Creep Behavior, Deformation Mechanisms, and Creep Life of Mod.9Cr-1Mo Steel

Abstract

The creep behavior, deformation mechanisms, and the correlation between creep deformation parameters and creep life have been investigated for Mod.9Cr-1Mo steel (Gr.91, 9Cr-1Mo-VNb) by analyzing creep strain data at 723 K to 998 K (450°C to 725°C), 40 to 450 MPa, and t r = 11.4 to 68,755 hours in NIMS Creep Data Sheet. The time to rupture t r is reasonably correlated with the minimum creep rate _ e min and the acceleration of creep rate by strain in the acceleration region dln_ e/de, as t r = 1.5/[_ e min (dln_ e/de)], where _ e min and dln_ e/de reflect the creep behavior in the transient and acceleration regions, respectively. The _ e min is inversely proportional to the time to minimum creep rate t m , while it is proportional to the strain to minimum creep rate e m , as _ e min = 0.54 (e m /t m ). The e m decreases with decreasing stress, suggesting that the creep deformation in the transient region becomes localized in the vicinity of prior austenite grain boundaries with decreasing stress. The duration of acceleration region is proportional to the duration of transient region, while the dln_ e/de is inversely proportional to the e m . The t r is also correlated with the t m , as t r = g t m , where g is a constant. The present creep life equations reasonably predict the degradation in creep rupture strength at long times. The downward deviation takes place in the t r vs _ e min curves (Monkman-Grant plot). At the same _ e min , both the e m and t m change upon the condition of t m µ e m . The decrease in e m with decreasing stress, corresponding to decreasing _ e min , causes a decrease in t m , indicating the downward deviation of the t r vs _ e min curves.

Development of creep property equations of 316FR stainless steel and Mod.9Cr-1Mo steel for sodium-cooled fast reactor to achieve 60-year design life

“…This change was considered to be caused by the occurrence of homogeneous recovery in the elastoplastic creep in the short term and heterogeneous recovery in the elastic creep in the long term just as the creep strength abruptly decreased in the long term (Kimura, 2004). This change in the inclination of the Monkman-Grant plot was also reported by Abe (Abe, 2015). Therefore, the creep strain equation of Mod.9Cr-1Mo steel should be improved considering the changes in creep characteristics at high temperatures in the long term.…”

Evolution of Lath Substructure and Internal Stresses in a 9% Cr Steel during Creep

“…1,[3][4][5][6][7][8][9] The acceleration of tertiary creep due to extensive migration of low-angle boundaries is considered to be the main reason for the creep strength breakdown, i.e., a sudden drop of the allowed stress in the range of long-term creep. 1,[4][5][6]13) In general, the evolution of TMLS during creep have been addressed in several studies. 3,10,14,15) However, limited attention was paid to the evolution of lath boundaries with strain.…”

“…2(c)) that is indicative of the significantly slow processes at tertiary creep. 4,13,[22][23][24] The stress dependence of minimum strain rate is commonly expressed by a power law relationship of applied stress, σ :…”

“…However, the lath structure does not completely replaced by subgrains with stress-free boundaries and the creep strength breakdown was not observed under the studied conditions. [4][5][6]13,38) A dispersion of secondary phase particles plays an important role in the recovery of the dislocation lath structure. 1,3,4,7,11,14,16,22,28) A gradual detachment of the lath/subgrain boundaries from the boundary particles during creep promotes the knitting reaction and the release of internal stress and thus accelerates the creep rate.…”

Low Cycle Fatigue and Relaxation Performance of Ferritic–Martensitic Grade P92 Steel

“…Generally, these studies confirmed the above-mentioned trends and indicated qualitatively similar behaviors for all ferritic-martensitic steel grades. However, the number of studies dealing with fatigue and creep-fatigue of these materials is still quite limited, compared to the great efforts spent on characterizing the creep behavior [12], especially when considering the large parameter fields which would need to be covered for a thorough analysis of the material behavior, under the different combined loadings. Consequently, the amount of available fatigue data is quite restricted, as is reflected, for example, in the number of creep data and fatigue data sheets provided by NIMS in Japan [13].Recently, combined loading involving creep and cyclic fatigue loads has received increased attention due to the more flexible operation of power plants in many countries.…”