Overlay welding technique is one of methods used to improve metal mechanical properties such as strength, toughness and corrosion resistance. Generally, Inconel 625 alloy is used for overlay welding layer on low carbon steels for economic consideration. However, the method produces some problems in the microstructure of the cast structure and some defects, caused by the elevated temperatures of the overlay welding process. To resolve these problems, heat treatments are required. In this study, Inconel 625 alloy was welded on a low carbon steel by the overlay welding process to investigate the resulting microstructure and mechanical properties. A double heat treatment was performed to improve the mechanical properties of the welding and substrate layers. It was found that Inconel 625 alloy had an austenite microstructure after the first heat treatment, but the low carbon steel had a ferrite-pearlite microstructure after the second heat treatment. After the double heat treatment, the sample showed the optimum hardness because of grain refinement and homogenization of the microstructure. (Received October 23, 2015; Accepted February 29, 2016)