Mechanism Of Weld Forming And Deformation Control Of ITER Correction Coil BCC Case

Fully superconducting controlled magnetic confinement thermonuclear fusion power generation technology will be one of the sustainable development strategic new energy technologies of mankind in the future.On the background of the ruly construction for the international cooperation project-ITER(International Thermonuclear Experimental Reactor),it is of great significance to promote the development of physics basis science and engineering technology of controlled fusion energy.Under the fund support of ITER correction coils(CC),and the serious welding deformation of the ITER CC case,the mechanism of weld forming of 316LN austenitic stainless steel thick plate was investigated.The effect of the post weld heat treatment(PWHT)on the microstructure and mechanical properties of TIG welding joint of ITER CC sub-case was analyzed,and the key technology of laser enclosure welding of the case after the coil insertion was developed.Based on the effect of the PWHT on the microstructure and mechanical properties of TIG welding joint of ITER CC sub-case,the heat treatment technology for stress relief of large-scale case was developed,and the fracture mechanism of TIG welding joint of 316LN thick plate was revealed.Only single austenite was found on the TIG welding joint of sub-case after PWHT at 760℃/1h.However,the passivation zone range was significantly decreased of the 760℃/1h PWHT weldment,thus,the PWHT has decreased the corrosion resistance of the weld joint.The tensile strength and J-integral fracture toughness of the PWHT weldments are measured to be 97.4%and 74%of those of the as-welded weldments at 4.2 K,respectively.The stress(strain)induced martensitic phase transformation was happened near the crack propagation area.Differ to the ductile fracture mode of the as-welded joints,after the PWHT at 760℃/1h,the secondary crack and cleavage facets were observed on the fracture surface that reveal a mixture of ductile and brittle fracture mechanism.Base on the "sandwich"online monitoring method,the mechanism of defects formation during the deep penetration laser welding of 316LN thick plate was explored.And the relationship between the microstructure and mechanical properties of the laser enclosure welding of the case was established.The results show that the violent fluctuation of the rear keyhole leads to the collapse at the bottom of the keyhole which causes the formation of bubbles,and the bubbles gradually merging and growing results in the formation of key-induced porosity.The instability of the rear keyhole also leads to the periodic change of the size of the keyhole mouth,the liquid metal in the molten pool on the rear keyhole wall was rapid flowed upwards under the action of the metallic vapor pressure result in formation of the spatter.And the microstructure and mechanical properties of deep penetration laser welding were also investigated.The microstructure of the case laser enclosure welding joint was composed of single austenite and was characterized by a coarse grain with a weak orientation,the secondary phase was also found in the weldment.The tensile strength of the laser welding joints was 1522MPa,to be 98%of that of the BM at 4.2 K.The impact toughness of the HAZ and weldment of the laser welding joint were 72%and 59%of that of the BM at 4.2K,respectively.The fracture toughness K(J)Ic of the weldments was 240MPa·m1/2,decreased to 92%of that of the BM at 4.2 K.The fracture surfaces of tensile and impact test specimens were characterized by ductile fracture features with multiple dimples and microvoids.The crack propagating rate of fracture surface of fracture toughness specimens of the laser weldment along the crack propagating direction was increased at the end period.The dimples became small and shallow and some secondary particles can be found at the bottoms of dimples of the laser weldment fracture surface.The stress induced martensitic phase transformation was happened near the crack propagation location of both BM and weldment,the martensite fraction in weldment is much lower than that of BM.The combination of double ellipsoidal with 3D conical heat source for BCC case welding numerical analysis model was established.The effective control of the welding deformation and the prediction and evaluation of the fatigue life of the case welding joints are realized.Base on the adjust of the stress and strain field during the large-scale structural components of BCC case enclosure welding,the welding deformation of the BCC case was controlled within ±2mm.The technical difficulty of welding deformation control of large-scale case thick plate was solved successfully.And based on the numerical simulation results and the modification by Goodman’s formula,the equivalent stress amplitude Seq of the weldment is 211MPa,which is much lower than the stress amplitude of 410MPa corresponding to the design requirements of the fatigue life of CC coil.Therefore,the fatigue life of BCC case enclosure welding joints meets the design requirements.