: With the prices of raw materials as well as final products growing a significant tendency to search for weight reduction of the machine components through improving their quality can be observed at present. To achieve as good mechanical properties as possible, efforts are made to reach the finest microstructure possible in steels. Besides the fine grain microstructure the required phase volume fraction must also be obtained in case of AHS steels to ensure the possibility of utilizing the TRIP effect during the final cold deformation. Their structure usually consists of ferrite, bainite and retained austenite, which transforms to deformation induced martensite during final cold deformation. With an appropriate volume fraction of these phases the materials show good combination of strength and ductility. In the experiment eight and twenty step incremental deformation was applied within identical thermomechanical sequences in the deformation temperature range of 900 – 720°C. At the same time the influence of twenty step incremental deformation on the development of microstructure was examined down to 600°C. Comparison of the results was carried out by means of light and electron microscopy. The volume fraction of structural phases including the fraction of retained austenite was determined by image analysis.
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Authors: Libor Mrňa, Jan Řiháček
Abstract: One of the ways to increase efficiency of the solar absorber is modification of absorption area in the system of pyramidal cavities, where the incident radiation is absorbed by multiple reflections. The paper deals with the technology of production of the flow solar absorber with a structured surface in a single technological operation using hydroforming. Two laser-welded sheets are inserted into the forming chamber. Then the space between them is pressurized with hydraulic fluid which causes their form to the desired shape. The first part of this article describes hydroforming device. The second section deals with theoretical simulation of forming a different structured surface in the programming environment of ANSYS. As follows practical test of hydroforming is performed. Austenitic stainless steel X5CrNi18-10 was used as a material for the production of samples.
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Authors: Jiri Lahoda, Michal Zemko
Abstract: Dissipation normal form (DNF) is a special system structure which originally came from the signal and system theory. It was derived from natural requirements of some form of an abstract energy conservation law validity. Strong nonlinearities brought into the DNF parameters can cause complex irregular oscillations of the system. Such a system is used as a generator of numerical sequencies whose properties are useful for mathematical modelling of irregularities in real fine and ultra-fine grained metal microstructures. The particular metal grains are modelled as the Gaussian curve with a 2-dimensional domain and their partial properties such as their height, width, position in the surface, elongation in a certain direction etc. can be chaotically modified. A certain part of grains can be modified separately by irregular splitting their area etc. It allows defining the individual components surface percentage to meet important statistical properties of the real material with complicated structure. In the contribution there it is shown that the presented mathematical model approximates real multiphase microstructure quite well and using DNF with strong nonlinearities represents approach with good prospects to mathematical modelling of metal microstructures including complex ones.
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Authors: Maria Borisova, Sofia Yakovleva, S.N. Makharova
Abstract: Severe plastic deformation is a well known procedure for fabrication considerable grain refinement in metal materials. Equal channel angular pressing (ECAP) is one of these severe plastic deformation techniques which allows the receiving of the bulk ultrafine grained (UFG) metals. It is well known that the ECAP method leads to significant improvement of strength of investigated materials, but the fracture mechanisms of UFG metals at low temperature are not enough studied. The investigation was performed on low-alloy constructional steel 09G2S (GOST) after application of ECAP. Fracture toughness tests were performed on Charpy impact testing specimens at low temperature. The experiments demonstrate that ECAP treatment has the effect on increasing of fracture resistance of steel 09G2S at low temperature.
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