Quantitative Relations between S-N Curve Parameters and Tensile Strength for Two Steels: AISI 4340 and SCM 435

Abstract

S-N curves of AISI 4340 and SCM 435 steels were determined according to the qualifications and systematically investigated by Basquin relation. The change of parameters (fatigue strength coefficients and exponents) of these S-N curves with increasing tensile strength was analyzed and summarized. For the steels with the same type of S-N curves, as tensile strength increases, their fatigue strength coefficients linearly increase and fatigue strength exponents almost keep unchanging at first and then decrease linearly. In addition, the fatigue mechanisms controlling the parameters of SN curves with tensile strength increasing were discussed. It is briefly explained that the fatigue strength coefficient can be improved by strengthening mechanisms, and the change of fatigue strength exponent with tensile strength may be influenced by the change of fatigue crack initiation sites, the microstructures and fatigue fractographies of steels treated by different heat treatments. Through properly adjusting fatigue strength coefficients and fatigue strength exponents the fatigue strength can be improved in the course of the selection and preparation of steels.