Effects of microstructure and carbide spheroidization on fatigue behaviour in high V–Cr–Ni cast irons

This paper describes the effects of microstructure and spheroidization of vanadium carbides (VCs) on fatigue behaviour in high V–Cr–Ni cast irons. Rotary bending fatigue tests have been performed in laboratory air at ambient temperature using smooth specimens of materials with different C and V contents, i.e. microstructures and VC morphologies. The microstructure of the cast irons consisted of three distinct regions: austenitic matrix microstructure, lamellar microstructure consisting of the austenitic microstructure and (V, Cr)-carbides, and isolated VCs dispersed within them. In the materials with non-spheroidal VC, fatigue strength roughly increased with increasing C and V content, but it was found that there existed a good correlation between fatigue limit and the constituent phases of the microstructure, where fatigue limit increased as the area fraction of the lamellar microstructure increased, indicating that the lamellar microstructure was a most important microstructural variable controlling fatigue strength. The fatigue strengths of the materials with spheroidal VC decreased significantly compared with the corresponding materials with non-spheroidal VC, which was more remarkable in the material with large spheroidal VC in size. This indicated that fatigue strength was not improved by VC spheroidization. Regardless of VC morphology, crack initiation was always related to VC. Particularly, in the material with large spheroidal VC cracks were generated due to VC fracture and subsequently often grew with VC fracture and along the boundary between VC and microstructure, leading to considerably low crack initiation and growth resistance. Furthermore, it was found that the fatigue limit of the materials with spheroidal VC did not conform to the above relationship between fatigue limit and the area fraction of the lamellar microstructure.