The processing of helical-welded large diameter pipes of grade X80 with 23.7 mm wall thickness and their properties

In recent years the development of large diameter, thick wall helical-welded linepipe grade API X80 was a priority. Hot-coiled strips, spiral pipes and submerged arc welds have been produced and microstructure, texture and mechanical properties have been characterized. Good field weldability was proven. The favourable microstructure has been achieved by the selection of an appropriate chemical composition of low carbon content and an increased amount of niobium in combination with a thermo-mechanical hot rolling process. The addition of niobium in combination with the adjustment of other alloying elements allowed improved slab reheating conditions and increased the recrystallisation stop temperature (high temperature processing: HTP). A homogeneous microstructure across the strip gauge was formed during accelerated cooling on the run-out table of the hot rolling mill. All results met the requirements for API X80 pipes which were determined for a wall thickness of 23.7 mm x OD 813 mm and 23.7 mm x OD 1220 mm. The bainitic microstructure and mechanical properties of the pipe base material and welding zones are discussed. (AU) Copyright © 2013 Companhia Brasileira de Metalurgia e Mineração (CBMM) All Rights Reserved