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

Singon Kang,Chester Van Tyne,John Speer,Timothy Weeks

doi:10.3390/met8050354

Singon Kang, Chester Van Tyne + Show 2 more

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https://doi.org/10.3390/met8050354

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Abstract

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.

Highlights

The demand for plate steels having increased combinations of strength, toughness, and weldability for linepipe applications continues to rise
Steel FP exhibited a banded microstructure composed of polygonal ferrite regions along with lighter pearlite bands elongated parallel to the transverse direction (TD) (Figure 3b)
The effects of pipe flattening on the tensile properties of two American Petroleum Institute (API) X65 linepipe steels having either bainite or ferrite/pearlite microstructures were investigated

Summary

Introduction

The demand for plate steels having increased combinations of strength, toughness, and weldability for linepipe applications continues to rise. In the past several decades, multiple efforts have been made to meet these needs through alloying and thermo-mechanical control processing (TMCP) with accelerated cooling. These processes result in complex microstructures, mainly consisting of low temperature transformation products such as bainite and acicular ferrite [1]. Except for granular bainite, the different bainite descriptions are not used systematically and their mechanisms of formation are not well understood This situation reflects the complex character of bainitic microstructures in particular for the TMCP processed steels. Controlling these complex microstructures aims to achieve high strength without significant loss of toughness or weldability of the linepipe steels

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