Characterization of mechanical properties, fatigue-crack propagation, and residual stresses in a microalloyed pipeline-steel friction-stir weld☆
Graphical abstract
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
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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.
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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.
References (45)
- et al.
Friction stir welding and processing
Materials Science and Engineering: R: Reports.
(2005) - et al.
Recent advances in friction-stir welding — process, weldment structure and properties
Progress in Materials Science.
(2008) - et al.
Friction stir welding of carbon steels
Materials Science and Engineering: A.
(2006) - et al.
Microstructural evolution of ultrahigh carbon steel during friction stir welding
Scripta Materialia.
(2007) - et al.
Development of a process envelope for friction stir welding of DH36 steel — a step change
Materials & Design.
(2014) - et al.
Microstructure and mechanical properties of hard zone in friction stir welded X80 pipeline steel relative to different heat input
Materials Science and Engineering: A.
(2013) - et al.
Effect of tool centreline deviation on the mechanical properties of friction stir welded DH36 steel
Materials & Design.
(2015) - et al.
Fracture toughness of ISO 3183 X80M (API 5L X80) steel friction stir welds
Engineering Fracture Mechanics.
(2010) Fatigue analysis of welded joints: state of development
Marine structures.
(2003)- et al.
Systematic investigation of the fatigue performance of a friction stir welded low alloy steel
Materials & Design.
(2015)
The role of residual stress and heat affected zone properties on fatigue crack propagation in friction stir welded 2024-T351 aluminium joints
International Journal of Fatigue.
Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys
International Journal of Fatigue.
Effects of welding residual stresses on fatigue crack growth behaviour in butt welds of a pipeline steel
Engineering Fracture Mechanics.
Residual stress measurements in a thick, dissimilar aluminum alloy friction stir weld
Acta Mater.
Morphological aspects of martensite–austenite constituents in intercritical and coarse grain heat affected zones of structural steels
Materials Science and Engineering: A.
Effects of the UOE/UOC pipe manufacturing processes on pipe collapse pressure
Int. J. Mech. Sci.
Structure, properties, and residual stress of 304L stainless steel friction stir welds
Scripta Materialia.
Effect of welding residual stresses on fatigue crack growth thresholds
International Journal of Fatigue.
Macro and microscopic observations of fatigue crack growth in friction stir welded aluminum joints
Engineering Fracture Mechanics.
The role of inclusions in ductile fracture and fracture toughness
Engineering Fracture Mechanics.
Feasibility of friction stir welding steel
Sci. Technol. Weld. Join.
Friction Stir Welding of API Grade 65 Steel Pipes
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Present address: The Ohio State University, 1248 Arthur E. Adams Dr., Columbus, OH 43221, USA.