Creep-damage assessment of high chromium heat resistant steels and weldments

The microstructural changes during the creep damage process were investigated on 10Cr-1Mo-1W-VNbN steel forging and the weldment of Mod.9Cr steel-12Cr steel casting, using a scanning electron microscope equipped to take electron backscatter diffraction (EBSD) patterns, and by nano-indentation. Creep rupture and interrupted creep test samples were cut at mid-section along longitudinal direction and inverse pole figures (IPF) and Kernel average misorientation (KAM) maps were obtained from EBSD patterns. Grain size and block size were clearly identified by IPF maps and proved to be good measures of creep time fraction. Area-averaged KAM (KAMave) was shown to be effective for evaluating dislocation microstructural changes during creep and for creep damage evaluation of the forging material. Nano-indentation tests were conducted at the same position in EBSD measurement, which showed a good correlation between hardness value and the square root of KAMave. The creep damage estimation curves of KAMave were obtained as a function of creep time fraction. For the fine-grained, heat-affected zone, area fraction of creep voids and grain size measured by EBSD were good measures of creep damage, while KAMave was not such a good measure.