| Fretting wear refers to oscillating relative movement between two contact bodies with tiny amplitude(at micrometer scale),and it exists widely in"approximately fastening" structures.The fretting wear occurs in the corrosive environment is called fretting corrosion.Alloy 690TT,as the materials of steam generator heat transfer(SGHT)tubes in nuclear power plants(NPPs),would suffer severe fretting corrosion with the support plates or anti-vibration bars because of the flow-induced vibration of high temperature high pressure secondary water,and lead to the thinning or even the hole forming of the SGHT tubes and eventually cause the leakage of radioactive substances,and thus reduce the service life of the steam generator.The fretting corrosion of SGHT tube material would massively affect the safe operation of NPPs and the failure of SGHT tubes could cause serious economic loss.Therefore,the research of the fretting corrosion behavior of SGHT tube material is extremely significative both in the scientific and engineering fields.However,the service environment of SGHT tubes is high temperature high pressure secondary water.The simulation of secondary water is very difficult,and the precisely control of both water condition parameters and mechanical parameters in the same time is even more difficult.Thus the researches so far are mostly under the room temperature or in the high temperature air.Some existing researches under HTHP water are focused on short-term experiments with relatively low frequency.Whether the results of these researches can be used to evaluate the real long-term,high frequency fretting corrosion process in the NPPs is doubtful.Hence,it is no time to delay to carry out the studies on the fretting corrosion behavior of SGHT tubes material Alloy 690TT under HTHP water.This work will study on the effect of temperature,normal force and amplitude on the fretting corrosion behavior of Alloy 690TT tubes under HTHP water by high temperature high pressure fretting corrosion testing equipment and various analysis testing equipments,including Scanning Electron Microscope(SEM),Transmission Electron Microscope(TEM),3D-analyzer,Electron Backscattered Diffraction(EBSD)and etc.The influence of temperature on the fretting corrosion behavior of Alloy 690TT was studied in pure water.The coefficient of friction(COF)decrease with the increase of temperature and the evolution of COF could be divided into six stages.The worn scars widen with the increase of temperature and the severest wear appeared at 100 ℃.The wear at 200 ℃ is obviously milder than other conditions.The wear mechanism changes at different temperature.The adhesive wear is the main mechanism at room temperature,and the abrasive wear becomes main mechanism at 100 ℃ with the existence of adhesive wear and delamination.The abrasive wear dominates at 200 ℃.Wear induced damages,including nano-grain tribologically transformed structure layer,deformation slips with high residual strain concentration and micro-cracks,may have detrimental effects on the performance of the heat exchanger tubes in NPPs.The influence of normal force on the fretting corrosion behavior of Alloy 690TT was studied in HTHP water.The COF decrease with the increase of normal force,and the difference is relatively small at low fretting cycles,meanwhile the evolution of COF could be divided into four stages.The wear is mild and dominated by abrasive wear at 20 N.The wear condition is complicated with massive transfer materials occur at 60 N.The worn scar is a smooth sphere scar at 100 N.Only micro-slip occurs at the edge of contact area at 100 N.The severest wear appears at 60 N,and the wear degree at 100 N is a bit less.The wear degree at 20 N is obviously milder.TEM analysis of sectional sample has been carried out.The multi-layer structure is found.From the surface of worn scar to the matrix,there are multiple layers:wear debris layer or materials transfer layer,TTS and general deformation layer(GDL)until matrix.The influence of amplitude on the fretting corrosion behavior of Alloy 690TT was studied in HTHP water.The COF,maximum wear depth and wear volume both increase with the increase of amplitude.The wear mechanism is adhesive wear at 40μm and the wear mode is partial-slip;the wear mechanism includes adhesive wear,abrasive wear and delamination at 70 μm,and the wear mode is mix-slip;the wear mechanism is abrasive wear and delamination at 100 μm and the wear mode changes from gross-slip to mix-slip.TEM analysis of sectional sample has been carried out.Multi-layer structure is found with wear debris layer,TTS,GDL from surface to the matrix. |
