In this work microstructure characteristic, mechanical properties and corrosion behavior of dissimilar joint between AISI 321 austenitic stainless steel and ASTM A516 low carbon steel and also improvement of microstructure and mechanical properties of this joint with heat treatment proces, were investigated. For this purpose, two filler metals including ER309L and ERNiCr-3 were selected to be used during the gas tungsten arc welding (GTAW) process. Microstructures investigation with optical microscope, scanning electron microscope and X-ray diffraction was performed. Impact test and micro-hardness test was performed to obtain the mechanical properties of samples. Also, to investigate the corrosion behavior of the joints, the specimens were tested in 3.5% sodium chloride solution using cyclic polarization test. In the microstructure of ERNiCr-3 weld metal niobium carbide distributed in the matrix and (mostly) in grain boundary. ER309L weld metal includes dual solidification structure, so that, adjacent the 321 stainless steel base metal consist FA solidification structure and adjacent the A516 base metal consist A solidification structure. The micro-hardness test result indicates that a carbide layer with high hardness in the ER309L weld metal adjacent to A516 base metal existed. The impact test result indicates that except the ERNiCr-3 weld metal that has both brittle and ductile fracture surface, the other base metals and weld metal has ductile fracture surface. In the corrosion tests, the highest resistance to corrosion was related to ERNiCr-3 weld metal and 321 base metal. Heat treatment of samples was performed in three modes; heated to 600? and water quench, heated to 1050? and water quench and heated to 1050? and air cool to room temperature for 1 hour. Heat treatment improves the microstructure of base metals and ER309L weld metal. A516 base metal microstructure was change to fine ferrite-pearlite microstructure. Also, the delta ferrite present of the HAZ in 321 base metal and in ER309L weld metal was reduced. Mechanical investigation of heat treatment samples indicated that the best mode of heat treatment was heated to 1050? and air cool to room temperature. Heat treatment in this mode reduced the high hardness of carbide layer in ER309L weld metal. Also, the impact energy of weld metals and base metals was increased. Key Words: AISI 321 austenitic stainless steel, ASTM A516 low carbon steel, dissimilar welding, heat treatment
