## 3 Results and Discussion
### Stress Relaxation Behavior  
It is well known that stress relaxation can occur in the holding period during the strain-controlled CF test [9; 10; 11; 12]. The typical stress relaxation behavior of the welded specimens during the tensile holding period (300 s) is shown in Figure 8. The magnitude of relaxed  
Figure 6: The representative strain-stress hysteresis loop of the welded specimen with a holding time of 300 s.  
Figure 7: Variations in measured inelastic strain for welded and BM specimens with different holding times at (**a**) the 1 st cycle and (**b**) 50% of the lifetime fraction. stress is directly related to the amount of accumulated creep deformation, contributing to the variation in the inelastic strains [19; 20]. To describe the stress relaxation behavior under different holding periods, the stress relaxation range \(\sigma_{\text{rel}}\) and stress level \(\sigma_{\text{th}}\) at the end of holding period were defined, as shown in Figure 8. Figure 9 compares the development of the stress relaxation range \(\sigma_{\text{rel}}\) for welded and BM specimens, which were subjected to different holding times. The data of five cycles around 50% of the cyclic life were used to describe the stress relaxation range. It can be seen that the stress relaxation range increased as the holding time increased for both kinds of specimens. The stress relaxation ranges of welded specimens were similar to those of the BM specimens, except for the one with a holding time of 30 s. As the number of cycles increased to 50% of the lifetime fraction, the stress relaxation range gradually increased for both kinds of specimens. Compared with the welded specimens, the degeneration of stress relaxation for the BM specimens was more remarkable. Figure 9b shows that the \(\sigma_{\text{th}}\) (stress level at end of holding period) for all of the welded and BM specimens at the first cycle and 50% of the lifetime fraction. Similarly, the data of five cycles around 50% of the cyclic life were used to describe the stress level. The \(\sigma_{\text{th}}\) for the BM specimens were observed to decrease with increasing holding times, and they gradually became saturated at approximately 70 MPa, as shown in Figure 9b. As for the welded specimens, the similar trend of \(\sigma_{\text{th}}\) was also obtained at the first cycle. However, after a rapid reduction in \(\sigma_{\text{th}}\) for the specimens with short holding times, they were slightly enhanced for specimens with long holding times (600 and 900 s). Furthermore, the values of  
Figure 8: Typical stress relaxation curves of the welded specimen with a holding time of 300 s at 50% of the lifetime fraction. Figure 9: (**a**) Stress relaxation range and (**b**) stress level of all the welded and BM specimens at the 1 st cycle and 50% of the lifetime fraction. \(\sigma_{\text{th}}\) for the welded specimens were higher than the values for the BM specimens, especially for the specimens with long holding times. It has been speculated that the inhomogeneity in the welded specimen could be responsible for the higher values of \(\sigma_{\text{th}}\). Since the CF experiments were all strain-controlled, the FGHAZ and BM regions (lower hardness) could have promoted more remarkable local deformation during tensile or compressive loading. During the holding period, these two regions firstly began to undergo stress relaxation due to their relatively lower hardness. As outlined above, an excessive Mo content could reduce the ductility and stability of the WM region, leading to a more remarkable reduction in the hardness of the WM region during the CF tests. Therefore, the WM region (higher hardness) would be continuously producing inelastic strain to compensate for the local stress during the holding period. This process also results in a more remarkable reduction in the hardness of the WM region, which was observed in the previous study [10].