• KSCE Journal of Civil and Environmental Engineering Research
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• v.36 no.6
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• pp.979-990
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• 2016

This study presents a nonlinear finite element procedure involving a phenomenological model to validate the tensile strain capacity of the X80 line pipe developed for the strain-based design purpose. The procedure is based on the Gurson-Tvergaard-Needleman (GTN) model, which models nucleation, growth and coalescence of void volume fraction occurred inside a metal. In this study, the user-defined material module (UMAT) is implemented in the commercial finite element platform ABAQUS and is applied to the nonlinear damage analysis of steel specimens. Material parameters for the nonlinear damage analysis of base and weld metals are calibrated from numerical simulations for the tensile tests of round bar and full thickness specimens. They are then employed in the numerical simulations for SENT (Single Edge Notch Tension) test and CWPT (Curved Wide Plate Test) and in the simulations, the tensile strain capacities are naturally evaluated. Comparison of the numerical results with the experimental results and the conventional empirical formulae shows that the proposed numerical procedure can fairly well predict the tensile strain capacity of X80 line pipe. So, it is readily expected to be effectively applied to the strain-based design procedure.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1999.08a
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• pp.383-389
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• 1999

The purpose of this study was to investigate the effects of alloying and rolling condition on the mechanical properties and to develop high strength line pipe steels with good toughness. Tests were carried out by the laboratory experiments followed by mill trials and mass production. It was found that a small addition of microalloying elements, such as Nb, V with Mo or Ti remarkably increased the strength and toughness of hot strips. The optimum condition of thermomechanical rolling on low carbon microalloyed steel improved the toughness through the formation of a fine and uniform microstructure. Based on this mill trials following the fundamental research, the production technology of line pipe steels, grade X70∼X100 with high toughness, has been established. These grade steels exhibit excellent low temperature toughness (vTs= under -80$^{\circ}C$) and sufficient strength in both the base metal and the ERW seam weld position, respectively.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.12 no.2
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• pp.1-24
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• 2016

The original fracture criteria developed by Maxey/Kiefner for axial through-wall and surface-cracked pipes have worked well for many industries for a large variety of relatively low strength and toughness materials. However, newer line pipe steels have some unusual characteristics that differ from these older materials. One example is a test data that has demonstrated that X80 line-pipe with an axial through-wall-crack can fail at pressures about 30 percent lower than predicted with commonly used analysis methods for older steels. Thus, it is essential to review the currently available models and investigate the applicability of these models to newer high-strength line pipe materials. In this paper, the available models for predicting the failure behavior of axial-cracked pipes (through-wall-cracked and external surface-cracked pipes) were reviewed. Furthermore, the applicability of these models to high-strength steel pipes was investigated by analyzing limited full-scale pipe fracture initiation test results. Based on the analyzed results, the shortcomings of the available models were identified. For both through-wall and surface cracks, the major shortcomings were related to the characterization of the material toughness, which generally leads to non-conservative predictions in the J-T analyses. The findings in this paper may be limited to the test data that were consider for this study. The requisite characteristics of a potential model were also identified in the present paper.

• Transactions of Materials Processing
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• v.15 no.2 s.83
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• pp.118-125
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• 2006

API steel is used for line-pipe to transport the oil and natural gas. As the recent trends in the development of API steel are towards the use of larger diameter and thicker plate, many researches have been studied to achieve higher strength, higher toughness and lower yield ratio in API steel. However, the strength of API steel after pipe forming is changed depending on the competition of the Bauschinger effect and work hardening which are affected by the strain history during pipe forming process. So, the purpose of this study is to investigate the influence of microstructure on the Bauschinger effect for API steel. To change the microstructure of API steel we have changed the hot rolling condition and the amounts of V and Cu addition. The compression-tensile test and the microstructure observation by OM and TEM were conducted to investigate the yield strength drop and the correlation between the Bauschinger effect and microstructure of API steel. The experimental results show that the increase of polygonal ferrites volume fraction increases the Baushcinger effect due to the back stress which comes from the increase of mobile dislocation density at polygonal ferrite interior during the compressive deformation. The hot rolling condition was more effective on the Bauschinger effect in API steel than the small amount of V and Cu addition.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2014.11a
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• pp.155-155
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• 2014

The oxide film was formed in hydrogen induced cracking (HIC) environment by potentio-dynamic method. Corrosion potentials and rates of the X-65 and X-80 line pipe steels were -0.3495 $V_{SHE}$, $2.833{\times}10^{-3}A/cm^2$ and 0.2716 $V_{SHE}$ and $2.533{\times}10^{-3}A/cm^2$, respectively. Surface composition analysis of the oxide film contained sulfur. Thermodynamic analysis of the HIC solution chemistry suggested that the oxide phase consisted of iron sulfate. Dynamic nano-indentation method applied to determine nano-hardnesses of the oxide film and base metal hardness.

• Corrosion Science and Technology
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• v.15 no.1
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• pp.13-17
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• 2016

The aim of this study was to evaluate the use of iron-nickel oxide ($Fe_2O_3$.NiO) nanopowder (FeNi) as an anti-corrosion pigment for a different application. The corrosion protection ability and the mechanism involved was determined using aqueous solution of FeNi prepared in a corrosive solution containing 3.5 wt.% NaCl. Anti-corrosion abilities of aqueous solution were determined using electrochemical impedance spectroscopy (EIS) on line pipe steel (API 5L X-80). The protection mechanism involved the adsorption of metallic cations on the steel surface forming a protective film. Analysis of EIS spectra revealed that corrosion inhibition occurred at low concentration, whereas higher concentration of aqueous solution produced induction behavior.