Effect of nickel and molybdenum additions on weld metal toughness in a submerged arc welded HSLA line-pipe steel

The effects of alloy additions of nickel (Ni), molybdenum (Mo), and Ni and Mo together on the impact toughness of an API HSLA-70 steel by submerged arc welding in the laboratory were investigated and micro-structural factors which affect the impact toughness were discussed. Ni additions resulted in a low impact toughness and an increased fracture appearance transition temperature (FATT) in weld metal (WM). The above influences of Ni should be attributed to the formation of acicular ferrite (AF) suppressed by increasing the Ni content. Conversely, the combined presence of Ni and Mo in the WM decreased the volume fractions of grain-boundary ferrite (GBF) and promoted formation of high toughness of AF. The increase of Mo content created an acicular ferrite-predominant weld metal microstructure with impressively improved toughness. Mo addition of 0.881 wt.% in the WM gave the optimal impact toughness at −45 °C with a microstructure of 77% AF and 20% granular bainite (GB).