Effects of Niobium on Microstructure and Hardness of Base Plate and Coarse Grained HAZ of High Strength X70 Grade UOE Linepipe Steel

Abstract

Abstract The present study systematically evaluated base plates and Coarse Grained Heat Affected Zone (CGHAZ) properties of linepipe steels by using the controlled addition of increasing levels of niobium in a low carbon steel for comparison with other alloying combinations of Mn, Ni, Mo and V using laboratory melts and processed under simulated production conditions. The effects of niobium and other alloying elements on the mechanical properties and microstructural development, have been quantified with the intention of maintaining constant CGHAZ hardness in order that specific compositional effects can be directly compared. Characteristics of martensite and austenite (M-A) constituents in terms of size, shape and chemical composition has also been assessed. It is demonstrated that niobium additions up to 0.1 mass% in a low carbon steel design provide opportunities to improve pipeline mechanical properties, service performance and safety. For the CGHAZ, 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. Bainite and small volume fractions of MA (nearly equal 2%) was a characteristic feature of CGHAZ of the materials having constant CGHAZ hardness, irrespective of chemical compositions examined. Other MA characteristics, such as size and cementite fraction, were also very similar among the steels.