## 3 Results and Discussion  
### Cyclic Softening Behavior  
Figure 4 exhibits the cyclic life of CF specimens (P92 steel and welded joint) with different holding times. All of the CF specimens were ruptured in the BM regions. Furthermore, the distance of the main crack from the fusion line decreased from 7.2 mm to 1.7 mm for the welded specimens as holding time increased. These results show that the cyclic life of P92 steel decreases as holding time increases. This trend was also observed for welded-joint specimens, except for the specimens with a holding time of 30 s. Compared with the cyclic life of P92 steel, the premature failure only occurred in the welded specimens with holding times of 30 s and 900 s. This observation is different from the reported phenomenon that creep-fatigue interaction can result in the welded joints of 9% Cr steel prematurely fracturing [5; 6]. The differing definitions of cyclic life could be the reason for this observation. The previous studies [5; 6] proposed the definition of cyclic life, which was defined as a reduction of 10% of the peak tensile stress of the CF specimen at the secondary softening stage. In order to investigate the fracture mechanism, the behaviors of main-crack propagation should be considered in this work. Therefore, the cyclic life of the CF specimens that were used in this investigation was defined as the cycle at the time of occurrence of the actual fracture. The cyclic damage can be characterized by the measured peak tensile stress per cycle during the CF tests. Figure 5 shows the variations in the measured peak tensile stress values during the creep-fatigue test with different holding times for both specimens. Three distinct softening states were observed: the rapidly softening stage, the steadily softening stage and the failure stage. At the rapidly softening stage, the peak tensile stress rapidly decreased from 250 MPa to 170 MPa, indicating that enhanced dislocation annihilation and degenerated dislocation multiplication were simultaneously occurring. Then, the creep-fatigue interactive progress changed to the steadily softening stage, wherein the annihilation and multiplication of  
Figure 4: Cyclic life of CF specimens (P92 steel and welded joint) with different holding times. Figure 3: Schematic representation of creep-fatigue test. dislocations were gradually in dynamic equilibrium, which led to a slight decrease in peak tensile stress from 170 MPa to 150 MPa. Finally, macroscopic cracks were formed, and the peak tensile stress began to sharply decrease at the failure stage. In this stage, an interesting phenomenon occurred. At short holding times (30 and 120 s), the failure stage of the welded specimens occupied 45% of the lifetime fraction, while it only took up 20% of the lifetime fraction of the BM specimens, as plotted in Figure 5a. After the normalization of cyclic life (N/Nf), Figure 5a shows that the macroscopic cracks were prematurely initiated in the welded specimens with short holding times. Figure 5b shows the peak tensile stress of BM and welded specimens with long holding times (300, 600 and 900 s) during the CF test. It should be noted that only two softening stages were observed for both kinds of CF specimens with long holding times (300, 600 and 900 s). The absence of a third softening stage in long-held specimens indicates that the processes of macroscopic-crack initiation, propagation and rupture are too rapid to be measured through the variation in peak tensile stress per cycle. This observation also suggests that the fracture mechanism could be gradually changed by increasing holding times for both kinds of specimens. On the other hand, the peak tensile stress levels of the welded specimens were enhanced in comparison to the BM specimens with the same holding time, as shown in Figure 5b. The peak tensile stress levels of the welded specimens were observed to increase with increasing holding time, while they remained mostly the same for all of the BM specimens.