A 12％Cr ferritic/martensitic steel,HT-9,has been used as a primary core material for nuclear reactors.The microstructure and mechanical properties of gas tungsten arc butt welded joints of HT-9 in as-welded,and as-tempered conditions have been explored.In as-welded condition,the fusion zone(FZ)contained a fresh martensite matrix with delta(δ)-ferrite.The δ-ferrite was rich in Cr and depleted in C compared with the matrix.The heat-affected zone(HAZ)could be divided into three areas as the distance from the fusion line increased:δ-ferrite/martensite duplex zone,fully recrystallized zone,and partly recrystallized zone.Prior austenitic grains did not coarsen in the δ-ferrite/martensite duplex zone due to the newly nucle-ated δ-ferrite grains and incompletely ferritizing(δ-ferrite)during the welding thermal cycle.The weldment microhardness distributed heterogeneously with values above 600 HV1 0 in the HAZ and FZ and 250 HV1 0 in the base metal(BM).Solute C in the matrix,induced by the dissolution of carbide during the welding process,dominated the microhardness variation.Low toughness was observed in the FZ with a quasi-cleavage fracture tested from-80 to 20℃.The tensile fracture occurred in the relatively soft BM tested from 20 to 600℃.In as-tempered condition(760℃for 1 h),M23C6-type carbides precipitated within the martensitic laths,the lath boundaries,and the δ-ferrite/martensite interfaces.Moreover,V,Cr,Mo-rich nitrides with very small size also precipitated in the δ-ferrite/martensite interface.The tempering treatment improved the homog-enous distribution of weldment hardness significantly.Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600℃.The impact toughness improved significantly,but the ductile-brittle transaction temperature was-12℃which was higher than that of the normalized and tempered(N&T)BM.δ-ferrite was considered to be one of the major factors aggravating the impact toughness in the FZ.