Study On The Mechanism Of Steam Generator Tube Under Impact-tangential Fretting Wear

The service reliability of steam generator(SG)tube plays a crucial role in the safe and normal running of the whole nuclear power plant.During the operation of the nuclear power plant,the coolant flowing out of the steam generator at a certain flow rate caused the heat transfer tube vibrating.As a result,the impact and wear between the SG tube and its supporting components(anti-vibration and support plate)occur,and finally resulting in fretting damage.Due to the randomness of vibration and the existence of support gap,the mode of fretting wear is impact,tangential,radial and composite fretting wear.At present,the investigation of the SG tubes fretting damage is focused on the single mode of tangential fretting wear and impact wear.Study on the impact-tangential fretting wear of heat transfer tubes has rarely been carried out.According to the literature,tangential displacement amplitude,impact velocity,temperature and contact mode all are important factors to fretting wear.Therefore,in view of the above conclusions,this paper used 690TT alloy tube and 405 stainless steel block as the test material,and thoroughly investigated the tangential and impact-tangential fretting wear of SG tubes under different fretting conditions,contact modes and ambient temperatures based on line contact tangential fretting wear,point contact impact-angential fretting wear,point contact tangential fretting wear,line contact impact-tangential fretting wear four different types.The main conclusions are as follows:1.Investigation of tangential fretting wear mechanism at vourious temperatureThe fretting running state of 690 TT alloy tube under 40N-100μm at various temperatures are all the gross slip region.According to the characteristics of friction coefficient and friction dissipation energy,the temperature can be divided into three sections:low temperature sections(25℃～90℃),secondary temperature sections(110℃～150℃)and elevated temperature sections(200℃～450℃).In the same sections,the dynamic response evolution law of 690TT alloy tube is similar.The wear loss first increases and then decreases with temperature increased.The wear damage is the most serious at 90℃and the damage is the least at 450℃.At 25℃,the wear mechanism of 690TT alloy tube is mainly delamination,with certain abrasive wear.When the test temperature is between 90℃and 150℃,the main wear mechanism is delamination.When the temperature is higer than 285℃,the main wear mechanisms are adhesive wear,abrasive wear and delamination,and adhesive wear plays a leading role.2.Impact-tangential fretting wear under different contact time at room temperatureThe peak impact force is independent to the contact mode used in the test process.The response of tangential force decreases with the decrease of contact time.Line contact test has better tangential response behavior than point contact.The wear volume and damage area decrease with the decrease of contact time.Under the same conditions,the wear volume of point contact test is lower than that of line contact,but the maximum wear depth is greater.In the impact-tangential fretting wear,the dominant damage mechanism of 690TT alloy tube is delamination,accompanied by abrasive wear and adhesive wear.With the decrease of contact time,the phenomenon of friction oxidation decreases.3.Wear mechanism of impact-tangential fretting wear at different temperature and high temperatureUnder different temperatures,the impact force of point contact test and line contact test is basically the same,and the test temperature and contact mode have no effect on the impact force.In the point contact test,the tangential force is the strongest at 285℃.In the line contact test,the tangential force at 25℃and 100℃is higher than 285℃and 200℃.In the point contact test,the wear volume and damage area first decreased and then increased with the temperature increased.The damage at 100℃is the lightest and 285℃is the most serious.In the line contact test,the damage is the most serious at 100℃,and the least at 200℃.The wear mechanism of point contact test at various temperatures is mainly delamination and abrasive wear.With the increase of temperature,the effect of tangential force increases and promotes the delamination damage of materials.The impact-tangential fretting wear mechanisms at different temperatures under line contact include delamination,abrasive wear and adhesive wear.The delamination phenomenon first increases and then decreases with the increase of temperature.In point contact test,the friction oxidation increases with the increase of temperature.In line contact test,the friction oxidation first intensifies and then decreases with the increase of temperature.The friction oxidation phenomenon is the most significant at100℃.When impact velocity V_i=30 mm/s at 285℃,the tangential force increases with the increase of displacement amplitude.In the whole wear process,under the same displacement amplitude,the smaller the impact velocity is,the more steady the evolution of peak tangential force is.The wear mechanism of 690TT alloy tube includes delamination,adhesive wear and abrasive wear,in which delamination and adhesive wear are dominant.With the increase of displacement amplitude,the phenomenon of delamination and adhesive wear is more obvious.With the increase of impact velocity,the wear debris is pressed into the wear pit,and the delamination phenomenon is weakened.When V_i=30 mm/s,the wear loss increases with the increase of displacement amplitude,and the more wear debris is accumulated on the surface of wear scars,the length and width of wear scars increase with the increase of displacement amplitude.The ratio of length and width decreases with the increase of displacement amplitude.With the increase of impact velocity,the wear loss continues to rise,and the length and width of wear scars also increase significantly,and the ratio of length and width also increases with the increase of impact speed.The friction oxidation is directly proportional to the amplitude of tangential displacement and inversely proportional to the impact velocity.4.Comparsion of damage property between tangential and impact-tangential fretting wearIn the impact-tangential fretting wear,the material is absolutely affected by the impact stress.The wear debris will form a solid debris layer,few of broken wear particles.In tangential fretting wear,there is no impact force between the contact surfaces,and the wear debris will be fully rolled into wear particles.In tangential fretting wear,delamination is an absolute dominant damage mechanism.In impact-tangential fretting wear,delamination mainly occurs in the center area of wear scars with high stress.The higher the impact velocity,the weaker the effect of tangential force,and and the slighter of delamination and adhesive wear.The degree of friction oxidation in the process of tangential fretting wear is much higher than that of impact-tangential fretting wear.The impact energy has little effect on friction oxidation,and the effect of tangential force is an important factor to promote friction oxidation.The effect of temperature on impact-tangential damage of 690 TT alloy tube is less than tangential fretting wear.With the change of temperature,the point contact impact-tangential fretting wear coefficient is the most steady,while the wear coefficient of line contact tangential fretting wear is the most discrete.90℃(100℃)is the temperature point with the highest fretting damage rate.5.High temperature and high pressure multiple phase flow impact fretting wear test systemA High temperature and high pressure multiple phase flow impact fretting wear test system is developed,which can simulate the random impact fretting wear under the service conditions of nuclear power.Aiming at the extreme service conditions of nuclear power and other major projects,the fretting damage test service and evaluation method under extreme conditions are established.High temperature and high pressure random impact fretting wear shows that the distribution area of bidirectional orthogonal loading is wider than that of unidirectional impact damage.Under random conditions,the wear scar damage has no obvious characteristics,and the wear mechanism includes delamination and adhesive wear.