Microstructural evolution in T24, a modified 2(1/4)Cr-1Mo steel during creep after different heat treatments

A crept modified 2(1/4)Cr-Mo steel (T24) was investigated in order to understand differences in creep behavior which are related to different cooling rates after austenitizing (air-cooling and water quenching). Two 823 K creep stress levels were compared, 150 MPa (rupture times of the order of: 50,000 h) and 200 MPa (rupture times of the order of: 500 h). The air cooled specimens showed lower rupture times at 200 MPa (short term creep conditions) while the differences in rupture times were not pronounced at 150 MPa (long term creep tests). The minimum creep rates were always faster for the air cooled material state. Microscopic techniques, documenting length scales from the mm (optical microscopy) to the nm range (transmission electron microscopy) were used to study microstructures. Water quenching results in the formation of martensite while air cooling produces bainitic microstructures. After creep, the water cooled specimens still show a finer microstructure (tempered martensite which is equivalent to a fine bainitic structure) than the air cooled specimens (coarse bainite). This mesoscopic microstructural difference can be documented using optical microscopy and scanning electron microscopy, and accounts for the differences in minimum creep rates and rupture times. No significant differences were found when using transmission electron microscopy (TEM) to study the two microstructures (fine and coarse bainite) after creep. M23C6 (on internal interfaces), MX precipitates (on internal interfaces and in the matrix) and M6C carbides were detected in both microstructures. TEM observations suggest that interactions between small and stable MX precipitates and dislocations provide the high creep strength. Long term creep conditions result in the formation of molybdenum-rich M6C carbides which were not observed after short term creep exposure. These M6C carbides form close to M23C6 particles. Moreover, it was found that the Mo-content of the MX precipitates increases during long term creep.