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2 Prace Instytutu Metalurgii Żelaza

2 2009

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Numerical model of heterogeneity of the ductile cast-iron

100%

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

2009

tom T. 61, nr 5

137-141

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.

A concast billet caster and electromagnetic stirring of the melt

86%

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

2009

tom T. 61, nr 5

142-145

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.