A Study of Microstructure and Mechanical Properties for the Autogenous Single-Pass Butt Weldment of a Ferritic/Martensitic Steel Using Gas Tungsten Arc Welding

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 (delta)-ferrite. The delta-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: delta-ferrite/martensite duplex zone, fully recrystallized zone, and partly recrystallized zone. Prior austenitic grains did not coarsen in the delta-ferrite/martensite duplex zone due to the newly nucleated delta-ferrite grains and incompletely ferritizing (delta-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 degrees C. The tensile fracture occurred in the relatively soft BM tested from 20 to 600 degrees C. In as-tempered condition (760 degrees C for 1 h), M23C6-type carbides precipitated within the martensitic laths, the lath boundaries, and the delta-ferrite/martensite interfaces. Moreover, V, Cr, Mo-rich nitrides with very small size also precipitated in the delta-ferrite/martensite interface. The tempering treatment improved the homogenous distribution of weldment hardness significantly. Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600 degrees C. The impact toughness improved significantly, but the ductile-brittle transaction temperature was - 12 degrees C which was higher than that of the normalized and tempered (N&T) BM. delta-ferrite was considered to be one of the major factors aggravating the impact toughness in the FZ.