NIOBIUM EFFECT ON BASE METAL AND HEAT AFFECTED ZONE MICROSTRUCTURE OF GIRTH WELDED JOINTS

The development of steels for line pipes during the last decades has been driven by the need to obtain improved combinations of high strength, toughness, weldability on industrial scale at affordable prices. The effect of niobium content on the heat affected zone (HAZ) microstructure is reported in this paper. Niobium, for its specific thermodynamic and kinetic attitude to form carbide and nitride precipitates, played a key role in the development of modern HSLA steels Results show that niobium addition is able to refine both the bainitic packet and cells size in the heat affected zone during welding. This implies that niobium addition leads to an improvement of both toughness and hardness of welded joints manufactured by Nb micro-alloyed steels.