Effects of interlaminar microstructural inhomogeneity on mechanical properties and corrosion resistance of multi-layer fiber laser welded high strength low alloy steel

The T-joints of high strength low alloy steel with 14 mm were made by fiber laser welding. Full penetrated welded joints without visible flaws such as pores, cracks and lack of fusion was obtained. The former weld was remelted and heat treated by the latter weld during multi-layer welding, which resulted in microstructure transformation and performance variation of the welded joint. The heat treatment caused by the latter welding process lead to not only grain growth but also carbide precipitation within the former weld and the heat-affected laser weld with inhomogeneous microstructures was formed in the interlaminar zone. Microhardness of the heat-affected laser weld was lower than that of the unaffected laser weld and reduced from the phase transformation line to the re-fusion line. The microstructural inhomogeneity of the interlaminar zone was regarded as the main reason of the microhardness variation and the uneven residual stresses distribution within the heat-affected zone was considered to be a secondary reason of this phenomenon. The corrosion resistance of the welded joint was better than that of the rolled base metal and the inhomogeneous microstructures resulted in the potential difference which exacerbated the electrochemical reaction and weakened the corrosion resistance of the welded joint.