| Due to their admirable capabilities of substantial power generations overcoming the intermittency of solar resource,concentrating solar thermal power(CSP)generations have attracted much attentions from researchers,power companies and governments.Combining with thermal storage systems,CSP exhibits improved tunable properties and environmental adaptability,which becomes a hot point in the field of international renewable energies.This paper focuses on two actual engineering problems presently confronted by AISI 321 stainless steel applied as CSP heat exchanger tubes:corrosion damage from molten Al-12Si environment at 620℃ and creep-fatigue(C-F)bearing issue due to frequent start-ups and shut-downs of the system,and aims at prolonging its service lifespans through alleviating corrosive influences and ameliorating mechanical properties in a manner of pack aluminizing and prior C-F deformations,respectively.The effect of prior C-F exposure with different lifetime fractions on remnant tensile and creep behaviors ofA ISI 321 before and after aluminizing were investigated.Next,the microstructural evolutions of samples subjected to prior C-F exposure with different lifetime fractions were characterized by transmission electron microscope and electron backscattered diffraction.Afterward,potential reasons for residual property variations of AISI 321 and aluminized steel encountered by different prior C-F exposure were systematically revealed through several angles,such as the precipitate behavior of carbide,the evolutions of twinning and dislocation cellar network,as well as coating structural developments during cyclic deformation.This article aims at providing useful references for the optimizations of service properties of aluminized steel employed as CSP heat exchanger tubes.The main contents and resultant conclusions obtained were summarized as follows:(1)Cyclic strain responses of AISI 321 present initial hardening and followed softening.The maximum extent of hardening emerges at 50%.Clear increase in remaining tensile strength and creep life with prior C-F lifetime fraction rising from 10%to 50%are observed at AISI 321,and then further increase in prior C-F lifetime leads to decreases in remnant properties.In particular,12.6%ultimate tensile strength(UTS),11.5%yield strength(YS)and 241.96%creep life improvements in AISI 321 are achieved from 50%lifetime fraction prior C-F interaction.(2)Within 0%-50%lifetime fraction,pinning effects caused by fine M23C6 precipitated at austenite interior can restrict dislocation motion.Meanwhile,gradually increased TB suppresses slip band evolutions.Hence,remarkable improvement of residual properties of AISI 321 are obtained.During 50%-80%lifetime fraction,carbide coarsening leads to the nucleation of secondary cracks.On the other hand,the reduced dislocation density and TB fraction are also responsible for degraded remnant mechanical properties.(3)Cyclic strain responses of aluminized steel present initial hardening and followed softening.The maximum extent of hardening emerges at 30%.As lifetme fraction of prior CF exposure progresses,residual tensile strength and creep life of aluminized steel exhibits initial increase and followed decline,whose maximums appear at 30%lifetime fraction.Note that,10.0%UTS,11.5%YS and 241.96%creep life enhancements in aluminized steel are achieved from 30%lifetime fraction.At early stage of C-F deformation(0%-30%lifetime fraction),the increased tensile and creep behavior is ascribed to the activation of secondary twinning,as well as the synergic strengthening mechanisms between dislocation cellar networks and twinning behavior.Afterwards(30%-80%lifetime fraction),carbide coarsening successive occurs and the impingement of dislocation into carbide accelerates the formation of slip bands inside the carbide.Most importantly,dynamic recovery occurs at the equiaxed dislocation cells.Once dislocation cells are produced,the flow stress of polycrystal materials is mainly associated to the cell size rather than the grain size.As a result,the remnant tensile and creep resistances of aluminized steel are impaired.(4)With lifetime fraction of prior C-F exposure proceeding,dynamic recovery of Fe3Al grains and degraded thickness of total coatings are observed.In detail,the thickness of Fe3Al intermetallic layer is raised,whilst those of FeAl intermetallic and α-Fe(Al,Cr)solid solution diffusion layers are reduced at this process.At early stage of C-F deformation(0%-30%lifetime fraction),dynamic recovery of Fe3Al grains effectively accommodate partial plastic strains at coating,postponing the initiations and propagations of coating microcracks.At later stage of C-F deformation(30%-80%lifetime fraction),The aspect ratios of the fine grains in this layer have reached critical value.As a result,defects generate and residual properties are degraded. |
