In this research, the properties of the dissimilar welding between Inconel 617 superalloy and AISI 321 AISI 321 austenitic stainless steel were investigated by Gass Tungdten Arc Welding (GTAW) method. The bonding of these two alloys was carried out with ERNiCrCoMo-1 filler metal and at three different heat inputs of 4.418, 5.126 and 6.22 Kj/mm . Microstructure study of base metals, welding metals and interface between base metals and weld metal investigated by using optical microscope, Scanning Electron Microscope (SEM) and X-ray diffraction analysis. The mechanical properties of the weld metal were investigated using microhardness and impact tests. Finally, Tafel corrosion test was performed in 3.5% sodium chloride solution and the effect of heat input on corrosion behavior of the weld metals were studied. According to the investigations on optical microstructure and the result of the the analytical software, Transition Zone (TZ) and Unmixed Zone (UZ) extended as the heat input increased. Also, the size of solidification grains has increased, but solidification subgrains decreased. Therefore increasing heat input leads the microstructure to finer morphology and finally mechanical properties improved, but corrosion behavaior declined. At the weld metal of the specimen with the highest input, type ? boundary observed locally. While in two other cases, normal epitaxial growth occured. The occurrence of this phenomenon in the specimen with the highest input temperature caused the local hardness to increase due to the formation of titanium carbonitrides at the fusion boundary. Also, the SEM micrographs showed that with the increase in the heat input, the failure mechanism remains soft and does not change, and only the fracture toughness increases. As a result of increasing the heat input, fracture toughness of samples A and C improved by 18% and 13%, respectively. Therefore, due to the improvement of mechanical properties and the deterioation of corrosion properties by increasing the heat input, it is not possible to determine the optimal heat input and the welding parameters should be considered in accordance with the conditions of service.
