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Utilization of thermal analysis to thermo physical properties study of real steel grades

Klus P., Zaludova M, Gryc K., Smetana B., Michalek K., Dobrovska J., Tkadleckova M., Socha L.

Archives of Materials Science and Engineering

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2012

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Vol. 58, nr 2

116--119

EN

Purpose: This paper deals with determining the temperatures of phase transformations in real steel grades. It also includes the study of industrially produced steel grades using the methods of thermal analysis by experimental equipment STA 449 F3 Jupiter made by NETZSCH and Setsys 18TM made by SETARAM. Design/methodology/approach: Selected methods of thermal analysis (DSC and DTA) enable to obtain the temperatures of phase transformations taking place in steel during the linear heating/cooling. Within the casting technology of steel, thermal analysis is used to determine the solidus temperature and especially the crucial liquidus temperature. Findings: Experimentally obtained solidus and liquidus temperatures are higher in the DSC method (max. 3.8°C). The difference between the temperatures of phase transformation (T) running between the solidus temperature (TS) and liquidus (TL) for both methods (DTA and DSC) differ by a maximum of 3.2°C. The results from experimental measurements were compared with theoretical calculations of liquidus and solidus temperatures by different authors and with the computed results from thermodynamic database COMPUTHERM and also with temperatures from the equilibrium phase diagram of Fe-Cr-C. Experimentally obtained solidus temperatures are lower than the calculated equilibrium solidus temperatures. Experimentally obtained liquidus temperatures are in the range of temperatures obtained using computational relations. The temperatures mentioned in the equilibrium phase diagram (diagram for a particular steel grades was not found) are higher than experimentally obtained temperatures. Research limitations/implications: The results of experimental studies can be used to refine the knowledge of basic physical properties of steel and for example replacement of the tabulated values or estimated values of phase transformation temperatures and thermal capacity. Furthermore, the obtained data will be implemented in the material databases of numerical programs used for the simulation of metallurgical processes. Originality/value: On the basis of applied research in close collaboration with industry companies, the obtained data can contribute significantly to optimize the operating conditions, thereby increasing the efficiency of the steelmaking technology and final quality of cast steel.

2

Numerical model of heterogeneity of the ductile cast-iron

Stransky K., Kavicka F., Dobrovska J., Stetina J., Sekanina B.

Prace Instytutu Metalurgii Żelaza

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2009

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T. 61, nr 5

137-141

EN

An original three-dimensional (3D) model of solidification is used to describe the process of solidification and cooling of massive 500×1000×500 mm cast-iron castings. The castings are cast in sand moulds. The calculated mode of the kinetics of the temperature field of the casting is verified during casting with temperature measurements in selected points. The sizes and positions (xi, yi, zi, where i = 1,2,3 is the number of samples taken) of the experimental samples are exactly defined and corresponding with the decreasing rate of solidification. The experimental samples – 15 mm in diameter and 12 mm high – are metallographically analysed and also in terms of heterogeneity of the chemical composition. The coordinates xi, yi, zi characterise approximately – within an accuracy of +/- 5 mm – the centres of the samples. Successively, the local solidification time (i.e. the time the specified position of the casting, defined by the coordinates xi, yi, zi, remains within the temperature range between the liquidus and solidus) is also calculated using the 3D model. The following dependences are later determined according to the experimental and calculated data: the average size of the graphite spheroids rg, graphite cells Rb and the average distances among the particles of graphite Lg – always as a function of the local solidification time [xi, yi, zi]. Furthermore, it has been found out that the given basic characteristics of the structure of the cast iron rg, Rb and Lg are directly proportional to the logarithm of the local solidification time. The original spatial model of solidification can therefore be used in its first approximation for the assessment of the pouring structure of massive cast-iron castings.

3

A concast billet caster and electromagnetic stirring of the melt

Stransky K., Kavicka F., Sekanina B., Stetina J., Dobrovska J., Stransky L.

Prace Instytutu Metalurgii Żelaza

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2009

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T. 61, nr 5

142-145

EN

Electromagnetic stirring (EMS) suppresses the growth of columnar crystals of billets and reduces the tendency to cracking during casting and at low temperatures. A caster was used for the testing of two induction stirrers – one on the actual mould and the other beneath the mould – to determine the effect of EMS on the formation of the structure of non-alloy steel. As part of these tests, certain parts of the billets had been cast without the use of stirrers and other parts underwent alternate switching on and off of the stirrers for as many as nine combinations of modes. Samples were taken from the sections of these billets, fine-ground and etched in order to make the dendritic structure visible. The mode with the highest effi ciency was when both stirrers ran simultaneously. The growth of the columnar crystals, which pointed inward, was limited to 1/4 -to- 1/3 of the length of the case when there was no stirring. Experimental research was also confronted with results acquired from the application of the models of the temperature field and chemical heterogeneity and the physical-similarity theory. Statistical monitoring of the quality of concast billets has proven that stirring significantly reduces the occurrence of defects – in this case cracks.

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Study of cooling rate effect on mikrosegregation behaviour and structural heterogeneity of in 738LC superalloy

Dobrovska J., Docekalova S., Dobrovska V., Stránský K.

Zeszyty Naukowe. Mechanika / Politechnika Opolska

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2007

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Vol. 321, z. 89

95-96

EN

Nickel based alloys are characterised during solidification by intensive dendritic segregation of elements [1-3]. Nickel based superalloys are widely used in turbine blades and other engine parts. IN 738LC is a modified version of superalloy IN 738 with enhanced carbide stability, castability and ductility, achieved by lowering the carbon content. IN 738LC alloy is well known alloy, however, not many studies have been reported on microsegregation of elements during solidification and on effect of cooling rate on microsegregation. The paper deals with the effect of cooling rate on solidification behaviour of IN 738LC nickel based superalloy and on resulting structural and chemical microheterogeneity of this alloy.

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Numerical model for the calculation of the temperature field of a concast billet

Stetina J., Kavicka F., Sekanina B., Bazan J., Dobrovska J.

Prace Instytutu Metalurgii Żelaza

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2006

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T. 58, nr 4

102-104

EN

Solidification and cooling of a continuously cast steel billet is a very complicated problem of transient heat and mass transfer. The solving of such a problem is impossible without numerical model of the temperature field, not only of the concasting itself, while it is being processed through the caster but of the mold as well. This process is described by the Fourier-Kirchhoff equation. An original 3-D numerical off-line model of the temperature field of a caster has been developed. It has graphical input and output - automatic generation of the net and plotting of temperature fields in the form of color iso-therms and iso-zones, and temperature-time curves for any point of the system being investigated. This numerical model is capable of simulating the temperature field of a caster as a whole, or any of its parts. Experimental research and data acquisition have to be conducted simultaneously with the numerical computation - not only to confront it with the actual numerical model, but also to make it more accurate throughout the process. After computation, it is possible to obtain the temperatures at each - node of the network, and at each time of the process. The utilization of the numerical model of solidification and cooling of a concasting plays an indispensable role in practice. The potential change of technology - on the basis of computation - is constantly guided by the effort to optimize, i.e. to maximize the quality of the process.

PL

Krzepnięcie i chłodzenie wlewków ciągłych o przekroju kwadratowym stanowi bardzo złożony problem, jeżeli chodzi o wymianę ciepła i przenikanie masy. Rozwiązanie takiego problemu jest niemożliwe bez posiadania numerycznego modelu obliczania rozkładu temperatury nie tylko w trakcie procesu odlewania ciągłego, gdy pasmo stali przechodzi przez urządzenie do odlewania, ale również w samym krystalizatorze. Ten proces opisuje równanie Fouriera-Kirchkoffa. Opracowano autorski trójwymiarowy model numeryczny do obliczania w trybie off-line rozkładu temperatury w urządzeniu do odlewania. Posiada on graficzne wejście i wyjście - automatyczne tworzenie sieci i nanoszenie rozkładów temperatury w postaci kolorowych izoterm i izostref oraz tworzenie krzywych zależności temperatury od czasu dla każdego punktu badanego układu. Ten model numeryczny może symulować rozkład temperatury urządzenia do odlewania w całości lub dla dowolnej jego części. Wraz z obliczeniami numerycznymi należy jednocześnie przeprowadzić badania eksperymentalne oraz pobieranie danych - nie tylko po to, aby je skonfrontować z rzeczywistym modelem numerycznym, ale również w celu zapewnienia większej dokładności obliczeń w trakcie całego procesu. Po wykonaniu obliczeń możliwe jest uzyskanie temperatur dla każdego węzła sieci w dowolnym momencie procesu. Zastosowanie modelu numerycznego krzepnięcia i chłodzenia wlewków ciągłych w praktyce staje się niezbędne. Potencjalna zmiana technologii - na podstawie obliczeń - nieustannie wiąże się z działaniami mającymi na celu zoptymalizowanie, t j. zmaksymalizowanie jakości procesu.

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Effect of Al203 addition on selected properties of slags

Dudek R., Dobrovsky L., Dobrovska J.

Prace Instytutu Metalurgii Żelaza

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2006

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T. 58, nr 4

76-78

EN

The intention of the presented elaborate is the research of chosen real slag system from the point of view of some physical chemical properties. On the basis of the surface tension changes research during the temperature loading there were analyzed the rules of structure changes in connection with the precipitation, contingently with the new phases destruction. We assumed that these properties were influenced mainly by the Al2O3 amount which might participate in the polyanionic nets creation. As the real metallurgical slags represent complicated poly component systems, it is not possible to define the phase changes rules, and the type and construction of the solid phase from the state diagrams only. Because of this reason the experimental results were confronted with the theoretical ideas about the structure of the oxidic systems.

PL

Celem pracy było zbadanie wybranego rzeczywistego układu żużlowego pod kątem niektórych właściwości fizykochemicznych. W oparciu o badania zmian napięcia powierzchniowego w czasie nagrzewania analizowano zmiany struktury związane z wydzielaniem i rozpadem nowych faz. Przyjęto, że na własności głównie wpływ miała ilość Al2O3, który mógł uczestniczyć w tworzeniu siatek polianionowych. Ponieważ rzeczywiste żużle metalurgiczne stanowią skomplikowane układy wieloskładnikowe, nie jest możliwe określenie mechanizmów zmian fazowych oraz typu i budowy fazy stałej tylko na podstawie wykresów równowag w stanie stałym. Z tego powodu wyniki eksperymentalne zostały skonfrontowane z danymi teoretycznymi o budowie układów tlenkowych.

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Zagadnienia związane z przepływem stali przez filtry ceramiczne

Stransky K., Bazan J., Buzek Z., Dobrovska J., Kavicka F.

Hutnik, Wiadomości Hutnicze

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2006

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Vol. 73, nr 11

486-490

PL

Znaczenie filtracji ciekłych metali filtrami ceramicznymi. Wielkości hydrauliczne przy przepływie cieczy przez filtr. Równanie Bernoulliego i jego zastosowanie w opisie filtracji ciekłych metali. Zjawiska fizyczne związane z przepływem cieczy przez kapilarę. Prawo Hagena-Poiseuilla i Darcy’ego. Charakterystyka przepływu laminarnego i turbulentnego. Bezwymiarowe kryteria charakteryzujące przepływ ciekłych metali przez filtr ceramiczny i podstawowe mechanizmy filtracji ciekłych metali.

EN

Importance of metal melts filtration by ceramic filters. Hydraulic conditions at melt flow through filter. Bernouilli equation and its use for description of metal melts filtration. Physical regularities of melt flow in capillary. Hagen-Poiseuille law and Darcy’s law. Characteristics of laminar flow. Attributes of turbulent flow. Dimensionless criteria characterized melt flow through ceramic filter. Principal mechanisms of metal melts filtration.