The effect of filler metal composition on the microstructure and mechanical properties of four wire tandem submerged arc welded API 5LX65 steel was investigated. Microhardness, Charpy impact, and all weld longitudinal round tensile tests were applied to evaluate the mechanical properties of the weld metals. The microstructure of specimens was studied by optical and scanning electron microscopes. The microstructural observations showed that the more acicular ferrite was formed in the microstructure, the better mechanical properties were achieved. The impact energy and strength values were higher for the specimen containing more amounts of titanium and boron. The ductile to brittle transition temperature could be decreased by increasing the amount of acicular ferrite. For the microstructure with 80% acicular ferrite, the impact energy was about 70 J at –60°C. The addition of titanium dispersed fine oxide inclusions and facilitated the nucleation of acicular ferrite in the microstructure. The oxide particles with a size of 2 ìm acted as the nucleation sites of acicular ferrite.

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