Fretting Wear Behavior And Damage Mechanism Of Nuclear Grade Inconel 690TT Alloy

Steam generator(SG)heat transfer tube is the interface between the radioactive first-loop system and non-radioactive second-loop system in pressurized water reactor nuclear power plants.The high temperature water flow-induced vibration and the stress variation resulted in the fretting wear between the tube and its supporter,which could cause tube thinning and rupture,eventually reduce service life of tube and threaten the safety of nuclear power plants.Therefore,the investigation on fretting wear behavior and damage mechanism of the tube had great significance to extend the service life of heat exchange tubes and improve the efficiency and safety of nuclear power plant.The systematic studies were conducted on the fretting wear behavior of Inconel 690TT in different environments(room-temperature air,high-temperature air,high-temperature controlled oxygen content,high temperature high pressure pure water).The fretting running characteristics,coefficient of friction and wear volume,gradient structure evolution and crack initiation and propagation in the worn subsurface induced by fretting wear were carefully analyzed by means of the optical microscopy,laser scanning confocal microscopy,scanning electron microscopy,energy dispersive spectroscopy,X-ray photoelectron spectroscopy,Raman spectroscopy,focused ion beam and transmission electron microscopy.Meanwhile,the models of formation of gradient structures in different environments were also systematically established.The main obtained results were listed as follows:(1)Room temperature air environment:Fretting running condition changed from partial slip regime(PSR),to mixed fretting regime(MFR),finally to gross slip regime(GSR)with the increase of displacement amplitude and the decrease of normal force.In PSR,oxidative wear occurred in micro-slip region.The unidirectional material transfer from 304SS to Alloy 690TT happened in the sticking and micro-slip region.The initiation and propagation of fatigue crack occurred in the junction of sticking region and micro-slip region.In MFR,wear damage mechanism was the combination of delamination,fatigue crack,oxidative wear and the reciprocal material transfer.In GSR,wear mechanism was dominated by the delamination,oxidative wear and the unidirectional material transfer.The strain gradient coupled with temperature gradient resulted in the gradient nanostructure in the subsurface.The aggregation of dislocations appeared when the matrix changed to plastic deformation layer(PDL).The farther from the worn surface,the weaker was the plastic strain.The formation of tribologically transformed structure(TTS)was due to the dynamic recrystallization of PDL.Mixed layer was produced by the insufficient oxidation and mechanical effect.Continuous oxidation of mixed layer impelled the formation of oxide layer.On one hand,the unidirectional material transfer resulted in the formation of double-layer TTS containing of the mixture of Ni-base and Fe-base nanostructure in the upper and only the Ni-base nanostructure in the lower.On the other hand,the unidirectional material transfer resulted in the oxidation on 304SS rather than 690TT alloy with Fe-rich oxides third body layer(TBL).(2)High temperature air environment:The friction coefficient and wear volume at high temperature was lower than that at room temperature,which was due to the formation of glaze layer.In PSR,TTS was produced due to the dynamic recovery.The grain size of TTS in PSR is larger than in GSR.The oxides in TBL mainly consisted of(Ni,Fe)Cr204,irrespective of the slip regime.In PSR,the content of Fe-rich oxides(Fe2O3 and Fe3O4)in TBL increased,which was induced by the material transfer.(3)High temperature controlled oxygen content:Under a low oxygen content(5 vol%),the TBL mainly contained large amount of Cr2O3 due to the limited oxidation and preferential oxidation of Cr.The farther from the worn surface,the weaker was the Cr2O3 concentration.While under a high oxygen content(21 vol%),the TBL mainly consisted of spinel oxides due to the sufficient oxidation during fretting wear.Only a small quantity of microcracks perpendicular to the worn surface were observed in the Cr2O3-rich TBL under a low oxygen content(5 vol%).However,the plenty of microcracks without specific orientation and voids appeared in the TBL under a high oxygen content(21 vol%).The TBL under a low oxygen content(5 vol%)resulted in lower friction coefficient,wear volume and dissipated energy,showing a better wear resistance with comparison to the TBL under a high oxygen content(21 vol%).(4)High temperature high pressure pure water environment:Under tube-on-flat contact,wear mechanism was the combination of delamination,abrasive wear,the reciprocal material transfer and oxidative wear.Wear area and depth increased with the increase of normal force and frequency.The gradient structure was observed in the worn subsurface.That is,the oxide layer,TBL,TTS layer,PDL and the matrix.The formation of TTS was attributable to the stress-strain induced dynamic recrystallization.Internal oxidation occurred within TTS layer because the nanograins provided the short-circuits for oxygen diffusion,resulting in the ellipsoid and stripe-like Cr2O3 with dispersive distribution.Oxide layer had a multilayered structure which was caused by the solid oxidation and ion deposition:the outer part consisted of Ni(1-x)Fe(2+x)O4;the inner part contained the discontinuously distributed Cr2O3.Under ball-on-flat contact,the thickness of oxide scale in sticking and micro-slip regions increased with the increase of dissolved oxygen(DO).The damage mechanism in micro-slip region was mainly micro-pitting induced by the oxide particles.The size of oxide particle increased with the increase of DO,which accelerated the pitting wear.