API X70 Pipeline Steel Research Articles

Retraction notice to “Applying a novel heat treatment cycle to modify the microstructure of welded API X70 pipeline steel” [MLBLUE 98C (2013) 178–81

Retraction notice to “Applying a novel heat treatment cycle to modify the microstructure of welded API X70 pipeline steel” [MLBLUE 98C (2013) 178–81

Influence of temperature on the corrosion behavior of API-X100 pipeline steel in 1-bar CO2-HCO3− solutions: An electrochemical study

This paper addresses on the influence of temperature, elucidated with a number of electrochemical methods and immersion tests, on the corrosion behavior of API-X100 steel in CO2-saturated bicarbonate solutions. Investigated by cyclic potentiodynamic polarization, the corrosion rates, which showed a sensible increase with 10 g L−1 (0.16 mol L−1), 30 g L−1 (0.5 mol L−1) and 50 g L−1 (0.82 mol L−1) bicarbonate concentrations, increased from about 500, to 1500 and 1800 μA cm−2 at 20, 50 and 90 °C, respectively. Passivation at 50 and 90 °C showed resistance to deteriorate against 100 ppm chloride ions, of which anodic 0.5 V vs. SCE peaks exclusively appeared. Moreover, transpassivation occurred at 0.9 and 0.7 V vs. SCE, respectively, unlike with the 20 °C cases whose chloride-induced-pitting-vulnerable, gradually-forming passive films transpassivated at 1 V vs. SCE. At different potentials, the potentiostatic currents increased with temperature, but their profiles suggested more effective passivation, accordingly. The charge transfer resistance, calculated by electrochemical impedance spectroscopy, decreased with temperature at the open circuit potentials and 0.6 V vs. SCE, where the interfacial interactions were governed by adsorption, and diffusion-limited processes, respectively.

Anisotropy of Mechanical Properties of API-X70 Spiral Welded Pipe Steels

The effects of microstructure and texture on the toughness anisotropy of two API-X70 pipeline steels were investigated. One steel contained no nickel (0Ni) and the other contained 0.3 wt pct nickel (0.3Ni). Charpy V-notch impact testing was conducted on plate samples for both steels in three directions: longitudinal (L), transverse (T), and diagonal (D) with respect to the rolling direction. The microstructures of both steels were mixed and consisted of acicular ferrite, granular bainite, and small amounts of polygonal ferrite, with martensite-austenite and retained austenite islands as secondary phases. The ductile to brittle transition temperatures (DBTT) for the Charpy impact test were higher in the D direction for both plates, with a pronounced increase in the 0Ni steel. The anisotropy in toughness was mainly attributed to the crystallographic texture.

RETRACTED: Applying a novel heat treatment cycle to modify the microstructure of welded API X70 pipeline steel

RETRACTED: Applying a novel heat treatment cycle to modify the microstructure of welded API X70 pipeline steel

Effects of Microstructure on Tensile, Charpy Impact, and Crack Tip Opening Displacement Properties of Two API X80 Pipeline Steels

The effects of microstructure on tensile, Charpy impact, and crack tip opening displacement (CTOD) properties of two API X80 pipeline steels were investigated in this study. Two API X80 pipeline steels consisting of acicular ferrite and granular bainite, and a small amount of hard phases such as martensite and secondary phases have elongated grains along the rolling direction, so that they show different mechanical properties as the specimens’ directions change. The 90 deg specimens have high tensile strength due to the low stress concentration on the fine hard phases and the high loads for the deformation of the elongated grains. In contrast, the 30 deg specimens have less elongated grains and larger hard phases such as martensite, with the size of about 3 μm, than the 90 deg specimens. Hence, the 30 deg specimens have low tensile strength because of the high stress concentration on the large hard phases and the low loads to deform grains. In the 90 deg specimen, brittle crack propagation surfaces are even since cracks propagate in a straight line along the elongated grain structure. In the 30 deg specimen, however, brittle crack propagation surfaces are uneven, and secondary cracks are observed, because of the zigzag brittle crack propagation path. In the CTOD properties, the 90 deg specimens have maximum forces of higher magnitude than the 30 deg specimens, because of the elongated grain structure. However, CTODs of the 90 deg specimens are lower than those of the 30 deg specimens because of the low plastic deformation areas by the elongated grains in the 90 deg specimens.

Corrosion Study of Pipeline Steel Weld Immersed in Sour Solution

This work presents an electrochemical study of API X52 pipeline steel weld immersed in NACE solution saturated with hydrogen sulphide (H2S). The electrochemical techniques, polarization curves and linear polarization resistance are used in the corrosion test of the steel weld samples with the focus on the three different zones of the weldment, heat affected zone (HAZ), weld bead (WB) and base metal (BM). In addition, a brief analysis was made in order to identify the corrosion product film. It was found that the increment of temperature and H2S dissolved in the NACE solution increase the corrosion rate (CR) of the three different zones. HAZ was significantly affected by the corrosion test and the phases mackinawite, troilite and pyrrhotite were identified in the film of corrosion products.

Firefly algorithm assisted optimised NN to predict the elongation of API X65 pipeline steel

Selection of a proper neural network structure is a complicated work that is often done through the trial-and-error method. In this research, a three layer neural network with feed forward topology and back propagation algorithm is considered as a basic neural network model. The number of neurons in hidden layer, the activation function, the training algorithm and the normalisation procedure are considered as the parameters of the optimisation problem. A proper fitness function is defined and the process of finding the best combination of theses optimisation parameters, based on firefly algorithm, is presented. The proposed procedure is used to find an optimised neural network to predict the elongation of API X65 pipeline steel. The optimised neural network is used to investigate the effects of Ni and microalloying elements on the elongation of the tested steel. The results are in general agreement with the other published works.

Investigating the Corrosion of API-X100 Pipeline Steel in Aerated Carbonate Solutions by Electrochemical Methods

Investigating the Corrosion of API-X100 Pipeline Steel in Aerated Carbonate Solutions by Electrochemical Methods

Effect of crystallographic texture on the anisotropy of Charpy impact behavior in pipeline steel

The current study systematically investigated the anisotropy of fracture behavior in commercial API X100 pipeline steel. The experimental results showed that the temperature for transition from ductile to brittle fracture was significantly different in samples with different orientations, defined as anisotropy of fracture behavior in this paper. The inhomogeneity distribution of microstructure (grain size) and the crystallographic texture were systematically analyzed. The difference in grain size of the samples with different orientations can be reasonably ignored in this study. Therefore, the anisotropy of fracture behavior is attributed to the crystallographic textures in which the deformation and transformation texture components were included. The effect of crystallographic texture on the anisotropy of ductile to brittle fracture transition temperature was theoretically interpreted based on the relationship among yield strength, fracture strength, and their average orientation factor.

Effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel

In the present work, a heat treatment process was used to modify corrosion behavior of heat affected zone (HAZ) and weld metal (WM) in welded pipe steel of grade API X70. A one-step austenitizing with two-step quenching and subsequent tempering treatment was performed to alter the microstructure of HAZ and WM. The hardness and strength values were controlled to be in the standard range after the heat treatment process. In order to investigate the effect of the heat treatment on the corrosion properties of welded joint, the samples were immersed in a mixture of naturally aerated 0.5M sodium carbonate (Na2CO3) and 1M sodium bicarbonate (NaHCO3) solution with pH of 9.7 for 45days. The electrochemical impedance spectroscopy (EIS) measurements were carried out then to study the protective properties of the corrosion products layer. The X-ray diffraction (XRD) investigation depicted that the corrosion products layer composition includes FeCO3, FeO(OH), Fe3O4 and Fe2O3. The EIS results showed that, the corrosion resistance of HAZ and WM increased after heat treatment. This can be attributed to formation of uniformly distributed polygonal ferrite (PF) and to the decrease in the volume fraction of bainite (B) after heat treatment.

Hydrogen diffusion and hydrogen influenced critical stress intensity in an API X70 pipeline steel welded joint – Experiments and FE simulations

An API X70 pipeline steel has been investigated with respect to hydrogen diffusion and fracture mechanics properties. A finite element cohesive element approach has been applied to simulate the onset of hydrogen-induced fracture. Base metal, weld simulated heat affected zone and weld metal have been investigated. The electrochemical permeation technique was used to study hydrogen diffusion properties, while in situ fracture mechanics testing was performed in order to establish the hydrogen influenced threshold stress intensity. The average effective diffusion coefficient at room temperature was 7.60×10−11m2/s for the base metal, 4.01×10−11m2/s for weld metal and 1.26×10−11m2/s for the weld simulated heat affected zone. Hydrogen susceptibility was proved to be pronounced for the heat affected zone samples. Fracture toughness samples failed at a net section stress level of 0.65 times the yield strength; whereas the base metal samples did not fail at net section stresses lower than the ultimate tensile strength. The initial cohesive parameters which best fitted the experimental results were σc=1500MPa (3.1·σy) for the base metal, σc=1800 MPa (3.0·σy) for weld metal and σc=1840 MPa (2.3·σy) for heat affected zone. Threshold stress intensities KIc,HE were in the range 143–149MPa√m.

Interpretation and significance of reverse chevron-shaped markings on fracture surfaces of API X100 pipeline steels

Fracture surfaces of X100 pipeline steels were examined with optical and electron microscopy after crack tip opening angle fracture testing. Some fracture surfaces exhibited chevron-shaped fracture patterns that are markedly different from classic chevron fracture. The chevron-shaped markings on the X100 fracture surfaces point in the direction of crack growth, rather than towards the location of fracture initiation, as observed in classic cases of chevron fracture. Existing models, predicting formation of chevron fracture patterns, do not explain the fracture behavior observed for X100 steel. A mechanism is proposed where reverse chevron-shaped patterns are developed due to the shape of the crack front itself. The chevron shape forms as a result of crack tunneling, and the overall pattern is developed on the fracture surface due to intermittent crack growth, resulting in alternating regions (bands) of fast fracture and slower, more ductile fracture. The contrast between these bands of alternating fracture defines the chevron. Care should be taken during interpretation of intermittent chevron markings on fractures of ductile materials, as they may point away from rather than towards the origin of fracture.

Variations in DBTT and CTOD within weld heat-affected zone of API X65 pipeline steel

Abstract The fracture resistance of heat-affected zones (HAZs) in girth welded joint of API X65 steel pipeline was systematically investigated. While the change in Charpy impact energy has been typically evaluated in previous studies, here the variations in ductile-to-brittle temperature (DBTT) and crack-tip opening displacement (CTOD) within HAZ were explored. A series of experiments revealed that both values vary dramatically (i.e., DBTT increases and CTOD decreases) as the location approaches the fusion line (FL) and thus the region adjacent to FL exhibited the lowest CTOD and highest DBTT, possibly due to the increasing portion of coarse-grained HAZ. Interestingly, however, even the FL regions still showed moderate toughness at −40 °C ∼ room temperature. Microstructural analysis and additional impact tests using simulated HAZ specimens suggested a possibility that fine-grained HAZs with higher toughness may suppress the brittle fracture from neighboring coarse-grained region.

Electrochemical investigations on the corrosion behavior and corrosion natural inhibition of API-X100 pipeline steel in acetic acid and chloride-containing CO2-saturated media

The purpose of this experimental work was to investigate selected electrochemical aspects of the corrosion behavior of API-X100 in CO2-saturated, multivariable-controlled corrosion media. Utilizing potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), the corrosion rates, anodic dissolution, cathodic regimes, and free interfacial interactions were discussed. The tests were performed with respect to the environmental factors of 10, 20, 30, 40, 50, and 60 g L−1 chloride and of 10, 20, 30, 40, 50, and 60 mL L−1 acetic acid at 20 and 90 °C in the absence and presence of 10 vol% crude oil. The corrosion rates exhibited a peak value with respect to the chloride content while they increased continuously with the acetic acid content irrespectively from temperature. The corrosion behavior was nearly independent from chloride in the presence of acetic acid and oil demonstrated an effective inhibition in all conditions. EIS results showed an agreement with the polarization findings and indicated adsorption-controlled mechanisms.

Assessment of the In-Service Degradation of Pipeline Steel by Destructive and Nondestructive Methods

We analyze the influence of the long-term operation of API X52 pipeline steel on the degradation of its mechanical properties. It is established that, after operation for 30 years, its strength and plasticity characteristics decrease substantially, and the fatigue toughness decreases particularly abruptly, which is connected with the development of the damageability. We use indentation methods for comparing the states of the operated and nonoperated material. It is shown that the indices of spread of the results of measurements (the coefficient of variance) of the mechanical properties (hardness, wear, etc.) are more sensitive to the damage of the material in the initial state and the damage after operation than their averaged values. Some correlations between the Vickers hardness, cyclic hardness, strength, and plasticity of the steel are established.

Effects of finish rolling temperature on inverse fracture occurring during drop weight tear test of API X80 pipeline steels

In this study, drop-weight tear tests (DWTT) were conducted on API X80 pipeline steels fabricated with different finish rolling temperatures in order to analyze abnormal fracture appearance, i.e., inverse fracture, occurring in the region impacted by a hammer. Area fractions of fracture modes were measured from fractured DWTT specimens, and the measured data were analyzed in relation to microstructures, DWTT absorbed energy, and strain hardening of the hammer-impacted region. As the finish rolling temperature decreased, the volume fraction of fine-grained acicular ferrite increased, while that of large-grained upper bainite or granular bainite decreased. According to the DWTT results, the absorbed energy tended to increase with increasing volume fraction of acicular ferrite (with decreasing finish rolling temperature). A large area of inverse fracture of cleavage type was found in the hammer-impacted region of the steels fabricated with high finish rolling temperatures, but the area fraction of inverse fracture was reduced in the steels fabricated with low finish rolling temperatures. Since the area fraction of inverse fracture was closely related with strain hardening of the hammer-impacted region, it could be successfully reduced by lowering strain hardening and by promoting the formation of acicular ferrite via low finish rolling temperatures.

Electrochemical evaluation of the corrosion behaviour of API-X100 pipeline steel in aerated bicarbonate solutions

This work is devoted to evaluating electrochemical corrosion aspects of API-X100 in bicarbonate solutions. They were prepared to be aerated; chloride free and 3wt.% chloride containing solutions of 0.1, 0.5, and 0.8M bicarbonate at 20, 40, and 60°C. Open circuit potentials, polarization potentiodynamics, and impedance spectroscopy were utilized. The behaviour exhibited a cathodic dependence on bicarbonate and temperature where the corrosion rates consequently increased. Chloride prevented stable passivation and increased the anodic sensitivity. EIS and polarization results were in agreement and impedance in the presence of chloride was temperature dependent.

Hydrogen effect on fracture toughness of pipeline steel welds, with in situ hydrogen charging

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm 2. A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J 0 integral. In all cases a slight change in toughness was measured in terms of K Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/cm 2) and a larger reduction for the third case (10 mA/cm 2). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferrite–pearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.

Effect of bainitic microstructure on the susceptibility of pipeline steels to hydrogen induced cracking

Hydrogen induced cracking (HIC) of API X80 and API X100 pipeline steels have been investigated in high pH carbonate–bicarbonate environment using slow strain rate testing (SSRT) method. It has been found that while both steels are highly susceptible to HIC, and diffusible hydrogen content is higher in API X80 than in API X100, the later steel is more vulnerable than the former at high (more negative) cathodic potential. This higher susceptibility can be primarily attributed to the combined effect of (1) separation of bainitic lath boundaries due to hydrogen trapping in these locations, (2) mobile hydrogen, and (3) stress. The charging–discharging experiments followed by SSRT experiments in air suggest that, the cracks that appeared due to lath boundary separation did not cause the reduction of ductility by themselves, rather it was the diffusible hydrogen that forced these cracks to propagate and, ruptured the steel with very low percent reduction of area (%RA). Despite the fact that the mobile hydrogen content plays a key role in causing the embrittlement, the large number of cracks in API X100 steel, resulting from the bainitic lath boundary separation at high cathodic potential, superseded the effect of higher diffusible hydrogen content in API X80 steel. The general conclusion is that bainitic lath type microstructure is more vulnerable to HIC at high cathodic potential than the ferritic/granular bainitic ones. It has been also found that applying cathodic protection can lead to excessive hydrogen embrittlement in both of the abovementioned steels in high pH carbonate–bicarbonate environment and, therefore, efforts need to be invested in developing nobler (more positive corrosion potential) and better HIC resistant steels.

Improvement of Charpy impact properties in heat affected zones of API X80 pipeline steels containing complex oxides

The present study aimed at investigating the improvement of Charpy impact properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of steels were fabricated by adding Ti, Al and Mg to form complex oxides and their microstructures, tensile properties and Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al and Mg was included in the steels. After the HAZ simulation test, the steel having a number of oxides contained about 72 vol.-% of acicular ferrite in the simulated HAZ, together with bainitic ferrite and granular bainite, whereas the HAZ microstructures of the conventional steels consisted of bainitic ferrite and granular bainite with a small amount of acicular ferrite. The volume fraction of acicular ferrite increased as the amount of complex oxides increased and the heat input decreased, and thus, Charpy impact properties of the oxide containing steel HAZs were greatly improved over those of the conventional steel HAZs.