Effects of microstructural instability on the fatigue behaviour of quenched and quench-aged steels

Abstract

The fatigue behaviours of low carbon and mild steels in quenched and quench-aged conditions have been explored by means of mechanical tests, metallography and observations of temperature rises due to hysteresis. The steels were hardened after quenching from below the critical temperature range, either by precipitation of the metastable carbide before fatigue or by dynamic strain ageing during fatigue. With both these hardening mechanisms, the arrangement of dislocation barriers that was most effective in increasing monotonic flow strength was susceptible to dissolution under the influence of cyclic plastic strains. The resultant local softening promoted strain concentration within narrow active slip bands and it led to low ratios of fatigue strength/monotonic flow strength. Mechanically stable dispersions of precipitates were associated with diffuse slip and high fatigue ratios. In the conditions of tests at room temperature, the precipitation hardened structures that provided the best compromise between the requirements of fine dispersion and stability were developed by ageing in the range 90–130°C, but it is concluded that the best choice of microstructure for fatigue applications will depend on the fatigue variables that influence structural stability.