Nitrogen segregation and blister formation of 316LN austenitic steels during electron beam welding

Full text of publication follows: High nitrogen austenitic stainless steel (316LN) has been selected as the structure material in shield blanket and the gravity support system in ITER due to its excellent erosion/corrosion resistance, high strength and toughness. However, most of nitrogen in this steel exists in the form of solid solution. The nitrogen can segregate from the matrix material and form small blisters or defects in the welding area, resulting in mechanical property reduction, which should be considered in the design and manufacture processing. In this study, we have investigated the blister and defect formation processing during electron beam welding. Focused electron beam with 100-150 kV high voltage, 300-500 mA beam current has been applied to weld the 316LN austenitic stainless steel components under vacuum condition. The blister formation in the welding area has been observed by both SEM and TEM directly, and was further confirmed by the micro-area composition analysis. The size and density distribution of blisters and defects with the welding depth, the vacuum condition and electron beam parameters has been investigated. At the same time, the tensile strength of the welded components was examined and compared with that of the matrix material. In this report, the mechanism of nitrogen blisters formation and its effects on the mechanical property of the welding components has also been discussed. (authors)