Effect of the cooling rate on the hot ductility and the mechanism of the phase transformation in solidification of tool steels

The presently used high-speed and other alloyed tool steels are characterized by low hot ductility due to peculiarities of their structure in the cast slate resulting from the complex multistage crystallization, L => delta-ferrite; L + delta-ferrite = austenile; L => austenite + K, The principal peculiarity of the solidification is the peritectic reaction. The investigations carried out using original quenched installation show that in this reaction a thin interlayer of liquid metal appears on the d/g interface, and that this interlayer is connected with the interdendritic melt through gaps in the austenitic envelope. At increased cooling rate fuller completion of the peritectic reaction and smaller austenite grains are obtained, since the diffusion exchange of the alloying components occurs principally through the liquid phase, which penetrate deeply in primary grains. The optimization the cooling rate in the stage of peritectic reaction increases the hot ductility of high-speed steels up to 1.5 times.