Roles of microstructure and carbides in fatigue crack propagation in high V-Cr-Ni cast irons

This paper describes the roles of microstructure and carbides during fatigue crack propagation (FCP) in high V-Cr-Ni cast irons, with varying C and V contents from 1 to 3% and 3.5 to 10%, respectively. FCP tests have been performed using CT specimens in laboratory air at ambient temperature, and FCP behaviour and fracture mechanisms were discussed on the basis of crack closure, crack path profile and fracture surface analysis. In the materials with non-spheroidal vanadium carbide (VC), the effect of C and V contents, i.e. microstructure, was seen when the data were characterized in terms of nominal stress intensity factor range. Also in the materials with spheroidal VC, the effect of microstructure was recognized. After allowing for both crack closure and elastic modulus, the intrinsic FCP resistance still became lower with increasing C and V contents, particularly remarkable in the materials with 3% C and 10% V regardless of VC morphology. In these materials, there existed the mutually competitive mechanisms: one was that accelerated FCP rates such as preferential growth into VCs and VC fracture, and the other was that decelerated FCP rates such as crack deflection, crack closure, secondary cracks, and uncracked-ligament bridging. The former exerted much larger influence on the overall FCP rates than the latter did, thus resulting in the lower intrinsic FCP resistance.