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

Abstract

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 nucleated 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 °C. The tensile fracture occurred in the relatively soft BM tested from 20 to 600 °C. In as-tempered condition (760 °C 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 homogenous distribution of weldment hardness significantly. Tensile fracture still occurred in the BM of the weldment specimens tested from 20 to 600 °C. The impact toughness improved significantly, but the ductile–brittle transaction temperature was − 12 °C 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.