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.     

Combination of radiation and hydrogen damage of reactor pressure vessel materials

The hydrogen embrittlement of low-alloyed base steel, austenitic cladding and heat affected zone (HAZ) of a reactor pressure vessel was measured for both unirradiated and irradiated materials. The fracture toughness decreased with both hydrogen charging and neutron irradiation; the shift of the fracture toughness-temperature transient curve is influenced by both damage processes. The plastic zone in hydrogen-charged material becomes smaller. The total elongation of both CrMoV and CrNiMoV HAZ decreases with increasing hydrogen content. This influence is pronounced in the HAZ after a weld process without subsequent annealing, a total loss of plasticity being observed in this case. The properties of the austenitic layer are not influenced at comparable hydrogen contents.     

Hydrogen permeation and distribution at a high-strength X80 steel weld under stressing conditions and the implication on pipeline failure

Hydrogen permeation and distribution at pipeline welds is critical to integrity maintenance of the pipelines, especially for those made of high-strength steels. The situation becomes even more important under stressing conditions. In this work, metallographic characterization and micro-hardness measurements were conducted at an X80 steel weld. Potentiodynamic polarization and electrochemical hydrogen permeation testing were performance at various zones at the weld, along with numerical modeling of hydrogen distribution at the zones. The X80 steel contains a microstructure of bainite bundles and polygonal ferrite. There are more polygonal ferrite, fewer bainite and some segregated cementite at heat-affected zone (HAZ). The weld metal is featured with acicular ferrite and some grain boundary ferrite. HAZ softening occurs at the weld. The hardness of the weld metal, HAZ and base steel is about 290, 248 and 261 HV_0.2, respectively. There is the greatest corrosion current density, i.e., corrosion rate, at HAZ under both elastic and plastic stresses. An applied stress further increases the corrosion current density. Under the plastic stress of 1.1σ_ys (σ_ys is yield strength), the corrosion current densities of HAZ, base steel and weld metal are 41.04, 17.03 and 25.49 μA/cm², respectively. There are always the greatest hydrogen trapping density and the smallest hydrogen diffusivity at HAZ. Hydrogen, once penetrating the welded steel, tends to accumulate at the HAZ, compared with other two zones. When the welded steel is under stresses, especially a plastic stress (i.e., 1.1σ_ys), the hydrogen diffusivity and permeability decrease, while the subsurface hydrogen concentration and hydrogen trapping density increase remarkably. Plastic deformation favors the hydrogen permeation and trapping at weld, especially the HAZ, to elevate the susceptibility to hydrogen damage. The hydrogen distribution at different welding zones can be evaluated and determined by a developed modeling method.     

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.     

Weld HAZ embrittlement of Nb containing C–Mn steels

The adverse influence of Nb on weld HAZ properties is still an active issue of discussion between construction companies and steel manufacturers. Some controversy exists in the literature concerning the influence of Nb on HAZ properties under certain conditions, and this investigation was subsequently performed over a range of C and Nb compositions, typical of the steels concerned, and for three different single cycle heat inputs ranging from 1.5 to 6 kJ/mm. Simulated thermal cycles were employed, using a Gleeble 1500 thermomechanical simulator, followed by CVN testing. It is shown that Nb additions can have a detrimental or beneficial effect at low heat inputs, depending on the C level, but a severe detrimental effect of Nb on HAZ toughness is observed at high heat inputs and C levels.     

On the role of hydrogen in the stress corrosion cracking behavior of Q345R steel in HF vapor environment

The stress corrosion cracking (SCC) behavior of Q345R steel in hydrofluoric acid (HF) vapor environment was investigated. It is shown that Q345R has a high susceptibility to SCC in HF vapor environment, which is negatively correlated with the strain rate. Several different crack morphologies and cracking factors are verified: flat cracks in ferrite are associated with anodic dissolution triggered by micro-galvanic corrosion, and porous cracks at the pearlite and pearlite-ferrite interfaces are mainly influenced by hydrogen. The results of hydrogen charging tests show that pre-charging has little effect on the hydrogen embrittlement of Q345R steel, while in-situ charging leads to severe brittle fracture of the material, because hydrogen interacts with large number of moving dislocations generated by in-situ stretching process and penetrates more readily into the material. The synergistic relationship between hydrogen and dislocation motion is found to be the main mechanism for the transition from ductile to brittle fracture.     

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.     

Effect of hydrogen on pitting corrosion of 2205 duplex stainless steel under alternating dry/wet marine environment

Stainless steels play an extremely vital role in the field of marine engineering equipment. However, stainless steel products in service are still subject to corrosion from severe environments such as alternating dry/wet condition and damage caused by hydrogen introduced during heat treatment or cathodic protection. Under alternating dry/wet marine environment, the synergistic effect of hydrogen and corrosion can influence the corrosion resistance of stainless steels dramatically. In this work, the corrosion behavior and mechanism of 2205 duplex stainless steel under alternating dry/wet marine environment are investigated before and after hydrogen charging using electrochemical testing, component characterization and morphological observation. The results show that the open circuit potential, film resistance and breakdown potential all reduce and the passive current density increases after 12 h hydrogen charging. The ratio of Fe³⁺ and O²⁻/OH⁻ decreases. The components of the hydrogen charged passive film alter and the performance deteriorates. The number of surface pits increases after 12 h hydrogen charging and additional 10 d alternating dry/wet corrosion. The pitting potential drops much lower. Consequently, The synergistic effect of high concentrations of Cl⁻ in the thin liquid film and hydrogen accelerates the destruction of the passive film, further reducing the corrosion resistance of stainless steels.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

Hydrogen absorption by metallic thin films detected by optical transmittance measurements

The hydrogen absorption by bilayers of Pd/Nb and Pd/Ti, grown on glass substrates, was studied by measuring changes in optical transmittance and reflectance in the visible range (wavelengths between 400 nm and 1000 nm) of the films at hydrogen pressures between 3.99 × 10² and 4.65 × 10⁴ Pa. The electrical resistance of the films was also measured during absorption to correlate with the optical data. All the films were grown by a controlled sputtering technique in high vacuum. Pd films ranging in thickness between 4 nm and 45 nm were also characterized when the films were exposed to a hydrogen pressure. The resistance and transmittance of all the Pd samples increased with the uptake of hydrogen until saturation occurred. For Pd/Ti bilayers, fast uptake of hydrogen was deduced from a transmittance increase, indicating hydrogen absorption in the Ti layer. In the case of the Pd/Nb bilayer, a decrease in transmittance was observed, indicating that hydrogen was not absorbed in the Nb layer. The transmittance decrease could be explained by a reduction of Nb native oxide by the hydrogen at the surface.     

Hydrogen embrittlement of super duplex stainless steel in acid solution

Super duplex stainless steel (SDSS) is a good choice of material when resistance to harsh environments is needed. Despite the material’s excellent corrosion resistance and high strength, a number of in-service failures have been recorded. The root cause of these failures was environmentally induced cracking initiated at manufacturing and in-service metallurgical defects. In this study the hydrogen embrittlement of pre-strained super duplex stainless steel specimens was investigated after 48 h cathodic charging in 0.1 M H₂SO₄. The metallurgical changes that resulted from four levels of cold work (4, 8, 12, and 16% plastic strain) were considered and their effect on the embrittlement of the SDSS alloy was investigated. After hydrogen charging, the specimens were pulled immediately to failure and the mechanical properties evaluated. The obtaining fracture morphology was investigated using low and high magnification microscopy. Experimental results indicated that charging the super duplex stainless steel alloy with hydrogen caused varying degrees of embrittlement depending on cold work level. Increasing cold work resulted in a reduction of the elongation to failure. Microscopic investigation confirmed the significant effect of cold work on the hydrogen embrittlement susceptibility of the super duplex stainless steel alloy investigated.     

Observing hydrogen in steel using cryogenic atom probe tomography: A simplified approach

This work demonstrates a new method to enable cryogenic atom probe tomography (cryo-APT) for the investigation of hydrogen in a high-strength steel, specifically to detect hydrogen localised to V–Mo–Nb carbides finely dispersed in the matrix. Prior cryogenic experiments required highly customised atom probe instrumentation to enable samples to be kept at cryogenic temperatures throughout the vacuum transfer process. Here we use an alternative approach without modification of the atom probe instrument itself, whilst still achieving hydrogen mapping. Additionally, we use this method to investigate the roles of solvent and solutes within the charging electrolyte, and we demonstrate that deuterated solute is not required when using heavy water as solvent, expanding the range of electrolytes that can be utilised in APT hydrogen charging experiments. This work reduces the experimental requirements for cryo-APT and makes the technique accessible to all APT equipped laboratories.     

Highly hydrogen permeable Nb–Ti–Co hypereutectic alloys containing much primary bcc-(Nb,Ti) phase

In order to develop alloys combing high hydrogen permeability with large resistance to the hydrogen embrittlement, microstructures and hydrogen permeability (Φ) have been investigated for the as-cast alloys on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase in the Nb–Ti–Co system. The alloys on the above-mentioned line consist of the TiCo compound and the (Nb, Ti) solid solution. The value of Φ increases with increasing temperature and volume fraction of the primary (Nb, Ti) phase. The most Nb-rich Nb₆₀Ti₂₁Co₁₉ alloy shows the highest Φ value of 3.99 × 10⁻⁸ (mol H₂m⁻¹ s⁻¹ Pa^−0.5) at 673 K, which is 2.6 times higher than that of pure Pd. The present work demonstrates that highly hydrogen permeable alloys are obtainable in the Nb rich Nb–Ti–Co ones on the straight line connecting the eutectic {TiCo + (Nb, Ti)} phase and the Nb-rich primary (Nb, Ti) phase.     

Hydrogen embrittlement behavior in interstitial Mn–N austenitic stainless steel

The susceptibility to hydrogen embrittlement behavior was investigated in an interstitial Mn–N austenitic steel HR183 and stainless steel 316L. Hydrogen was introduced by cathodic hydrogen charging at 363 K. HR183 has stronger austenite stability than 316L despite its lower nickel content, the addition of manganese and nitrogen inhibited martensitic transformation during the slow strain rate tensile deformation. Due to the diffusion of hydrogen being delayed by the interstitial solution of nitrogen atoms and the uniform dislocation slips, hydrogen permeates more slowly in HR183 than 316L, contributing to an 84.79 μm thinner brittle fracture layer in HR183 steel. Hydrogen charging caused elongation losses in both 316L and HR183 steels associated with the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanism. However, the hydrogen embrittlement susceptibility of HR183 is 3.4 times lower than that of 316L according to the difference in elongation loss between the two steel after hydrogen charging. Deformation twins trapped a lot amount of hydrogen leading to brittle intergranular fracture in 316L. The multiple directions of slip in HR183 steel suppressed the strain localization inside grains and delayed the adverse effects conducted by HELP and HEDE mechanism, eventually inhibiting server hydrogen embrittlement in the HR183 steel. This study is assisting in the development of low-cost stainless steel with excellent hydrogen embrittlement resistance that can be used in harsh hydrogen-containing environments.     

Suppression of hydrogen absorption into 304L austenitic stainless steel by surface low temperature gas carburizing treatment

Effect of low temperature gas carburizing (LTGC) on hydrogen absorption and hydrogen embrittlement (HE) susceptibility of 304L metastable austenitic stainless steel was investigated. The LTGC treatment imparted carburized layer on the steel surface with supersaturated solute carbon atoms (namely expanded austenite or S-phase) and more than 1 GPa surface compressive stress. Carburized layer thickness, carbon concentration level, residual compressive stress and hardness increased but hydrogen absorption decreased with increasing LTGC treatment time. Carburized surface layers had much higher austenite stability. The HE susceptibility of carburized steel was reduced due to the reduction of hydrogen absorption and the increment of austenite stability. The specimens whose residual compressive stresses were eliminated by tensile plastic straining also exhibited low hydrogen absorption during hydrogen charging, indicating that, besides the residual compressive stress, the supersaturated solute carbon atoms also have the ability to reduce hydrogen absorption. In addition, the results indicate that the supersaturated solute carbon atoms in the LTGC case can suppress hydrogen solubility without affecting diffusivity.     

Effects of hydrogen-charging on the susceptibility of X100 pipeline steel to hydrogen-induced cracking

In this work, the hydrogen-induced cracking (HIC) behavior of X100 pipeline steel was investigated by a combination of tensile test, electrochemical hydrogen permeation measurement and surface characterization techniques. The effect of inclusions in the steel on the crack initiation was analyzed. Results demonstrated that the amount of hydrogen-charging into the X100 steel specimen increases with the charging time and charging current density. Hydrogen-charging will enhance the susceptibility of the steel to HIC. The cracks initiate primarily at inclusions, such as aluminum oxides, titanium oxides and ferric carbides, in the steel. The diffusivity of hydrogen at room temperature in X100 steel is determined to be 1.04 × 10⁻⁸ cm²/s.     

Oxidation behavior and electrical conductivity of MAXs phase (Ti,Nb)3SiC2 as a novel intermediate-temperature solid oxide fuel cell interconnect material in anode environment

(Ti,Nb)₃SiC₂ possesses high oxidation resistance and electrical conductivity in cathode side, endowing it potential application as intermediate-temperature solid oxide fuel cell (IT-SOFC) interconnects. However, the performances of (Ti,Nb)₃SiC₂ in anode side must be well understood before the application comes true. In this paper, the oxidation resistance and electrical conductivity of (Ti,Nb)₃SiC₂ in simulated anode reducing atmosphere are systematically investigated. The oxidation kinetics follows parabolic law with a k_P value of 7.57 × 10^?14 g² cm^?4 s^?1. The formed single oxide layer is composed of uniformly distributed Nb-doped rutile TiO₂ and amorphous SiO₂, without carbon deposition. High partial pressure of H₂ and CO in simulated anode reducing atmosphere inhibit the oxidation by H₂O and CO₂. Nb doping with strengthen the Ti–O bond can also slow down the oxidation rate. ASR of (Ti,Nb)₃SiC₂ after 605 h cyclic oxidation is 1.6 and 3.7 mΩ cm² at 800 °C and 500 °C, respectively. The low ASR comes from the induced extra electrons by Nb doping and the dissolution of H₂O and H₂ in oxide, and the integrated TiO₂ conductive network in the scale. (Ti,Nb)₃SiC₂ exhibits superior performances in simulated anode reducing atmosphere, making it an promising candidate for SOFC interconnect.     

Effect of welding process parameters on embrittlement of Grade P92 steel using Granjon implant testing of welded joints

Precipitation of Cr-rich carbides, diffusible hydrogen content and heterogeneous microstructure formation across the weldments makes heat-affected zone (HAZ) susceptible to intergranular cracking and makes weldability of creep strength enhanced ferritic (CSEF) Grade P92 steel a critical issue. In the present research work, the Granjon implant test and mercury method (for diffusible hydrogen measurement) have been performed on Grade P92 steel welded specimens to study the effect of welding parameters on diffusible hydrogen levels and their subsequent effect on hydrogen-assisted cracking (HAC). The weld metal was deposited by a shielded metal arc welding process on Grade P92 steel samples using P92 matching filler. The three different welding conditions are used to measure the diffusible hydrogen level in the deposited metal. Granjon implant test was performed to evaluate HAZ HAC susceptibility with similar welding conditions which were used in the mercury method. Lower critical stress (LCS) was also evaluated using the Granjon implant test. The higher susceptibility of CSEF Grade P92 steel welded plate towards HAZ HAC was noticed in case of lower heat input or higher diffusible hydrogen content. However, by considering LCS, fracture mode and diffusible hydrogen content, the weld deposited using the highest heat input (condition III) offers great resistance to HAZ HAC.     

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.     

Electrochemical study on the effect of hydrogen on the passive film of selective laser melted 316L stainless steel in a proton exchange membrane water electrolyzer environment

The effect of hydrogen on the passivation behavior and electrochemical characteristics of selective laser melted (SLMed) 316L stainless steel in a simulated anode environment for a proton exchange membrane water electrolyzer (PEMWE) was studied. The results indicate that hydrogen charged into the sample increased the ratio of superficial Fe²⁺/Fe³⁺ and OH⁻/O²⁻, increased the concentration of point defects, reduced the film thickness, and weakened its protective effect. The film near 0.6 V_SCE showed n-type semiconductor behavior. Hydrogen charging resulted in a higher defect density and thinner space charge layer in the film, which promoted the invasion of aggressive ions.