Effect of Cr:Fe ratio on the mechanical properties of (Cr,Fe)7C3 ternary carbides in abrasion-resistant white cast irons

Abstract

(Cr,Fe)7C3 ternary carbides constitute the majority of eutectic carbides in abrasion-resistant white cast irons. Density functional theory models have predicted these carbides to have a combination of metallic, covalent and ionic bonding, in proportions depending on the carbide’s Cr:Fe ratio. However, experimental research to validate these predictions has been lacking. This study investigates the characteristics of the carbides as a function of Cr:Fe ratio, which was manipulated from Fe-rich to Cr-rich by varying the Cr:C ratio of the bulk alloy. The carbides’ crystalline structure, hardness, Young’s modulus, fracture toughness and abrasion performance have been assessed through techniques including nano-indentation, HR-TEM and the inner circumference abrasion test (ICAT). Fe-rich M3C formed at very low bulk Cr:C ratio was found to have an orthorhombic crystal structure. In all other alloys, with Cr:C ratios above 2.7, M7C3 was formed and found to have a hexagonal structure. Hardness, Young’s modulus and calculated fracture toughness of M7C3 all increase with Cr:Fe ratio, from (Fe5,Cr2)C3 up to a maximum for (Cr4,Fe3)C3 (in 18Cr–6.8Cr:C WCI). This gave the highest hardness (22.9 GPa) and Young’s modulus (315 GPa), but also the highest fracture toughness (4.5 MPa.m0.5). The peak fracture toughness at carbide composition of (Cr4,Fe3)C3 in this study is consistent with the prediction of DFT models in the literature; while the peak hardness at the same carbide composition shows a marginal deviation from the predictions. Abrasion performance generally increased with carbide hardness and fracture toughness, with one exception: (Cr4.3,Fe2.7)C3. Although (Cr4.3,Fe2.7)C3 showed marginally lower inherent fracture toughness than (Cr4.0,Fe3.0)C3, the higher Cr:Fe carbides imparted the highest abrasion performance, associated with modified eutectic morphology.