Microstructural evolutions and cyclic softening of 9%Cr martensitic steels

Detailed TEM and EBSD measurements were carried out to quantify the microstructural evolutions and to identify the physical mechanisms taking place during fatigue and creep-fatigue at 823 K on a P91 martensitic steel. The coarsening of former martensitic laths is shown to be heterogeneous for low applied strains, whereas for higher applied strains and longer holding periods the whole microstructure coarsens. Based on these observations and on a careful study of the stress partition (backstress, isotropic and viscous stress), the softening effect in creep-fatigue is found to be mainly related to the cumulated viscoplastic strain at a given fatigue strain range. The microstructural coarsening taking place during cyclic loadings is shown to increase significantly the minimum creep rate of this steel.