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.Keyword: welded joints, microstructure, precipitation
IntroductionThe 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. A similar situation occurred in other fields of application of structural steels like offshore steel structures or ships, with broadly similar objectives, even though the balance of requirements could vary depending on the specific design or operational needs. 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 [1][2][3][4][5][6][7][8]. These steels have certainly made possible the efficient and cost-effective design and the development of construction technologies in a variety of applications. In the field of transportation pipelines, for example, the increase in the available strength level of linepipe that have taken place during the last forty years has produced cumulative benefits valued in the billion-dollar range. The effect of niobium on the microstructure and the properties of the heat affected zone (HAZ) of a girth weld is a very complex issue because of a number of different interrelated mechanisms depending on the chemical composition of the steel and on the welding parameters. In particular it is known that: undissolved precipitates (typically complex (Ti,Nb)(C,N)) have a significant effect on austenite grain size [9][10][11][12]. On the other hand, it is well known that austenite grain size affects the hardenability of the steel and has consequently an influence on the toughness through the packet size of the final microstructure [13][14][15][16]; Nb in solid solution has a direct impact on the hardenability by reducing the transformation temperature of the austenite [17][18];