The 316L thick plate electron beam welding (EBW) has been widely used in fusion test reactor manufacturing. Therefore, the numerical simulation of the 50 mm 316L austenitic stainless steel by two heat sources and experimental on microstructure and residual stress have been studied in this article. In the simulation study, the traditional heat source model (3D Gaussian heat source) and composite heat source (double ellipsoid heat source superimposed on the 3D Gaussian heat source) were proposed to simulate the welding of local joint. Weld cross-section, temperature curve, and residual stress after welding obtained by simulations were investigated. The experimental study involved residual stress tests and microstructure analysis. It turned out that the result of the composite heat source was closer to the actual joint. The residual stress distribution of simulation was validated and in accordance with experimental measurement. Moreover, the microstructures were studied by electro backscattered diffraction (EBSD) and compared with the temperature curve. The formation mechanism of microstructural heterogeneity was caused mainly by different thermal cycles at different positions of the thick plate. The top of the joint was more prone to stress concentration.

With the progress of the large-scale scientific facilities, the vacuum system and the stability of materials have shown the need for a higher level of reliability to ensure the stable operation of the beam and provide an ultrahigh vacuum environment. The key component such as the vacuum vessel of the Tokamak fusion experimental reactor, has a high requirement for its relative magnetic permeability, and the steel used must meet the relative magnetic permeability of less than 1.05 or even 1.03; therefore, the control of the magnetic permeability is of great significance for the development of nuclear fusion testing devices and various accelerators. This article conducts research on the issue of magnetic permeability exceeding standards in engineering applications of 316L (N) austenitic stainless steel plates, bolts, and other components. It is found that nickel element composition plays a decisive role in controlling magnetic permeability. In addition, bolts with relative magnetic permeability exceeding standards were demagnetized at 1050 $^{\circ}$ C, and it was found that their magnetic permeability was controlled, but their sealing performance decreased.