ASME STP-NU-013-2008 pdf download.IMPROVEMENT OF ASME NH FOR GRADE 91 NEGLIGIBLE CREEP AND CREEP FATIGUE.
5.3 Creep Fatigue Tests in Vacuum Continuous fatigue tests and fatigue tests with hold times under vacuum at 593˚C are reported in . The benefit of the vacuum environment in fatigue life is more marked in continuous fatigue tests. With tensile hold times of 1 hour or more, the increase in fatigue life under vacuum is small, if any. Nevertheless, the fatigue life is reduced by tensile hold times when compared to continuous fatigue results in vacuum and in air. Other tests under vacuum at 600˚C are reported in  and . A marked reduction in fatigue life is observed only for unsymmetrical cycles with tension hold time or with longer tension going time than compression going time. The frequency effect is small in the case of symmetrical cycles. Compressive hold time cycles show a slight life reduction from a symmetric continuous cycle, bringing a transgranular type of cracking in the specimen . At 593˚C and 600˚C under vacuum, which eliminates or drastically reduces the oxidation effect, the observed reduction in fatigue life in Mod 9Cr-1Mo from creep-fatigue tests seems to be similar to that found in austenitic stainless steels. In addition, metallographic indications of creep cavities and intergranular path for cracks are observed in the case of fatigue life reducing cycles. At such temperatures, true creep fatigue interaction can probably be studied, after elimination of tests with compressive hold times in air. The analysis of this latter kind of tests should be performed in the frame of clarification of environment effects on fatigue. At lower temperatures (550˚C and 500˚C) there is a lack of creep fatigue results under vacuum (or under protective environment) to direct the selection of relevant data for analysis of true creep fatigue interaction.
5.5 Effects of Prior Aging Long term aging (50,000 hours and 75,000 hours at 538˚C and 482˚C) was applied to Mod 9Cr-1Mo samples to evaluate its effect on impact toughness, material microstructures and tensile properties . Complex and various changes in microstructures (carbides M23C6, Laves phases, NbC, VC, recovery and recrystallization zones adjacent to grain/subgrain boundary) result in degradation of impact transition temperature and increase in intergranular cracking tendency. The effect of aging on tensile properties is moderate at 482˚C with, perhaps, a tendency to hardening. Softening is detected after aging at 538˚C and 593˚C (50–60 MPa reduction in yield strength, and 80–90 MPa reduction in tensile strength). At 538˚C, the major part of the aging effect is obtained in 50,000 hours; tests after 75,000 hours do not show significant further evolution. The reduction in the tensile properties after aging is taken into account in ASME Subsection NH in accordance with NH-2160 and Table NH 3225-4 for Mod 9Cr-1Mo. Corresponding provisions are to be introduced in RCC-MR. The sudden occurrence of cycles of fatigue or creep fatigue after long times at temperatures as high as 538˚C or 593˚C is not a very relevant situation for the design of HTRs. Nevertheless, it is interesting to investigate why such pre-aging affects continuous fatigue, and creep fatigue, life. In continuous fatigue at 538˚C, with 0.7% strain amplitude, the aging effect is not clearly detectable. For 0.5% straining, the results of aged materials is on the lower part of the scatter band of unaged material. After 50,000 hours of aging at 593˚C, continuous fatigue life is clearly lower than the lowest life for unaged material.
The softening effect due to aging also reduces creep time to rupture . The data on aged materials are not sufficient to quantify this aging effect in term of stress to rupture which can be used to re- evaluate the creep damage accumulated during creep fatigue tests with hold times. But it is of interest to note that: • In the case of evaluation of test results any kind of softening can explain the underestimation of creep damages when the reference data are those of non-softened materials; this produces low representative points in the creep fatigue interaction diagram. • In the case of creep fatigue design works, it is not necessary, in principle, to take account of the stress to rupture after aging if the creep fatigue interaction diagram is defined based on tests including this material condition. It should be ensured, however, that procedures used in design assessment and interpretation of experimental test results are consistent in terms of how aging is taken into account. In contrast to design for normal service conditions, for which aging is already taken into account, in the long term creep stress to rupture, emergency and faulted situations may require the use of short term creep stress to rupture (presumably short term creep data of maximum 1000 hours). For such situations, smaller values due to reduction by aging (or softening in general) are to be considered as it is the case for tensile strength.