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.
Highlights
Due to the attempt to reduce carbon dioxide emissions, fossil fuel and nuclear power plants will be operating at higher temperatures, leading to an increase in the requirement of material properties and structural-integrity assessments
Advanced alloys like 9% Cr steel have been extensively utilized in power units due to their excellent creep resistances, their welded joints are regarded as being weak regions that could be the sources of creep-fatigue failure during service
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
Summary
Due to the attempt to reduce carbon dioxide emissions, fossil fuel and nuclear power plants will be operating at higher temperatures, leading to an increase in the requirement of material properties and structural-integrity assessments. Advanced alloys like 9% Cr steel have been extensively utilized in power units due to their excellent creep resistances, their welded joints are regarded as being weak regions that could be the sources of creep-fatigue failure during service. 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. 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. Based on the observation of fracture surfaces, the effects of the holding period on the transformation of fracture mechanisms of welded joints under creep-fatigue loading were systematically studied. Sci. 2021, 11, 9983 the BM and FGHAZ regions, while coarser prior austenitic grains were seen in the CGHAZ and WM regions
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