Studies On Tangential Fretting Wear Mechanisms Of Steam Generator Tubes In Nuclear Power Systems

Energy problem has been limiting the sustainable development of domestic economy and society, as a strategically severe problem, the solution of which lies in safe and efficient use of nuclear energy. Steam generator (SG) tube is an important component in nuclear power plants. SG (Steam Generator) is one of the major equipment of nuclear power systems, and heat exchanger tube is a key component of SG. Inside the SG of nuclear power plants, the flow-induced vibrations usually cause an oscillatory movement with the relatively small amplitude between heat exchanger tubes and their supports, where fretting wear might occur due to the combined effect of stress and media. The phenomena would lead to a reduction of U-tubes life time. Therefore, it is of great importance to prevent the damages of the heat exchanger tubes in order to enhance the safety and service life of the nuclear power equipment. Thus the research on the tangential fretting under alternating loads is not only of significance to realize and understand the mechanisms of the fretting damages, but also can provide theoretical supports and practical guides for engineering application of nuclear power equipment in the resistance against fretting.On the basis of hydraulic fretting test system, a new-style device for the fretting wear test rig, by employing the radial cyclic loading device and control system, has been developed, which can actually simulate the fretting wear process under alternating loads in high temperature and controlled atmosphere environment. The tester presents high stability, and its test results indicate a better comparability and repeatability.The fretting tests have been carried out by the new-style device. The systematic studies have been conducted on the fretting wear behavior of two typical heat exchanger tubes (Inconel690and Incoloy800) in different environments (air, high-temperature air, high-temperature nitrogen, water, and alkaline ionized water). The fretting running behaviors and damage mechanisms of the fretting have been studied systematically, based on the frictional dynamic behavior analyses and by means of the optical microscopy (OM), laser confocal scanning microscopy (LCSM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and triboindenter in-situnanomechanical test system. The main research work and obtained conclusions are listed as follows:1. The running and damage mechanisms of the fretting for Inconel690under alternating lo-ads conditions in high temperature (300℃) and controlled atmosphere environment(1) The running behaviors of the fretting of Inconel690alloy have been investigated in detail. The research revealed that the fretting running behaviors were closely related to the radial frequency. In parallelogram shaped Ft-D curves, the friction fluctuates periodically, and accordingly, the fretting was running in the slip regime (SR).(2) Five stages of friction force curves could be observed in different environments, i.e. initial stage, ascending stage, peak value, descending stage and steady stage. Under the same conditions, the oxygen content of the atmosphere and temperature had significant effect on the friction force. The steady-state friction force in air at room temperature was higher than that at300℃, while much lower than that in nitrogen at300℃.(3) The damage behaviors of the fretting for Inconel690strongly depended on the normal load, displacement amplitude, temperature, atmosphere and radial frequency in different environments. By observing the damage morphology, the radial frequency and atmosphere had a major impact on the morphology, chemical composition and structure of the wear debris. The delamination phenomenon became severer due to the superposition effect of the fretting under alternating loads. Specifically, compared with the single mode of the tangential or radial fretting, more severe material failure would be induced under the combination of the two modes. There were significant differences in the fretting damage mechanisms for Inconel690in different environments:The abrasive wear and delamination were the major mechanisms of Inconel690in air at room temperature. The wear mechanisms were mainly abrasive wear, oxidation wear, and delamination in air at300℃while in nitrogen at300℃, the abrasive wear and delamination were the main mechanisms.2. The running and damage mechanisms of fretting wear for Inconel690in water and alkal-ine ionized waters(1) In gross regime, the friction coefficient was reduced because of the lubricating effect of water and alkaline ionized water. It was also found that friction coefficient depended on the medium property and test parameters. The friction coefficient was lower in water than that in alkaline ionized water, while the friction coefficient increased with the increase of solution temperature.(2) The fretting wear behavior of Inconel690in alkaline ionized water was affected by the displacement amplitude, normal load and especially the temperature. The increasing temperature promoted the absorption reaction of hydrazine and dissolved oxygen, and obviously reduced oxidative corrosion rate, but the compactness and stability of the oxide films were weakened. Consequently, fretting wear became more serious. The abrasive, oxidative wear and delamination were the main mechanisms of Inconel690in water. In alkaline ionized water, whereas abrasive wear and delamination were the dominant mechanisms.3. The running and damage mechanisms of fretting wear for Incoloy800in air (1) The fretting running regimes and damage behaviors of fretting were strongly dependent upon the normal load, displacement amplitude, temperature and material properties. It was found that temperature had no significant effect on the fretting running regime.(2) In the partial slip regime (PSR), the wear scar appeared in shape of the annularity, and little damage was observed in the contact center due to the sticking. The micro-slip, slight wear and microcracks occurred in the oval ring at the contact boundary zone. The damage mechanisms were mainly slight abrasive wear and microcracks.(3) The mixed fretting regime (MFR), the debris of the crushed metallic detached particles at the contact zone were formed due to intense plastic deformation, and the debris congregated at the contact zone at room temperature. As the increase of test temperature, a large number of debris layers, produced during the fretting wear processes, could easily congregate and adhere at the contact zone. The plastic flow characteristics accompanied by adhesion traces and high temperature oxidation could be observed in wear scars at400℃. In the MFR, the damage mechanisms were mainly abrasive wear, oxidation wear and delamination.(4) In the slip regime (SR), the particles were detached by delamination mechanism at room temperature, causing severe damages. However, some phenomena of the layered structure and plastic flow characteristics could be observed on the wear surface at high temperature. The damage mechanisms were mainly abrasive wear, oxidation wear and delamination in the SR.