Fretting Wear Mechanism of TP316Ti in High Temperature Dry Condition and Liquid Lead-Bismuth Environments

The lead cooled fast reactor, which uses Lead-Bismuth eutectic (LBE) as a high transfer media is considered to be the most promising fourth generation reactor type for commercial application. However, there are also various material service damage issues that affect the safety of equipment operation, especially fretting damage. In order to solve out the typical fretting wear problem in Lead bismuth fast reactor, a self-developed liquid lead bismuth environmental fretting wear testing system was made out. Futhor more, the motion above was used to conduct fretting wear experiments at different temperatures and displacement in both dry and liquid lead bismuth environments. After completing the experiments, various methods such as scanning electron microscopy (SEM) and white light interferometer were used to characterize the fretting damage, analyze its wear behaviors, and study the fretting damage mechanism of TP316Ti stainless steel. The results showed that the Ft-D-N curves exhibited different characteristics at different temperatures and environments. The temperature rise caused the fretting towards the slip condition in LBE environment, possibly due to the involvement of liquid metal in loading and lubrication, especially at higher displacement and higher temperature. In the same environmental conditions, compared to partial slip fretting condition, the degree of damage to the fretting interface was significantly more severe under the complete slip fretting condition. This was because the frictional materials in the partial slip zone did not experience relative slip, the friction coefficient was small, and the contact surface damage was slight. But the relative displacement of the frictionals material in the complete slip zone was relatively large. The frictional materials germinated a large number of microcracks and propagated parallel to the surface until peeling off under the continuous action of normal stress and shear stress. As the experiment progressed, the peeled materials were repeatedly crushed and crushed to form a layer of debris, and the degree of damage to the fretting interface was relatively severe. In the dry condition, elevated-temperature enhanced the plasticity of the materials, leading to adhesion and material transfer, and main forms of damage transform from fatigue wear to adhesive wear and intensify oxidation wear. When the temperature rised to 450 ℃, adhesion points, which were caused by severe oxidation of the fretting interface were sheared and fractured on the surface of the weak contact area. As a result, the extent of material transfer diminished, leading to a reduction in the severity of damage. In the LBE environment, the flowing metal helped to expel debris and expanded the actual contact area with the grinding material. At the same time, local adhesion leaded to material transfer. Besides, under the influence of the LBE, the dissolution and corrosion of the materials intensified the wear degree, and the main wear mechanisms of the materials included adhesive wear, fatigue wear, and oxidation corrosion. LBE exhibited different dynamic viscosities at two different temperatures (250 and 450 ℃). Compared to the dry condition, the enhancement of LBE boundary lubrication resulted in a decrease of 24.4% and 30.9% in the friction coefficient. However, the dissolution corrosion generated by LBE intensified the removal of the matching material. At the same time, during the fretting wear process, it infiltrated into microcracks, accelerating the propagation of cracks along the contact surface and causing the material to peel off, resulting in an increase in wear rates of 31.3% and 29.6% compared to dry environments, respectively.