Thermomechanical Fatigue Behaviour of a Modified 9Cr-1Mo Ferritic-Martensitic Steel

Thermomechanical fatigue (TMF) tests were carried out on a modified 9Cr-1Mo ferritic steel (P91) under a mechanical strain control mode using a strain amplitude of ±0.4% and a strain rate of 1.2×10−4 s−1. In-Phase (IP) and Out-of-Phase (OP) strain-time waveforms were employed for the tests which were performed under different temperature ranges in the interval, 573-923 K. For the sake of comparison, isothermal LCF (designated as IF) tests were also carried out at the maximum temperatures (Tmax) of TMF cycles on similar specimens and using the same strain amplitude and strain rate. Isothermal cycling was observed to be the most detrimental while IP TMF yielded the highest lives. However, with an increase in the Tmax of TMF cycling, the difference in lives was seen to narrow down. Also, lives under IP TMF and IF cycling were seen to reduce more drastically compared to OP cycling on account of a greater creep damage accumulation. A continuous cyclic softening characterized the stress response of the alloy under all testing conditions. The lower lives observed under OP cycling were rationalised in terms of oxidation damage and mean stress development. The observed behaviour was explained on the basis of detailed TEM investigations.