Understanding the weldability of niobium-bearing HSLA steels

The last four decades have seen remarkable progress in the development of High Strength Low Alloy (HSLA) steels for structural applications in pipelines, offshore structures, ships and buildings. These developments have been driven by the need to obtain improved combinations of weldability, toughness and strength in tonnage quantities at affordable prices. They have engaged all sectors of the industry; steelmakers, manufacturers, constructors, operators and safety regulators, and have spanned disciplines from theoretical modeling to manufacturing and site construction activities. At the heart of these developments has been the understanding of the physical and chemical metallurgy of the steel product, how the joining process influences it and how this in turn determines the performance, integrity and reliability of the final installation. Interest has focused particularly on the weld heat affected zone, its microstructure and properties, especially the effect of steel chemistry, weld process and heat input, preheat and post-weld heat treatment on weldability and toughness. While significant improvements in weldability have been achieved over the years through reductions in the carbon equivalent, a consequence of this has been the increased reliance on thermo-mechanical processing and a shift to microalloy-dominated strengthening mechanisms, away from the carbon-dominated mechanisms on which the original understanding was based. In turn, improvements in toughness in one region of the multi-pass weld heat affected zone have exposed other regions as the weakest remaining links. A resurgence of interest in yet higher strength, especially for pipelines, has prompted closer examination of many engineering-critical performance parameters associated with the integrity of the weld zone. The paper examines recent and current research that is directed towards obtaining a more coherent understanding of the effects of welding parameters and steel composition on both weldability and toughness, focusing particularly on the parent metal heat affected zone. It explores how the different influences are kept in balance, thereby ensuring the effective construction and reliable operation of nextgeneration pipelines and structures. (AU)