 The use of higher strength pipeline steel grades has enabled the economic development of oil and gas fields in hostile and remote locations. Steel alloy design for these steels are the subject of continual debate in terms of cost of production and the mechanical properties afforded to the base pipe and weld heat affected zone. Debate continues in the use of various alloying combination. Niobium is indispensable in terms of austenite grain size control during steel fabrication and following weld fabrication during pipe production and field construction. Contradictory evidence exists as the role of small concentrations of niobium in the control of weld HAZ properties, primarily because of the influence of other alloying additions as the pipe strength grade increases.

The present study systematically evaluated the controlled addition of increasing levels of niobium in comparison with other alloying combinations of Mn, Ni, Mo and V using laboratory melts and processed under standard production conditions

 

 It is demonstrated that niobium additions up to 0.1mass% in a low carbon steel design provide opportunities to improve pipeline mechanical properties, service performance and safety. For the hot-rolled plates, increasing niobium content resulted in grain refinement with a concomitant increase in YS and TS. Charpy impact toughness was also improved. Bainite and small volume fractions of secondary phase was a characteristic feature of all steels, irrespective of chemical composition and also niobium content. For the coarse grained HAZ, austenite grain size was limited as the niobium content increased. Weld HAZ microstructures were relatively similar with little influence of niobium content on MA character, although the hardness was noted to increase with increasing niobium content, which would be beneficial to ensure adequate resistance to weld zone softening. The results of these laboratory trials were confirmed with comparison of niobium bearing production pipe steels.

 In summary, the effects of niobium and other elements on the mechanical properties and microstructural development, including hardness has been quantified. The role of martensite and austenite in terms of size, shape and chemical composition has also been assessed.


Effects of Niobium on Microstructure and Hardness of Coarse Grained Haz of High Strength X70 Grade Uoe Linepipe Steel

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

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