Hydrogen-Assisted Crack Growth in the Heat-Affected Zone of X80 Steels during in Situ Hydrogen Charging

Herein, the hydrogen embrittlement of a heat-affected zone (HAZ) was examined using slow strain rate tension in situ hydrogen charging. The influence of hydrogen on the crack path of the HAZ sample surfaces was determined using electron back scatter diffraction analysis. The hydrogen embrittlement susceptibility of the base metal and the HAZ samples increased with increasing current density. The HAZ samples have lower resistance to hydrogen embrittlement than the base metal samples in the same current density. Brittle circumferential cracks located at the HAZ sample surfaces were perpendicular to the loading direction, and the crack propagation path indicated that five or more cracks may join together to form a longer crack. The fracture morphologies were found to be a mixture of intergranular and transgranular fractures. Hydrogen blisters were observed on the HAZ sample surfaces after conducting tensile tests at a current density of 40 mA/cm2, leading to a fracture in the elastic deformation stage.

Environmentally Assisted Cracking Behavior of S420 and X80 Steels Containing U-notches at Two Different Cathodic Polarization Levels: An Approach from the Theory of Critical Distances

This paper analyzes, using the theory of critical distances, the environmentally assisted cracking behavior of two steels (S420 and API X80) subjected to two different aggressive environments. The propagation threshold for environmentally assisted cracking (i.e., the stress intensity factor above which crack propagation initiates) in cracked and notched specimens (KIEAC and KNIEAC) has been experimentally obtained under different environmental conditions. Cathodic polarization has been employed to generate the aggressive environments, at 1 and 5 mA/cm2, causing hydrogen embrittlement on the steels. The point method and the line method, both belonging to the theory of critical distances, have been applied to verify their capacity to predict the initiation of crack propagation. The results demonstrate the capacity of the theory of critical distances to predict the crack propagation onset under the different combinations of material and aggressive environments.

Pipeline Plain Dent Fatigue: A Comparison of Assessment Methodologies

A collection of results of fatigue tests undertaken on full scale pipeline specimens containing unconstrained plain dents is presented. A total of 61 test results are reported, comprising dents of different shapes and depths, introduced in pipe sections with nominal diameters of 12″ and 24″, D/t ratios varying from 18.6 to 77.2 and made of API 5L X42, X46, X52, X60, X70 and X80 steels. The fatigue test results are used to compare five plain dent fatigue assessment methodologies. The assessment methodologies most widely used in the industry, namely, API 1156 and EPRG, are evaluated; each one of these has two different versions, evaluated independently. A dent assessment methodology previously proposed by the authors is also is included in the study. The classic Palmgren-Miner’s cumulative fatigue damage rule is employed in order to correlate the results, since all tests were carried out with two different pressure cycles. Only two of the evaluated methodologies presented a correlation with the fatigue tests that justifies recommending their application in the industry.

Effect of Microstructures on Corrosion of X80 Pipeline Steel in an Alkaline Soil

The influence of microstructures with different heat treatments to stimulate the weld fusion zone and HAZ on corrosion properties of X80 pipeline steel was investigated in alkaline sand soil using electrochemical measurement and surface analysis(SEM, EDS and XRD). The results showed that the microstructure of X80 steel affected the properties of corrosion product layers. Generally, X80 steels with heat treatments had a higher corrosion rate than the as-received steel. The increase of pearlite content enhanced the corrosion of ferrite through a galvanic effect. The appearance of upper bainite and martenite increased further the activity of the steel. The corrosion product layer formed on as-received X80 steel was compact and complete, provided an effective protection to the underneath steel. However, the corrosion product layers on the heat-treated X80 steels were generally inhomogeneous, loose, porous and defective, and provided minor protectiveness. The cathodic/ anodic reactions of X80 steel are dominated by the oxygen reduction and formation of iron oxides that deposit on the steel surface which was through a physical block effect to afford the protection. We shall be able to publish your paper in electronic form on our web page http://www.scientific.net, if the paper format and the margins are correct. Your manuscript will be reduced by approximately 20% by the publisher. Please keep this in mind when designing your figures and tables etc.

The Influence of Microstructures on the Corrosion Properties of X80 Pipeline Steel in Near-Neutral pH Soil

The influence of microstructures with different heat treatments on corrosion properties of X80 pipeline steel was investigated in near-neutral pH soil using electrochemical measurement and surface analysis(SEM, EDS and XRD). The results showed that the microstructure of X80 steel affected the properties of corrosion product layers. Generally, X80 steels with heat treatments had a higher corrosion rate than the as-received steel. The increase of pearlite content enhanced the corrosion of ferrite through a galvanic effect. The appearance of upper bainite and martenite increased further the activity of the steel. The corrosion product layer formed on as-received X80 steel was compact and complete, provided an effective protection to the underneath steel. However, the corrosion product layers on the heat-treated X80 steels were generally inhomogeneous, loose, porous and defective, and provided minor protectiveness. The cathodic/anodic reactions of X80 steel are dominated by the oxygen reduction and formation of iron oxides that deposit on the steel surface which was through a physical block effect to afford the protection.

Experimental Characterization of Hydrogen Embrittlement in API 5L X60 and API 5L X80 Steels

The present work aims to study the hydrogen embrittlement process in API 5L X60 and API 5L X80 steels. The tests were performed using two kinds of hydrogen sources to work with two conditions of hydrogen damage: environmental hydrogen embrittlement and internal hydrogen embrittlement. The mechanical behavior of API 5L X60 and API 5L X80 steels in tensile tests, with and without hydrogen, were studied. Under environmental hydrogen embrittlement conditions, the API 5L X60 steel presented a softening process observed by the decrease in yield strength and increase in its deformation. The API 5L X80 steel was more susceptible to the phenomenon due the deformation decrease of hydrogenated samples. In notched samples, both steels were susceptible to embrittlement as shown by the decrease in elongation. Under internal hydrogen embrittlement conditions, in both steels the changes in deformation were significant and can be attributed to changes in the hydrogen trapping due to the hydrogenation process used, the chemical composition and microstructure. It was observed that the fracture surface morphology of hydrogenated samples of both steels was ductile by microvoids coalescence, and that the distribution of dimples per unit area was higher in the API 5L X60 steel. It can be concluded, as reported in the literature, that the reversible hydrogen trapping observable in environmental hydrogen embrittlement is more damaging than irreversible hydrogen trapping, observable in internal hydrogen embrittlement.