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Open AccessArticle

Effect of Pipe Flattening in API X65 Linepipe Steels Having Bainite vs. Ferrite/Pearlite Microstructures

Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401, USA
Department of Materials Science and Engineering, Dong-A University, Busan 49315, Korea
Applied Chemicals and Materials Division, National Institute of Standards and Technology (NIST), Boulder, CO 80303, USA
Author to whom correspondence should be addressed.
Metals 2018, 8(5), 354;
Received: 10 April 2018 / Revised: 2 May 2018 / Accepted: 8 May 2018 / Published: 15 May 2018
(This article belongs to the Special Issue Development of Bainitic Steels)
PDF [4346 KB, uploaded 15 May 2018]


The influence of microstructure on pipe flattening response was assessed using two different commercially produced U-ing, O-ing, and expansion (UOE) pipes from API X65 steels having either a bainitic microstructure (steel B) or a ferrite/pearlite microstructure (steel FP). A four-point bending apparatus and distinctive procedure were used to minimize strain localization during flattening. The flattened specimens were sectioned at different positions through the thickness, and tensile tested in both the longitudinal (LD) and transverse directions (TD) to assess the through-thickness variation in properties. Yield strength (YS) distributions in the LD show V-shaped profiles through thickness in both steels, whereas the YS in the TD nearest the outside diameter (OD) surface is reduced. These variations in YS are due to the Bauschinger effect associated with the compressive flattening pre-strain. The uniform elongation (UE) of steel FP is almost independent of specimen position through the thickness, but for steel B there is a substantial reduction of the UE at both the inside and outside diameter positions and this reduction is greater in the LD. This work confirms that flattened pipe mechanical properties exhibit an important dependence on their microstructure type and it is postulated that the flattening procedure also influences the mechanical properties. View Full-Text
Keywords: bainite; ferrite/pearlite; steel; pipe flattening; tensile test; pre-strain bainite; ferrite/pearlite; steel; pipe flattening; tensile test; pre-strain

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Kang, S.; Speer, J.G.; Van Tyne, C.J.; Weeks, T.S. Effect of Pipe Flattening in API X65 Linepipe Steels Having Bainite vs. Ferrite/Pearlite Microstructures. Metals 2018, 8, 354.

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