Incoloy 800 superalloy and HP heat resistant steel are alloys having excellent strength and corrosion resistance at high temperature. Dissimilar welds of both alloys are widely used at environments such as steam generator materials, reformer and pyrolysis tubes in the refinery and petrochemical industries. In this research, first microstructural and mechanical properties changes of two alloys were characterized before and after the aging. The microstructures were identified using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS) analysis. The aging effects of two alloys on its weldability, microstructure and properties of weld metal and its heat affected zone were then studied. It was found that the aging causes crack formation in the heat affected zone of two alloys resulting in decreasing of the weldability. The effects of heat input, interpass temperature, type of filler material and a pre-weld solution annealing were then investigated. It was found that the most important step to improve weldability and to reduce cracking susceptibility is solution annealing. A suitable annealing treatment was then proposed. Hot cracking susceptibility of base and filler materials was investigated by Varestraint test. The results showed that the aging increases hot cracking susceptibility, while the solution annealing treatment can reduce this susceptibility. It was concluded that dissimilar welding of alloy 800 to HP steel can be successfully carried out under the conditions of erforming a suitable pre-weld solution annealing and selecting low heat input, low interpass temperature with using Inconel 82 or 617 filler materials.
