The Fretting Wear Behavior And Damage Mechanism Of Heat Exchange Tube Materials In Air And Solution Environments

With the advent of the global energy crisis,nuclear energy has become an important means for countries to respond the energy crisis.Efficient and safe use of nuclear energy has become the focus of research.Steam generator(SG)is one of the most important components in pressurized water reactor(PWR)nuclear power plants as the junction of primary loop and second loop.However,fretting wear is likely to take place at the contact of steam generator(SG)tube and tube support plate due to flow-induced vibrations,which could cause tube thinning and rupture,eventually reduce service life of tube and threaten the safety of nuclear power plants.Therefore,the researches of fretting wear behavior and damaged mechanism of heat exchange tube have great significance to extend the service life of heat exchange tubes and improve the efficiency and safety of nuclear power plant.There were many basic research works on the fretting wear behavior of SG tube in China and abroad.However,the correlations among environment and fretting wear as well as damage mechanism were not clear.The systematic studies had been conducted on the fretting wear behavior of Inconel 600/690TT in different environments(room-temperature air condition,room-temperature solution condition,applied potential condition).By means of the optical microscopy(OM),3D surface profilermeter,laser confocal scanning microscopy(LCSM),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDX),X-ray photoelectron spectroscopy(XPS),focused ion beam(FIB)and transmission electron microscopy(TEIM),the structural evolution and crack propagation as well as oxidation behaviors in the worn subsurface induced by fretting wear were firstly in-depth analyzed.Then,the fretting wear laws and damage mechanisms of material in different environments were systematically revealed.Finally,the models of damaged structures in different environments as well as the relationships among environment,wear property and damage mechanism were also systematically established.The main obtained results were listed as follows:(1)In room termpertature air environment:In gross slipping regime(GSR),the wear mechanism was mainly oxidation and delamination wear.There was a stratified structures including wear debris layer(WDL),tribologically transformed structure(TTS--composed of equiaxed nanocrystals)and generally deformed structure(GDL--composed of elongated grains)in the worn scar subsurface from outmost to the matrix.The transformation mechanism from GDL into TTS layer was dynamic recrystallization.Then,TTS was oxidized and mixed with the transformed 304SS to form the WDL mainly consisting of nano-sized metal oxides and Nickel-based alloy grains.In partial slipping regime(PSR),the wear mechanism was mainly oxidation and adhesive wear.The annular fatigue cracks on worn surface induced by stress concentration propagated along the direction of 30-50° into the deep material in a mixed mode of transgranular and intergranular.Compared with the case of PSR,the formation of delamination cracks exacerbated the material loss in GSR.The effect of grain size on fretting wear and damage mechanism indicated that the increase of grain size was not conducive to the formation of TTS layer,which caused the formation of longer delamination cracks and eventually the reducing of fretting wear performance as well as the increase of wear volume.(2)In room termpertature solution environment:The effects of solution and chloride ion on fretting wear were mainly reflected in the following aspects.Firstly,due to the low oxygen concentration in solution,the preferred oxidation of chromixum occurred in TTS.Oxidation plugged into the TTS layer at the interface of WDL/TTS and resulted in formation of the Cr-rich oxide zone surrounded by Ni-rich zone.Then,the further oxidation led the formation of WDL,while the content distributions of(Cr,O)felled,the content distributions of(Ni,Fe)rose in WDL layer.Secondly,in pure water condition,the lubrication of solution prompted the transformation of fretting regime at the contact surface from PSR to GSR as well as the formation of fatigue crack in GSR and furrows in PSR on worn scar surface.With the combined effect of low oxygen concentration and lubrication,the wear volume(except the case of 80 N)and cofficient of friction(COF)decreased.Finally,compared with pure water condition,on one hand,the corrosion of chloride ion further prompted the transformation of fretting regime at the contact surface from PSR to GSR and caused the increase of wear volume but decrease of COF in 3.5%NaCl solution condition.On the other hand,the corrosion of chloride ion caused the formation of a larger size of Cr-rich oxide zone in TTS layer and a completely oxidized WDL layer in GSR as well as lots of oxidies on worn scar center and the shorter fatigue crack beneath worn scar in PSR in 3.5%NaCl solution condition.(3)In applied potential environment:The fretting wear behavior and damaged mechanism strongly depended on the polarization condition.The applied potential played little impact on wear volume and COF in cathodic polarization condition(CPC)but significant impact on above two in anodic polarization condition(APC).The wear mechanism was mainly abrasive wear in CPC,and became oxidation wear and abrasive wear in CPC.In CPC,neither TTS nor WDL was oxidized as the oxidation behaviors were inhibited by cathodic current.In passivation condition,on one hand,both TTS and WDL were oxidized.On the other hand,compared with the condition without applied potential,the anodic polarization caused a larger size of Cr-rich oxide zone.In the transpassive condition of 3.5%NaCl solution,the over-passivation current resulted that TTS associated with the peeled material were directly oxidized into the soluble oxides,thus led that few corrosion products was observed on TTS without WDL.Anode current promoted the oxidation of the deformed layer and exacerbated the material loss,while the cathode current inhibited the oxidation and reduced the material loss.(4)The effects of environment on damage mechanism and wear volume were strongly dependent on the oxidation degree of TTS in worn subsurface.