The novel hybrid concept on designing advanced multi-component cast irons: Effect of boron and titanium (Thermodynamic modelling, microstructure and mechanical property evaluation)

The novel hybrid concept of multi-component high‑boron cast irons for wear applications is proposed in this work. The effect of 1.59–3.57 wt% boron on the crystallization path and resulting microstructural state and hardness of (wt.%) 1.1C–1Si–1Mn–5W–5Mo–5V–10Cr alloy enriched with 2.5 wt% Ti is herein investigated. At 1.59 wt% B, the alloy presents a eutectic microstructure consisting of three different eutectics: a predominant Ferrite + (Fe,V,Cr,Mo,W,Ti)2(B,C)5 eutectic of "Chinese Script" morphology, a Ferrite + (Fe,Cr,V,Mo,W,Ti,Mn)7(C,B)3 eutectic of "Rosette" morphology, and a Ferrite + (Fe,Cr,W,Mo,V)3(C,B) eutectic of Ledeburite morphology. With boron content increasing, the alloys shift to a hyper-eutectic domain due to the crystallization of coarse primary carboboride (Fe,V,Cr,Mo,W,Ti)2(B,C)5 (with a hardness of 2200–2600 HV). With increasing boron content, the total volume fraction of carboborides increases from 38.5 vol% to 65.7 vol% while the bulk hardness increases from 387 to 580 HV, accordingly. The formation of primary carboborides has consumed most part of the alloying elements resulting in a (W,Mo,V,Cr)-depleted melt, which further transforms to a Ferrite+M3(C,B) eutectic of "Coarse-network" morphology. With boron content increasing, the matrix of the eutectic hard phases has changed from fully ferritic at 1.5 wt% B, to martensitic at 2.5 wt% B and to Ferrite + Pearlite at 3.5 wt% B. Irrespectively of boron content, all alloys contained about 9 vol% of a TiC-type carboboride ((Ti,V,W,Fe,Mo)(C,B)), containing 54–61 wt% Ti and having an equiaxed facetted shape. These compounds have been crystallized at high temperatures, being further rejected by the growing primary carboborides (as eutectic colonies) into the melt; thereby, clusters of Ti-based M(C,B) particles at the periphery of primary phases or in the gaps between the eutectic colonies have resulted. The equilibrium crystallization path in the alloys is being analysed based on Thermo-Calc modelling.