Experimental determination of the nuclei number in the deeply undercooled and rapidly solidified powder particles of high-alloyed steel

Abstract In metallurgical practice, solidification of metallic materials usually starts by heterogeneous nucleation. Homogeneous nucleation can occur in deeply undercooled melts. Gas atomization represents one of the methods which can be used to achieve deep melt undercooling. In the gas atomization process, a certain number of the melt droplets can even reach hypercooling, which may provide suitable conditions for homogeneous nucleation in such particles. The paper presents an original approach to the quantitative analysis of the number of crystallization nuclei originated in deeply undercooled melt by the process of homogeneous nucleation using the particles of rapidly solidified (RS) powder of high alloyed Cr-Mo-V tool steel of a ledeburite type prepared by nitrogen gas atomization. For microstructural analysis, RS powder particles were sieved into six standard-sized granulometric fractions ranging from 50 μm to 125 μm. The maximum volumetric density of homogeneously formed crystallization nuclei was identified using the methods quantitative stereology in the granulometric fraction of RS powder from 56 μm to 63 μm with the value approximately 1.5 × 10 6  mm −3 .