(in lingua inglese)
A work duration of 5000 h at the maximum service temperature resulted in the enlargement and elimination of a large quantity of intra-dendritic carbides in the Ex-service parts. Moreover, a significant increase of Nb was detected in the intermetallic MC carbides. The microstructural changes during 5000 h indicated that the service temperature was more than the standard working temperature of this material. Decarburization and precipitate enlargement resulted in the reduction of mechanical properties that led to the deformation of ring at high temperature due to thermal stresses. Furthermore, enlarged Nb rich precipitates were proper sites to generate voids in front of the crack path that joining of the voids resulted in crack propagation.