## 3 Results and Discussion
### Cyclic Softening Behavior
In the previous study [10], the WM region had a higher value of nanoindentation hardness than the BM and FGHAZ regions. Therefore, the average values of hardness for welded specimens were higher than those of the BM specimens. Since the creep-fatigue experiment is strain-controlled, the peak tensile stress of the welded specimen could be higher than that of the BM specimen at the same strain (0.20%). Furthermore, this change in peak tensile stress level can be related to the discrepancy of inelastic strain between BM and welded specimens. Figure 6 shows the typical strain_stress hysteresis loop of the welded specimen with a holding time of 300 s. It schematically indicates that the inelastic strain amplitude can be directly obtained from the strain-stress hysteresis loop. The creep-fatigue behaviors at the first cycle and the cycle equivalent to 50% of the lifetime fraction are widely reported to be important to the investigation of the damage mechanisms [4; 19; 20; 23]. Figure 7 shows the variations in measured inelastic amplitude during the creep-fatigue test for all of the BM and welded specimens. The inelastic strain amplitude of BM specimens was observed to increase from 0.190% to 0.230% with increasing holding time at the first cycle. Compared with the BM specimens, the enhanced inelastic strain amplitude (0.250% to 0.290%) was estimated for the welded specimens at the first cycle, as shown in Figure 7a. In the previous study [13], the local hardness of the WM and CGHAZ regions were higher than that of the BM and FGHAZ regions before CF testing. Hence, the BM and FGHAZ regions in welded specimens could be subjected to more severe deformation compared with  
Figure 5: Variations in measured peak tensile stress during the creep-fatigue test with (**a**) short holding times (30 and 120 s) and (**b**) long holding times (300, 600 and 900 s). the BM specimens at the first cycle of the strain-controlled CF tests, resulting in enhanced inelastic strain amplitudes. Figure 7b shows that the inelastic strain amplitude increased to 0.255% (30 s), 0.262% (120 s), 0.270% (300 s), 0.280% (600 s) and 0.292% (900 s) for the BM specimens at 50% of the lifetime fraction. Furthermore, the inelastic strain amplitude of the welded specimens was almost unchanged at 50% of the lifetime fraction, except the one with a 900 s holding time, which slightly decreased to 0.273%. In the previous study [10], the reduction in the hardness of the WM region was more remarkable than in the BM and FGHAZ regions, even though the WM region still had a higher value of hardness due to its high initial hardness. This reduction can be related to the Mo content in the alloy. The chemical composition of the WM zone is dominated by the P92 wire, with a higher Mo content (0.940%) than the P92 steel (0.374%). However, excessive Mo content could reduce the ductility and stability of alloys at high temperatures [24], leading to a remarkable reduction in the hardness of the WM region. The reduction in the hardness of the WM region could gradually induce more deformation to compensate for the local deformation of the FGHAZ and BM regions during the strain-controlled CF test. This process could result in the unchanged inelastic strain amplitudes of welded specimens at macroscopic levels.