Fatigue Behavior of Welded API 5L X70 Steel Used in Pipelines
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TECHNICAL ARTICLE—PEER-REVIEWED
Fatigue Behavior of Welded API 5L X70 Steel Used in Pipelines ¨ zlem Turhan . Ali Motameni . Rıza Gu¨rbu¨z S¸ ermin O
Submitted: 21 April 2020 ASM International 2020
Abstract In this study, fatigue failure behavior of welded X70 pipeline steel was investigated by rotating bar bending fatigue tests performed at room temperature. S–N curves of base metal, weld and heat-affected zone (HAZ) were plotted. Tension tests, hardness measurements and Charpy V-notched impact tests were carried out for mechanical characterization. Samples were examined with optical microscope and scanning electron microscope (SEM) microstructurally. Fracture surfaces were examined with SEM. In addition to fractographic analysis, striation counting method was used to estimate the crack growth rate of base metal, weld and HAZ. Stress intensity factor (DK) ranges were also calculated. Tensile properties and hardness values of base metal and weld were found to be almost the same. Charpy impact test results show that base material has approximately twice impact energy than weld and HAZ at room temperature. However, Charpy impact energies of HAZ and weld decreased noticeably at 30 C, while very slight decrease was observed for base metal. The comparison of high-cycle fatigue and crack growth behaviors shows that base metal has better fatigue behavior of all from the point of crack initiation and propagation. Moreover, our findings also support the idea that striation counting can be used as a fractographic approach to estimate the crack growth behavior of materials.
¨ . Turhan (&) R. Gu¨rbu¨z S¸ . O Metallurgical and Materials Engineering Department, Middle East Technical University, 06800 Ankara, Turkey e-mail: [email protected] A. Motameni Faculty of Engineering, Middle East Technical University, 06800 Ankara, Turkey
Keywords Pipeline steels Fatigue S–N curve X70 steel Striation spacing
Introduction In recent years, with the increase in the production and consumption of crude oil and natural gas worldwide, the transport of materials used in energy and petrol sectors is of strategic importance [1, 2]. This has made production, installation and operation of large diameter pipes used at very long distances and under high-pressure conditions important in terms of economy and safety [3, 4]. The 70% of energy transportation worldwide are carried out by pipelines [2]. There are also other ways of transportation on land such as trucks and trains for crude oil. However, when they are compared with transportation by pipelines, it is seen that transport through the pipelines is more economical and safer [5].The transportation of iron ore to long distances with pipelines has brought a solution also for mining sector [2]. In addition to these materials, pipelines also transport hydrogen and dangerous liquids such as LPG, gasoline and refined petroleum products [1, 6]. When considering the sectors where the pipes are used, the pipes could work at high pressures and have necessarily large diameters in
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