| In the system of ship,the fixing bolt of impeller is acting as an important part for safety operation.The higher requirement of corrosion resistance,toughness and plasti.city were asked due to corrosive envirment and complex stress of working condition.Besides,hydrogen brittle is a common form of failure mode for fixing bolt under the effect of cathodic protection.Thus,to develop proper fixing bolt for the applied on bolt has important application value.Inconel 718 alloy with high strength,high toughness and good property of resisting hydrogen brittleness was researched in this thesis.Inconel 718 alloy is a typical precipitation strengthening alloy with a outstanding combination of strength,ductility,toughness,processability,structural stability and good fatigue resistance,oxidation resistance,corrosion resistance during-253? to 700?.Additionally,this alloy has a low sensitivity to hydrogen for its face-centered cubic(FCC)lattice.Therefore,Inconel 718 alloy is suitable for manufacturing bolt with complex loading in marine environment.Mechanical properties,precipitation kinetic of second phase,mechanism of pitting and intergranular corrosion,partitioning characteristics of alloying elements,et al were investigated to reveal microstructure stability and relevant properties of Inconel 718 alloy,employing transmission electron microscopy(TEM),scanning transmission electron microscopy(STEM),slow strain rate testing(SSRT),two-stage interrupted compression method,dynamic electrochemical impedance spectroscopy(DEIS),X-ray photoelectron spectroscopy(XPS),thermodynamics calculation of Thermo-Calc software and other experiment methods and test means.The designs scheme for optimizing the chemical composition of this alloy were presented according to the relevant experiments and theoretical calculations,and innovative research results were obtained.It was showed in this thesis that the mainly strengthening mechanism of Inconel 718 alloy was the precipitation strengthening of ?'/?" phase and grain refining strengthening,respectively,and the grain size had a close relationship with the precipitation amount and distribution of 8 phase.? phase is almost completely soluble in matrix phase after isothermal holding at 1050? for 1 h,the grain size increased significantly with the dissolution of ? phase above 1025?,and the static nucleating mechanism of deformed alloy was characterized by bulging at grain boundary,accompanying with the merging of sub-grain.On strength properties,the yield ratio decreased with the coarsening of ?'/?" particles,and ? phase went against the improving of strength for this alloy.In addition,the corrosion and hydrogen resistance increased with the dissolving of 8 phase,and those properties got worse when the granulated 8 phase turned into block and continuous state.By analyzing the precipitation-time-temperature(PTT)curves of 8 phase,the nose point of start and finish precipitation were about 5s at 920? and 2815s at 940? dislocation was main nucleation site,but grain boundary would became additional nucleation site with decreasing temperature.At lower temperature(<900?),? phase was formed mainly through the transformation of ?'/?" to 8 phase.Research indicated that the precipitation of block MC particles acted as corrosion source,increasing the pitting and intergranular corrosion sensitivity in Inconel 718 alloy.And during precipitating process of ?'/?" phases,the stability of passive film of Inconel 718 alloy increased due to the higher chromium concentration in the matrix phase.But when the pitting corrosion happened,it was promoted for the existence of ?'/?" phase,and its sensitivity was increased.On the other hand,the intergranular corrosion was mainly controlled by the mechanism of selective dissolution of 8 at grain boundary.The precipitation of ? phase could be controlled by the heat treatment technology to improve the intergranular corrosion resistance.Moreover,through the thermodynamic calculation of Thermo-Calc software during 600—1100? on Inconel 718 alloy,the results showed that strengthening phase forming elements(Nb,Ti,Al)were mainly distributed in ?(?")and ?' phases.At 620?,there were about 73.5%Nb,18.6%Ti and 13.5%A1 distributing in 8 phase,and 23.4%Nb,78.2%Ti and 79.2%Al in ?' phase,while only about 2.5%Nb,3.2%Ti and 20.8%Al in matrix(y)phase.And carbon mainly distributed in carbides.Decreasing carbon content could be able to decrease the precipitation quantity and size of carbides dramatically.Therefore,appropriate heat treatment technology could be effective to minimize the precipitation of MC and M23C6 phase and to avoid the precipitation of M6C phase.In view of the anti-corrosion and low hydrogen embrittlement applied characteristic of fixing bolt materials,And combined with the anlysis of corrosion mechanism and thermodynamic calculation results,the mechanical properties,corrosion resistance and hydrogen resistance of Inconel 718 alloy were optimized by reducing the content of carbon,adjusting the content of niobium and adding tungsten,which aims to decrease the effect of precipitation strengthening and increase the effect of solution strengthening.Finally,the modified alloy,which had a better mechanical properties,higher corrosion resistance and lower hydrogen sensitivity than that of the traditional Inconel 718 alloy,had been developed successfully.And its strength drop rate of slow strain rate testing was about 8.79%in 0.5 mol/L H2SO4+0.25 g/L As2O3 solution after charging at-100 mA/cm2 for 4 h,and its ductility was in a high level. |
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