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Dissimilar welded joints of reduced activation ferritic/martensitic (RAFM) steel and 316L austenitic stainless steel were prepared by friction stir welding with different butt joining modes and welding parameters.The weld quality of the joint was improved by placing the 316L steel on the advancing side and the RAFM steel on the retreating side,and using a relatively high rotational speed of 400 rpm.The microstructure of the stir zone on the 316L steel side consisted of single-phase austenite,and the microstructure of the stir zone on the RAFM steel side mainly consisted of lath martensite and equiaxed ferrite.A mechanical mixture of the two steels and diffusion of Cr and Ni could be detected near the bonding interface.Diffusion of Ni from the 316L steel to the RAFM steel resulted in the formation of a dual-phase structure consisting of austenite and ferrite.The as-welded joints showed good strength and ductility at room temperature and 550 ℃,which were nearly equal to those of the 316L base material.The heat-affected zone on the RAFM side had the lowest impact toughness throughout the weld with a value of 13.2 J at-40 ℃,～ 52％ that of the RAFM base material.