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Tuesday, June 3, 2008 - 2:00 PM

Research Concerning the Friction Stir Welding of Linepipe Steels

D. P. Fairchild, A. Kumar, S. J. Ford, N. E. Nissley, ExxonMobil Upstream Research Co., Houston, TX; R. Ayer, A. Ozekcin, ExxonMobil Research and Eng., Corp. Strategic Research, Annandale, NJ

Advances in the Friction Stir Welding of steels are attributable to the development of suitable tool materials, notably tungsten-rhenium alloys and polycrystalline boron nitride.  This study was initiated to investigate the effect of tool material on the mechanical properties of Friction Stir Welds in linepipe steels.  Specifically, several specimens of X65 and X80 from various manufacturers were welded.  A primary focus was to explain the stir zone toughness properties through detailed microstructural examination using optical, scanning electron, and transmission electron microscopy.  To aid the research, a computational fluid dynamics model was developed which takes into account the variation of material properties, process parameters, and contact conditions arising due to slipping and sticking between the plasticized material and the tool.  The results of this study indicate that microstructure and properties have relatively little dependence on the tool material, while significant variations were observed with different steels (i.e., chemistry and manufacturer variation).  In general, the toughness properties were low.  While typical linepipe steels can easily achieve CTOD values greater than 0.4 mm at -20°C, the welds of the current study typically produced CTOD values less than 0.1 mm and often less than 0.05 mm.  The low toughness welds are characterized by prior austenite grain sizes of 40-60 microns which indicate higher than anticipated peak temperature, extended time at peak temperature, and/or the influence of strain.  The results emphasize the need for a detailed understanding of microstructural evolution during FSW as influenced by the steel chemistry, weld thermal cycle, strain, and strain rate.

Summary: FSWs in X65 and X80 were investigated. In general, the toughnesses were low and grain sizes larger than expected. The results emphasize the need for a detailed understanding of microstructural evolution during FSW of steel as influenced by the steel chemistry, weld thermal cycle, strain, and strain rate.