Formation of Eutectic Borocarbides in High Boron High Speed Steel during Non‐Equilibrium Crystallization

Abstract

In this work, the microstructure, the formation characteristics, and the growth mechanism of eutectic borocarbide during solidification in high boron high speed steel with 2.0 wt% B–0.4 wt% C–6.0 wt% Cr–4.0 wt% Mo–2.0% Al–1.0 wt% Si–1.0 wt% V–0.5 wt% Mn are investigated utilizing liquid quenching technology. The experimental results illustrate that the as‐cast microstructure of high boron high speed steel consists of α‐Fe matrix, eutectic borocarbide M2(B,C) with continuous network structure, M3(B,C) boron‐cementite, and M7(C,B)3 (M = Fe, Cr, Mo, V, Mn). Different precipitating sequences of eutectic borocarbides with various compositions are revealed. During solidification, order of precipitation is Cr‐rich borocarbide and Mo‐rich borocarbide, respectively. Mo‐rich borocarbide precipitates from the existing Cr‐rich borocarbide rather than forming independently. Additionally, the growth mechanism of different borocarbides is systematically studied, which clearly explains the reason of continuous reticular formation of borocarbide. Cr‐rich borocarbide is the proeutecitc phase, which grows in the form of faceted‐nonfacetedgrowth. As the significant finding of this work, the “hexagon” growth mechanism of Mo‐rich borocarbide is discovered. This mechanism feasibly describes the paralleled lamina and faveolated structure of Mo‐rich borocarbide. These results render a consideration for the elimination of elemental segregation through reasonable foundry process designing.