Solidification and microstructural evolution in hypoeutectic Fe–C–B–Cr–V alloys

The objective of this study was to characterize the microstructural effects of Vanadium (V) additions on the hypoeutectic region of the quaternary Fe–C–B–Cr system. Special attention was given to determining the stabilization of carbides, carboborides, or borides by V. Thermodynamic equilibrium calculations were performed to fundamentally understand the alloying system. Furthermore, multiphase-field simulation was employed to analyze non-equilibrium solidification processes. Experimentally, four alloys of the Fe0.4C1B2.5Cr–V system with 0, 1, 2 and 4 wt% V were cast, swaged and heat treated. Microstructural analysis and phase identification were conducted by energy dispersive X-ray spectrometry, wavelength dispersive X-ray spectrometry, X-ray diffraction, and electron backscatter diffraction. It was found that V contents ≥0.9 wt% thermodynamically stabilized the eutectic formation of B-rich (V,Fe) 3 (B,C) instead of M 2 B, as reported in the Fe–Cr–B–C system. The reason for this was found in the high V solubility of M 3 (B,C) compared to tetragonal M 2 B. Further, VC-type carbides formed during the eutectic solidification at V contents of 0.9 wt%. Nevertheless, a carbon content >0.3 wt%, sufficient for martensitic hardening, was achieved in the Fe-matrices. All alloys exhibited excellent hot formability without cracking, with significant deformation observed in the hard phases. The results are promising, as, despite of high carboboride fractions, these hard phases are B-rich and therefore are formed and open a path towards the development of secondary hardenable high boron tool steel grades.