Abstract
In continuously cast line-pipe steel, solute elements segregate near the centerline of the slab. The extent of centerline segregation is compared with reference Mannesmann images either by visual or computer-aided techniques and provided a Mannesmann rating (MR). Originally MR was devised to gauge the casting performance, however, recently it is used as a reliability/performance metric for line-pipe qualification. Centerline segregation phenomenon in the slabs is expected to translate to through-thickness microstructural heterogeneity and precipitate formation in rolled products affecting mechanical properties. Furthermore, weldability of line-pipe steels is impacted by the interplay of heterogeneous centerline microstructure, precipitates, and the utilized welding process parameters and consumable package. In this study, X70 grade steel slabs rated with MR of 2 (MR2) and 3 (MR3) were rolled and submerged-arc welded to form pipes. Microstructural analysis, and mechanical property and hardness measurements as per ASTM standards were performed on these pipes. Despite some microstructural indication, no substantial difference was observed in the mechanical properties of the manufactured MR 2 and MR3 pipes.
WIC (Welding Institute of Canada) and pipe through-thickness weldability testing using cellulosic E8010 electrodes showed no substantial influence of MR on weldability or hydrogen-assisted crack formation. This study highlights MR of slabs is not a good reliability/performance metric for the pipelines, and it is rather important to understand the effects of other downstream processes to establish a quality control criteria.
