| Reduced-activation ferritic-martensitic(RAFM)steels and 316L austenite stainless steels have been developed as the candidate structural materials for the Test Blanket Module(TBM),and the dissimilar butt joint between the two materials(RAFM-316L)has been joined by.Friction stir welding(FSW)in this paper.The defect free joint with well formed has been obtained via optimized process parameters.The microstructure evolution of RAFM-316L joints was studied by means of optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),electron back scatter diffraction(EBSD)and transmission electron microscope(TEM).The hardness distribution,tensile properties,impact properties and high temperature creep resistance of the dissimilar joints were conducted to discuss the relationship among welding parameters,microstructure evolution and mechanical properties.The following conclusions have been made:Defect free joint can be obtained with 400rpm rotational speed,100mm/min welding speed.In addition,defect free joint can also be obtained when 316L steel was positioned on the advancing side(AS)and RAFM steel was positioned on the retreating side(RS)at 300rpm,75mm/min.Martensitic transformation was occurred during FSW process and the microstructure is mainly consisted of martensite phase in the heat-affected zone(HAZ),thermal mechanically affected zone(TMAZ)and stir zone(SZ)in RAFM side.Obviously recrystallization can be detected in 316L side,the microstructure is mainly consisted of austenite phase.TEM analysis showed that the microstructure of the interface in SZ is consisted of martensite,austenite and ferrite.The metallurgical bonding of 316L stainless steel and RAFM steel was achieved at the mixed area of the weld,and the tensile strength of dissimilar joint were equivalent to the BM of 316L.The precipitates of M23C6 can be detected in the base mateal(BM)on RAFM steel side,and the number of M23C6 decreased when the heat input increased in RAFM-HAZ.Precipitates of M23C6 would dissolve into the Fe matrix in RAFM-SZ.There are two kinds of precipitates of the ball-like MX particles and needle-like M3C particles observed in RAFM-SZ,and they have a significant effect on pining the mobile dislocations.At-40℃,the lowest impact absorbed energy is 15.809J in RAFM-HAZ.All the rupture samples fractured at RAFM-HAZ,the highest rupture time can reach 1941h at 600℃ and 100MPa.Two reasons could explain the creep fracture:Firstly,Laves phase formed in RAFM-HAZ and adjoined the M23C6 carbides,the coarsening of the precipitates particles was the main reason for the creep fracture of the samples;Secondly,the microstructure transformation of martensite to ferrite casued obviously softening in RAFM-HAZ during creep test,and the strength of RAFM-HAZ had significant decrease,which resulted in the creep failure.The rupture time could be affect by different heat input of joints,the rupture time decreased with the higher heat input.There was a risk of cracking while some small blocks of martensite formed in austenite matrix in SZ during creep test.However,there was no risk of cracking when palced RAFM in AS,and some small blocks of austenite formed in martensite matrix in SZ,the rupture time of joint can also be increased with this joint mode. |
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