Research On The Microstructure Properties And Stress Evolution Of TIG Weld Joint Of CLAM Steel

Reduced activation ferritic/martensitic(RAFM)steel was primary candidate structural material of fusion reactors with obvious advantages of reduced activation,irradiation resistence,high thermal conductivity as well as good industrial production ability.According to the current design of international thermonuclear experimental reactor(ITER),RAFM steel was chosen as the primary candidate structural material for test blanket module(TBM)and the material of cooling pipes directly connected to the TBMs is expected to be austenitic steel type 316L.Joints between CLAM and 316L steels need to be developed to achieve the reliable connection between TBMs and piping system.China Low Activation Martensitic(CLAM)steel,a type of RAFM steels developed by INEST for more than one and half decades under wide domestic and international collaboration,has been chosen as the primary candidate structural material for Chinese test blanket module(TBM)for ITER by China International Nuclear Fusion Energy Program Execution Center(CNDA).In this manuscript,microstructure,mechanical properties as well as stress evolution and residual stress distribution of CLAM similar and CLAM-316L dissimilar joints were investigated.Furthermore,the stress distribution among typical welding structures of TBM was predicted using FEM with purpose to support TBM fabrication and assembly.Firstly,microstructure and mechanical properties of CLAM similar and CLAM-316L dissimilar joints using tungsten inert gas(TIG)welding process were investigated.In order to improve the weldability of CLAM steel and suppress the formation of 8 ferrite which will deteriorate the impact property of the weld joints,a new type of filler wire was proposed and fabricated.Multilayer joint of CLAM steel was conducted using TIG process with preferred welding parameter based on series of investigation.The yield strength of the as welded joint was 492 MPa and the impact absorbed energy was 27.7 J with good plastic deformation and toughness performance.The impact absorbed energy of weld metal increased to 53.8 J after post weld heat treatment(PWHT)of tempering at 740? for 2h.Results of research on microstructure and properties of CLAM-316L dissimilar joints showed that CLAM heat affected zone(HAZ)and fusion zone(FZ)was easy to be deteriorated.The yield strength of the joint was only 60%of base metal with the rupture occurred at CLAM HAZ.After PWHT of tempering at 740? for 2h,yield strength was improved to 70%of base metal ruptured at 316L base metal.Impact absorbed energy of CLAM HAZ incresed from 110 J to 184 J,nearly equal to that of base metal.Secondly,the stress evolution and residual stress distribution of CLAM similar and CLAM-316L dissimilar joints were analysed.The weld stress was mainly produced by backing welding.When the heat source passed through,both sides of the base metal were subjected to 'compression' and then 'tensile' stress.Among CLAM-316L dissimilar joint,weld stress of CLAM steel side stress was significantly higher than that of 316L steel side.The residual stress distribution of CLAM similar joints was symmetrical with the stress peak value was 350 MPa,however,the residual stress distribution of CLAM-316L dissimilar joints was asymmetrical with the stress peak value of CLAM side was 450 MPa and that of 316L side was 360 MPa.After PWHT of tempering at 740? for 2h,the residual stress of CLAM similar joints and the stress in 316L side of CLAM-316L dissimilar joints were eliminated.The stress peak value decreased to lower than 90 MPa.Lastly,the stress distribution of typical welding structures of TBM,such as T-joint between first wall(FW)and back plate(BP),Frame structure weld,pipe-plate T-joint and pipe butt-joint,was simulated using FEM software named Sysweld.The effect of restraint condition,welding sequence on the stress distribution was analysed.Effect of restraint stress distribution is mainly manifested in avoiding stress concentration and reducing high stress area.The stress peak value of frame structure weld was almost the same under Four-point Clamp(FC)condition and Bilateral Clamp(BC)condition,but the welding deformation was lower under BC condition.For pipe-plate T-joint,the high stress area was larger under BC condition,and stress concentration occurred at the edges of the plate and the hole.For pipe butt-joint,when both the ends was rigid fixed,the high stress area expanded and the stress concentration is more obvious.Stress concentration both occurred at CLAM HAZ with one end or two ends rigid fixed,with the stress peak value of 333 MPa and 361 MPa respectively.For frame structure weld,pipe-plate T-joint and pipe butt-joint,the symmetrical discontinuous welding sequence can reduce high stress area,compared with that of integrated continuous welding sequence.In the work,the microstructure,mechanical properties and stress evolution of CLAM similar and CLAM-316L dissimilar joints was investigated,and the stress distribution of typical welding structures of TBM was analysed.The results will provide technical support for fabrication of TBM and its reliable service in the future fusion reactors.