Effect Of Microstructure On Fretting Wear Of 690 Alloy Tube At Room Temperature

Steam generator is the key equipment for heat exchange in primary and secondary sides of pressurized water reactor(PWR)in nuclear power plant(NPP).Heat transfer tube is an important part to ensure the heat exchange and pressure boundary integrity of primary and secondary sides of PWR.The reliability of heat transfer tube directly affects the economy,stability and safety of nuclear power plant.During the operation of the steam generator,the flow-induced vibration of high temperature and pressure medium causes fretting wear between heat transfer tube and supporting parts,which reduces thickness of tube wall,even breaking the tube wall and releasing the radioactive substance to the secondary circuit medium,which finally endangers the safe operation of the nuclear power plant.At present,alloy 690 has been the preferred material for the second and third generation heat transfer tubes in nuclear steam generator.The effect of microstructure of the material on its fretting wear performance at room temperature and the fretting wear failure mechanism provide theoretical support for improving the fretting wear performance of the heat transfer tubes in steam generator.Through solution annealing treatment and thermal treatment(TT)for 690 alloy tube,different microstructures were obtained by changing the grain size and the intergranular carbide morphology.Then,heat-treated alloy 690 tube and 405SS plate were subjected to fretting wear test to obtain friction coefficients and wear scars.The3D surface morphology and volume of wear scar of alloy 690 tube were obtained by3D non-contact surface profilometer.The wear scar morphology and cross-section morphology were observed by SEM,and the element distribution of wear scar of alloy690 tube was measured by EDS.It is found that with the increase of solution annealing treatment temperature and time,the grain size increases,the microhardness of matrix and grain boundary decreases,and the wear volume increases;With the increase of TT temperature and time,the size and spacing of intergranular carbide are increasing,and the increase of intergranular carbide size leads to the increase grain boundary microhardness and the decrease of wear volume;On the contrary,the increase of intergranular carbide spacing results in the decrease of grain boundary microhardness and the increase of wear volume;Compared with grain size,intergranular carbide size and spacing,grain boundary microhardness has little effect on wear;As TT temperature is 715℃and time is 5 h,the minimum wear volume is 2.85×10~7μm~3.During fretting wear,alloy 690 tube appears abrasive wear,delamination wear and oxidation wear;The trends of carbon content on wear scar for different heat treated specimens are consistent with the trends of wear volume;The oxygen and carbon content of wear debris on wear scar are higher than that of other areas on wear scar;Element transfer occurs between the wear pairs.Through the above analysis,the damage mechanism of the fretting wear process is revealed as follows:fretting lead to plastic deformation of the sub surface layer of alloy 690 tube,which lead to grain boundary deformation and the forming of twin boundary;With the increase of plastic deformation,the deformed grain boundary and twin boundary generate crack core,which expand to form delamination;Delamination falls off to form wear debris,which lead to fretting wear.The results can provide theoretical support for optimizing the microstructure of alloy 690 tube for reducing fretting wear.