Metallurgical Design of Ultra-High Strength Steels For Gas Pipelines

Abstract

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 −20°C. Ultra refinement involving careful chemistry design and thermomechanical controlled processing (TMCP) produced fine domains below about 2 m within prior austenite pancakes below about 6 m 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 Nb/V 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. INTRODUCTION Natural gas is an increasingly attractive energy source, but major reserves are often remotely located from potential markets. While high operating pressures and/or thin-wall pipes are a means to reduce gas transmission costs, conventional steels typically lack sufficient strength. To address these challenges, a significant advance in steel-making and plate-manufacturing technology is necessary. This paper describes the metallurgical design basis for X120-grade pipeline steel for low-temperature service, including Arctic environments. Over the past 30 years, the trend toward increased transportation efficiency has largely been achieved by increasing the diameter of pipelines. Currently, large systems that are installed on land consist of 1420-mm–diameter (56-in) pipe operating at pressures in the range between 70 and 100 bar (1015− 1450 psi). In such *ISOPE Member. Now at Chiba Institute of Technology. Received August 18, 2003; revised manuscript received by the editors January 22, 2004. The original version (prior to the final revised manuscript) was presented at the ISOPE Symposium on HighPerformance Materials in Offshore Industry, the 13th International Offshore and Polar Engineering Conference (ISOPE-2003), Honolulu, Hawaii, USA, May 25–30, 2003.