Long term ageing effect on fracture toughness of the GTAW welded joints for nuclear power main pipelines
Introduction
In nuclear industry, a large number of welded joints are utilized for connecting structural components. The shielded metal arc welding (SMAW) and the gas tungsten arc welding methods (GTAW) are the commonly used methods to achieve the main pipelines connection in nuclear power plants (NPPs) [1,2]. Since the simplicity and low cost of the filler, the SMAW technique was widely used in NPPs during the past decades, despite its defective quality and stability [[3], [4], [5]]. However, since the cross-sectional areas of welding grooves and the filling amount of welded metals can be both reduced [6,7], GTAW method has been broadly applied in coolant piping system of the second-generation plus and even third-generation NPPs. Furthermore, this method has a satisfactory efficiency with a low heat input during GTAW welding process [8,9].
It is common to choose ER316L/ER316LSi as the welding consumable materials in industry. Even though the wrought fillers are completely austenitic, duplex structure could form during the welding process and then austenite and ferrite phases both exist in the weldment [[10], [11], [12]]. The existence of ferrite phase could improve the material performance, such as the strength and the stress corrosion cracking resistance [[13], [14], [15]]. Nevertheless, thermal ageing embrittlement will be obtained in duplex steels during long term operation because of NPPs operation temperature [[16], [17], [18], [19]].
Important studies on the thermal ageing effect of the welded metal of the nuclear power main pipelines have been made by S. Hong et al. [20,21]. Comparing with the difference in welding process, various filler metals have an insignificant influence on the fracture properties of the stainless steel welds [22]. Generally, the same thermal ageing mechanism exists in both austenite stainless steel welds and casting austenite stainless steels (CASS). However, the variation of mechanical properties is mainly caused by the carbides at grain/phase boundary and second-phase inclusions owing to the limited ferrite phase content in welded joints [14,23,24]. Hence, it is meaningful to investigate the crack initiation and growth mechanisms owing to smaller ferrite grains, more grain/phase boundaries and inclusions, especially for long term operation. Furthermore, in view of the time limit analysis (TLAA) needs for the long term operation (LTO) of NPPs, especially on the condition of the end of life which plans to extend to 60–80 years, it is necessary to obtain the fracture toughness variation of welded joints during long term operation.
Therefore, a detailed study has been undertaken in the present paper to characterize the quasi-static fracture properties, such as J-resistance (J-R) curves for the long term thermal aged GTAW welded joints, to investigate fracture failure mechanisms and then examine whether the fracture toughness is still sufficient for the license renew or LTO requirements.
Section snippets
Material
GTAW method was used to complete the multipass circumferentially butt weld between two Z3CN20.09 M nuclear power main pipelines (Ø935 mm × 76 mm thickness × 250 mm length), in which ER316L/ER316LSi was used as weld metal. The chemical compositions and typical microstructure of various positions in the welded joints are shown in Table 1 and Fig. 1 respectively. Table 2 presents the specific welding specifications for each process. It is found that both base metal (BM) and welded metal (WM)
Results
Fracture toughness tests are performed on GTAW welded joints after different thermal ageing times and then its force-crack mouth opening displacement curves are shown in Fig. 4. Generally, the force capacity in WM is higher than that in BM. Tensile strength of BM was lower than WM for this type of welded joints. After thermal aging, both yield strength and tensile strength were increased, but ductility became poor [25]. Therefore, it also leads to a decrease for the force capacity in fracture
Crack initiation and growth
As shown in Fig. 6, the crack initiation is obtained at the crack tip of the welded joints without thermal ageing and after 30000 h thermal ageing, respectively. Austenite phases matrix suffers more welding residual stress than ferrite phases after GTAW welding process and thus cracks initiate mostly in austenite phases of the welded joints without thermal ageing. Obviously, the long-term thermal ageing has no significantly influence on the location of crack initiation at the crack tip, wherein
Conclusions
The accelerated thermal ageing tests were performed on GTAW welded joints of the nuclear power main pipelines in NPPs at 400 °C from 0 h to 30,000 h to investigate fracture properties at different locations of the welded joints. Based on the experimental results, the conclusions obtained in this study were summarized as follows:
- (1)
The fracture toughness JIC of both base metals and the welded metals in the GTAW welded joints decreases clearly with increasing thermal ageing time, but J-R curves
CRediT authorship contribution statement
Songwei Wu: Investigation, Writing - original draft, Resources. Zhe Zhang: Data curation, Writing - review & editing, Project administration, Supervision.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by the science and technology project of Quzhou (2017G11).
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2023, Materials Chemistry and PhysicsCitation Excerpt :It generally refers to incessant, non-reversible changes in the composition, and morphology of materials and their structures operating in high temperatures that they are likely to face while they are in operation. Generally in GTAW/MIG welded joints, the fracture toughness decreases with thermal ageing time [235]. Youn et al. [236] reported that during GTAW of 316 SS, ductility decreases while tensile strength increases with thermal ageing.