IMPROVEMENT OF WELD METAL TOUGHNESS IN HIGH HEAT INPUT ELECTRO-SLAG WELDING OF LOW CARBON STEEL

In recent years, important steel structures of high-rise buildings such as the box columns required high toughness of welded joints considering the shockproof against the large-scale earthquake. In Japan, there are some specifications that demand Charpy impact energy more than 70 J at 0 degrees C in any welded joints. However in high heat input electro-slag welding, the microstructure of the weld metal becomes very coarse and weld metal toughness is remarkably deteriorated. In this paper, authors tried to form fully acicular ferrite microstructure in the weld metal of high heat input electro-slag welding to improve the weld metal toughness. High heat input (approximately 100 kJ/mm) electro-slag welding was carried out in the 60 mm thickness steel plate joint. Applying high Ti bearing welding wire and low basicity welding flux provided the weld metal with acicular ferrite microstructure, because large amounts of Ti containing oxide inclusions which were known as effective nucleation sites for acicular ferrite formations were dispersed in the weld metal. B addition to the weld metal suppressed the formation of coarse grain boundary ferrite when B/N ratio in the weld metal ranged between 0.5 and 0.8. When B/N ratio in the weld metal exceeded 0.8, the weld metal toughness was deteriorated due to an increase of M-A constituent. Consequently it was possible that fully acicular ferrite microstructure was formed in the weld metal of high heat input electro-slag welding and achieved the excellent weld metal toughness (vE(0) > 100 J).