Creep-fatigue interaction in a bimodal 12Cr-ODS steel

Abstract The present study reports on the creep-fatigue (CF) behavior of a ferritic bimodal 12Cr-ODS steel. The tests were performed in air at 550 °C and 650 °C by introducing hold-time at peak tensile, peak compressive and both peak tensile as well as compressive strains of ±0.5%. The symmetrical loops under pure fatigue/continuous cycling (CC) became asymmetrical upon introducing hold-time at either peak tensile or peak compressive strain. On the contrary, hold-time on both sides had no influence regarding symmetry but exhibited minor variation in peak stresses. For the investigated hold-time durations, the increase in hold-time period had a negligible effect on peak stresses. Due to stress relaxation during hold-time, elastic strain gets partially converted into inelastic strain which is relatively lower at 650 °C than at 550 °C. This eventually led to lower mean stresses; and finally, a relatively lower reduction in lifetime at 650 °C than at 550 °C. In comparison to similar CC conditions, CF loading resulted in an expeditious and prominent microstructural evolution, which assists in accumulating additional inelastic strain. Nevertheless, microstructural evolution appeared independent of the nature of the applied hold-time waveform which became pronounced with increase in temperature. The dislocation arrangements varied from grain to grain and even within the grains as a consequence of different grains sizes, orientations, and oxide particle distributions. Furthermore, in comparison to CC, an extensive W-enriched Laves phase precipitation/coarsening in the form of continuous or semi-continuous grain boundary networks was realized under CF loading. The damage studies revealed a single surface initiated transgranular crack under CC and multiple surface initiated transgranular cracks under CF loading. The transgranular crack path under CC possessed an intergranular tendency under CF loading which is obviously associated with the lower cyclic life.