Experiences concerning RRR degradation and gas absorption of high purity niobium welded at different electron beam (EB) facilities are summarized. The oxygen and nitrogen content is increased as a rule at the welding seam. The absorption of hydrogen takes place not directly at the welding seam but in the heat-affected area. The RRR degradation is moderate, if the total pressure is better than 5×10 -5 mbar and the preparation is done under clean condition. A series of welding was done in the new

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... DESY EB facility, which was specifically designed to meet the requirements for the welding of niobium cavities for TESLA. High vacuum (between 2×10 -8 and 2×10 -6 mbar) allowed improving the niobium purity at the welding seam, in which the RRR were increased from 350 to ca. 370−380. The distribution of RRR, gases, grain size and thermal conductivity in the welding area are analyzed. The repair of a burned hole in the cavity is also discussed.