Investigation on microstructure and mechanical properties of 160mm thick 316L stainless steel electron-beam-welded joint

The maximum thickness of flanging position on the vacuum vessel (VV) window collar of China Fusion Engineering Test Reactor (CFETR) is 160 mm. Electron beam welding can achieve one-time penetration of the thick plate without groove and the deformation after welding is quite small. The cross section of weld was sampled for microscopic observation and mechanical properties tests to explore the difference in microstructure and performances of 160 mm thick 316 L stainless steel electron-beam-welded joints. The results showed that the weld depth-width ratio reached up to 16:1, and the microstructure of weld was composed of ferrite and austenite with different morphologies which was mainly coarse cellular crystal in heat affected zone (HAZ). The equiaxed grain orientation in the root region (denoted as "R region") was mostly [001] and the columnar grain orientation along the temperature gradient was mostly [101]. The microstructure along the depth direction was quite different, which was mainly related to the solidification mode and composition overcooling. The high strength (YS=715.22 MPa) of R region was due to the smaller temperature gradient and the larger cooling rate, the position of fracture suggested that the strength of weld was higher than that of the base metal. The microhardness measurement results showed that the hardness increased gradually along the depth direction of weld (Top:156.5 HV, Root: 238.7 HV) and fluctuated along the width direction. The tensile properties along the thickness direction exhibited a trend of decreasing first and then increasing, which was mainly related to the characteristics of electron beam deep penetration welding and the difference of grain structure along the thickness direction.