Material design of high strength/heavy gauge linepipes for sour service

Linepipe material for sour gas service primarily needs to have crack resistant properties. However, applications of sour linepipes are expanding toward deep water or cold regions, which require higher toughness and/or heavier wall thickness as well as higher strength. Steel plates for high strength linepipes, Grade X60 and higher, are produced by controlled rolling subsequently followed by accelerated cooling. Mechanical properties of linepipe steels are strongly affected especially by the plate rolling parameters. Higher cooling rates in the accelerated cooling process after rolling give tremendous benefit for obtaining higher strength and superior toughness even for thicker plates. This paper firstly summarizes the design concepts for controlling crack resistance and mechanical properties of high strength linepipe steels for sour gas service. Optimum conditions of controlled rolling and accelerated cooling that balance crack resistant properties and toughness were investigated. In order to fulfill the need for higher strength material, Grade X70 linepipe for NACE sour environments was developed by advanced plate manufacturing technology. An on-line heat-treatment process was applied after accelerated cooling, for precipitation hardening by fine carbides. Homogeneous microstructures without martensite-austenite constituents (MA) were also obtained through the thickness of the plate by this new process. It was proven that precipitation hardened ferrite-bainite steels have extremely high resistance against HIC for Grade X70. Mechanical properties and microstructural characteristics of this newly developed steel are introduced in this paper. (AU) © CBMM