Effect of Ce content on the hydrogen induced cracking of X80 pipeline steel

In this study, the effect of Ce content on hydrogen induced cracking (HIC) of X80 pipeline steel has been investigated. The results show that as the Ce content increased from 0 wt% to 0.0042 wt%, 0.016 wt% and 0.024 wt%, the HIC susceptibility of tested steels decreased first and then increased. The steel containing 0.016 wt% Ce possessed the lowest HIC susceptibility because Ce modified inclusions, promoted the formation of acicular ferrite, and decreased the number of hydrogen traps and intergranular cracks.     

Hydrogen permeation behavior of X70 pipeline steel simultaneously affected by tensile stress and sulfate-reducing bacteria

The hydrogen permeation behavior of submarine pipelines buried in anoxic sea mud and protected by cathodic potential is affected by both sulfate-reducing bacteria (SRB) and tensile stress. In this study, the individual and simultaneous effects of SRB and tensile stress on hydrogen permeation parameters were investigated using an electrochemical hydrogen permeation method together with mechanical tensile tests. Cathodic potentiodynamic polarization and surface morphology investigations were also conducted. Both elastic and plastic stresses were considered. Results showed that SRB enhanced the sub-surface hydrogen concentration significantly but had little influence on the diffusion coefficient. Elastic stress had a minimal effect on the hydrogen permeation behavior of X70 steel. Plastic stress reduced the diffusion coefficient and increased the sub-surface hydrogen concentration. The lattice trap produced by plastic deformation was responsible for the impact of plastic stress on hydrogen permeation. SRB and plastic stress not only enhanced the sub-surface hydrogen concentration independently, but also had synergistic effects accelerating the hydrogen accumulation on a steel surface.     

Comprehensive study on hydrogen induced cracking of electrical resistance welded API X52 pipeline steel

Different heat treatment cycles were designed in order to investigate the effect of microstructural changes on hydrogen induced cracking resistance (HIC) and mechanical properties of the electric resistance welded steel. The heat treating of the as-welded specimen improved the ductility and impact toughness. After heat treatment, the uniform hardness profile was obtained for the welded specimens. The removal of local hard zones reduced the risk of HIC. The chemical composition and clustering of inclusions have a deleterious effect on cracking resistance in the H₂S environment. Aluminosilicate compounds and MnS inclusions were favorite sites for HIC. The most promising post weld heat treatment for improving mechanical properties and the resistance to HIC was the application of two-cycle quenching followed by tempering.     

Hydrogen induced blister cracking and mechanical failure in X65 pipeline steels

The present work aims to investigate the role of hydrogen induced blisters cracking on degradation of tensile and fatigue properties of X65 pipeline steel. Both tensile and fatigue specimens were electrochemically charged with hydrogen at 20 mA/cm² for a period of 4 h. Hydrogen charging resulted in hydrogen induced cracking (HIC) and blister formation throughout the specimen surface. Nearly all the blisters formed during hydrogen charging showed blister wall cracking (BWC). Inclusions mixed in Al-Si-O were found to be the potential sites for HIC and BWC. Slow strain rate tensile (SSRT) test followed by fractographic analysis confirmed significant hydrogen embrittlement (HE) susceptibility of X65 steel. Short fatigue crack growth framework, on the other hand, specifically highlighted the role of BWC on accelerated crack growth in the investigated material. Coalescence of propagating short fatigue crack with BWC resulted in rapid increase in the crack length and reduced the number of cycles for crack propagation to the equivalent crack length.     

Hydrogen diffusion and trapping in X70 pipeline steel

In this investigation, the hydrogen permeation experiment was used to determine the parameters for diffusion, and the hydrogen microprint technique was used to visualize the diffusion path in X70 pipeline steels. The samples in mid-layer at the segregation zone and top-layer of the steel were used in this study. The diffusion parameters from the hydrogen permeation experiment enabled us to determine that the calculated density of total, reversible, and irreversible hydrogen trapping sites in the top layer of the steel decreases for larger grains. However, the irreversible and total trapping sites of the mid-layer showed an initial growth and subsequent decay with grain growth due to the inclusions in mid-layer. The observations from the hydrogen microprint experiment allowed us to conclude that the preferential hydrogen diffusion increases in the order of grains, grain boundaries, triple junctions and cementites with cementites being the easiest path for hydrogen diffusion.     

Effects of vanadium precipitates on hydrogen trapping efficiency and hydrogen induced cracking resistance in X80 pipeline steel

In this study, the number and size distribution of vanadium precipitates and their effects on hydrogen trapping efficiency and hydrogen-induced cracking (HIC) susceptibility were investigated in X80 pipeline steel. The results showed that as the vanadium content increased, the number of nanoscale vanadium precipitates clearly increased. Furthermore, the amount of hydrogen atoms trapped by vanadium precipitates gradually increased and the hydrogen diffusion coefficient decreased from 4.74 × 10^?6 cm² s^?1 in the vanadium-free V0 steel to 8.48 × 10^?7 cm² s^?1 in the V4 steel with 0.16% V, according to hydrogen permeation results. It also reduced the possibility of hydrogen atoms diffusing into the sites of harmful defects such as large-size oxides and elongated MnS inclusions, where cracks were caused more easily. In addition, the V3 steel with 0.12% V, containing the largest number of vanadium carbide particles of less than 60 nm, had the lowest HIC susceptibility.     

3D cohesive modelling of hydrogen embrittlement in the heat affected zone of an X70 pipeline steel

A three-dimensional finite cohesive element approach has been developed and applied in order to simulate the crack initiation of hydrogen-induced fracture. A single edge notched tension specimen of an X70 weld heat affected zone was simulated. The results were compared to similar two-dimensional plane strain model and the cohesive parameters were calibrated to fit the experimental results. The three dimensional simulations gave higher values in terms of opening stress at the stress peak, plastic strain levels at the crack tip and hydrogen lattice concentration when compared with two-dimensional simulations under the same global net section stress levels. Nevertheless a higher cohesive strength was needed for the 2D model for the onset of crack propagation. The best fit to the experimental data were obtained for a cohesive strength of 1840 MPa and 1620 MPa for the 2D and 3D simulation respectively. The critical opening was assigned to 0.3 mm for both models. The threshold stress intensities K_IC,HE were 142 MPa√m and 146 MPa√m for the 2D and 3D models, respectively.     

Measurement of lattice and apparent diffusion coefficient of hydrogen in X65 and F22 pipeline steels

The permeation through and desorption of hydrogen from cathodically charged micro-alloyed API 5L X65 and low alloy ASTM A182 F22 steels were investigated with the aim to analyse the trapping parameters by means of the electrochemical method developed by Devanathan and Stachurski. The lattice diffusivity of hydrogen as well as the amounts and distribution of the diffusible and trapped hydrogen were found by means of partial permeation transient.     

The significance of inclusion morphology and composition in governing hydrogen transportation and trapping in steels

In this paper, two steels with different morphology of inclusions are studied. Scanning electron microscopy, finite element calculations, and electrochemical hydrogen permeation tests were conducted. Furthermore, in-situ observations of hydrogen de-trapping from the surface of steels were achieved using a hydrogen charging device and optical microscope. Results show that stripe-shaped complex inclusions exhibit a higher level of residual stress, especially at the boundary of oxide components. The hydrogen molecule is also prone to emerge at the oxide composition and intensifies the risks of cracking. In contrast, the residual stress levels around spherical inclusions are comparatively lower due to discrete sites of stress concentration. Thus, the effect of dispersed tiny dioxide sphere inclusions can help relieve the local hydrogen pressure and promote the resistance against hydrogen-induced cracking.     

Effect of grain size on hydrogen embrittlement in stable austenitic high-Mn TWIP and high-N stainless steels

Two stable austenitic steels, 20Cr-11Ni-5Mn-0.3N (wt%) stainless steel (STS) and 18Mn-1.5Al-0.6C (wt%) twinning-induced plasticity steel (TWIP), were investigated to understand the effect of grain size on hydrogen embrittlement (HE). Grain refinement promoted HE in the STS but suppressed HE in the TWIP. These opposite effects occurred because the steel composition affected deformation mechanism. Cr-N pair enhanced short-range ordering (SRO) in STS, which promoted planar slip and delayed mechanical twinning. In contrast, TWIP exhibited mechanical twinning which was more active in coarser grains. Final dislocation density after tensile deformation was increased by grain refinement in STS, but was decreased in TWIP. The damaging effects of hydrogen on strain energy at interfaces and on interfacial bonding strength were controlled by dislocation density; therefore, increase in dislocation density led to increase in susceptibility to HE.     

Effect of tensile stress on the hydrogen permeation of MS X65 pipeline steel under sulfide films

The effect of the tensile stress on the hydrogen permeation of MS X65 pipeline with sulfide films was investigated through measuring the steady-state hydrogen permeation current (I_∞), permeability (J_∞L) and apparent diffusivity (D_app) and quantitatively analysing the hydrogen-permeable resistance factor (HPRF) of single tensile stress HPRF (stress), single sulfide film HPRF (film) and the two together HPRF (stress-film). The results indicated that J_∞L and sub-surface hydrogen concentration (c_o) greatly increase and that D_app decreases as the elastic stress increases. When applying plastic stress, J_∞L and D_app all reduce, while c_o continues to increase without the film but decreases with the film. While single tensile stress can promote hydrogen permeation, with the sulfide film, the value of HPRE (stress-film) is not a simple addition of the value of the HPRE (stress) and the HPRE (film), and the interaction results in the blocking effect of hydrogen permeation. The surface morphology of the sulfide films changes caused by tensile stress should be responsible for the HPRE (stress-film) reducing as tensile stress increases but increasing with plastic tensile stress.     

3D cohesive modelling of hydrogen embrittlement in the heat affected zone of an X70 pipeline steel – Part II

A three steps 3D cohesive element based modelling procedure for hydrogen embrittlement susceptibility prediction of an X70 weld thermal simulated Heat Affected Zone has been developed. Fracture mechanics testing both under electrochemical charging and pressurized hydrogen gas are used for simulations validation. A new approach for the cohesive parameter's choice, based on the initial cohesive stiffness entity, has been adopted. The best fit to the experimental data were obtained for cohesive strength of 14,450 MPa and for opening of 0.005 mm, yielding a reduced threshold stress intensity K_IC,HE of 49 MPa√m, for test in CP. Hydrogen boundary condition for simulations of test under hydrogen gas obtained by applying Sievert's law provided far too low concentrations for any appreciable influence on the cohesive energy. Toward the end of the paper, comparison between the two hydrogen sources is made by imposing the same initial hydrogen boundary conditions for the simulations. Results indicate that crack initiation is located at the hydrostatic stress peak and at the crack tip for cathodic protection and 0.6 MPa H₂ gas conditions, respectively. Simulations suggest that the hydrogen in lattice interstitials is mainly responsible for the embrittlement.     

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/cm². 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 J₀ 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/cm²) and a larger reduction for the third case (10 mA/cm²). 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 microstructure inhomogeneity on hydrogen embrittlement susceptibility of X80 welding HAZ under pressurized gaseous hydrogen

Hydrogen embrittlement (HE) of high-grade pipeline welded joint is a threat to hydrogen gas transport. In this research, slow strain rate tension (SSRT) tests in high-pressure hydrogen gas, combined with hydrogen permeation tests and microstructure analysis were conducted on X80 steel, intercritical heated-affected zone (ICHAZ), fine-grained heat-affected zone (FGHAZ) and coarse-grained heat-affected zone (CGHAZ). The change of HE susceptibility from high to low was CGHAZ, FGHAZ, ICHAZ, and base metal. Microstructure was the important factor influencing hydrogen permeation and susceptibility to HE. Susceptibility to HE was increased in the order of “fine-grained massive ferrite (MF) and acicular ferrite (AF)”, “fine-grained granular bainite (GB) and MF”, “coarse-grained GB and bainite ferrite (BF) embedded with martensite-austenite (M-A) constitute”. The fine-grained MF and AF in base metal with lower hydrogen diffusivity can impede the embrittlement behaviour, while the coarse-grained GB and BF with higher hydrogen diffusivity in CGHAZ increased its susceptibility to HE.     

Hydrogen embrittlement behavior in the nugget zone of friction stir welded X100 pipeline steel

Hydrogen-induced damage is an inevitable challenge in pipeline safety applications, especially, the fusion welded joints owing to microstructure heterogeneity caused by welding process. In this work, X100 pipeline steel was subjected to friction stir welding (FSW) at rotation rates of 300–600 rpm under water cooling, and the relationship among the microstructure, hydrogen diffusivity, and hydrogen embrittlement (HE) behavior of the nugget zone (NZ) were studied. The NZ at 600 rpm had the highest effective hydrogen diffusion coefficient (D_eff) of 2.1 × 10^?10 m²/s because of the highest dislocation density and lowest ratio of effective grain boundary. The D_eff decreased with decreasing rotation rate due to the decrease of dislocation density and the increase of ratio of effective grain boundary, and the lowest D_eff of 1.32 × 10^?10 m²/s was obtained at 300 rpm. After hydrogen charging, the tensile strength of all specimens decreased slightly, while the elongation decreased significantly. As the rotation rate decreased, the elongation loss was obviously inhibited, and ultimately a lowest elongation loss of 31.8% was obtained at 300 rpm. The abovementioned excellent mechanical properties were attributed to the fine ferrite/martensite structure, low D_eff, and strong {111}//ND texture dramatically inhibiting hydrogen-induced cracking initiation and propagation.     

Quantifying the hydrogen embrittlement of pipeline steels for safety considerations

In a near future, with an increasing use of hydrogen as an energy vector, gaseous hydrogen transport as well as high capacity storage may imply the use of high strength steel pipelines for economical reasons. However, such materials are well known to be sensitive to hydrogen embrittlement (HE). For safety reasons, it is thus necessary to improve and clarify the means of quantifying embrittlement. The present paper exposes the changes in mechanical properties of a grade API X80 steel through numerous mechanical tests, i.e. tensile tests, disk pressure test, fracture toughness and fatigue crack growth measurements, WOL tests, performed either in neutral atmosphere or in high-pressure of hydrogen gas. The observed results are then discussed in front of safety considerations for the redaction of standards for the qualification of materials dedicating to hydrogen transport.     

Influence of microstructure on the hydrogen permeation of alumina coatings

In this work, the influence of microstructure on the hydrogen permeation property of alumina coatings was investigated. The different microstructures were obtained by annealing coatings at 700 °C or 900 °C. The permeation measurements showed that the 700 °C annealed coating exhibited lower hydrogen permeability than the 900 °C annealed coating. The 700 °C annealed coating was amorphous alumina. While, the 900 °C annealed coating had the spinel MnCr₂O₄ phase and γ-alumina. This spinel MnCr₂O₄ formed a network on the coating surface compared with the fine and smooth surface of the 700 °C annealed coating. The MnCr₂O₄ network in the 900 °C annealed coating formed short-cut for hydrogen diffusion, and thus resulted in high permeability. Furthermore, apparent delamination of coating was illustrated on the 900 °C annealed coating after the permeation test, and this was another reason for the high permeability of coating.     

Damage associated with interactions between microstructural characteristics and hydrogen/methane gas mixtures of pipeline steels

There is no common standard for blended hydrogen use in the natural gas grid; hydrogen content is generally based on delivery systems and end-use applications. The need for a quantitative evaluation of hydrogen-natural gas mixtures related to the mechanical performance of materials is becoming increasingly evident to obtain long lifetime, safe, and reliable pipeline structures. This study attempts to provide experimental data on the effect of H₂ concentration in a methane/hydrogen (CH₄/H₂) gas mixture used in hydrogen transportation. The mechanical performance under various blended hydrogen concentrations was compared for three pipeline steels, API X42, X65, and X70. X65 exhibited the highest risk of hydrogen-assisted crack initiation in the CH₄/H₂ gas mixture in which brittle fractures were observed even at 1% H₂. The X42 and X70 samples exhibited a significant change in their fracture mechanism in a 30% H₂ gas mixture condition; however, their ductility remained unchanged. There was an insignificant difference in the hydrogen embrittlement indices of the three steels under 10 MPa of hydrogen gas. The coexistence of delamination along with the ferrite/pearlite interface, heterogeneous deformation in the radial direction, and abundance of nonmetallic MnS inclusions in the X65 sample may induce a high stress triaxiality at the gauge length at the beginning of the slow strain rate tensile process, thereby facilitating efficient hydrogen diffusion.     

Effect of nickel on hydrogen permeation in ferritic/pearlitic low alloy steels

Nickel offers several beneficial effects as an alloying element to low alloy steels. However, it is, in the oil and gas industry, limited by part 2 of the ISO 15156 standard to a maximum of 1 wt% due to sulfide stress cracking resistance concerns.Hydrogen uptake, diffusion, and trapping were investigated in research-grade ferritic/pearlitic low alloy steels with Ni contents of 0, 1, 2 and 3 wt% by the electrochemical permeation method as a function of temperature and hydrogen charging conditions.Qualitatively, the effective diffusion coefficient, D_eff, decreased with increasing Ni content. The sub-surface lattice hydrogen concentration, C₀, decreased with increasing Ni content in all charging conditions while the trend between the sub-surface hydrogen concentration in lattice and reversible trap sites, C_OR, and Ni content varied with the charging conditions. Irreversible trapping, evaluated by consecutive charging transients, was not observed for any of the materials. Lastly, the possible influence of an increasing fraction of pearlite with increasing Ni content is discussed.     

Effect of low partial hydrogen in a mixture with methane on the mechanical properties of X70 pipeline steel

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H₂. Overall, 10 MPa of 100% H₂ significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H₂ does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H₂ causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.