engleski

Effect of Cooling Rate on Hot Ductility and Mechanism of the Phase Transformation by Solidification of Tool Steels

The presently used high-speed and other alloyed tool steels are characterized by low hot ductility; this is due to peculiarities of their structure in the cast state which forms as result of complex multistage crystallization: L--->d-ferrite; L+d-ferrite--->austenite; L--->austenite+K. The principal peculiarity of the solidification transformations is the peritectic reaction. The special investigations carried out using original quenched installation show that this reaction has the following mechanism which differs from the others, existing at present days their is a thin interlayer of liquid metal on the d/g interface; this interlayer is connected with the interdendritic melt through gaps in the austenite envelope. An increse cooling rate causes fuller completion of the peritectic reaction and makes the austenite grains finer. This is due to diffusional exchange of the alloying components effected principally through the liquid phase, which penetrates deeply in primary grains. The optimization the cooling rate on the stage of peritectic reaction makes it possible to increase the hot ductility of high-speed steels up to 1.5 times.

cooling rate; phase transformation; tool steels

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