API X70 Pipeline Steel Research Articles

A focus on different factors affecting hydrogen induced cracking in oil and natural gas pipeline steel

In this research, hydrogen induced cracking (HIC) behavior of an API X70 pipeline steel has been studied. In order to create HIC cracks, an electrochemical hydrogen charging experiment was carried out on X70 steel by using 0.2 M sulfuric acid and 3 g/l ammonium thiocyanate for 8 h. Moreover, SEM, EDS and EBSD techniques were used to characterize the as-received (AR) steel and investigate the different aspects of HIC phenomenon as well. The results showed that the inclusions and precipitates which play a key role in HIC phenomenon have been distributed randomly through the cross-section of tested steel. However, the concentration of them was higher at the center of cross-section than other areas. All HIC cracks initiated and propagated through the center of thickness where center segregation of elements has occurred. It is also observed that HIC cracks were initiated from several special types of inclusions and precipitates such as manganese sulphide and carbonitride precipitates. EBSD results showed that the dominant local texture of center of thickness in RD-TD plane was {001}//ND and {111}//ND. Moreover, HIC cracks propagate through differently oriented grains where the local texture is random.

Indentation and erosion behavior of electroless Ni-P coating on pipeline steel

Electroless Ni-P coating has a potential use in oil and gas pipeline industry due to its good corrosion and wear resistance. However, the erosion and indentation behavior of the coating have not been fully explored yet. The objective of this study is to investigate the suitability of utilizing electroless Ni-P coating in the oil and gas industry by characterizing its mechanical damage under erosive and denting conditions. In this study, electroless Ni-P coating with different thicknesses were deposited on API X100 pipeline steel and characterized using XRD, SEM and EDS. Structural characterization indicates that as-deposited Ni-P coating has an amorphous structure with 9-11wt% phosphorus content. Hertzian-type indentation and solid particle erosion tests were performed in order to investigate the extent of the mechanical damage in the coatings. Optical microscopy and SEM observations of the indentation impressions show the presence of Hertzian, radial and bend cracks. It is found that the indentation behaviour of electroless Ni-P coating is significantly influenced by coating thickness. Eroded surfaces were examined and operative erosion mechanisms were identified. It was found that the erosion rate of electroless Ni-P coating is higher than the substrate.The suitability of utilizing electroless Ni-P coatings in the oil industry was assessed.

Effect of Microstructure on Hydrogen Diffusion in Weld and API X52 Pipeline Steel Base Metals under Cathodic Protection

The aim of this research was to evaluate the influence of microstructure on hydrogen permeation of weld and API X52 base metal under cathodic protection. The microstructures analyzed were of the API X52, as received, quenched, and annealed, and the welded zone. The test was performed in base metal (BM), quenched base metal (QBM), annealed base metal (ABM), and weld metal (WM). Hydrogen permeation flows were evaluated using electrochemical tests in a Devanathan cell. The potentiodynamic polarization curves were carried out to evaluate the corrosion resistance of each microstructure. All tests were carried out in synthetic soil solutions NS4 and NS4 + sodium thiosulfate at 25°C. The sodium thiosulfate was used to simulate sulfate reduction bacteria (SRB). Through polarization, assays established that the microstructure does not influence the corrosion resistance. The permeation tests showed that weld metal had lower hydrogen flow than base metal as received, quenched, and annealed.

Effect of Rolling in the Recrystallization Temperature Region Associated with a Post-Heat Treatment on the Microstructure, Crystal Orientation, and Mechanical Properties of API 5L X70 Pipeline Steel

In order to improve the mechanical properties of API X70 pipeline steel, a rolling schedule in the recrystallization temperature region associated with a post heat treatment was proposed. Optical and scanning electron microscopy, electron backscattered diffraction, hardness, and tensile tests were carried out to show the effects of microstructure and crystallographic orientation on the final mechanical properties. The normalized hot-rolled sample and the sample subjected to hot-rolling followed by quenching and consequent tempering at 700 °C exhibited an excellent combination of yield strength and elongation. This was attributed to the recrystallized ferrite microstructure and tempered martensite with martensite–austenite constituent dispersion, respectively. In addition, the development of (112)[111] and (111)[101] texture components and the high fraction of low angle boundaries have a significant effect on the improvement of the mechanical properties.Keywords:

Effect of the cathodic current density on the sub-surface concentration of hydrogen in X80 pipeline steels under cathodic protection

The effect of the cathodic current density (i) on the permeation of hydrogen through API X80 pipeline steels under cathodic protection was investigated using an electrochemical permeation technique. A proper equation was presented for determining the sub-surface concentration of hydrogen (c0). With the aim to enhance the predictive capabilities of the proposed equation, actual environmental parameters such as temperature were considered. The experimental results reported herein clarified the relationship between c0 and i. It was clearly shown that the ratio of c0 to the specimen thickness (L) was proportional to √i when i was lower than its critical value (iH). On the other hand, for i values above iH, c0 was equal to the saturation hydrogen content because of reaction equilibrium. Furthermore, iH was found to be dependent on temperature (i.e., iH increased with the test temperature).

Erosion Behaviour of API X100 Pipeline Steel at Various Impact Angles and Particle Speeds

Erosion is the gradual removal of material due to solid particle impingement and results in a failure of pipeline materials. In this study, a series of erosion tests were carried out to investigate the influence of particle speed and impact angle on the erosion mechanism of API X100 pipeline steel. A dry erosion machine was used as the test equipment, while the particle speed ranged from 20 to 80 m/s and impact angles of 30° and 90° were used as test parameters. The eroded API X100 steel surface was characterized using scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The weight loss and erosion rate were also investigated. The results showed that at a 90° impact angle, a ploughing mechanism was occurring on the tested specimens, while material removal through low-angle cutting was the dominant mechanism at lower impact angles. Embedment of alumina particles on the target steel surface, micro-cutting, and low-angle cutting were observed at low impact angles. Therefore, the scratches, cuttings, and severe ploughings observed on some failed oil and gas pipelines could be attributed to the erosion mechanism.

Effect of Microstructural Parameters on Fatigue Crack Propagation in an API X65 Pipeline Steel

In the current research, we investigate fatigue crack growth in an API X65 pipeline steel by using an Instron fatigue testing machine. To this, first the microstructure of steel was accurately investigated using scanning electron microscope. Since nonmetallic inclusions play a key role during crack propagation, the type and distribution of such inclusions were studied through the thickness of as-received X65 steel using energy-dispersive spectroscopy technique. It was found that the accumulation of such defects at the center of thickness of the pipe body was higher than in other regions. Our results showed that there were very fine oxide inclusions (1-2 µm in length) appeared throughout the cross section of X65 steel. Such inclusions were observed not at the fatigue crack path nor on both sides of the fatigue crack. However, we found that large manganese sulfide inclusions (around 20 µm in length) were associated with fatigue crack propagation. Fatigue experiments on CT specimens showed that the crack nucleated when the number of fatigue cycles was higher than 340 × 103. On fracture surfaces, crack propagation also occurred by joining the microcracks at tip of the main crack.

Fracture Toughness of Different Locations in API X80 Pipeline Steel on Low Constraint SENT Specimens

With the considerable use of high-grade pipeline steel in onshore and offshore project, welded joints are recognized as the weak link in pipeline because of the non-uniform microstructural regions induced by welding heat input. At first, the microstructural of different regions in API X80 pipeline welded joints was characterized and quantified by SEM, which indicate that the pipeline steel is a typical acicular ferrite steel. In this paper we investigated the J-integral resistance curve (J-R curve) in different locations of API X80 pipeline welded joints through low constraint SENT specimens with side grooves at room temperature. The effect of notch orientation (longitudinal-radial (L-R) and transverse-radial (T-R)) on resistance curve were investigated in base metal, which reveal the orientation almost have no effect on resistance curve. As the welded joints adopted in this study is two-pass steel arc welds, so the J-R curves of the inner surface, the outer surface and through-thickness surface notches specimens in the weld metal were investigated. The inner surface sample have the highest toughness through three samples because of the effect of second pass welding process. The effect of constraint on resistance curve was conduct between low constraint SENT specimen and high constraint SENB specimen, which found that the lower constraint corresponding to the higher resistance curve. After finishing the test, crack advancing plan of different positions were etched and observed by OM to demonstrate that the crack path always in the region which we would like to test.

Investigating the corrosion of the Heat-Affected Zones (HAZs) of API-X70 pipeline steels in aerated carbonate solution by electrochemical methods

The electrochemical investigations were carried out to link between the quantities of selected microstructural features, of the weld metal, near-fusion, coarse grain and fine grain HAZs, and their corrosion behavior. The weld and HAZs were made of API-X70 steels by the longitudinally submerged-arc welding process. The electrochemical tests were the potentiodynamic polarization and electrochemical impedance spectroscopy. They were used to evaluate the cathodic reduction, corrosion kinetics, dissolution, the passivation process, and the stability of the passive films – in comparison to the base API-X70 steels – in naturally-aerated solutions containing 0.5 M bicarbonate and 0.01 wt% chloride at 25 °C.

Numerical simulation of dynamic brittle fracture of pipeline steel subjected to DWTT using XFEM-based cohesive segment technique

In the past several numerical studies have addressed the ductile mode of fracture propagation. However, the brittle mode of pipeline failure has not received as much attention yet. The main objective of this study is to predict brittle fracture behaviour of API X70 pipeline steel by means of a numerical approach. To this end, the eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model Drop Weight Tear Test (DWTT) of X70 pipeline steel at -100°C. In this model the dynamic stress intensity factor and crack velocity are calculated at the crack tip at each step of crack propagation.

Investigation of temperature and residual stresses field of submerged arc welding by finite element method and experiments

This article reports on a numerical and experimental investigation to understand and improve computer methods in application of the Goldak model for predicting thermal distribution in submerged arc welding (SAW) of APIX65 pipeline steel. Accurate prediction of the thermal cycle and residual stresses will enable control of the fusion zone geometry, microstructure, and mechanical properties of the SAW joint. In this study, a new Goldak heat source distribution model for SAW is presented first. Both 2D and 3D finite element models are developed using the solution of heat transfer equations in ABAQUS Standard implicit. The obtained results proved that the 2D axi-symmetric model can be effectively employed to simulate the thermal cycles and the welding residual stresses for the test steel. As compared to the 3D analysis, the 2D model significantly reduced the time and cost of the FE computation. The numerical accuracy of the predicted fusion zone geometry is compared to the experimentally obtained values for bead-on-plate welds. The predictions given by the present model were found to be in good agreement with experimental measurements.

Finite element simulation of dynamic brittle fracture in pipeline steel: A XFEM-based cohesive zone approach

Leaking in a CO2 pipeline could escalate to sudden crack propagation, due to a large temperature drop. The resulting drop in fracture toughness together with the pressure stresses at the defect plane leads to pipeline brittle fracture. The main objective of this study is to monitor and predict brittle fracture behaviour of API X70 pipeline steel by means of experimental and numerical approaches, respectively. Dynamic fracture properties of CO2 line pipe steels are generally assessed using the Charpy impact test. To this end, Charpy V-notch tests are performed at different temperatures in order to study the resistance of materials subjected to impact loading conditions. The Charpy test provides valuable indications on the impact properties of materials. Using the experimental results the ductile to brittle transition temperature curve is presented. The extended finite element method based cohesive zone approach is introduced to model the brittle fracture at low temperature. After validation of the developed model against experimental observation significant results from the simulation are graphically presented and discussed.

Assessment of brittle fractures in CO2 transportation pipelines: A hybrid fluid-structure interaction model

In order to transport dense-phase CO2 captured from power and industrial emission sources in the Carbon Capture and Storage (CCS) chain, pressurised steel pipelines are considered the most practical tool. However, concerns have been raised that low temperatures induced by the expansion of dense-phase CO2, for example following an accidental puncture or during emergency depressurization, may result in a propagating brittle fracture in the pipeline steels.The present study describes the development of a hybrid fluid-structure model for simulating dynamic brittle fracture in buried pressurised CO2 pipelines. To simulate the state of the flow in the rupturing pipeline, a compressible one-dimensional Computational Fluid Dynamics (CFD) model is applied, where the pertinent fluid properties are determined using a thermodynamic model. In terms of the fracture model, an extended Finite Element Method (XFEM) is used to model the dynamic brittle fracture behaviour of the pipeline steel.Using the coupled fluid-structure model, a study is performed to evaluate the risk of brittle fracture propagation in a (real-scale) 1.22m diameter API X70 steel pipeline, containing CO2 at 0°C and 11MPa. The simulated results are found to be in good agreement with the predictions obtained using a semi-empirical model accounting for the pipeline fracture toughness. From the results obtained it is observed that a propagating fracture is limited to a short distance. As such, for the conditions tested, there is no risk of brittle fracture propagation for API X70 pipeline steel transporting dense-phase CO2.

Experimental and numerical investigation on combined girth welding of API X80 pipeline steel

API X80 pipeline steel is widely used in natural gas and oil transportation. In this study, X80 pipeline steels with 18.4 mm thickness and 1219 mm diameter, made in China, were welded by a combined girth welding technology. Experimental results showed that microstructures in weld metal and coarse grain heat affected zone are mainly composed of coarsening bainite grains with dimensions of 20–50 μm, and Vickers hardness in weld seam is lowered by the seven-pass welding process. Simulated results illustrated that repeatedly thermal cycles imposed by the combined multipass girth welding process easily result in softened weld joints. The predicted microstructures, phase compositions and hardness in weld metal are in agreement with the measured ones. The results could be applied to optimise this combined girth welding process and improve the weld quality of API X80 pipeline steel and even higher grade pipeline steels.

A comparison between fracture toughness at different locations of longitudinal submerged arc welded and spiral submerged arc welded joints of API X80 pipeline steels

In this paper, extensively experimental testing of fracture toughness for two different welded joints of API X80 pipeline steels after service were carried out. The microstructures and CTOD resistance curves of longitudinal submerged arc welded (LSAW) and spiral submerged arc welded (SSAW) joints were investigated at different locations of the base metal (BM), weld metal (WM) and heat affected zone (HAZ). The critical fracture toughness values were compared with each other. The optical microscope (OM), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were used to analyze the fracture mechanism and precipitates composition. This investigation will provide an important reference to engineering practice and evaluation of pipeline structures.

Correction of flow stress and determination of constitutive constants for hot working of API X100 pipeline steel

Uniaxial compression tests of X100 pipeline steel were conducted to investigate its hot working behavior. Cylindrical specimens were compressed over a wide strain rate range of 0.001–50 s−1 and a temperature range of 850–1100 °C. Due to the heat generated from deformation and friction, the measured temperatures of compressed specimens at higher strain rates (≥1 s−1) deviated from the preset temperatures. Therefore, their measured stresses were corrected to obtain the true stresses at the pre-set temperatures before they were used for calculating constitutive equations. The Arrhenius-type constitutive equation was selected to describe the hot working behavior on the basis of corrected stresses. Strain-dependent constitutive constants were determined. The constitutive equation and corrected stress-strain data were both verified by using the results of additional tests at a strain rate of 20 s−1.

Low Alloy X100 Pipeline Steel Corrosion and Passivation Behavior in Bicarbonate-Based Solutions of pH 6.7 to 8.9 with Groundwater Anions: An Electrochemical Study

This research investigates the fundamental corrosion and passivation processes occurring on API-X100 pipeline steels before, during, and after passive layer formation in bicarbonate-based simulated soil solutions of pH 6.7 to 8.9. Free corrosion potentials decrease exponentially with bicarbonate (and pH), owing to increased water and bicarbonate reduction in more alkaline conditions and the coupled iron oxidation reaction. Active corrosion rates at potentials slightly above open circuit potential increase with bicarbonate, until a concentration of 1.68 g L−1 sodium bicarbonate (plus dilute amounts of chlorides/sulfate) at which first signs of film formation appear. Thereon, increased bicarbonate concentration generally decreases current densities and resists ferrous-oxide product formations due to improved iron carbonate formation conditions precipitating more durable passive layers. Potentiodynamic polarization in the anodic regime reveals varying electrochemical processes involving interactions between hydroxide, carbonate, and oxide complexes with ferrous, with diverse pH and potential dependencies. The products of corrosion reactions were investigated by scanning electron microscopy, X-ray diffraction, and/or X-ray photoelectron spectroscopy.

Delamination effects on fracture behavior of a pipeline steel: A numerical investigation of 3-D crack front fields and constraint

This work addresses a numerical investigation of the crack front fields and effects of crack-tip constraint in conventional fracture specimens with prescribed transverse delamination cracks. One purpose of this study is to conduct a systematic evaluation of delamination effects in side-grooved and plane-sided C(T) and clamped SE(T) fracture specimens, which are commonly utilized in fracture toughness testing of pipeline steels. Another is to quantify the potential coupling influence of specimen geometry and delamination size on crack-tip constraint by means of the J−Q theory thereby providing valuable insight into the effect of delamination cracks on macroscopic fracture behavior in conventional fracture specimens. Laboratory testing of an API 5L X70 steel at room temperature provides the mechanical properties used in the numerical analysis incorporating delamination cracks with varying sizes. Nonlinear finite element analyses of very detailed 3-D finite element models of C(T) and clamped SE(T) fracture specimens for the API X70 pipeline steel enable assessing the effects of prescribed delamination cracks on the crack front fields and constraint with increased deformation levels as characterized by the J-integral. Overall, the present analyses reveal important features of 3-D crack front fields in fracture specimens with a crack-divider delamination that have a direct bearing on the often observed toughness increase in fracture testing of materials with through-thickness anisotropy in mechanical properties.

Fracture Toughness of Different Locations of Spiral Submerged Arc Welded Joints in API X80 Pipeline Steels

API X80 pipeline steels have been widely used in long-distance transportation of oil or natural gas under high pressure due to their high transportation efficiency, low energy loss and low production cost. While the fracture toughness of welded joints are harder to measured. In this paper, J-integral resistance curves of Spiral Submerged Arc Welded (SSAW) joints were examined at different locations of the BM, WM and HAZ. The grain coarsening induced by welding thermal cycling leaded to weaker toughness in WM and HAZ than the BM. The optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the fracture mechanism.

Biocorrosion of Bacterial Inoculation on the API X52 Pipeline Steel

ABSTRACTThe role of the initial bacterial inoculates on the biocorrosion of API X52 pipeline steel coupons was evaluated by electrochemical noise technique. The experiments were performed under laboratory conditions using an aerobic bacteria identified as Achromobacter xylosoxidans. Inoculations in the interval strain of 1x104 – 1x108 CFU/ml were evaluated. Environmental scanning electron microscopy (ESEM) analysis was carried out to evaluate the corrosive effects induced on the API X52 electrodes. The results show that all corroded surfaces show sites of localized corrosion, however, the density of de sites of localized corrosion have different grades depending of the initial inoculation used during the experiments. The maximum density sites of localized corrosion were obtained in the experiments with 1x105 CFU/ml. From inoculates of 1x106 CFU/ml the density sites of localized corrosion diminished constantly. The results show that with inoculates over 1x106 CFU/ml, the oxygen demand for the bacterial strain limits the presence of oxygen available into the metallic surface to maintain the corrosion reactions. The results were supported by the EDX analysis of the corrosion products formed on the metallic surfaces where the oxygen peaks diminished as the bacterial inoculation increases.