Microstructure And Mechanical Properties Of CLF-1/316L Steel Dissimilar Joints Welded With Fiber Laser Welding

China Fusion Engineering Experimental Reactor(CFETR)is the first fusion research project with full details and exceeds the International Thermonuclear Experimental Reactor(ITER)project.In CFETR,the service environment of the test blanket module(TBM)is expected to be very harsh.Meanwhile,the TBM is composed of a complex structure and its manufacturing process involves the welding requirements of reduced-activation ferrite/martenstic(RAFM)CLF-1 steel and 316L stainless steel.In this paper,fiber laser butt welding on the dissimilar joint between reduced-activation ferrite/martenstic CLF-1 steel and 316L stainless steel with a thickness of 10 mm was performed.Furthermore,the mechanical properties,microstructure,and phase of the joints before and after post welding heat treatment(PWHT)were investigated and the evolutionary mechanism of microstructure was explored in combination with temperature field simulation and element distribution characteristics.Through our previous technological test in laser beam welding dissimilar joints between CLF-1 steel and 316L stainless steel,and the welding reliability of dissimilar joints with a thickness of 10mm at a laser power of 10 k W was explored.According to the optimized processing parameters,well weld formation was formed by controlling the offset of the laser beam in a different position.Meanwhile,the microstructure of the joint was explored in combination with phase to obtain the reasonable offset.Finally,a PWHT study was performed on the joint with the reasonable offset.The experiments showed that the optimal defocusing amount was considered to be-2mm.The reliable welding speed of 10mm dissimilar joint thick plate was 2m/min,and the laser beam offset limit was 0.35mm,which could obtain the weld formation without obvious defects.As the laser beam offset toward the 316L base metal by 0.2mm,the dissimilar joint with better mechanical properties between CLF-1 steel and 316L stainless steel was obtained by performing PWHT at 710?for 2h.The results showed that as the laser beam offset toward the 316L base metal,the fusion zone near the 316L steel narrowed gradually and slender columnar austenite grains grew from the FZ to the weld center.Meanwhile,the phase of WM changed from a martensite phase to a mixture of a martensite and austenite phase,then a dendrite austenite phase.The element analysis showed that as the beam was shifted from the 316L side,the contents of element in the WM were close to the values of the316 L base metal,reflecting the increase of Ni and Cr contents.Depending on the equilibrium phase diagrams of the three WMs,the temperature range of high-temperature?-Fe became wider as the laser beam offset toward the 316L base.The A_C1and A_C3in the WM dropped gradually,and simultaneously,the?-phase increased gradually,the M_sdecrease gradually and the?-phase dropped gradually.As the laser beam offset toward the 316L by 0.35 mm,the weld was composed mainly of austenite?-phase.As the laser beam offset toward the 316L by 0.2 mm,the microstructure showed that a large number of residual austenite(RA)phases were distributed between the martensite laths.WM showed a gradient of Cr and Ni concentrations with increasing nickel content.The grain boundary extension was formed by solidification nucleation and epitaxial growth of 316L side interface.After PWHT,the size of the martensite lath in WM was finer;the metal diffusion in WM was further mixed,and the Cr and Ni elements;the fluctuation of Cr and Ni elements were greater;The martensite lathes precipitated M₂₃C₆carbide in the crystal and at the crystal boundary,and the carbide coarsened and had dislocation structure.The typical columnar and cellular austenite dendritic crystals were formed by the diffusion of alloying elements at the 316L fusion interface,and the grain size of the heat affected zone of CLF-1 decreased obviously.Three phases,namely,austenite,martensite,and ferrite,co-existed in the weld according to the prediction results of the Schaeffler-Schneider diagram.which was consistent with the identification results of phases obtained using TEM scanning.Meanwhile,XRD intensity diagrams showed that the welds were I-body center lattice martensite/ferrite phase and F-face center lattice austenite phase.When the laser beam offset toward the 316L base metal by 0.2mm,the room-temperature and 550?mean tensile strength under the as-welded condition and after PWHT were slightly higher than the values of the CLF-1 base metal.However,the yield strength of the weld was lower than those of the CLF-1 base metal.At room-temperature,the specimens were all fractured at the 316L base far away from the WM.At a high-temperature of 550?,the specimens were all fractured in the CLF-1 base.The weld under the as-welded condition had a great hardness and the sample was fractured during 180°transverse side bending.After PWHT,the microhardness of the WM decreased significantly.No obvious cracks were observed after 180⁰transverse side bending test,and mean impact ductility was enhanced from130 J under the as-welded condition to 169 J.Moreover,the weld exhibited a ductile fracture and favorable durability.