| As a type of driven fluid machinery that effectively increases gas pressure,centrifugal compressors have been widely used in the industry of petroleum petrochemical,electric power,metallurgy,fuel gas,national defense and military.After years of development,our country has been able to manufacture many compressors with our own technology,while the problems,including short life and poor reliability,still widely exist for the key components,such as impellers and crankshafts.Compressors are becoming much bigger and operating at higher parameters,the reliability of these compressor are also becoming more crucial.Every year tens of safety accidents happen in petrochemical companies,followed by enomerous economic loss,and great threat to both the hunman and the machines.Funded by the National Basic Research Program of China(The deterioration mechanism of compressor key components under extreme conditions and the key technology of life extending(Grant No.2012CB026003)),this paper studied the performance of FV520B martensite precipitation hardening stainless steel under the combined effect of multiple fields,including chemistry,temperature and stress,to clarify the failure mechanism of stress corrosion and corrosion fatigue,and establish a corrosion fatigue life prediction model.This work is supposed to provide both academic and technical supports for ensuring the life and reliability of centrifugal compressor impellers under extreme conditions.First of all,electrochemical corrosion of FV520B stainless steel immersed in Na Cl solutions,aqueous H2S solutions,and H2S+Cl-+H2O solutions were studied,in accordance with the actual service environment of the recycle hydrogen compressor impeller.Results show that FV520B steel immersed in Na Cl solution had an“activation-passivation-breakdown”characteristics and a relatively wide passive region,which indicates that FV520B steel in Na Cl solutions has a good corrosion resistance.With the increasing of Cl-,the corrosion current density increased slightly.In aqueous H2S solutions,the passivation behavior of FV520B steel changed with the H2S concentration,due to various corrosion products were produced.The corrosion current density of FV520B steel immersed in aqueous H2S solutions was higher than that in Na Cl solutions.In H2S+Cl-+H2O solutions,FV520B steel had a poor corrosion resistance under the combined effect of H2S and Cl-,which results in the acceleration of the passive film dissolution.With increasing H2S concentrations,the corrosion current density of the FV520B steel increased.And then,the performance of FV520B steel in corrosive and static stress environments was studied.The stress corrosion cracking susceptibility of FV520B steel in above-mentioned three corrosive environments was determined through slow strain rate tests.The result shows that the ranking for the stress corrosion cracking susceptibility of FV520B steel from good to bad was Na Cl solutions,aqueous H2S solutions and H2S+Cl-+H2O solutions.The stress corrosion cracking susceptibility of FV520B steel increased when the H2S or Cl-concentration increased.The fracture mechanism of FV520B steel in Na Cl solutions is ductile fracture caused by mechanical action.In aqueous H2S solutions,the fracture mechanism gradually transformed from ductile fracture to brittle fracture with the increasing of H2S concentration.Fracture mechanism of FV520B steel in H2S+Cl-+H2O solutions is brittle fracture.ISSRT of FV520B steel in H2S+Cl-+H2O solutions were higher than the sum of ISSRT in two single corrosive media,and this is due to the synergistic effect of H2S and Cl-will accelerate the stress corrosion cracking process of FV520B steel.Based on the research of stress corrosion cracking susceptibility,pre-cracked wedge-open loading(WOL)specimens were used to study the stress corrosion cracking behavior of FV520B steel immersed in H2S+Cl-+H2O solutions.It shows that the fracture toughness of FV520B significantly decreased compared with that in atmosphere environments,and the threshold stress intensity factor(KISCC)of FV520B decreased with the increasing concentration of hydrogen sulfide.Fracture morphology analyses showed that the fracture was characterized by quasi-cleavage mechanism.The SCC mechanism of FV520B exposed to H2S+Cl-+H2O solutions is hydrogen embrittlement(HE)type of SCC,and Cl-can accelerate the stress corrosion cracking process.Intermittent arc-cracks on the side surfaces around the tip of main crack was observed in the experiment.It can be concluded that the formation mechanism of the arc-crack is maninly resulted from the combined action of hydrogen embrittlement and anodic dissolution.At last,fatigue properties of FV520B steel in combined effect of stress,temperature and chemical fields were studied.The results indicate that fatigue properties of FV520B in mist,salt-spray and H2S+Cl-+H2O environment were lower than that in the atmospheric environment.The change of temperature below 150?had little effect on the fatigue performance of FV520B steel in the atmospheric environment.While in mist and salt-spray environment,corrosion fatigue properties of FV520B steel decreased with the increasing of temperature.The fatigue mechanism of FV520B steel in salt-spray environment is pitting corrosion fatigue,while it is a mixed mechanism of pitting corrosion and hydrogen embrittlement.Corrosion fatigue properties of FV520B steel in H2S+Cl-+H2O environment decreased with the increasing of H2S concentration.Based on the fatigue mechanism,modified pitting corrosion fatigue model for salt-spray environment was developed,and the influence of temperature on fatigue properties of FV520B steel was also considered.As for H2S+Cl-+H2O environment,pitting corrosion-hydrogen embrittlement fatigue model was established,which took the influence of H2S concentrations into consideration.The model predictions were in good agreement with the testing results.The fatigue life of the hydrogen circulating compressor impeller under typical service conditions was estimated with fatigue models.It indicates that fatigue life of the impeller in H2S+Cl-+H2O environment is shorter than that under mist,salt-spray and atmospheric environment,which will easily cause premature failure to an impeller.To avoid premature failure of an impeller caused by corrosion fatigue fracture,H2S+Cl-+H2O corrosive environment should be avoided. |
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