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To resist large electromagnetic loads, the ITER correction coils (CCs) are protected by a 20-mm thickness 316LN austenitic stainless steel case. Due to the strict tolerance requirement of the case manufacture, tungsten inert gas (TIG) welding technology was used for subcase assembly welding, whereas laser beam welding (LBW) technology was applied to enclosure welding of the cases. The case shall be operated at 4.2 K and shall undergo considerably cyclic stress due to magnet electromagnetization as well as interaction with the toroidal field (TF) and poloidal field (PF). This article focuses on the assessment of the mechanical properties of the welding joints at cryogenic temperature. Results indicated that the values of the ultimate tensile strength of the TIG welding joint and the LBW welding joint were 1555 and 1522 MPa, respectively, which reach to 99.7 and 97.6% of that of the base material (BM). The elongation at the fracture of both the TIG and LBW weldments was larger than 35%. The plane strain fracture toughness KIc(J) of the TIG and the LBW weldments was 210 and 241 MPa·m1/2, respectively. The fractographic analysis indicated that the fracture surface was composed of fine equiaxed dimples, which indicated that both welding joints fractured in a ductile manner. The welding joints survived the minimum of 30 000 cycles fatigue test at stress level and load ratio specified by the ITER document. In addition, the fatigue crack growth rates of both welding joints were lower than that of the BM. All mechanical properties of both the welding joints satisfied the ITER specifications. [ABSTRACT FROM AUTHOR]

Laser welding of modified 316LN steel with a thickness of 20 mm was conducted using a YLS-20000 fiber laser. The microstructure of the weld joint was characterized and tensile and fracture toughness tests were carried out. The microstructure evolution of the fracture specimens was studied systematically to elucidate the fracture mechanism. The weld was composed of the single austenite phase and was characterized by cellular and columnar grains, and the grain size became coarser with a weak orientation. The R m values of the laser welded joints were almost equal to that of the base metal both at RT and 4.2 K, and the fracture surfaces were featured by ductile fracture with quantities of dimples and microvoids. The fracture toughness of the weldments decreased to ˜84% of that of the parent metal at 4.2 K. Enhancement of the fracture toughness was attributed predominantly to the presence of twins and to the fine grain size, and the reduction was caused by the partially stress-induced phase transformation of the austenite to martensite. The synergistic effects of these factors result in a favorable improvement in the fracture toughness of the weldment. [ABSTRACT FROM AUTHOR]