Correlation between microstructure and mechanical properties of welding joint in 9% Ni steel with two types of Ni-based superalloy filler metals

Flux-cored arc welding (FCAW) joints with 9% Ni steel were prepared using Ni-based superalloy filler metals. The correlation between the microstructure and mechanical properties of the weld joint based on the type of filler materials was investigated. Owing to the heat transferred during the welding process, the heat-affected zone (HAZ) of the base metal primarily comprised martensite and exhibited higher hardness than the weld metal and existing base metal. To evaluate the toughness of FCAW joints against low-temperature fractures, Charpy impact tests at −196 °C were conducted; an absorbed impact energy of at least 55 J was observed in the weld metal, HAZ, and base metal regions, which was significantly higher than the standard specification. Comparing Alloys 709 and 609 as filler metals, a higher absorbed impact energy was observed when using Alloy 709 in both the weld metal and HAZ regions. This is because the quantity of precipitates, which can act as a point of crack initiation and propagation in weld beads, was smaller when Alloy 709 was used, and the γ phase was stably maintained even at low temperatures owing to the high content of elements that decrease the martensite start temperature (γ stabilizers) in the bead and transition areas. Furthermore, Alloy 709 obtained a low volume fraction and small grain size in the coarse-grained HAZ, which is known as the toughness degradation zone. This can contribute to the higher impact toughness compared with Alloy 609.