Improvement of weld metal toughness in HSLA line-pipe steel

High strength low alloy (HSLA) steels are designed to provide better mechanical properties (strength and toughness) and/or greater resistance to atmospheric corrosion than conventional carbon steels. They are preferred to similar carbon-manganese structural steels because of their very low alloy content, which distinguishes them with respect to the cost, weldability, corrosion resistance and strength to weight ratio. The higher strength and toughness of HSLA steels is the result of their acicular ferritic microstructure achieved by closely controlling their composition during steel making and the thermo-mechanical processing.; One of the drawbacks is that when HSLA steel is subjected to a thermal cycle such as welding, the weld region will no longer retain the base metal properties. The weld metal (WM) composition is altered; its structure resembles an as-cast structure and there is a severe degradation of impact toughness properties.; The research was confined to submerged arc welding (SAW) of API X70 line-pipe HSLA steel. The properties of a two-sided mill WM were analyzed and compared with simulated two-sided and single-sided laboratory WMs. Attempts have been made to improve the sub-zero temperature toughness properties of WM by using welding fluxes with slightly higher basicity indices (1.20 and 1.29 instead of 0.98), and by changing the WM composition by adding pure Ni (up to 3.75%) and Mo (up to 1%) metal powders in the laboratory welds.; The simulated two-sided laboratory weld demonstrated better impact toughness properties than the mill weld as a result of differences in the welding conditions. It was found that the fluxes of different basicity indices resulted in different, but comparable impact toughness properties. Nickel addition was harmful and Mo addition was beneficial in improving the sub-zero temperature impact toughness of the WM.; The difference in impact toughness was a result of compositional differences, amount of grain boundary (GB) phases and fineness of the acicular ferrite (AF) in the WM. Within a group of WMs of similar compositions, the impact toughness was reduced with increased GB phases and by coarser AF. Maximum improvement over the standard WM of impact energy of 50J (50%) at -45°C accompanied by 75MPa (12%) higher yield strength was noted for laboratory WM with 0.881% Mo.