Compatibility of ferritic-martensitic steel T91 welds with liquid lead-bismuth eutectic: Comparison between TIG and EB welds

The 9 wt% chromium ferritic-martensitic steel T91 is being considered as candidate structural material for a future experimental accelerator driven system. This material and its welded connections would need to be used in contact with liquid lead-bismuth eutectic, under high irradiation doses. Both unirradiated tungsten inert gas and electron beam welds of T91 have been examined by means of metallography, scanning electron microscopy, transmission electron microscopy, Vickers hardness measurements and tensile testing in both gas and liquid lead-bismuth environment. The TIG weld was commercially produced and post weld heat treated by a certified welding company while the post weld heat treatment of the experimental EB weld was optimized in terms of the Vickers hardness profile across the welded joint. The mechanical properties of the T91 TIG and EB welds in contact with LBE have been examined using slow strain rate tensile testing in LBE at 350°C. All welds showed good mechanical behavior in gas environment but total elongation was strongly reduced due to liquid metal embrittlement when tested in liquid lead-bismuth eutectic environment. The reduction in total elongation due to LME was larger for the commercially TIG welded joint than for the EB welded joint.