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 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 then-no-mechanical controlled processing (TMCP) produced fine domains of less than about 2 gm within prior austenite pancakes of less than about 6 mum in thickness. To achieve the target properties in the pipe body, seam weld, and girth weld heat-affected-zones (HAZ), three low carbon microstructure designs have been produced. Low carbon chemistry and Nb/V microalloying combined with boron additions were used to impart sufficient HAZ cold cracking resistance and limit softening in the seam weld HAZ. Both the lower bainite and the dual phase microstructures provided superior property combinations.