Development of Advanced Ferritic Steels for High Efficiency Power Generation Plant

E911 creep samples exposed to temperatures of 600˚C, 625˚C and 650˚C at differing stress levels were supplied by CORUS. The hardness of the gauge length that experienced both creep strain and temperature was found to be lower than that of the head where thermal softening only can be assumed. The changes in the morphology and size of precipitates were observed qualitatively by optical microscopy and Scanning Electron Microscopy. A creep fracture mechanism map of E911 steel was constructed with two modes of creep (transgranular and intergranular ). A fitted ellipse shape was used to characterise the irregular block shape by Electron Backscattered Diffraction (EBSD). It showed that the width of the block inside a prior austenite grain increases more rapidly in the gauge length than in the head; subgrain growth was also observed by EBSD. Transmission Electron Microscopy studies indicate that at 600°C E911 steel can reach up to 75647 hours creep rupture life (108MPa), which is due to the relatively small size of M23C6, Laves and M2X phases. However, Z phase precipitation results in a drop in creep resistance owing to the dissolution of fine MX phase and the transformation of M2X phase. At 625°C and 650°C, the creep rupture life decreases owing to the coarsening of Laves, M23C6 and M2X phases. Four experimental steel casts were prepared with varying levels of Ni and Cr to investigate the effect of these elements on Z phase formation. After 10,000 hours exposure, there was little evidence of Z phase in the samples studied and therefore it is difficult to draw definitive conclusions about the role of Ni or Cr in promoting Z phase formation. It is possible that the casts studied here will allow better conclusions to be drawn after exposing the samples to longer durations.