- Original Article
- Open Access
Impact Fretting Wear Behavior of Alloy 690 Tubes in Dry and Deionized Water Conditions
© The Author(s) 2017
- Received: 29 November 2015
- Accepted: 20 April 2017
- Published: 26 May 2017
The impact fretting wear has largely occurred at nuclear power device induced by the flow-induced vibration, and it will take potential hazards to the service of the equipment. However, the present study focuses on the tangential fretting wear of alloy 690 tubes. Research on impact fretting wear of alloy 690 tubes is limited and the related research is imminent. Therefore, impact fretting wear behavior of alloy 690 tubes against 304 stainless steels is investigated. Deionized water is used to simulate the flow environment of the equipment, and the dry environment is used for comparison. Varied analytical techniques are employed to characterize the wear and tribochemical behavior during impact fretting wear. Characterization results indicate that cracks occur at high impact load in both water and dry equipment; however, the water as a medium can significantly delay the cracking time. The crack propagation behavior shows a jagged shape in the water, but crack extended disorderly in dry equipment because the water changed the stress distribution and retarded the friction heat during the wear process. The SEM and XPS analysis shows that the main failure mechanisms of the tube under impact fretting are fatigue wear and friction oxidation. The effect of medium(water) on fretting wear is revealed, which plays a potential and promising role in the service of nuclear power device and other flow equipments.
- Impact fretting wear
- Alloy 690
- Oxidative wear
- Fracture appearance
As a special type of damage occurring at the contact surface, fretting can cause rapid crack formation of working components and lead to premature service failures . According to the directions of the relative motions, four basic fretting modes exist, namely, tangential, radial, rotational, and torsional [2, 3]. These four models belong to the condition matched interface. Nevertheless, only a few studies focus on the separation of fretting wear modes, particularly of impact wear. Impact wear is a subtle amount of vibration that occurs on the surfaces of two objects. Numerous studies have reported that repeated impact fretting produces cracks and that the cracks propagate and cause the objects to fail. Zhao, et al , postulate delamination theory of wear, which is mainly concerned with deformation below the surface. Xin, et al , investigate that there are five layers: oxide layer, mixed layer, TTS layer, plastic deformation layer and base materials in the fretting wear subsurface. Sato, et al  describe the significant differences in impact-fretting wear by comparing impact-fretting wear with pure impactor with fretting, which show that dynamic corrosion followed the parabolic law of oxidation of metals and the thermal activation process.
Numerous fretting damages exist at various parts of nuclear power systems , such as reactor fuel assembly, control rod assembly , reactor component, steam generator, pressure vessel , main pump, and coolant pump. The steam generator is a key equipment in nuclear power systems, and fretting damage is one of the main reasons  causing its failure. With high thermal strength, good corrosion resistance, anti-oxidation, and other characteristics, alloy 690 is extensively used in the nuclear power and aerospace fields. In nuclear power plants, U-tubes in the steam generator are supported by structures called egg crates. Flow-induced vibration of the U-tubes causes wear to occur on the zone of contact and generates combinations of impacting and sliding motions between the U-tube against the support [11–14]. Recently, most of the domestic and foreign studies on wear have concentrated on the sliding wear or bending behavior of the steam generator tubes. In another study, bending (four-point or three-point bending) has been used to determine the damage behavior of tubes or rods . Gueout, et al  report that wear of pure sliding is larger than impact wear in the case of anti-vibration bar testing. In particular, the wear amount of impact sliding increase more than that of pure sliding or impact wear test. Jeong, et al , indicate that the friction coefficient in air is higher than that in water. The friction coefficient and wear rate increased as the temperature of water increase in the water environment. Chung, et al , conclude that the wear coefficient in ambient room temperature is lower than 80 °C in water conditions and explain that the protective nature of the tribologically transformed layers could decrease the wear volume. Most of study focus on the tangential fretting wear of alloy 690 tubes, and research on impact fretting wear of alloy 690 tubes is limited. The impact fretting wear behavior of alloy 690 is significant in the evaluation of the life of steam generator tubes in nuclear power plants and in better understanding the wear mechanisms of steam generator tube materials.
In this study, an impact fretting wear simulator was demonstrated to elucidate the impact wear behavior under the dry and deionized water conditions at room temperature.
2.1 Specimen Preparation
Chemical composition of wear tested materials (wt%)
2.2 Impact Fretting Test
As shown in Fig. 1, alloy 690 tube was fixed in a “V” groove fixture, the load sensor was connected to the vibrator, and the plate specimens were installed under the load sensor by the upper fixture. During the test, the impact wear test used a force control mode. The impact force could be changed by adjusting the magnitude of the current. When the tube was impacted by the plate, the impact contact force can be measured by the load sensor. The impact block was raised when the peak force was reached. Two types of lubrication conditions, namely, dry and water(deionized water), were obtained at room temperature. A frequency of 5 Hz of the 1 mm impact distance and 106 cycles were selected in all the tests, and the applied normal peak loads were set as 20 N, 30 N, 40 N, 45 N, and 50 N. A visual monitoring device observed the status of the interface during the test. If a macroscopic crack was observed on the tube surface, then the test was stopped and the computer recorded the cycle number.
2.3 Analysis Methods
After the wear test, the wear scar was observed by using the white light interferometer(Contour GT type) and the wear area was calculated. Various surface imaging and chemical analysis techniques were conducted to reveal the wear and damage mechanisms. These techniques included optical microscopy (OM, OLYMPUS BX50 Japan), scanning electron microscopy (SEM, JEOLJSM-6610LV), energy-dispersive X-ray spectroscopy(EDX, OXFORD X-MAX50 INCA-250), and X-ray photoelectron spectroscopy(XPS, Thermofisher Scientific, ESCALAB 250Xi).
3.1 Wear Behavior
Number of impact cycles in different loads and tube (× 105)
Tube length L/mm
Impact load F n /N
With the increase in the applied normal load, the time or fracture is shorter.
The 10 mm long tubes easily crack in the same condition. For example, after increasing the length of the tube, the number of fracture increases from 3 × 105 cycles to 9 × 105 cycles because long tubes have better flexibility and can absorb more energy from wear and impact processing.
- (3)Compared with the impact of tube in dry and deionized water, when the tube cracks in dry water, the deionized water lubrication can significantly delay the cracking time. The cycle of crack appearance increases from 1.3 × 105 to 1.8 × 105 when the impact load set as 50 N. Figure 2 shows the OM and 2D profiles of the wear scar of the 15 mm long tube in dry water. Increasing the impact loads results in serious wear of the morphology of the tube. Particularly when the load increases to 45 N, the tube cracks until the end of the test. Wear depth increases from 1 µm to 10 µm when the impact load increase from 20 N to 50 N. The uplift phenomenon is observed at the edge of the wear surface. This phenomenon indicates that the extrusion deformation of material occurs during impact fretting wear and remains negligible in smaller load.
3.2 Fatigue Behavior
The impact wear gradually increased with the increases in impact load from 20 N to 50 N. Oxidative wear and delamination are the dominant mechanisms of wear. The oxidative wear decrease apparently because of the protection of the fluid.
Cracking occurs at high loading and the tube with a short length. However, the deionized water lubrication can significantly delay the cracking time.
The main failure mechanism during impact fretting wear is fatigue wear in water and oxidation wear in the dry condition. The crack of the wear surface is different in the dry and deionized water condition. Stress corrosion has an effect on the cracking behavior in water.
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