Abstract
The effect of cyclic thermal loading on the creep behaviour and microstructure evolution of three creep strength enhanced 9%Cr ferritic-martensitic steels (P91, P92 and E911) was investigated. The temperature cycles consisted of cooling the specimen after creep exposure from the creep testing temperature of 600 °C to room temperature and then heating it up again to the creep testing temperature. Standard creep tests at the same creep testing temperature were carried out simultaneously for comparison. The results of the thermal cycled and standard creep tests showed no detrimental effect of intermittent heating on the creep behaviour of the specimens during the cycled tests. Microstructural investigations revealed no substantial differences in the types and chemical compositions of precipitating phases for the specimens after standard or cyclic creep in the three steels examined. There is no reason to expect substantially different creep behaviour for specimens experiencing standard or cycled creep, provided that the specimens possess similar microstructures.

Key words
9%Cr creep-resistant steels, cyclic thermal loading, creep, microstructure evolution, precipitation, fractography

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