Optimum specification of Ti & N for control of HAZ properties in high strength pipeline steels
High strength line pipe steels exhibit a combination of excellent toughness and high strength through microalloy additions and thermomechanical controlled processing (TMCP). During welding, the severe thermal cycles experienced in the coarse grained heat affected zone (CGHAZ) result in precipitate coarsening / dissolution and ultimately grain growth with a concomitant reduction in impact toughness. Addition of Ti is employed to control grain growth in the HAZ through the grain boundary pinning action of TiN precipitates. The optimum levels however are the subject of much debate both in the open literature and that specified in international standards for the procurement of line pipes. This paper reports the findings of a detailed weldability assessment of a range of API 5L X70 grade steels of fundamentally the same base composition but with different Ti and N contents. The as-deposited double submerged arc welded (DSAW) pipes and thermally simulated HAZ were evaluated to determine the effect of Ti and N levels and their ratio on the CGHAZ impact toughness. The results of the present study showed slight improvement of low temperature toughness of simulated CGHAZ at close to stoichiometric ratio (3.42). Quantitative metallographic examination and detailed Charpy testing suggests that an optimum Ti/N ratio exists and has the potential to better clarify specification requirements for end users of high strength line pipe.