Abstract
The work deals with the influence of thermal treatment on the properties of rapidly solidified tool steel alloyed with niobium. Powder alloy was prepared by pressure nitrogen atomisation. Changes in microstructure, phase occurrence and hardness were evaluated as a function of solidification rate and annealing temperature at chosen size fractions.
Transition from hypoeutectic to hypereutectic microstructure arising from increasing solidification rate was observed in the examined powder. At annealing temperatures of 400-600 °C rapid precipitation of very fine carbides was detected in hypoeutectic microstructure. It results in an increase of hardness, phase transformations, and microstructure changes. Hypereutectic microstructure contains stable phases, transformation of which takes place slowly in wide range of annealing temperatures.

Key words
tool steel, powder metallurgy, niobium, rapid solidification, thermal treatment

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