INFLUENCE OF CARBON CONTENT AND TEMPERING TEMPERATURE ON FATIGUE THRESHOLD CHARACTERISTICS OF A LOW-ALLOY STEEL

This paper attempts to describe the effect of carbon content on the fatigue threshold characteristics DELTA-K(th) in various heat treated conditions. Essentially it has been shown that a tempering treatment increased DELTA-K(th) while increasing the carbon content of steels from 0.13% to 0.8% significantly decreased the DELTA-K(th) value by over 100%. At intermediate fatigue crack growth rates all the data show a linear relationship with DELTA-K level. In terms of yield strength sigma-y, the threshold stress intensity level could be given by the expression: DELTA-K(th) = 8.74 - 3.42 x 10(-3)(sigma-y). At near threshold fatigue crack growth levels significant amounts of isolated intergranular failure were observed in the 400-degrees-C tempered condition. In the other heat treated microstructures only a flat transgranular ductile striated failure mode was evident. A maxima in the amount of intergranular facets occurred at DELTA-K values approaching 15 to 20 MPa square-root 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.