###### Abstract  
To understand the premature-fracture mechanisms of long-term service damage of an advanced alloy's (Chinese P92 steel) welded joint, the creep-fatigue (CF) experiments with holding times of 30, 120, 300, 600 and 900 s were individually performed at 923 K. The cyclic softening, inelastic-strain amplitudes and stress-relaxation behaviors were compared between welded and base-metal (BM) specimens. From the results, the failure stage of the welded specimens occupies 45% of the lifetime fraction, while it only takes up 20% of the lifetime fraction in BM specimens with short holding times (30 and 120 s). Furthermore, only two softening stages were observed for both kinds of CF specimens with long holding times. The absence of a third softening stage in longer-held specimens indicates that the processes of macroscopic-crack initiation, propagation and rupture were accelerated. Based on the observation of the fracture surfaces, the fracture mechanism shifted from fatigue-dominated damage to creep-fatigue interaction when the holding period was increased. P92 steel; welded joint; creep-fatigue interaction; fracture +
Footnote †: journal: Applied Sciences  
the homogeneous base metal. [PERSON] et al. [8] studied the formation mechanism of various microstructures in different regions of a P92 steel welded joint and found that the welding process produced a heterogeneous microstructure in the heat-affected zone (HAZ) of the welded joint. Those regions have different sensitivities to the macroscopical CF loading [9; 10; 11; 12]. Relying on nanoindentation, [PERSON] et al. [13] investigated the remnant creep behavior of the base metal (BM), fine-grain heat-affected zone (FGAZ), coarse-grain heat-affected zone (CGHAZ) and weld metal (WM) for a CF-tested P92-welded joint, and they found remarkable degradations in the creep resistances of the BM and FGAZ regions. Furthermore, the nucleation and propagation of creep voids are more likely to occur in the FGAZ [14; 15; 16], which was defined as the main reason for the premature failure of the welded joint. On the other hand, creep and fatigue are two competitive factors, which can result in the fatigue-dominated fracture mechanism rather than the creep-fatigue damage mechanism. When the factors of creep-fatigue loading such as holding time, strain rate, strain amplitude and holding mode are changed, the dominant fracture mechanism and the ruptured position in the welded joint are varied. The complexity of creep-fatigue loading and the inhomogeneity of welded joints cooperatively impede the development of the life-prediction method for CF-tested welds. The life-prediction methods for fatigue fracture and creep-fatigue fracture are totally different [17]. Based on the linear damage rules, the nonlinear damage curve and two-stage linearization approaches, the life-curve modification methods, the approaches based on crack-growth concepts, the continuum-damage mechanics models and energy-based theories, the fatigue life-prediction methods have been widely used for metals and their alloys [18]. As for the life prediction of creep-fatigue fracture, [PERSON] and [PERSON] have each proposed a modified strain energy density exhaustion model for creep-fatigue life prediction, which can accurately predict the cyclic life of CF-tested high-Cr steels [19; 20]. However, if the fatigue life-prediction method is selected to predict the cyclic life of the creep-fatigue-ruptured welds, the accuracy of life prediction can be significantly reduced. Therefore, it is important to study the transformation of the fracture mechanism under different creep-fatigue loadings, so as to provide the experimental data support to the investigation into creep-fatigue life-prediction methods for welds. This paper focuses on the fracture mechanism of Chinese P92 steel welded joint under CF loading. The strain-controlled CF experiments with holding times of 30, 120, 300, 600 and 900 s were individually performed on welded specimens at 923 K, as well as on base-metal (BM) specimens.