Microstructure and properties of pipeline steel with a ferrite/martensite dual-phase microstructure

In order to satisfy the transportation of the crude oil and gas in severe environmental conditions, a ferrite/martensite dual-phase pipeline steel has been developed. After a forming process and double submerged arc welding, the microstructure of the base metal, heat affected zone and weld metal was characterized using scanning electron microscopy and transmission electron microscopy. The pipe showed good deformability and an excellent combination of high strength and toughness, which is suitable for a pipeline subjected to the progressive and abrupt ground movement. The base metal having a ferrite/martensite dual-phase microstructure exhibited excellent mechanical properties in terms of uniform elongation of 7.5%, yield ratio of 0.78, strain hardening exponent of 0.145, an impact energy of 286 J at − 10 °C and a shear area of 98% at 0 °C in the drop weight tear test. The tensile strength and impact energy of the weld metal didn't significantly reduce, because of the intragranularly nucleated acicular ferrites microstructure, leading to high strength and toughness in weld metal. The heat affected zone contained complete quenching zone and incomplete quenching zone, which exhibited excellent low temperature toughness of 239 J at − 10 °C.

Research Highlights
► The pipe with ferrite/martensite microstructure shows high deformability.
► The base metal of the pipe consists of ferrite and martensite.
► Heat affected zone shows excellent low temperature toughness.
► Weld metal mainly consists of intragranularly nucleated acicular ferrites.
► Weld metal shows excellent low temperature toughness and high strength.