Fretting Fatigue Test and Damage Mechanism Analysis of Nuclear Power 690 for Alloy Heat Transfer Tube

In the nuclear reactor system, the steam generator is one of the most critical main equipments of the nuclear power plant. The core and weakest part of the steam generator is the heat transfer tube underneath it. The heat transfer tube of the steam generator undertakes the important function of energy exchange between the primary and secondary circuits and ensuring the integrity of the pressure boundary of the primary circuit under high temperature and high-pressure conditions, and requires high reliability. The heat transfer tube of the nuclear power steam generator suffers fretting fatigue under the action of slight abrasion and alternating load, which leads to the initiation and propagation of surface cracks on the heat transfer tube. In severe cases, the heat transfer tube will rupture and affect the safety of the reactor. To study the effect of radial load and axial alternating stress on the fretting fatigue life and damage mechanism of 690 alloy tubes, a fretting fatigue test was carried out on 690 alloy tubes. The research obtained the fretting fatigue life curve of 690 alloy tubes. Comparing the fitting curve and the empirical formula, in the fretting fatigue experiment, the variation of radial load had a greater impact on fatigue life. Comparing the fretting fatigue test data with the ANL test results, it was found that all the test data were above the design curve, reflecting the better fretting fatigue performance of the 690 alloy tube. Three-dimensional morphology and scanning electron microscopic observation on the wear scar surface of the 690 alloy tube specimens under different loads were performed to analyze the damage mechanism of the worn surface. From the scanning electron microscope image of the wear scar surface, the wear scar was covered by a discontinuous layer of wear debris, and there were furrows around the wear debris layer. The variation in axial load had no significant effect on the microstructure of the wear scar. As the normal load increased, the wear and the area of the wear scar significantly increased, and delamination cracks appearred. The macroscopic and microscopic morphology of the 690 alloy tube specimen fracture under different loads were characterized. The crack initiation, crack propagation process, and damage mechanism under fretting fatigue were analyzed. The results showed that the wear mechanism of the 690 alloy tube against the 403 anti-vibration strip was delamination and abrasive wear. In the fretting fatigue test, the variation of radial load had a greater impact on the fretting fatigue life. From the perspective of macroscopic fracture morphology of the 690 alloy tube at the fretting wear under the action of radial load, the nucleation and propagation of the cracks under the action of axial load expansion eventually led to fracture. From the microscopic fracture morphology, the fretting fatigue failure mode of the 690 alloy tube was cleavage fatigue fracture. There were obvious river-like stripes in the fatigue crack propagation area. To reduce energy consumption during the cleavage crack propagation process, the pattern of rivers tended to merge into large rivers. At constant radial load, increasing the axial load produced more cracks in the fatigue source area, and the formation of crack groups prompted the crack growth rate and thereby reduced the fatigue life. Under the same axial load, increasing the radial load increased the oxidation of the sample fatigue source area and the degree of plastic deformation. However, the local plastic deformation under a larger contact pressure generated residual stress, which improved the distribution of local stress.