API X70 Research Articles

Fractographical Insight into Heat-Treated Low Carbon Microalloyed API X65 Steel Fracture Mode

Fractographical Insight into Heat-Treated Low Carbon Microalloyed API X65 Steel Fracture Mode

Hydrogen Embrittlement Behavior of API X70 Linepipe Steel under Ex Situ and In Situ Hydrogen Charging.

This study investigates the hydrogen embrittlement behavior of API X70 linepipe steel. The microstructure was primarily composed of a dislocation-rich bainitic microstructure and polygonal ferrite. Slow strain-rate tests (SSRTs) were performed under both ex situ and in situ electrochemical hydrogen charging conditions to examine the difference between hydrogen diffusion and trapping behaviors. The ex situ SSRTs showed almost the same tensile properties as air and a limited brittle fracture confined to near the surface. In contrast, the in situ SSRTs showed an abrupt failure after the maximum tensile load, leading to a brittle fracture across the entire fracture surface with stress-oriented hydrogen-induced cracking (SOHIC). The crack trace analysis results indicated that SOHIC propagation paths were influenced by localized hydrogen accumulation due to high-stress fields. As a result, the dominant hydrogen embrittlement mechanisms, such as hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE), changed. These findings provide critical insights into the microstructural factors affecting hydrogen embrittlement, which are essential for the design of hydrogen-resistant steels in hydrogen infrastructure applications.

Experimental study of natural fiber Jute reinforced by metal wires and its using as patch on semi-elliptical defected of the API X52 pipelines corroded according to ratio a/t

The aim of this study is to evaluate the efficiency of a hybrid patch of Jute with/without Glass fiber reinforced with Steel wire for repairing defective API X52 pipeline. Five specimens were tested using Jute fiber, Glass fiber and Steel wire to enhance the mechanical properties according to ASTM 3039 and D790 standards. The experimental results showed that the effects of hybrid materials (synthetic and natural) for repairing API X52 pipeline defect depend on the nature of the patch and the properties of the adhesive (polyester resin) by increasing the tensile strength and bending load. In the first step, specimen S5 was selected due to its distinctive mechanical properties, which revealed maximum values of tensile strength, toughness and bending load respectively 47.42014 MPa, 2357.42 kJ/m3 and 488.3 N. In the second step, the selected sample (S5) was used as a patch to repair API X52 pipeline, which has semi-elliptical defects where the bending load is 31.4246 kN. According to the obtained results, the patch efficiency is 103% for defects with a/t < 0.3, which can be, restored 100% of the mechanical properties. There fore, repairing API X52 pipeline using hybrid patch (Jute reinforced with Steel wires) with shallower defects can be a successful technique.

Local and global mechanical properties of orbital friction stir welding on API X65 PSL2 steel / Inconel 625 clad pipes

Orbital friction stir welding (FSW) is a promising approach to joining clad pipes. In this work, the influence of individual process parameters on the material flow even as, mechanical properties in orbital friction stir welded clad pipes is investigated. Due to the local heterogeneous microstructures within the weld, joints were evaluated by optical and scanning electron microscopy (SEM), microhardness, and digital image correlation (DIC). A microhardness increase was observed, with a maximum of 380 HV0.5 within the Inconel 625 and 265 HV0.5 within the X65 steel stir zones. Sound joints had an average ultimate tensile strength (UTS) exceeding up to 13% of the calculated theoretical tensile strength, which is obtained considering the minimum tensile strength of both materials. Additionally, failure occurred in the base material during the tensile testing, where the local strain in the stir zones of the X65 steel and Inconel 625 were only 3.3% and 10%, respectively. In summary, this investigation shows friction stir welded joints of API X65 steel and Inconel 625 clad pipes with high mechanical properties.

Effect of Heat Treatments on the Mechanical and Microstructure Properties of Welded API X70 Steel

Effect of Heat Treatments on the Mechanical and Microstructure Properties of Welded API X70 Steel

Realizing the outstanding strength-toughness combination of API X65 steel by constructing lamellar grain structure

The development of pipeline steels with exceptional cryogenic toughness is deemed imperative for their applications in various engineering fields. In this work, API X65 steel with lamellar grain (LG) structure is constructed through warm deformation techniques. Comparative analysis with raw API X65 steel revealed notable enhancements in both yield strength (YS) and ultimate strength (UTS), exhibiting increments of 315 MPa and 164 MPa, respectively, while retaining substantial fracture toughness (KIc = 251.14 MPa m0.5). Remarkably, the LG steel exhibits a distinct absence of a ductile-brittle transition behavior over a wide temperature range, maintaining an exceptionally high impact energy from room temperature (RT) to liquid nitrogen temperature (LNT), and the Charpy impact energy exceeding 330 J at LNT, nearly 16 times higher than the raw API X65 steel. This performance can be primarily attributed to the superior plastic deformation capability, coupled with the delamination toughening mechanism. The resultant tortuous path of crack propagation effectively facilitates the absorption of substantial energy, decreases notch tip sensitivity depending on the temperature, stress triaxiality, and strain rate, thereby fostering excellent cryogenic toughness.

Finite Element Ductile Fracture Simulation of Charpy and Drop Weight Tear Tests for API X52

This paper presents a systematic procedure for performing finite element (FE) impact ductile fracture simulation of Charpy (CVN) and Drop Weight Tear Tests (DWTT) with validation using test data of API X52. For deformation and fracture models, the Johnson-Cook (J-C) model is used, of which seven parameters are determined by analyzing (1) round bar tensile test data at three different temperatures (two parameters), (2) tensile test and fracture toughness test at room temperature (three parameters) and (3) instrumented Charpy test (load–displacement) data at room temperature (two parameters). FE impact fracture simulation results with the determined parameters show good agreement with instrumented CVN test data at three different temperatures (0 °C, −30 °C and −60 °C) and DWTT data at temperatures of RT and −30 °C. For DWTT simulation, an analysis of the pre-strain due to flattening is included. Additionally, sensitivity analyses for the effect of adiabatic heating and strain rate on simulation results show that, although both phenomena should be considered in simulation, the strain rate effect is more significant than the adiabatic heating effect.

Experimental Study on Microalloyed Steel with Layers Subjected to Diesel

This work studies the mechanical behavior of microalloyed steels (API X60 and API X70) with boride layers using a boriding process and immersion in diesel. First, the microalloyed steels were borided using dehydrated boron paste at a temperature of 1273 K for 6 h, and then the borided microalloyed steels were immersed in diesel for one year. The characterization of the layers on the specimens subjected to diesel used scanning electron microscopy (SEM), energy dispersive spectroscopy, and X-ray diffraction (XRD). The evaluation of the mechanical properties was performed with tensile tests according to ASTM E8, and then the fracture surface was observed by SEM. This work contributes to the understanding of the changes in the mechanical properties of borided microalloyed steel immersed in diesel for possible potential applications in the storage of fuels, oils, hydrogen, and biofuels.

Modeling 3D finite element analysis of a semi-elliptical crack on stress corrosion cracking of API X52 pipeline

In this study, an external semi-elliptical crack was modeled in a 3D API 5 L X52 pipeline with stress corrosion cracking (SCC). To make the crack, the finite element method (FEM) was used to obtain the failure pressure (Pf) and its mechanical behavior. The longitudinal crack had a constant length (2c) and varied with depth (a). The properties of X52 steel subjected to SCC using the slow strain rate technique (SSRT) were considered. True stress-strain curves were obtained in air and in an NS4 solution with pHs of 3.5 and 9.5 at temperatures of 25 and 50 °C. According to the SCC index calculated from the mechanical properties of the SSRT, the X52 steel is susceptible to SCC at pH 3.5 and 50 °C. The mechanical properties of the tensile test using the stress-strain curves decreased as the pH and temperature changed, compared to those carried out at room temperature. This was due to the corrosion and hydrogen embrittlement produced by the solution on the X52 steel. The failure pressure is sensitive to the stress-strain curve; if the stress-strain curve decreases, the failure pressure also decreases. Corrosion defects, such as longitudinal cracks in X52 steel, which could be susceptible to SCC in an NS4 solution, decrease the failure pressure (Pf) and its capacity to withstand pressures of up to 75 % in pipe-grade steel that transports hydrocarbons.

Characterizing pearlite transformation in an API X60 pipeline steel through phase-field modeling and experimental validation

This study explores the microstructural characterization of pearlite phase transformation in high-strength low-alloy API X60 steel, which is used in pipelines. Understanding the formation, phase percentages, and morphology of the pearlitic phase is crucial since it affects the mechanical properties of the considered steel. In this research, a phase-field model, particularly the Cahn–Hilliard approach, was used in order to simulate the formation and morphology of the pearlite phase in response to different heat treatments. Both double- and triple-well potentials were considered for comprehensively studying pearlite’s morphology in the simulations. The simulation results were then compared with experimental outcomes obtained by metallography and field-emission scanning electron microscopy analyses. Considering the double-well potential can help simulate only two phases, ferrite and cementite, which is less compatible with the experiment results than the triple-well potential, which gives the possibility of simulating a three-phase microstructure, ferrite, cementite, and austenite, and a better match with experimental data. The study revealed that as the cooling rate increases, the interlamellar spacing and layer thickness decrease. Additionally, the difference between experimental and simulation results using triple-well potential was approximately ∼10%. Therefore, triple-well potential formulation predictions have better agreements with experimental results for the development circumstance of pearlitic structures.

Electrochemical and theoretical evaluation of dihydro-benzoimidazoquinazolines as corrosion inhibitors for API X-120 steel in acid medium

In this paper, the corrosion inhibition of three dihydro-benzoimidazoquinazolines, namely 2´,6´diFDHBIQz 1, 2Br2´,6´diFDHBIQz 2 and 2,4diBr2´,6´diFDHBIQz 3 was determined for an API 5 L X120 steel in 0.5 M HCl. Electrochemical techniques such as Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarization curves were used obtaining inhibition efficiencies greater than 90 %. In addition to this, Quantum chemical calculations using the DFT technique was used to correlate the inhibitors efficiency and their electronic properties. Results have shown that the three organic compounds are good, mixed type of corrosion inhibitors, with an efficiency that increased with their concentration but decreased with the temperature. The inhibitor efficiency decreased in the order 2´,6´diFDHBIQz 1 > 2Br2´,6´diFDHBIQz 2 > 2,4diBr2´,6´diFDHBIQz 3. All of them were adsorbed on to the steel surface following a Langmuir type of adsorption isotherm, however 2´,6´diFDHBIQz 1and 2Br2´,6´diFDHBIQz were adsorbed in a mixed chemical and physical way dominated by a physical type of adsorption, whereas 2,4diBr2´,6´diFDHBIQz 3 was physically adsorbed. Quantum chemical results revealed that 2´,6´diFDHBIQz 1 was the most efficient because it had the highest HOMO energy.

Experimental Investigation on Corrosion Behaviour of Heat-Treated API X70 Pipeline Steel

Experimental Investigation on Corrosion Behaviour of Heat-Treated API X70 Pipeline Steel

Fracture analysis of hydrogen-embrittled API X52 pipes at low temperature

This paper quantifies the effect of temperature (ranging from room temperature (RT) to -90 °C) on fracture toughness for hydrogen-embrittled API X52 by combining small punch (SP) test data and finite element (FE) damage analysis. For the FE damage analysis, a multi-axial fracture strain damage model was used, and the parameters were determined by analyzing the tensile and SEN(T) test data in air at RT. The hydrogen-enhanced ductile fracture was considered using the hydrogen-embrittlement constant, which was determined by analyzing the SP test results in hydrogen. Due to the effect of the interaction between strength and hydrogen-induced ductility loss on fracture toughness, the predicted fracture toughness in terms of temperature does not show a monotonic decrease up to -90 °C; rather, it decreases up to -30 °C, then increases slightly before decreasing again. Fracture mechanics analysis of a hydrogen-embrittled pipe with an axial surface crack using the determined fracture toughness values showed that the maximum pressure decreased slightly (less than 9 %) with decreasing temperature up to -90 °C, suggesting that the effect of temperature on the maximum pressure would not be so significant for API X52.

Coupled slow strain rate and acoustic emission tests for gaseous hydrogen embrittlement assessment of API X65 pipeline steel

Hydrogen embrittlement (HE) sensitivity of API X65 pipeline steel was evaluated in this study through the simultaneous application of slow strain rate tests (SSRT) and acoustic emission (AE) monitoring on hollow dog-bone samples. Exposure to hydrogen gas decreased the elongation to fracture percentage with a significant rise in embrittlement index for the higher pressure of 10 MPa. SEM images of the fracture surfaces demonstrated that the fracture mechanism changed from ductile to ductile–brittle under 1 MPa pressure, and to predominantly brittle with quasi-cleavage characteristic when testing under 10 MPa hydrogen gas. Classification of the AE events based on waveform analysis revealed a mainly shear mode fracture for 10 MPa nitrogen sample, while a dominant tensile mode fracture was identified for the hydrogen charged samples.

Solvent selection and studies of the anti-corrosion efficacy of Heliotropium curassavicum leaves extract on API L X60 steel in 5 wt% hydrochloric acid solution

Corrosion is a huge challenge in the manufacturing industry during sticky oxides, grease, and rust removal from metal surfaces. It is often managed by the use of inhibitors and current interest is on sustainable inhibitors with less to zero harmful effects. This study explores the efficacy of Heliotropium curassavicum leaves extracts (HCLEs) in retarding the deterioration of API X60 grade steel in 5 wt% HCl solution and the influence of extracting solvent namely acetone (ACE), ethanol (ETH), water (WAT), butanol (BUT), methanol (MET), isopropanol (ISO), and ethyl acetate (ETA) on performance. The characterization of the HCLEs was achieved by Fourier-transform infrared (FTIR) and ultraviolet–visible (UV–vis) spectroscopies while the weight loss, electrochemical, and surface examination approaches were used for anticorrosion property evaluation. The results disclose that irrespective of extracting solvent, the extracts could suppress the dissolution of the substrate under the studied conditions. The best performance is, however, noted for ethanolic HCLE where 300 mg/L protected the substrate surface by 51.46% at 25 °C. Hence, ethanolic HCLE was selected for detailed studies and results indicate that the steel surface can be protected by 78.42% if 1000 mg/L of extract is used. Furthermore, the performance of the extract improves upon an increase in the temperature of the corrodent. The findings suggest that ethanolic HCLE could be considered for material protection during HCl pickling.

The Effects of Post Weld Heat Treatment on Microstructure and Mechanical Properties of API X70 Linepipe using Submerged Arc Welding

API X70 steel requires high strength and toughness for safety in extreme environments like high pressure and low temperature. Submerged Arc Welding (SAW ) is effective for manufacturing thick steel pipes. However, the welding heat input during SAW alters the microstructure and mechanical properties of the heat affected zone (HAZ). Therefore, investigating the correlation between microstructure and mechanical properties in welded X70 pipes is important to address potential degradation of HAZ and weld metal (WM). In this study, post weld heat treatment (PWHT) was performed to improve mechanical properties of HAZ and WM and to reduce residual stress caused by the welding process. We performed PWHT at 640°C for 15 hours and followed by air cooling. After heat treatment, we observed the microstructure through OM and SEM analysis, and investigated the mechanical properties through tensile test, hardness test, and Charpy impact test.

Assessing fracture toughness of vintage and modern X65 pipeline steels under in situ electrochemical hydrogen charging using advanced testing methodology

The susceptibility of pipeline steels to hydrogen embrittlement, which can significantly impair mechanical performance, especially fracture toughness, is a critical issue in infrastructure integrity. This study evaluates the fracture toughness of modern and vintage API X65 steels under cathodic polarization (CP) conditions, employing methods such as rising displacement testing, stepwise rising constant load testing, and constant load testing. Tearing crack growth was detected at crack tip opening displacements (CTODs) of 0.032 to 0.055 mm while subsequent crack arrest under constant force loading was observed with CTOD thresholds of 0.148 mm and 0.255 mm for modern and vintage X65, respectively. Electron channelling contrast imaging demonstrated that arrested cracks in both steels exhibited reduced plasticity and a higher propensity for crack branching compared to propagating cracks. These findings highlight the need for tailored strategies to mitigate the effects of hydrogen embrittlement in pipeline materials and underscore the differences in damage tolerance between steel generations.

The Effect of Notch Depth on the Absorbed Energy of High-Toughness Steels in the DWTT

Abstract Because of the larger specimen dimensions and more significant ligament for crack growth, the drop-weight tear test (DWTT) has become more widely used than other impact tests. On the other hand, very little information is available from DWTT experiments on weld metal (WM). In the present study, in addition to the standard chevron notch (CN) DWTT specimen with a notch depth (ND) of 5.1 mm, API X65 specimens with 10- and 15-mm ND were tested on the WM and also the base metal (BM). Fracture energies versus the ND were plotted, based on which tests can be performed successfully with lower-capacity DWTT machines to predict the absorbed energy for each ND, by specimens with increased ND and reduced absorbed energy. The standard requires thickness reduction for specimens thicker than 19 mm. With the method proposed in the present study, high-grade specimens can be tested with their original thickness. In the DWTT, regardless of specimen materials (BM or WM), about 44 % of the absorbed energy is spent on crack initiation, and 56 % is spent on crack growth. In all specimens, the BM absorbed energy was more than that of the WM. Therefore, this study proposes that the API standard, which does not include DWTT on the WM, should require performing DWTT on WM specimens. Hence, the results of the current study can form the basis for further studies that can initiate the development of new standards on the WM.

Performance of two high-strength steels under electrochemical corrosive and stressed conditions

The corrosion behaviors of stressed API X-80 and API X-120 pipeline steels were investigated using electrochemical and slow strain rate tensile tests after being immersed in a saline solution containing 3.5% NaCl. Scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive x-ray spectroscopy (EDX) were used to create images of the flawed steel samples. Corrosion rates for API X-120 pipeline steel were found to be much lower than those of API X-80 pipeline steel in both stressed and saline environments. While different oxide layers were observed on the two steels, the result showed the oxide film was of iron carbonate. It was observed that the API X-120 steel surface was latently smooth in a saline solution environment which reduced the corrosion rate. As a result, the API X-120 pipeline steel was protected and had a delayed time-to-failure in comparison to the API X-80 pipeline steel when it was tested in steel under stress and in solution medium. This failure mechanism will not only be significant to the petroleum sector, but also to the industry that produces pipelines in terms of the choice of fitting material for pipe design. Specifically, this mechanism will have an impact on the design of fittings for pipes.

Evaluation of corrosion behaviour and susceptibility to hydrogen-induced cracking in high-strength low-alloy X70 steels under sour gas conditions

High-strength low-alloy (HSLA) steels find extensive usage in the oil industry for manufacturing pipes due to their requirement for materials with high mechanical and corrosion resistance. Among these steels, the HSLA X70 steels are particularly of great interest to the petroleum industry because of its high mechanical resistance, ductility and weldability. In this study, it was aimed to assess the corrosion behaviour of HSLA API X70 pipelines using electrochemical impedance spectroscopy (EIS) technique and evaluate its resistance to hydrogen-induced cracking (HIC). The analyses were conducted in non-sour and sour media, as per NACE TM0284 standard. Microstructural characterisations of polished and corroded samples were also conducted using scanning electron microscopy (SEM) and optical microscopy (OM). The results indicate that the resistance to HIC in API 5L X70 steel is compromised when exposed to surface corrosion in sour gas media. This susceptibility may be attributed to the presence of microconstituents MnS and M/A along the grain boundaries, which were found in the crack trajectory. The identified microstructural features are deemed to exert a significant influence on the propagation of cracks and the consequent susceptibility to HIC. Moreover, the EIS tests revealed the lowest corrosion resistance when exposed to H2S media, as MnS inclusions accelerate the active regions in the matrix, consequently leading to the formation of a plethora of corrosion products and the dissolution of iron.