Influence of chemical composition on the microstructural evolution and mechanical properties in dissimilar weld between Incoloy 825 superalloy and high strength low alloy API X70 steel was investigated in this research. Samples of base metals were welded with NiCrMo-3 and 309L austenitic stainless steel fillers using gas tungsten arc welding process. The microstructure of different regions including base metals, fusion zone and heat affected areas was characterized by optical microscopy (OM), scanning electron microscopy (SEM), linear scan technique of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis. The mechanical properties of the specimens were assessed by tensile, impact and micro-hardness tests. Microstructural studies showed that the structure of NiCrMo-3 weld metal was mostly austenitic, and segregation of Mo and Nb to the inter-dendrite spaces and solidication grain boundaries led to the deposition secondary phases. It was also demonstrated that 309L weld with its austenite + ferrite (FA) solidification mode has a dendritic microstructure with skeletal ferrite, segregated with a skeletal or worm morphology at the boundaries between dendrites or cells. The results of tensile test showed that NiCrMo-3 weld had the highest tensile strength and the rupture occurred in ferritic base material. Fractography examinations of tensile test specimens revealed that the fractures were mostly soft-type. Results of impact test indicated that NiCrMo-3 weld had the highest toughness. Examination of fracture surfaces revealed that the fractures were mostly soft-type in impact test. This was due to the austenitic structure of NiCrMo-3 weld alloy. This is also true for the soft-type failure of the tensile test samples. According to the micro-hardness test results, the NiCrMo-3 weld the highest hardness due to the formation of hard Nb and Mo-rich phases, as well as Lava. Also, the lowest hardness obtained was related to the heat affected zone of API X70. It can be attributed to the dissolution of precipitations and grain growth in this area because of the heat cycles applied during welding. key word Incoloy, high strength low alloy steel, dissimilar welding, transition zone, heat input, dendrite
