Microstructure And Mechanical Properties Of Laser Welded Joints Of Reduced Activation Ferritic/Martensitic Steel Heavy Plate With A Thickness Of 35mm

Reduced activation ferritic martensitic?RAFM?steel is chosen as a structural material for test blanket modules?TBMs?to be used in International Thermonuclear Experimental Reactor?ITER?and China Fusion Engineering Test Reactor?CFETR?.Chinese specific RAFM steel named with CLF-1 has been developed for CFETR.In this paper,a narrow-gap groove laser multi-pass welding of CLF-1 steel with thickness of 35mm is conduced by YLS-15000 fiber laser.The results show that the butt weld joint of CLF-1 steel with a thickness of 35 mm was well-formed using the optimal narrow-gap laser filler wire welding and no obvious defects were found,such as incomplete fusion cracks and pores.Further,the microstructures and mechanical properties of welding joints under optimal welding conditions were systematically investigated.Moreover,the changes of the microstructure and mechanical properties with the post-welding heat treatment were compared.The mechanisms were also discussed.Through our previous technological test in thick-plate narrow-gap laser filler-wire welding,the thickness of weldable truncated edge in CLF-1 steel was determined at the laser power of 15 kW.Then the thickness of blunt edge of narrow gap filler wire welding groove was determined.The microstructure of laser autogenous welding joint was analyzed.According to the optimized processing parameters,the upper groove size was estabilished.Best weld formation was also formed by controlling the defocus of energy density distribution.The low defocussing amount helps to achieve adequate energy distribution in laser wire-filling welding process under medium and small power conditions and ensures the input of tremendous heat to welding wires and BM.The pores and incomplete fusion defects in laser filler-wire welding of CLF-1 steel were most effectively controlled and layer-to-layer transitions became smooth.Further,this welding condition exhibits a wide technological adaptability.Therefore,the optimal defocusing amount was determined to be+20 mm.The macroscopic metallographic morphology of the welding joint after laser filler-wire welding where the backing layer of the welding joint was formed by laser autogenous welding using a truncated edge of 13 mm and the layer-by-layer filling layer was filled by 17 weld beads was obtained.The results show that the microstructures of the weld metal in filling and backing layer of the joints are almost the same.All of them are coarse lath martensite,but the coarse columnar crystals appear in the center of the welds of the filler wire.And the grain size of filling layer is significantly larger than that of the backing layer.The microstructures of FZ closed to the fusion line were mainly composed of martensites and a few of carbides.The metallographic structures of HAZ around the fusion line were mainly composed of two-phase hybrid structures including secondary-tempering sorbites and martensites formed through weld thermal cycles.The metallographic structures of HAZ near BM still remain original fine tempered sorbites.After PWHT with different holding time for the welded joints,the base material zone still remain original fine tempered sorbites.And the grain size of the heat affected zone gradually decreases from the fusion line to the direction of the base metal.After PWHT,more lath martensites are found and polygonal fragmentation has occurred at the grain boundaries along with forming metastable dislocation sub-grain-boundary.The precipitated phase mainly consists of elliptic M₂₃C₆ carbides with nano-scale.With the quenching time further extending,the grain size of the weld zone tends to grow,and the martensite laths increase.At the same time,the intragranular carbide particles diffuse into the grain boundaries,and the carbide particles aggregate toward the grain boundaries.The microhardness of weld joints of CLF-1 steel laser welded joints was significantly higher than that of the base metal,and no obvious softening was observed in the heat affected zone.After PWHT,the overall hardness distribution trend was similar to that of as-welded joints,and the weld metal hardness was significantly decreased.With the quenching time extending,the microhardness of the weld is not significantly reduced.The tensile strength of laser welded joint is very high.And it will decrease slightly with PWHT.What's more,the tensile strength of the joints will be further reduced with the quenching time is further extended.But it is still equivalent to the level of the base metal.The tensile specimens at room temperature and high temperature were both fractured in the parent metal far from the weld.The impact toughness of welded joints are very poor.It can significantly improve after PWHT.But the lath martensite grain size will increase,and the impact toughness will decrease slightly with the quenching time is further extended.