Comparison of various 9–12%Cr steels under fatigue and creep-fatigue loadings at high temperature (original) (raw)

In the present study we investigate the evolution of the microstructure of a 12% Cr tempered martensite ferritic steel under conditions of long-term aging and creep (823 K, 120 MPa, t R = 139,971 h). We show how subgrains coarsen, that the close correlation between car-bides and subgrain boundaries loosens during long-term creep and that the frequency of small-angle boundaries increases. All these elementary deformation processes have been discussed in short-term creep studies. The present study shows that they also govern long-term creep. However, during long-term creep, precipitation and coarsening reactions occur that are not observed during short-term creep. Three types of particles (M 23 C 6 , VX and Laves-phase) were identified after long-term creep. M 23 C 6 particles coarsen at constant volume fraction and establish their equilibrium concentration after 51,072 h; VX particles are stable; and the Laves-phase particles never reach thermodynamic equilibrium.

The creep resistance of two high chromium ferritic power plant steels (grades P91 and P92) is shown to be considerably influenced by microstructure changes and microstructure stability during long-term isothermal annealing and/or creep over a range of dislocation (power-law) creep. It is suggested that the change in the dislocation substructure is much more important than the indirect effect caused by particle evolution. By contrast, no significant effect of microstructure stability on creep was found in a regime of viscous creep that is inherent to real service loading conditions of the steels of interest. Finally, no deterioration of the creep properties was found for the steels P91 and P92 under cyclic creep (nonsteady stressing and heating) in a regime of power-law creep.

Five Co-modified P92-type steels with different contents of Cr, W, Mo, B, N, and Re have been examined to evaluate the effect of the chemical composition on the evolution of Laves phase during creep at 650 °C. The creep tests have been carried out at 650 °C under various applied initial stresses ranging from 80 to 200 MPa until rupture. An increase in the B and Cr contents leads to a decrease in the size and volume fraction of M23C6 carbides precipitated during tempering and an increase in their number particle density along the boundaries. In turns, this affects the amount of the nucleation sites for Laves phase during creep. The (W+Mo) content determines the diffusion growth and coarsening of Laves phase during creep. Susceptibility of Laves phase to coarsening with a high rate is caused by the large difference in Gibbs energy between fine and large particles located at the low-angle and high-angle boundaries, respectively, and can cause the creep strength breakdown. The addition ...

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. M 23 C 6 (on internal interfaces), MX precipitates (on internal interfaces and in the matrix) and M 6 C 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 M 6 C carbides which were not observed after short term creep exposure. These M 6 C carbides form close to M 23 C 6 particles. Moreover, it was found that the Mo-content of the MX precipitates increases during long term creep. Crown

We document the evolution of dislocation densities in tempered martensite ferritic steels during long-term aging and creep. Scanning transmission electron microscopy in combination with a high-angle annular dark-field detector is used to study dislocations in a 12% Cr steel. During aging, the dislocation density quickly decreases by a factor 2 and then remains constant. Long-term creep results in an initial decrease by a factor 10, and after this sharp drop, the dislocation density continues to decrease.