Isothermal and thermomechanical fatigue studies on a modified 9Cr–1Mo ferritic martensitic steel

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue (TMF) tests were carried out on a modified 9Cr–1Mo ferritic martensitic steel under a mechanical strain control mode employing a strain amplitude of ±0.4%. Different temperature ranges in the interval, 573–923 K were employed for the tests which were carried out using a constant strain rate of 1.2 × 10−4 s−1. Isothermal low cycle fatigue tests were also performed concurrently at the maximum temperatures (Tmax) of TMF tests on similar specimens and employing the same strain amplitude and strain rate. The life variation was seen to follow the sequence: IF < OP TMF < IP TMF. However, the difference in lives narrowed down with an increase in the Tmax of TMF/test temperature of IF cycling. The cyclic stress response behaviour of the alloy was characterized by a continuous softening under all testing conditions. The lower lives observed under OP TMF in comparison with IP cycling was attributed to a combined influence of oxide cracking and a higher tensile hysteresis loop energy in the former. Cyclic life under IP TMF was observed to reduce more drastically compared to OP cycling with increase in Tmax, owing to the increasing prominence of dynamic recovery effects coupled with creep deformation. Transmission electron microscopy (TEM) investigations revealed that IF cycling at the Tmax resulted in a significantly greater substructural recovery compared to that produced by TMF cycling as a consequence of which, the former led to lower lives compared to both the IP and OP TMF. Dynamic strain ageing (DSA) associated with serrated flow was observed as the deformation temperature under TMF cycling traversed the regime of 300–400 °C.