Influence of dynamic sodium environment on the creep–fatigue behaviour of Modified 9Cr–1Mo ferritic–martensitic steel

The use of liquid sodium as a heat transfer medium for sodium-cooled fast reactors (SFRs) necessitates a clear understanding of the effects of dynamic sodium on low cycle fatigue (LCF), creep and creep–fatigue interaction (CFI) behaviour of reactor structural materials. Mod. 9Cr–1Mo ferritic steel is the material of current interest for the steam generator components of sodium cooled fast reactors. The steam generator has a design life of 30–40 years. The effects of dynamic sodium on the LCF and CFI behaviour of Mod. 9Cr–1Mo steel have been investigated at 823 and 873 K. The CFI life of the steel showed marginal increase under flowing sodium environment when compared to air environment. Hence, the design rules for creep–fatigue interaction based on air tests can be safely applied for components operating in sodium environment. This paper attempts to explain the observed LCF and CFI results based on the detailed metallography and fractography conducted on the failed samples.