Microstructure Formation in Hypoeutectic Alloys in the Fe–C–B–Cr–W System

This work investigates the influence of tungsten (W) on the microstructure of alloys in the system Fe–C–B–Cr. The main goal is to examine the effect of W-additions on the microstructure and especially the potential stabilization of primary or eutectic borides in alloys with the compositions of Fe–0.4C–1B–2.5Cr + W. Thermodynamic equilibrium and Scheil–Gulliver calculations are performed to identify microstructurally significant alloy-compositions which are also devoid of borocarbides at austenitizing temperature. Accordingly, laboratory melts are cast, samples are swaged, austenitized, quenched and tempered. Additionaly, a near-equilibrium state is created by a diffusion annealing step to validate the equilibrium calculation. The resulting microstructures are characterized by X-ray diffraction, scanning electron microscopy. It is shown that swaging can effectively convert the eutectic network into a powder-metallurgical-like spherical and isotropic hard phase dispersion without introducing microscopic or macroscopic defects. W-additions lead to the stabilization of eutectic FeWB and B-rich M₂₃(B,C)₆ carboborides as opposed to C-rich M₂₃(C,B)₆ formed in the quarternary Fe–C–B–Cr system. Thus, the chemical binding of B in M₂₃(B,C)₆ leads to a significant destabilization of the M₂B-type borides FeWB and (Fe,Cr)₂B. The results show that W-addition strongly influences the solidification reaction of Fe–Cr–C–B–W alloys and thus the present phases, even after diffusion annealing.