Study of high toughness ferrite wire for submerged arc welding of pipeline steel

With the development of steel plate and the need of high toughness pipeline, it is necessary to develop wire and flux for high efficiency submerged arc welding to produce weld metal, which can match the properties of the base metal. The weld metal is designed to mainly consist of acicular ferrite. The variations of elements between wire and weld metal, and the effects of Mo, B and Ti on the microstructures and mechanical properties are discussed. Fine and uniformly distributed inclusion particles in the weld metal can promote the formation of acicular ferrite. TiN particles can pin the prior austenite grain boundaries of the heat-affected zone (HAZ) and inhibit the growth of austenite grains. B segregates in the boundaries of prior austenite grains and suppresses the growth of proeutectoid ferrite. The toughness of weld metal is high and stable. The hardness of the weld metal is between Hv 185 and 214, which is in the same level of the hardness of base metal, Hv 190. The welded joint has a higher strength than that of the base metal, good ductility, and a low ratio of σ0.5/σb.