PSI - Issue 71

K.M.K. Chowdary et al. / Procedia Structural Integrity 71 (2025) 188–195

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early onset of the tertiary stage of creep deformation. One of the main strengthening mechanisms in IN-RAFM steel is precipitation hardening, by two main types of precipitates: Cr-rich M 23 C 6 and Ta or V-rich MX particles (where M stands for Cr, V, Ta, and X for C or N). Figure 9 shows the TEM microstructures of IN-RAFM steel, emphasizing that the thermal stability of fine MX precipitates which in turn contributes to enhanced long-term creep resistance (Vanaja et al., 2012).

Fig.9 TEM micrographs revealing sub grains and coarse M 23 C 6 carbides mostly on sub grain boundaries (arrow marked) and fine globular MX precipitates within sub grains pinning the dislocations (circled) after (a) thermal exposure and (b) creep test at 823 K for 1947 h. (Vanaja et al., 2012). Tertiary creep has also been examined using a concept of time to reach Monkman-Grant Ductility t MGD and its relationship with rupture life tr (Phaniraj et al., 2003). Based on this concept and using CDM approach, the creep damage criterion interrelating t MGD and t r that depends only on creep damage tolerance factor [ = ∕( ̇ ) ] is given as t MGD / t r = 1- [( − ]1) = constant = f CDM (3) Damage tolerance factor, P of IN-RAFM steel is in line with other ferritic steels with an average value of 4(Vanaja et al., 2012). The IN-RAFM steel obeyed damage criterion relating t MGD and t r with constant value of f CDM = 0.68. The time to reach Monkman-Grant Ductility t MGD and its relationship with rupture life (shown in Fig.10) depends only on damage tolerance factor.

Fig.10 . Variation of time to reach MGD (t MGD ) as a function of rupture time (t r )

4. Conclusions: Based on the creep deformation and FE-CDM analysis (Sin- Hyperbolic model) of creep deformation of IN RAFM steel at 823 K, the following conclusions have been drawn.