document: The need for power generation industry to improve the thermal efficiency of power plant has led to the development of 9–12% Cr martensitic steels. The development of and research on P91 steels started since late 1970s and early 1990s, respectively [1]. The work has focussed on their creep strengths due to its intended application at high temperature. Recently, the introduction of more cyclic operation of power plant has introduced the possibility of fatigue problems. Bore cracking due to the effects of varying steam warming has been reported [2]. The temperature cycling causes thermal gradients between the inside and outside of components and this can cause cyclic stress levels to be of concerns. Recently, research on thermal–mechanical analysis of P91 has been carried out including the characterisation of the cyclic behaviour of the material using the two-layer and unified visco-plasticity models [3,4].
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