Abstract
	The study investigated the influence of cobalt addition to the PM high speed steel focusing on physical - technological and microstructure properties in comparison with the reference PM high speed steel STN 4 19 830. Development of microstructure was analysed in rapidly solidified particles from tool steel produced by nitrogen gas atomization. Three types of solidified structures were observed: dendritic, cellular, and a mixture of the two mentioned types. The microstructure of powder particles was formed by martensite, retained austenite and uniformly distributed fine carbide particles ~ 0.5 ?m in diameter. The portion of individual types of microstructure was analysed on minimum 200 particles. The portion of the 160 - 100 ?m fraction in the PM HSS material with 5% addition of Co was 1.5 times higher in comparison with the reference PM HSS while the Vanadis 30 material had the lowest portion of the fraction below 71 ?m. An equation describing the cooling rates of particles during atomisation, obtained by measurement of distances of secondary dendrite arms, was calculated for the investigated PM materials. The presence of cobalt increased the cooling rate of atomised, rapidly solidified powder particles.