Effect of Nb on Mechanical Properties of HAZ for High-Nb X80 Pipeline Steels

The mechanical properties of heat affected zone (HAZ) of two commercial high-Nb X80 grade pipeline steels with different alloy elements were investigated using thermal simulation performed on a Gleeble-3500 thermal simulator. The results showed that the high-Nb steels have excellent weldability. Embrittlement regions appear in coarse grain heat affected zone (CGHAZ) and intercritically heat affected zone (ICHAZ); Softening region appears in fine-grain heat affected zone (FGHAZ), and the strength here was even lower than 555 MPa as required in the standard. Meanwhile, with the increase of heat input, the strength and the toughness of HAZ of steel with high Nb, C and lower alloy decrease notably. Therefore, take into account the welding procedure during manufacture of weld pipe, suitable amount of alloy elements, such as Cr, Ni, Cu, Mo and so on, is necessary for high Nb X80 heavy-thick steel plate.