Metallurgical design of ultra high-strength steels for gas pipelines

This paper describes the metallurgical science used to develop a new, weldable, high-strength linepipe (X120) for the transport of natural gas. A domain-based microstructure design has been used to ensure ductile fracture behavior at temperatures down to -20degreesC. Ultra refinement involving careful chemistry design and thermomechanical controlled processing (TMCP) produced fine domains below about 2 mum within prior austenite pancakes below about 6 mum in thickness. To achieve the target properties in the pipe body, seam weld and girth weld heat-affected zones (HAZ), 3 low-carbon microstructure designs have been produced. Low-carbon chemistry and NbN micro-alloying combined with boron additions were used to impart sufficient HAZ cold cracking resistance and to limit softening in the seam weld HAZ. Both the lower bainite and the dual-phase microstructures provided superior property combinations.