Influence of repair welding numbers on microstructure and mechanical properties of S355J2G4 steel welded joints

Abstract

In this study, original welds of 12 mm thick S355J2G4 steel used for railway vehicles were repaired through Metal Active Gas (MAG) welding to determine influence of repair welding numbers on microstructure and mechanical properties of welded joints. The results indicated that the weld macrostructure did not significantly change with increasing repair welding numbers, the phase composition of the microstructure in the heat affected zone were the same while the grain size slightly increased. The microhardness values on upper weld surface between the repaired and original weld fusion lines gradually reduced as repair welding number increased. Both the average yield and tensile strengths reduced with increasing repair welding numbers, while the average elongations remained nearly constant. The tensile specimens all fractured in base metal. There were no visible cracks appeared on surface of the bending test specimen. The average impact energy and median fatigue strength both presented gradual downward trends as repair welding number increased. The fatigue specimens of the 0R (original), 1R (repaired once), and 2R (repaired twice) welded joints all fractured in weld metal, while the fatigue specimens of the 3R (repaired thrice) welded joints fractured near fusion line, this change was primarily due to the serious joint softening as repair welding number increased. The above results indicated that the microstructure and mechanical properties of the repair-welded joints still could meet the application requirements even the repair welding number reached up to three, which could provide practical guidance for developing repair strategies in industrial applications.