Papers by Author: Jana Dobrovská

Abstract: Crystallization of metallic materials occurs during their casting and following solidification. The influence of directional material solidification, the size of grains, chemical heterogeneity and existence of any non – compactness, can negatively affect properties of the final product. Microstructural analysis of its longitudinal section was performed to study heterogeneity in a continuously cast steel billet. Three typical areas of the cast structure of the billet were monitored (the chill zone, columnar crystals area and central area of equiaxed crystals). EDX microanalysis and micro-structural analysis were used for evaluation of chemical heterogeneity. Via X-ray microanalysis, concentrations of the following elements were observed: manganese, chromium, silicon, phosphorus, molybdenum and iron. The microanalytical measurements were completed by microhardness measurements in the investigated areas. Based on the measured data it was proven that central areas were enriched with all the substitutional elements including a considerable increase in phosphorus concentration.

Abstract: This article summarises results of structure stability investigation of cast Ni-base and Co-base alloys after prolonged high temperature exposure at 900-1100 °C. Cast Ni (Co)-Cr-W-C alloys are resistant to high-temperature corrosion, due to high chromium content. Their heat resistance is caused by presence of carbides, which are stable at very high temperatures. Carbides precipitate in shape of large plate-like particles or carbide eutectics at casting cell boundaries, thus forming carbide skeleton of the alloy. Carbide morphology and temperature stability depends on chemical composition of the alloy, e.g. carbide content, type and content of carbide-forming elements. Microstructure changes were evaluated by stereological analysis and X ray-spectral microanalysis.

Abstract: Thepaper deals with an experimental measurement of the transformation temperatures of Co-base alloy. Temperatures were determined by means of DTA-method during controlled heating and cooling. The samples in an as-received state were analysed at heating/cooling rates of 2, 5, 10 and 20 °C/min with the use of the equipment Setaram SETSYS 18TM (DTA-method). The samples after various heat treatments were analysed at heating/cooling rate of 5 °C/min by Setaram SETSYS 18TM (DTA-method). On the basis of evaluation of the results the influence of heating/cooling rate on shift of the transformation temperatures was determined. The influence of heat treatment on shift of the transformation temperatures was also studied. The samples in an as-received state and the samples after heat treatment were alsosubjected to the phase analysis by scanning electron microscopy using the microscope JEOL JSM-6490LV equipped with an energy dispersive analyser EDAX (EDS INCA x-act). The individual phases were identified by semi-quantitative X-ray microanalysis.

Abstract: The paper deals with investigation into segregation behavior of selected elements in longitudinal cut of continuous steel slab, in the breakout area. The breakout occurred after a flying change of tundish in order to begin casting of another steel grade. Altogether 11 samples were taken from the part of a solidified slab. Concentrations of selected elements (Al, Si, P, S, Cr, Mn, Ni and Mo) were measured in these samples using scanning electron microscope and energy dispersive spectroscopy. Using the original mathematical models the basic micro-segregation characteristics and the parameter of macro-heterogeneity were further determined for each analyzed element. Then a quantitative measurement of inclusions ("micro-purity") in the samples was performed using a metallographic microscope. Then method of differential thermal analysis was used for the measurements of temperatures of phase transformations. The following main results were found: - magnitude of micro-segregation of the analyzed elements in the measured sections of 1000 μm is approximately the same in all the analyzed samples, - chemical macro-heterogeneity is very high across the analyzed slab section, - very uneven mixing of melts of both steels was probably one of the main causes of formation of the breakout.

Abstract: The quality of the working rollers from ductile-cast-iron used for rolling rails is determined by the chemical and structural composition of the material of the rollers and the production technology. The requirements of the quality cannot be ensured without perfect knowledge of the course of solidification, cooling and heat treatment of the cast rollers as well as the kinetics of the temperature field of the casting and mould. An original application of ANSYS simulated the forming of the temperature field of the entire system comprising the casting, the mold and ambient. In the experimental investigation of temperature field, an original methodology for the measurement of the distribution of temperatures and heat flows in the roller-mould system had been developed and verified in the operation. The kinetics of the solidification has a measurable and non-negligible influence on the chemical and structural heterogeneity of the investigated type of ductile-cast-iron. Tying on to the results of the model of the temperature field of the cast rollers, an original methodology was developed for the measurement of chemical microheterogeneity. The chemical and structural heterogeneity of the cast roller is therefore a significant function of the method of melting, modification and inoculation and the successive procedures of risering, casting and crystallization after cooling.

Abstract: Presented paper deals with investigation of selected ternary oxidic system with high content of silicates from the viewpoint of surface properties. Research was focused not only on experimental investigation, but mostly on theoretical modelling of surface tension. This approach was chosen due to absence of compact set of information from the area of heterogeneous processes at high temperatures. It is moreover very often difficult to access appropriate data. Surface properties were investigated by sessile drop method. Afterwards calculation was made with use of selected semi-empirical models and obtained outputs were confronted with experimentally obtained data. Modelling comprised also temperature dependence of surface tension and influence of SiO₂ on its values.

Abstract: This paper deals with the causes of a transversal crack in a steel slab with an l300×l45 mm cross-section by means of results from two models. Samples were taken from and around the crack in order to analyze the concentration (chemical heterogeneity) of the selected elements. Simultaneously, the concentration of elements at the surface of the crack was measured after the crack was opened. The chemical heterogeneity of elements was analyzed by means of the JEOL JXA 8600/KEVEX analytical equipment. The measurement results were processed using mathematical statistics procedures. The results proved that there was an internal crack initiating immediately below the curve of the solid-state temperature and consecutively propagating.

Abstract: In the secondary-cooling zone, where the slab is beginning to straighten out the breakout of the steel can occur in points of increased local chemical and temperature heterogeneity of the steel, from increased tension as a result of the bending of the slab and also from a high local concentration of non-metal, slag inclusions. Especially dangerous are the changes in the chemical composition of the steel during the actual concasting. In the case of two melts one immediately after the other, this could lead to immediate interruption in the concasting and a breakout. The material, physical, chemical and technological parameters, which both melts differed in were determined. If the dimensionless analysis is applied for assessing and reducing the number of these parameters, then it is possible to express the level of risk of breakout as a function of five dimensionless criteria.

Abstract: This paper deals with the quantitative evaluation of temperature and time effect on the concentration distribution of the substitutive elements in the vicinity of the welded joint. The calculations were performed for the weld of two different steels. The measurement of the concentration profiles of the elements in the weld was evaluated by the numerical optimalisation methods using the 2nd Fick’s law. The output of calculations defines quantitatively two terms: diffusion and penetration layers.

Abstract: The paper deals with the effect of cooling rate on solidification behavior of IN 738LC nickel based superalloy and on resulting structural and chemical microheterogeneity of this alloy. Samples taken from as-received state were heated with controlled ramp rates (1, 5, 10 and 20 °C min-1). Immediately after melting they were cooled with the same controlled ramp rate with the help of the laboratory experimental system SETARAM SETSYS 18TM TG/DTA/TMA. Then the microanalysis of minority phases was conducted with use of X-ray spectroscopy and microstructure of the individual samples was documented by scanning electron microscopy. Chemical microheterogeneity was determined on the basis of measured concentration data of selected elements (Al, Ti, Cr, Co, Ni, Nb, Mo, Ta and W) in representative areas of the individual samples structure.