Weld Haz properties in modern high strength niobium pipeline steels
A microalloy addition of Ti is currently the option of choice to minimise austenite grain coarsening in the weld HAZ through the grain boundary pinning action of TiN precipitation. High thermal stability of the TiN precipitates provides effective control of boundary migration compared to other microalloy additions but steelmaking controls required to produce the optimum precipitate size distribution can be difficult to achieve consistently. Increased additions of Nb in modern high temperature processed (HTP) pipeline steels have demonstrated increased control of HAZ microstructures with improved fracture toughness. The present paper details the microstructure property relationship of two pipe steel grades with different alloy designs. Thermo-mechanical simulation techniques were utilized to evaluate the critical coarse-grained HAZ (CGHAZ), and the inter-critically (IC) reheated CGHAZ. Simulations were calibrated using real weld thermal cycles, to quantify the influence of alloy design and specifically the role of Nb in weld zone properties. The results reveal that the fracture toughness of the simulated CGHAZ in the HTP steel is superior to that of a lower Nb, Ti microalloyed pipeline steel grade. Toughness was related to the subtle difference in the bainitic HAZ microstructure and most importantly a difference in austenite grain size. As a result, improvements in the ICCGHAZ are expected. Further work is required to elucidate the mechanisms involved.