Microstructural Characterization of X80 Steel Underwater Friction Stir Welded

  API steels are largely used in petroleum industry, like offshore pipeline, due to the combination of high tensile strength, good weldability and low inclusions level. Friction Stir Welding (FSW) is a solid state joining process, which eliminates the melting and solidification associated problems, such as liquation and solidification cracking, with several advantages like good dimensional stability, repeatability and a huge potential for application in critical union involving high melting point alloys. Underwater friction stir welding has been demonstrated to be available for the strength improvement of regular FSW joints. In the present study, a 12 mm thick X80 steel was friction stir welded in two passes using polycrystalline cubic boron nitride (PCBN)-WRe tools. The process was performed at a fixed rotation speed of 350 rpm and welding speed of 100 mm/min in order to evaluate the effects of the underwater process in the X80 microstructure.

The aim of this work was evaluate the microstructural evolution of HSLA ISO 3183 X80M (API 5L X80) friction stir welded joints manufactured through TMCP process without accelerated cooling, exploring parameters that results good weld joints free from macrodefects and with microstructural characteristics that results in suitable mechanical properties.

The microstructural characterization of underwater friction stir welded samples was compared to friction stir welded samples done in air, using Vickers microhardness, optical microscopy and scanning electron microscopy. Microscopy analyses presented microstructures composed by ferritic bainite, martensite-austenite (M-A) and carbides dispersed in the heat affected zone. Stir zone presented also ferritic bainite, M-A and dispersed carbides.

Keywords: Underwater FSW, X80 steel, microstructural characterization