Microstructural Evolution and Mechanical Properties of FCAW Joints in 9% Ni Steel Prepared with Two Types of Ni-Based Weld Metals

The microstructural and mechanical evaluation of 9% Ni steel with flux-cored arc welding (FCAW) was performed with two different Ni-based weld metals: Inconel 625 and Hastelloy 609. The weld metals showed microstructural changes depending on the temperature gradient and crystal growth rate for each region during the cooling after welding. A cellular/planar growth was exhibited at the bottom of the weld metal, which was rapidly cooled in contact with the cold base metal. Columnar dendrites were exhibited in the central region that cooled relatively slowly, and precipitates were observed in the interdendritic region. The weld joints between the base metal and weld metal have a compositional transition region due to dilution. The transition region comprised a martensite layer and a γ-phase cellular/planar layer, according to the compositional distribution. In the low-temperature toughness test, the absorbed impact energies were 89 and 55 J for Inconel 625 and Hastelloy 609, respectively. When Inconel 625 is used as the weld metal compared to Hastelloy 609, the high content of the γ-stabilizer and martensite start temperature decreasing elements leads to the formation of a thicker γ-phase layer and thinner martensite layer in the transition region. In addition, the high content of these elements suppresses the martensite transformation and maintains the stability of the weld joint interface even at low temperatures, resulting in the higher absorbed impact energy. The microstructure of weld joints and its influences on mechanical properties help to improve the practical application of 9% Ni steel FCAW.