Creep-Fatigue Interactions in a 9 Pct Cr-1 Pct Mo Martensitic Steel: Part II. Microstructural Evolutions

A multiscale characterization of the microstructural evolutions taking place in 9 to 12 pct Cr martensitic steels subjected to fatigue and creep-fatigue (CF) loadings is presented. Specimens of a P91 steel subjected to high-temperature cyclic loadings are examined using several experimental techniques. Bright-field transmission electron microscopy (TEM), electron backscattered diffraction (EBSD), and TEM orientation mapping are used to characterize and quantify the microstructural evolutions. A recovery phenomenon consisting of the coarsening of the subgrains and a decrease of the dislocation density is observed. This coarsening is heterogeneous and depends on the strain amplitude and on the applied hold time. The size distribution of subgrains and the dislocation density are measured from bright-field TEM observations. Orientation mapping on scanning electron microscopy (SEM) and TEM show that, even though a correlation between the crystallographic orientation and the recovery phenomenon is highlighted, a complex dependency related to the orientation of neighboring blocks exists. These microstructural observations are consistent with the very fast deterioration of creep properties due to cyclic loadings (reported in the first part of this study).