Quantitative observations of precipitation in 2.25Cr–1Mo steel exposed to different creep conditions in a power station

Samples of 2.25Cr–1Mo steel in the form of tubes have been examined in the new condition and after extended service in a power station. The differences in microstructure in terms of precipitates have been characterized using a TEM; the various precipitates present have been identified individually by using extraction replicas and energy dispersive spectroscopy analysis. The area fraction of each type of precipitate was calculated. The results have shown that the quantity of M₇C₃ precipitated within grains as well as on grain boundaries decreased markedly with the time in service while the quantity of M₆C increased in both regions. It was observed that precipitates of Mo₂C and (CrMo)₂CN, detected after aging respectively on the samples with the shorter and the longer aging times, were present only within grains. The procedure used was not accurate for identification and quantification of small VC and (VNb)C precipitates. The results of the relative quantity of M₇C₃ and M₆C, as a function of time in service, particularly on grain boundaries, showed a well defined interdependence. These results can be used as a complementary analysis for creep damage and remaining life estimation for this class of steel operating at high temperature under steady state conditions.