Influence of carbon content and tempering temperature on fatigue threshold characteristics of a low alloy steel

Abstract

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics ΔK th in various heat treated conditions. Essentially it has been shown that a tempering treatment increased ΔK th while increasing the carbon content of steels from 0·13% to 0·8% significantly decreased the ΔK th value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with ΔK level. In terms of yield strength σ y, the threshold stress intensity level could be given by the expression: ΔK th = 8·74 − 3·42 × 10 −3(σ y) . At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400°C tempered condition. In the other heat treated microstructures only a flat trans-granular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at ΔK values approaching 15 to 20 MPa√ m . It has been shown the existence of intergranular failure resulted from environmentally induced fracture (through the diffusion of hydrogen) that occurred within the crack tip enclave.