Mechanical Properties of Welded Steel Joint in Molten Zinc

Abstract

It was reported previously by one of the authors that steel became brittle by “Liquid Metal Embrittlement” when it was stressed in tension in molten zinc. Cracks formed in a welded steel structure in molten zinc grow mainly in the region adjacent to the welded jotnt. Therefore, it is necessary to study the characteristics of the welded joints in molten zinc to prevent the initiation of cracks.
In this study, tensile and bending tests were carried out in molten zinc using both SM50A and STK55 steel welded joints and the results were compared with those of the base metals. The welding methods used were both manual and semi-automatic CO₂ arc welding. The types of joints used were butt welded and fillet welded ones. Fractographic features of the ruptured surface of the welded joints in molten zinc were examined.
The following conclusions were obtained:
(1) The welded joints became brittle by tensile stress and elongations were reduced in the same way as the base metal in molten zinc. The tensile strength of the butt welded joint was also reduced to 75-78% and that of the cruciform fillet welded joint became 57-68% compared with those at room temperature in air, respectively.
(2) The tensile strength of the cruciform fillet welded joint in molten zinc was reduced by 7-16% compared with that of the butt welded joint because of stress concentration at the weld bead.
(3) The bending angle, at which cracks initiate, for the butt welded joint without reinforcement of weld, was smaller than that of the base metal in molten zinc.
(4) The intergranular facet patterns were observed in the crack initiating position of both the weld metal and the bonding portion fractured in molten zinc, as in the case of the base metal.