Fatigue behavior of welded API 5L X70 pipeline steel

Date

2020

Author

Turhan, Şermin Özlem

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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. Microstructural examination was conducted by optical microscope and scanning electron microscope (SEM). Fracture surfaces were examined with SEM. In addition to fractographic analysis, striation spacing was used to estimate the crack growth rate of base metal, weld and HAZ. Stress intensity factor (∆K) 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 ℃ while very slight decrease was observed for base metal. The comparison of high cycle fatigue and crack growth behaviors shows that base metal has a better fatigue behavior than weld and HAZ from the point of crack initiation and propagation. Moreover, our findings also support the idea that striation spacing can be used as a fractographic approach in order to estimate the crack growth behavior of materials.

Subject Keywords

Metallurgy, Pipeline Steels, Fatigue, S-N Curves, X70 Steel, Striation Spacing

URI

http://etd.lib.metu.edu.tr/upload/12625389/index.pdf
https://hdl.handle.net/11511/45625

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Graduate School of Natural and Applied Sciences, Thesis