Elsevier

Materials & Design

Volume 88, 25 December 2015, Pages 632-642
Materials & Design

Characterization of mechanical properties, fatigue-crack propagation, and residual stresses in a microalloyed pipeline-steel friction-stir weld

https://doi.org/10.1016/j.matdes.2015.09.049Get rights and content

Highlights

  • Structure–property relationships of friction-stir welded pipelines are investigated.

  • Microstructures in the friction-stir welds vary significantly from base metal.

  • Residual stresses are quite low compared to the base metal yield strength.

  • Fatigue crack propagation is improved in weld material due to residual stresses.

  • A simple engineering model of friction-stir weld strain hardening is provided.

Abstract

The influence of the friction-stir welding process on microstructure and mechanical properties of API 5L X80 skelp was investigated. Friction-stir welds were produced using welding parameters optimized to promote weld toughness. The solid-state welding process produced microstructures that significantly varied from those observed in the base metal, namely the redistribution and resizing of Martensite–Austenite constituent in the heat-affected zone and stir zone regions of the welds. Mechanical properties of the welds and base metal were evaluated with uniaxial tension testing and microhardness testing revealing overmatching welds and a hard zone within the weld stir zone. Residual stresses were determined in several directions with respect to the joint revealing that stress in the longitudinal direction is highest, yet well below material yield strength. Fatigue-crack propagation behavior was characterized in the different weld regions and base metal by testing with the compact tension specimen configuration showing that welds have impeded fatigue-crack growth compared to the base metal mostly due to welding-induced residual stress fields interacting with the crack.

Introduction

Friction-stir welding (FSW) is a solid-state joining process that eliminates solidification-induced shrinkage strains, thus excluding hot cracking as a potential weld defect and reducing weld distortion compared to conventional arc welding [1]. The FSW process was initially employed to join low melting-temperature materials such as aluminum alloys; however, the development of advanced tool materials with better resistance to high-temperature wear has resulted in wider application of this joining technology. Materials that possess higher melting temperatures than aluminum alloys, including copper alloys, steels, and titanium alloys have been successfully welded since the inception of advanced tool materials [2].

Steels [3], [4], [5], [6], and pipeline steels in particular [7], [8], [9], [10], have exhibited good weldability with the FSW process, and have seen microstructural refinement with friction-stir processing [11]. One particular advantage of welding steels with FSW is the elimination of hydrogen-induced cracking [1], which is accomplished through removal of the source of hydrogen by eliminating the arc welding consumable. This is particularly important in the higher strength grades or those with high carbon equivalent because of their susceptibility to hydrogen-induced cracking.

Tool wear and subsequent short tool life is detrimental to more widespread implementation of the process. Tool materials such as tungsten alloys and ceramic or metal-matrix composite materials reinforced with polycrystalline cubic boron nitride (PCBN) have been employed to weld ferrous materials, although their wear is readily apparent. Even when considering a finite tool life, an evaluation of economic incentives showed that construction cost savings of about 7% and 25% could be realized for onshore and offshore (J-lay) pipeline construction, respectively when replacing gas metal arc welding with FSW [12]. Other considerations are necessary to realize widespread implementation of FSW for joining pipeline steels, such as a better understanding of weld mechanical performance.

The successful application of FSW to steels is precluded by the lack of mechanical property characterization to the extent typically required for structural applications [13]. Much of the data published on friction-stir welded steel consists of simple mechanical test results such as hardness, tensile properties, and Charpy impact toughness. Regarding fracture in particular, low alloy steel friction-stir welds have exhibited brittle fracture behavior due to drift of the welding tool from the weld centerline [14]. Crack tip opening displacement (CTOD) tests have been performed to some extent on friction-stir welded pipeline steels [10], [15]. The former study found that CTOD toughness values of FSW pipeline steels (API 5L X65 and API 5L X80) were typically low due to large prior austenite grain size indicative of extended time at peak welding temperature, and the influence of strain induced by the welding process. The latter study showed improved CTOD toughness of X80 grade steel after optimizing ceramic-matrix PCBN-reinforced tool rotational speed to reduce heat input.

Aside from toughness considerations, fatigue damage accumulation is an important consideration for pipelines experiencing internal pressure fluctuations and cyclic wave loading action in offshore applications [16]. Metal fatigue susceptibility has been studied in low-alloy structural steel with stress-life approach showing that initiation of fatigue damage tends to occur at surface-breaking features such as those associated with the welding tool shoulder [17]. Fatigue crack propagation behavior has been characterized extensively in aluminum alloy friction-stir welds [18], [19] and in steel welds produced by conventional fusion welding processes. The microstructure, and perhaps more importantly, the residual stress distribution in the welds plays a significant role in influencing the fatigue crack propagation behavior. Fusion welds typically exhibit tensile residual stresses in the weld in the longitudinal direction [20], whereas FSW residual stresses can be compressive in the weld zone in the same orientation [1]. Therefore, the potential difference in behavior necessitates further study of residual stresses, in particular, with respect to fatigue crack propagation.

Building upon a prior study that optimized welding parameters to enhance fracture toughness [15], the goal of this study was to more fully characterize mechanical variables controlling the mechanical performance of FSW girth welds in X80 steel, namely the stress–strain characteristics, fatigue crack propagation behavior, and post-welding residual stress. Simulated pipeline girth welds are investigated since that is a potential scenario of interest with application of the friction-stir welding process [21].

Section snippets

Materials

Skelp material used to manufacture American Petroleum Institute (API) 5L X80 grade pipeline steel was obtained from a Brazilian steel manufacturer. Skelp was selected for the experiments instead of formed pipeline material because the FSW system employed here was not capable of orbital welding, whereas skelp is readily welded using a butt weld configuration. The chemical composition of the microalloyed steel base metal is shown in Table 1. The X80 steel is a microalloyed grade (containing Nb,

Welding process variability

Variations in FSW parameters can influence thermal history and the resulting phase transformation behavior, which subsequently influence mechanical integrity [15]. Weld heat input variability can provide some sense of thermal variations that might influence the microstructure and properties. To evaluate the variation of heat input over three separate welds, a simple torque-based model [26] described by Eq. (1) was used to determine the weld heat input (η), where Ω is the tool rotational speed

Conclusions

  • 1.

    The welding process results in a fine distribution of massive MA constituent in the heat-affected zone and finer and elongated MA particles in the stir zone. Also, a hard zone developed in the stir zone where peak hardness was achieved.

  • 2.

    The friction-stir welds produced on the microalloyed X80 steel skelp were mechanically sound and overmatching to the base metal, while suffering only a slight decrease in overall ductility. Plastic strain hardening was significantly greater in the stir zone than

Acknowledgments

We gratefully acknowledge the technical assistance of Dr. Nicholas Barbosa III (of NIST) while performing scanning electron microscopy on the fatigue fracture surfaces. We also thank Dr. Robert Amaro for helpful discussions that improved the manuscript.

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    Present address: The Ohio State University, 1248 Arthur E. Adams Dr., Columbus, OH 43221, USA.

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