Processing and the Effect of Nb–Mo on the Microstructure, Mechanical and Tribological Behavior of High-Cr Cast Iron

In this research, high-Cr cast irons alloys were developed and cast. The compositions were modified by the addition of niobium (≈ 3 wt pct Nb), molybdenum (≈ 3 wt pct Mo), and a combination of both (≈1 wt pct Mo–2 wt pct Nb) to evaluate the production of reinforcing carbides. Through quantitative image analysis, the size, volume fraction, and mean free path of M₇C₃ eutectic carbides were determined and correlated to the microstructural characteristics, as well as to the mechanical and wear properties of the alloys. The thermodynamic evaluation predicted that the Nb nuclei precipitated in the molten liquid previously to the other phases and that the higher Nb content offered higher nucleation and growth potential for their nuclei. The microstructural examination showed that the morphology of Nb-rich carbides varied from nodular (homogeneous dispersion) to a flake-like shape morphology (inhomogeneous dispersion) as the Nb content decreased. The Nb–Mo combination produced MC carbide composed mainly of Nb-containing bound Mo. Fracture surfaces indicated that this carbide worked as a stress concentrator, causing a noticeable matrix/carbide debonding, reducing the impact energy. NbC carbide had a beneficial effect on wear resistance, due to its better distribution than the other reinforcing carbides M₂C (Mo-rich) and MC.