Mechanical properties and microstructure of longitudinal submerged arc welded joint of a new C–Mn steel sheet cladding fine grained ferrite in surface layer for a long distance transport pipeline

Abstract A new type of C–Mn pipeline steel plates with the ult5ra-fine grained ferrite in surface layer and the fine-grained ferrite in the central layer has been developed successfully. In order to manufacture the long distance oil pipelines using this new ultra-fine grain pipeline steel sheet, the microstructure and mechanical properties of the welded joints by the Longitudinal Submerged Arc Welding (LSAW), especially the impact toughness, need to be evaluated. Tensile tests, impact tests, are used to study the mechanical properties in this paper according the Chinese National standards. The metallographic technique and SEM are adapted to analyze the microstructure and impact fracture morphology of the LSAW joint. The research results show that the HAZ near the fusion line is a weak part of impact toughness of LSAW joint. However, the base metal is of the fine-grained and ultra-fine grained microstructure itself, and the heat treatment effects between the post-process welding seam and pre-process welding seam on the multi-layer and multi-pass submerged arc welding process, the impact toughness of the HAZ is greatly improved. The whole welded joint is better than the performance of ordinary pipeline steel, which is satisfied with the welding performance requirements of steel sheet engineering application, and can be used in long distance pipeline manufacturing projects.