The effect of mean stress on the high-temperature fatigue behaviour of SAE 1045 steel

Abstract

Specimens of the mild steel SAE 1045 in normalized condition were subjected to stress-controlled push-pull fatigue tests at temperatures ranging from 20 °C to 375 °C. Up to about 250 °C, the steel shows a decrease in cyclic strength and in the number of cycles to failure Nf with increasing temperature. A further increase in temperature leads to an increase in strength due to dynamic strain ageing (DSA) which reaches its maximum effect at a temperature TDSA,max of about 325 °C. As a consequence of the strongly reduced plastic strain amplitude, a maximum of the cyclic lifetime could be observed at a temperature close to TDSA,max. Tests with superimposed mean stresses varying from σm = − 80 MPa to σm = +60 MPa showed that a positive (tensile) mean stress generally increases the plastic strain amplitude and reduces the fatigue life and vice versa. The study of the microstructure revealed dislocation arrangements in the form of debris/bundle structures at TDSA,max and cell structures below and above the temperature range of DSA. At a given temperature in this range, positive mean stresses promote the formation of a cell structure because of the superimposed cyclic creep deformation taking place.