Microstructure and Mechanical Properties of the Enclosure Welding Joint for ITER Correction Coils Cases

Abstract

To resist the alternating electromagnetic loads, the International Thermonuclear Experimental Reactor (ITER) correction coils are protected by a 20-mm-thick 316LN austenitic stainless steel case. According to the strict tolerance requirement of the case manufacture, laser welding was applied to enclosure welding of the cases with a 20-kW high-power laser for root pass and Tungsten Inert Gas welding was applied for filler and cover passes. In this study, the microstructure and mechanical properties of enclosure welding joint were investigated to evaluate the quality of the joints. No porosity, crack or lack of fusion was found on the cross-section of the joints. The microstructure in the weld zone is single austenite phase and the morphology of the joint consists of cellular crystal, columnar crystal, and small amount of the equiaxed grain. The average tensile strength of the joint is 590 MPa, which is 98.3% of the strength of the base material of 600 MPa. The fracture position is located at the base material with an apparent necking morphology, which confirmed the good plasticity. The side bend test specimens were subjected to significant plastic deformation, which bent through 180° and no surface crack can be found. The impact toughness of the weld joint at 4.2 K is larger than 180 J/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and the fracture surface is composed of fine equiaxed dimples and tearing ridges, which indicates that the specimens failed in a ductile manner. All of the test results satisfy the ITER requirements.