Strength and deformability of welded joints of 780 MPa grade steel plates

Seven hundred and eighty megapascal grade low C–Mn steel plates with two different chemical compositions were manufactured under various rolling conditions. Three types of welded joints, in which the strength of the weld metal was smaller than (under-matching), larger than (over-matching), or equal to (even-matching) that of the base metal, were fabricated from the developed steel plates. Investigation of the welded joints reveals that all of the joints softened in the heat-affected zone (HAZ) and the tensile specimens of the welded joints ruptured from the HAZs. However, by utilizing strength over-matching weld metal, welded joints can be produced with almost the same strength of the base metal. Furthermore, the deformability of the welded joints can be improved by lowering the yield ratio (yield strength/tensile strength) of the base metal.