| As one of materials with excellent cryogenic performance,9%Ni steel has been the most important structural material in liquefied natural gas(LNG)industry.Due to extremely harsh service environment,it has high demand in cryogenic properties of 9%Ni steel welded joint,especially the fatigue property and fracture toughness at cryogenic temperature.With the increasing demand of LNG,the safe service of 9%Ni steel welded structure has attracted more and more attention.However,there is a lack of clear understanding of cryogenic fatigue performance and fracture toughness of9%Ni steel welded joint.The influence mechanism of cryogenic temperature on the microstructure and performance of the joint is not completely clear is still unclear.It is necessary to study the cryogenic fatigue performance and fracture toughness of 9Ni steel joint,which is of great significance in the process of getting our own Intellectual Property rights of the welding technology in the manufacture in LNG storage and transportation industy.The object of this research is of 9%Ni steel joints welded by flux cored arc welding(FCAW).The low-cycle fatigue behavior of the joint at cryogenic temperature is investigated by low-cycle fatigue test at cryogenic temperature(80K).The influence mechanism of cryogenic environment on fatigue failure of the joint was clarified.On this basis,the microstructure characteristics and cryogenic fracture toughness of weld center and fusion boundary are systematically studied.The study intends to provide effective information and fundamental for fatigue lifetime evaluation and safe service of LNG containers made by low temperature steel under cryogenic environment.Firstly,low-cycle fatigue(LCF)test was performed at 296 K and 80 K to investigate the effects of cryogenic environment on the LCF behavior of 9Ni steel FCAW joint.The comparative analysis was performed on the differences of fracture location,fatigue crack propagation behavior,and their relation with microstructure evolution.The test results indicated that under small total strain,the lifetime of specimens tested in cryogenic environment was remarkably higher than that tested at room temperature.The amplitude of plastic deformation decreased in cryogenic environment under the same total strain.The gap of fatigue lifetime decreased with the increment of strain amplitude.The LCF specimens underwent cyclic hardening and cyclic softening before fracture at room temperature.The duration of cyclic hardening became longer,but no obvious cyclic softening was observed at cryogenic temperature.It could be attributed to the alternate of dislocation slip and deformation twinning in cryogenic LCF process,which was the fatigue deformation mechanism of 9Ni steel FCAW joint under cryogenic environment.It turned out that the fracture location was at PMZ at room temperature,but changed to weld center in cryogenic environment.The competition between the weakening effect of precipitates and the strengthening effect of brass texture transformation ultimately determined the fatigue life and fracture location.For the specimens tested at room temperature,the content of brass texture in weld center increased remarkably after LCF test.In cryogenic environment,the transformation of brass texture mainly occurred in PMZ.Under periodic load in cryogenic environment,microvoids formed near precipitates,which resulted in the interdendritic propagation of fatigue cracks in weld center.Research results contributed to safe service and fatigue lifetime estimation of LNG containers made by 9% Ni steel in cryogenic environment.Subsequently,based on the failure location in LCF deformation,the microstructure characteristics and cryogenic fracture toughness of weld metal was systematic studied.9% Ni steel was welded by FCAW using Ni–Cr–Mo–Nb alloy as filler material.Different welding parameters were designed for two groups of joints.Their fracture toughness was investigated using Crack Tip Opening Displacement(CTOD)test at room temperature(296K)and at cryogenic temperature(80K),respectively.It was demonstrated that the precipitates are very sensitive to welding parameters.With the increase of welding heat input,the austenitic fraction of the weld increased but the number of precipitated phases decreased.CTOD results showed that the weld metal with less precipitates had higher CTOD values no matter the testing temperature.By investigating the crack propagation paths,it was fount that most cracks propagated through dendritic region at room temperature and propagated along interdendritic region at cryogenic temperature,and there were three interaction modes between crack and precipitates.Finite element analysis was carried out to study the effect of precipitate quantity,precipitate morphology and precipitate distribution on micro-regional force and deformation(MRFD)feature in the weld.In stretching process,with the increment of precipitate quantity,high stress concentration and local plastic deformation occurred around precipitate no matter at room temperature or at cryogenic temperature,which could decreased the resistance of crack propagation.Rod-shaped Laves phase had a far greater effect than globular-shaped carbide.The size change of carbide had little effect on MRFD.Extended finite element method(XFEM)was used to simulate the formation of microcrack at cryogenic temperature.It was found that microcrack was formed inside Laves phase firstly and then around Laves phase.It appeared around carbide.High local plastic deformation and peak stress of precipitated phase were the main factors for the formation of microcracks.It provode the theoretical basis and technical support for the welding process design and cryogenic fracture toughness anslysis of 9% Nickel steel.Finally,the interface morphology and forming mechanism of 9%Nickel steel FCAW joint were systematic studied.It was found that sharp fusion boundary(SFB)and widened fusion boundary(WFB)alternately appeared on the cross section of weldment.A dilution layer(DL)with homogeneous composition was found at WFB.High viscosity of nickel base alloy and 9Ni steel and local decrease of flow rate of molten pool are possible reasons for the formation of DL.Inside DL,martensite is found near base metal while austenite near weld,and they were followed the KS relationships.The martensite in DL is lenticular-like shaped,different from the lath martensite in adjacent HAZ.Compared with HAZs,the martensite in DL has more M-A constituents and less HAGBs.The stress occurred at weld interface in cooling stage of welding process and during cryogenic treatment is the determinant of martensitic transformation.Charp impact test was carried out to investigate cryogenic toughness of different fusion boundaries.The cryogenic impact absorbed energy of fusion boundary descends 18.7% when approximate 1/5 length of fusion line was DL.FCGR test result showed that the propagation rate of fatigue crack increased in DL.DL had an adverse effect on the cryogenic properties of fusion boundary.This study provides a theoretical basis and guidance for improving the microstructure characteristics and cryogenic performance of the interface of dissimilar metal in 9% Nickel steel welded joints. |
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