Investigation of the hydrogen induced cracking behaviour of API 5L X65 pipeline steel

The effect of microstructural features on the hydrogen induced cracking (HIC) susceptibility of two API 5L X65 pipeline steels were investigated by cathodic charging, hydrogen permeation and hydrogen microprint experiments. Microstructural evaluation after hydrogen charging revealed cracks at the mid-thickness (segregation zone) of both plates. However, more severe cracks were observed in the plate with higher dislocation density and residual stresses. The plate with lower plastic strain and more {111}-oriented grains had less severe cracks. Inclusions found along the crack path, comprising of Si-enriched oxides and carbides contributed to the initiation and propagation of cracks. The variation of the trapping behaviour and hydrogen diffusion through the plates were examined. The results confirmed that a higher ratio of reversible to irreversible traps contributes to increasing HIC severity in steels. Additionally, hydrogen transport through the steels was most prominent along the grain boundaries, indicating the importance of grain boundary character to HIC.     

Hydrogen induced cracking susceptibility in different layers of a hot rolled X70 pipeline steel

Hydrogen induced cracking (HIC) behaviour was investigated in three layers of RD–ND plane. HIC test showed that all cracks initiated from the mid-thickness of the RD–ND plane and propagated in the rolling direction of the steel plate. Hydrogen permeation test results showed a lower permeability and diffusivity coefficient for the third layer resulting in the highest density of traps and consequently HIC susceptibility. Considering the HIC test and crystallographic texture measurements, cracks initiated from the grain boundaries associated with {100} grain orientation and arrested in regions with some strong texture components, such as {110}//ND, {112}//ND, and possibly {332}//ND. The role of HABs and CSLs boundaries was important in crack propagation.     

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.     

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.     

Evolution of microstructure and texture in pipeline steels at different TMCP procedures with regard to hydrogen induced cracking

SEM and EBSD techniques are used to evaluate hydrogen induced cracking susceptibility in API X70 pipeline steels produced by thermo-mechanical controlled process (TMCP) in laboratory scale. Based on the observations, there is no dominant texture in the specimens and the grains are randomly distributed. Different TMCP parameters and rolling processes generates different grain size, and grains are often elongated along the rolling direction. The results also show that cooling rate is another factor affecting the grain size. A high cooling rate does not allow the grains to grow. The reason for the transgranular type of cracking might be the strong grain boundaries in ambient temperatures which prevents the intergranular cracking. Based on experiments, the hydrogen environment does not have permanent effects on the mechanical properties of the investigated specimens. The electrochemical hydrogen charging experiment shows that the grain refinement improves the resistance to hydrogen embrittlement.     

Evaluation of electrochemical hydrogen absorption in welded pipe with steel API X52

The assessment of ability to absorb hydrogen by welds components of API grade pipeline steel X52 has been done. The factors of cathodic hydrogen charging, time of exposure on hydrogen concentration in base metal, heat affected zone and metal of weld were taken into account. It has been shown that all components of weld demonstrate the sensitivity to hydrogenating in deoxygenated, near-neutral pH NS4 solution under relatively “soft” cathodic polarisation, although the efficiency of hydrogen permeation in metal is relatively low and depends on time of exposure. The ability to absorb hydrogen decreases in the following sequence: heat affected zone – base metal – weld. The sensitivity to hydrogenation is higher for heat affected zone in comparison with base metal and weld.     

Effect of tensile stress on the hydrogen adsorption of X70 pipeline steel

The effect of stress on the cathodic hydrogen evolution behavior of X70 pipeline steel was investigated by electrochemical tests, tensile tests, and microstructural characterization. The results indicated that the tensile stress enhanced the activity of hydrogen adsorption sites on the metal surface, which was considered as the dominating factor a?ecting generation, adsorption, and permeation of hydrogen atoms. The subsurface hydrogen atom concentrations quantified by Cyclic voltammetry (CV) tests and the data calculated by hydrogen permeation experiments showed a good correspondence. The results indicated that the tensile stress enhanced the adsorption of hydrogen atoms on the surface and an inhibitory effect on the Tafel and Heyrovsky reaction, thereby leading to the increase of the subsurface hydrogen atom concentration, enhance the hydrogen embrittlement susceptibility of the X70 steel material as demonstrated by plasticity loss in the tensile tests.     

The effect of hydrogen content and welding conditions on the hydrogen induced cracking of the API X70 steel weld

The self-restraint testing was used to investigate the influence of hydrogen content, preheating, and post-heating on the sensitivity of welding of API X70 pipeline steel to hydrogen induced cracking (HIC). The variation of hydrogen content was applied using a low hydrogen electrode E8018-G and a high hydrogen (cellulosic) electrode E8010-P1. Diffusible hydrogen of these electrodes was measured by mercury displacement method. The average diffusible hydrogen content of cellulosic electrode E8010-P1 and low hydrogen electrode E8018-G were 43.6 and 1.1 ml/100 g of weld metal, respectively. The results of visual inspection, penetrant test, and macroscopic examination showed that welding with cellulosic electrode leads to cracking unless both preheating and post-heating are applied. However, in the case of low hydrogen electrode, cracking occurs only if no preheating or post-heating is applied. The microstructure of the welded specimens in different conditions by optical and scanning electron microscopy (SEM) showed that the dominant phase in the weld zone of all specimens is bainite. The microhardness profile displayed that hardness limitation (350 HV) cannot predict the sensitivity to cold cracking; therefore, other parameters such as hydrogen content should also be considered.     

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.     

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.     

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.     

Effects of internal hydrogen and surface-absorbed hydrogen on the hydrogen embrittlement of X80 pipeline steel

Effects of internal hydrogen and surface-absorbed hydrogen on hydrogen embrittlement (HE) of X80 pipeline steel were investigated by using different strain rate tensile test, annealing and hydrogen permeation tests. HE of X80 pipeline steel is affected by internal hydrogen and surface-absorbed hydrogen, and the latter plays a major role due to its higher effective hydrogen concentration. The HE susceptibility decreases with increasing the strain rate because it is more difficult for hydrogen to be captured by dislocations at the high strain rate. Annealing at 200 °C can weakened HE caused by internal hydrogen, while it has little effect on HE caused by surface-absorbed hydrogen. HE of X80 pipeline steel is mainly determined by the behavior of dislocation trapping hydrogen, which can be attributed to the interaction between hydrogen and dislocation.     

Hydrogen-induced cracking and corrosion behavior of friction stir welded plates of API 5L X70 pipeline steel

The use of friction stir welding (FSW) has proven to be an excellent alternative to join engineering components. Although FSW has had a significant development in recent years, challenges for new applications have been raised, such as offshore steel parts suffering hydrogen embrittlement in the gas and oil industry. Therefore, in this work, the microstructure, corrosion, and hydrogen-induced cracking were investigated in a two-pass FSW welded joint of API 5L X70 pipeline steel. The electrochemical results indicate an inhibitory effect on corrosion reaction because of a carbonate product generation in the steel surface. The polygonal ferritic and degenerated pearlite bands microstructure in the base metal fixed carbonate deposits in the steel surface. In the welded regions, the bainitic microstructure and the carbide particle distribution are less efficient in setting the weld surface carbonate deposit. HIC tests showed cracks initiation and propagation to be more prone in hard phases.     

Hydrogen trapping and hydrogen induced cracking of welded X100 pipeline steel in H2S environments

Hydrogen induced cracking (HIC) susceptibility of the welded X100 pipeline steel was evaluated in NACE “A” solution at room temperature according to the NACE TM0284-2011 standard. Both the kinetic parameters of the permeability $\left(J_{\infty }L\right)$, the apparent diffusivity (D_app) and the concentration of reversible and irreversible hydrogen in the base metal and welded joint of X100 pipeline steel were quantitatively investigated by hydrogen permeation test. The results showed that the welded joint with an inhomogeneous microstructure had a higher trap density and more susceptible to HIC due to two orders of magnitude larger in the concentration of irreversible hydrogen than that of base metal, though all presenting poor HIC resistance for both base metal and the welded joint. The HIC cracks initiated from the inclusions enriching in Al, Ca, Si, Mn. The cracks are primarily transgranular, accompanying with limited intergranular ones.     

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.     

Hydrogen embrittlement behavior of high strength low carbon medium manganese steel under different heat treatments

Hydrogen embrittlement (HE) behavior was investigated in a low carbon medium Mn steel with three different volume fraction of retained austenite (RA), which was obtained after different heat treatments. The hydrogen permeation test showed a higher permeability for directly water quenched specimen compared to quench-tempered specimens. Melt extraction test showed hydrogen concentration increased with hydrogen charging current density in the order of directly quenched specimen, QLA, quenched with low-temperature annealed specimens and QHA quenched with high-temperature annealed specimens. Slow strain-rate tensile test was employed to examine the HE behavior, the HE indices decreased with the increase of RA irrespective of increased hydrogen concentration. HE susceptibility can be suppressed by raising intercritical annealing temperature because Mn enrichment increases the stability of RA.     

Role of crystallographic texture on the improvement of hydrogen-induced crack resistance in API 5L X70 pipeline steel

The purpose of this work was to improve the resistance of hydrogen-induced cracking in API 5L X70 steel by engineering the crystallographic texture and grain boundary distributions via different rolling temperatures. Hydrogen-induced cracking and electrochemical hydrogen charging tests were carried out in two different conditions: commercially produced and isothermally rolled at 850 °C in laboratory. The results showed that the development of dominant {011} grains parallel to the normal direction, and a small number of {001}//ND grains obtained by isothermal rolling at 850 °C, increased the hydrogen-induced crack resistance; while the hot rolled sample with sharp {001}//ND textures was highly susceptible to cracking.     

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.     

Effect of cyclic plastic deformation on hydrogen diffusion behavior and embrittlement susceptibility of reeling-pipeline steel weldments

The hydrogen embrittlement (HE) susceptibility and hydrogen permeation behavior of reeling-pipeline welded joint with/without cyclic plastic deformation (CPD) were studied using the electrochemical hydrogen charging technique. Results indicated that the surface of welded joint emerged hydrogen-induced damage containing cracks and blisters. The degree of hydrogen-induced damage increased with the increase of hydrogen charging time and current density. When the hydrogen charging current density and time was 50 mA/cm² and 4 h, respectively, the area ratio of hydrogen-induced damage of overall welded joint with CPD process was reduced from 6.61% to 2.28%, and the damage ratio of different sub-zones in welded joint was also decreased. The oxidized inclusions enriching Al–Mg–Ca elements acted as the initiation sites for hydrogen-induced damages. The effective diffusion coefficient of as-welded joint was 2.63 × 10⁻⁶ cm²/s, while that of welded joint with CPD showed a smaller value of 1.36 × 10⁻⁶ cm²/s. The welded joint with CPD process presented better resistance to HE, which was attributed to the increased density of hydrogen traps and the formation of dislocation cells to disperse hydrogen uniformly and reduce the possibility of local accumulation and recombination of diffusible hydrogen. Sub-zones in welded joint without CPD process were considerably more sensitive to hydrogen-induced damage, which indicated the important role of microstructure and dislocation density in HE mechanisms. The order of HE susceptibility from low to high was weld metal, base metal and heat affected zone.