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Low alloy steels combine relatively low cost with exceptional mechanical properties, making them commonplace in oil and gas equipment. However, their strength and hardness are restricted for sour environments to prevent different forms of hydrogen embrittlement. Materials used in sour services are regulated by the ISO 15156-2 standard, which imposes a maximum hardness of 250 HV (22 HRC) and allows up to 1.0 wt% Ni additions due to hydrogen embrittlement concerns. Low alloy steels that exceed the ISO 15156-2 limit have to be qualified for service, lowering their commercial appeal. As a result, high-performing, usually high-nickel, low alloy steels used successfully in other industries are rarely considered for sour service. In this work, the hydrogen stress cracking resistance of the high-nickel (3.41 wt%), quenched and tempered, nuclear-grade ASTM A508 Gr.4N low alloy steel was investigated using slow strain rate testing as a function of applied cathodic potential. Results showed that the yield strength and ultimate tensile strength were unaffected by hydrogen, even at a high negative potential of −2.00 VAg/AgCl. Hydrogen embrittlement effects were observed once the material started necking, manifested by a loss in ductility with increasing applied cathodic potentials. Indeed, A508 Gr.4N was less affected by hydrogen at high cathodic potentials than a low-strength (yield strength = 340 MPa) ferritic-pearlitic low alloy steel of similar nickel content. Additionally, hydrogen diffusivity was measured using the hydrogen permeation test. The calculated hydrogen diffusion coefficient of the ASTM A508 Gr.4N was two orders of magnitude smaller when compared to that of ferritic-pearlitic steels. Hydrogen embrittlement and diffusion results were linked to the microstructure features. The microstructure consisted of a bainitic/martensitic matrix with the presence of Cr23C6 carbides as well as Mo- and V-rich precipitates, which might have played a role in retarding hydrogen diffusion, kept responsible for the improved HE resistance.

Hypereutectoid martensitic steels possess excellent hardness levels which make them attractive materials for specific industrial applications. However, they can contain cracks and/or retained austenite after quenching which show a particular interaction with hydrogen (H). Hence, this work evaluates the interaction between H and a martensitic Fe-1.1C alloy by combining a wide variety of (H) characterization techniques with a systematic approach for specially designed H charged and heat treated samples. A detailed analysis of the microstructure for every condition serves as the basis for the interpretation. The results show that the presence of H leads to additional cracking and branching or growth of pre-existing quench cracks. Moreover, it is shown that when the temperature exceeds the retained austenite decomposition temperature while the austenitic grains contain H, additional cracking occurs which increases the amount of reversible H trapping sites thus raising the HE susceptibility.

This work evaluates the effect of hydrogen on the mechanical integrity of a weld in a martensitic base metal. Different regions in the heat-affected zone (HAZ) are reproduced to investigate the int...

Thermal desorption spectroscopy (TDS) was performed on ultra-low carbon (ULC) steel with various degrees of hydrogen-induced damage and deformation-induced defects. First, the extent to which hydro...

This work is part of a project that aims to develop a micromechanics based damage law taking into account hydrogen assisted degradation. A 'vintage' API 5L X56N and a 'modern' API 5L X70M pipeline steel have been selected for this purpose. The paper focuses on an experimental calibration of ductile damage properties of the well known complete Gurson model for the two steels in absence of hydrogen. A basic microstructural characterization is provided, showing a banded ferrite-pearlite microstructure for both steels. Charpy impact tests showed splits at the fracture surface for the X70 steel. Double-notched round bar tensile tests are performed, aiming to provide the appropriate input for damage model calibration. The double-notched nature of the specimens allows to examine the material state at maximum load in the unfailed notch, and the final material state in the failed notch. Different notch radii are used, capturing a broad range of positive stress triaxialities. The notches are optically monitored for transverse necking in two perpendicular directions (transverse to rolling and through thickness) to reveal any anisotropy in plastic deformation and/or damage. It is explained how the occurrence of splits at the segregation zone, and anisotropy complicate the calibration procedure. Calibration is done for each steel and acceptable results are obtained. However, the occurrence of splits did not allow to evaluate the damage model for the highest levels of tested stress triaxiality.

The influence of the austenite (gamma) phase fraction on the hydrogen embrittlement of duplex stainless steel is investigated. Heat treatments are performed to create two duplex stainless steel specimens, containing 50% and 44% of austenite, respectively. Mechanical testing with and without hydrogen charging reveals that significant embrittlement occurs regardless of the austenite fraction. A higher austenite fraction results in a reduced ductility loss under the presence of hydrogen. Samples with a higher ferrite fraction are embrittled more due to their higher hydrogen diffusivity. In-situ tensile tests, interrupted at the ultimate tensile strength, show hydrogen-assisted cracks on the specimen surface both in austenite and ferrite and across the alpha/gamma interface.

This work evaluates the impact of different organic acids on the corrosion sensitivity and stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. For all organic acids, potentiodynamic measurements shows linear relationships between corrosion rate and hydrogen proton concentration between pH 2.4 and 3.9. For solutions with the same pH, i.e., similar conductivity, the corrosion rate differs depending on the type of organic acid. The anodic dissolution in formic acid is the highest, followed by acetic, propanoic and nonanoic acid. The acid dissociation reaction is identified as the rate determining step in the corrosion process. Nonanoic acid, alternatively, clearly acts as a corrosion inhibitor. In situ four-point constant-extension tests in formic acid, acetic acid and nonanoic acid, at a pH value of 3.4 were performed to evaluate their impact on the SSC sensitivity. The general degradation followed the trend of the corrosion rate, although the synergetic effect of corrosion and stress resulted in a higher degradation depth. Though nonanoic acid induced little visible corrosion, still stress-corrosion cracks were still detected. It was shown that solutions of different organic acids with the same pH do not have the same influence on stress-induced degradation.

This study evaluates the electrochemical hydrogen charging behavior and interaction between hydrogen and the microstructure of a duplex stainless steel. A saturation level of approximately 650 wppm is reached after 10 d of charging. The data are compared with a model resulting in a diffusion coefficient of 2.1 x 10(-14) m(2)/s. A two-step increase of the concentration is observed and ascribed to saturation of ferrite followed by charging of austenite grains. Microstructural changes are observed during charging, i.e., formation and interaction of dislocations, as a result of the high residual stresses inherent to the production process of duplex stainless steels.

The present study evaluates the hydrogen induced damage by in-situ hydrogen plasma charging in dual phase (DP) steel. Cold deformation of 15% is applied on the material to change microstructural defects, such as dislocation density. The susceptibility to hydrogen embrittlement is hence evaluated for two material conditions, i.e. DP 0% and DP 15%. Small scale tensile tests are done inside an ESEM chamber for which in-situ hydrogen plasma charging is compared with electrochemical hydrogen charging while uncharged samples serve as a reference. Generally, the hydrogen effect on the ductility and stress level is increased when deformation is applied, due to the hydrogen trapping ability of the deformation induced defects, as confirmed by thermal desorption spectroscopy. Complementary in-situ electrochemical nanoindentation tests verify the more pronounced hardness increase due to hydrogen when cold deformation is applied. A slightly increased ductility loss is observed when the samples are charged electrochemically, although similar tendencies are found for both hydrogen charging procedures. These observations are confirmed by the fractographic analysis, where the detrimental role of MnS inclusions in the segregation line on hydrogen induced cracking is demonstrated as well.

Notched TRIP tensile samples were strained until fracture and until the tensile strength under variable hydrogen charging conditions in order to assess the influence of hydrogen present in the samples (internal hydrogen) vs. hydrogen charged during tensile testing (environmental hydrogen) on the hydrogen assisted cracking behavior. Three types of charging conditions were applied: 1) pre-charging with hydrogen till saturation followed by tensile testing with in-situ electrochemical hydrogen charging during the test, 2) pre-charging with hydrogen till saturation and tensile testing without additional hydrogen charging during the test, and 3) tensile testing with in-situ electrochemical hydrogen charging, but without pre-charging. Environmental hydrogen was found to embrittle the material more than internal hydrogen, even though less hydrogen was present in the material. Fracture surface and secondary crack analyses indicated the importance of continuous hydrogen supply to critical zones during loading and the importance of the hydrogen/dislocation interaction in the failure process.

Hydrogen (H) induced mechanical degradation is studied in DP steel by performing tensile tests under static and dynamic conditions. Tensile specimens were electrochemically H charged and tensile tests were done ex-situ after charging. Different H contents were charged into the samples by modifying the current density. The strain rate is increased from static (1.67*10−2 and 1.67 s−1) to dynamic (450 and 900 s−1) conditions to verify the effect of H diffusivity during the tensile tests on the hydrogen embrittlement (HE) susceptibility. Therefore, a reproducible methodology was established by using a standardized tensile machine for static testing and split Hopkinson bar experiments for dynamic conditions. The HE degree increased with current density due to higher amount of H, as confirmed by melt extraction. The HE% also increased with slower strain rates since H was able to diffuse to a crack tip, hence accelerating failure. Even at the highest strain rate (900 s−1), the material lost about 10% of its ductility due to H present in the sample and not because of H diffusion during the test. This was concluded since H induced brittle failure initiated at the edges of the samples at slow strain rates. Though at a strain rate of 1.67 s−1, fracture initiated in a ductile way from the center similarly as for tests performed without charging. Fractographic visualization of the fracture surfaces revealed an embrittled central line when charged with H, which evolved into a major crack. MnS inclusions were found in this central line accounting for the H induced crack initiation.

Understanding the interaction of hydrogen with a steel microstructure is key toward further material development and a potential future hydrogen based economy. A reliable determination of the hydrogen diffusivity and trapping are crucial hereto. Electrochemical permeation is mainly used for the former, while thermal desorption spectroscopy (TDS) is opted for the latter. Combination of both, together with a detailed microstructural evaluation, provides fundamental insights for the engineering of suitable hydrogen traps and diffusion barriers. Finally, in-situ mechanical testing enables to evaluate the hydrogen embrittlement susceptibility and identifies the active deformation mechanism. This chapter summarizes some recent experimental advances and developments of our research group. At first, focus lies on the electrochemical permeation technique, which has been expanded to apply a constant load to the materials to evaluate the influence of stresses on hydrogen diffusivity during in-situ hydrogen permeation. Elastic load on dual-phase steel increases hydrogen diffusion due to crystal lattice expansion, while plastic deformation decreases diffusion due to the formation of lattice discontinuities, e.g., dislocations. Next, the ability to assess hydrogen trapping in austenite-containing materials is critically assessed for a duplex stainless steel, revealing that the TDS data are dominated by hydrogen diffusion in these low diffusivity materials. In-situ interrupted tensile testing on these materials is further complemented with scanning electron microscopy—electron backscatter diffraction analysis. \(\varepsilon \)- and \(\alpha \)’-martensite are found in austenite of the hydrogen-charged tensile specimens, while these martensitic transformation do not show in the uncharged samples. This is, among others, explained by a reduction in stacking fault energy due to the presence of hydrogen. Hydrogen-assisted cracks also initiate in these materials, mainly in the austenite phase. Finally, the hydrogen sensitivity is evaluated for high strength-low ductility Fe–C steels. Due to their brittle nature, a novel in-situ three-point bending setup is developed to evaluate their sensitivity to hydrogen. Hydrogen causes a transition from a microvoid (Fe–0.2C), intergranular (Fe–1.1C), or mixed (Fe–0.4C) fracture surface (air-tested samples), to a hydrogen-induced cleavage fracture appearance. This is accredited by the Hydrogen Enhanced Plasticity Mediated Decohesion mechanism, proposing that hydrogen is preferentially trapped at packet or block boundaries in high carbon steels, while lath martensitic boundaries play a minor role in the crack development.

The hydrogen induced damage of generic Fe-C-Ti and Fe-C-V ferritic alloys was investigated to assess the influence of precipitates on the hydrogen sensitivity of a material. The precipitates, formed during heat treatment, were evaluated by scanning transmission electron microscopy (STEM). The hydrogen/material interaction was evaluated by: 1) melt and hot extraction to determine the total and diffusible hydrogen content, respectively, 2) permeation experiments to calculate the diffusion coefficient, 3) thermal desorption spectroscopy to determine the hydrogen trapping characteristics of the materials. Furthermore, two different types of hydrogen induced damage were evaluated, i.e. hydrogen assisted cracking and blistering, resulting from electrochemical hydrogen charging with and without the application of an external load, respectively. Evaluation of the hydrogen induced damage and the role of the precipitates was performed by combining optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). An important though divertive role of diffusible hydrogen is observed in both damage mechanisms for the investigated microstructures. On the one hand, a large amount of diffusible hydrogen compared to strongly trapped hydrogen promotes hydrogen assisted cracking of materials, while on the other hand, the blistering phenomenon is delayed under such conditions.

A new thermal desorption spectroscopy (TDS) apparatus was used to identify the hydrogen trapping peaks, and to measure the hydrogen concentrations in 3.5NiCrMoV steel after hydrogen charging electrochemically, and in gaseous hydrogen. The hydrogen concentration increased with (i) increasingly negative charging potential and (ii) increasing hydrogen gas pressure. The equivalent hydrogen fugacity versus charging overpotential was derived. There was a difference in the diffusible hydrogen traps activated during electrochemical and gas phase charging, attributed to the difference in the hydrogen fugacity. A two-site model for Sieverts' Law explained the positive Y-intercept, as the density of already filled hydrogen traps.

The hydrogen trapping efficiency of different types of W based precipitates is considered in three generic Fe C W alloys with increasing carbon content and a stoichiometric amount of W. A martensitic microstructure is prepared and two conditions are compared; i.e. an as-quenched (as-Q) state and a quenched and tempered (Q&T) state in which W based carbides are formed. The tempering time and temperature are modified to change the carbide characteristics. Hence, the hydrogen trapping characteristics are evaluated for the different carbides. Melt extraction is done to determine the hydrogen content, whereas thermal desorption spectroscopy (TDS) is performed to verify whether the tempered induced carbides are able to trap hydrogen efficiently. The trapping capacity is found not only to be size dependent, but also reliant on the morphology and type of the carbides, as investigated by transmission electron microscopy. TDS revealed that the tempered induced W2C in the Q&T at 600 °C for 1 h condition do not trap hydrogen due to their rather large size (>20 nm) and hence incoherent nature. Moreover, the hydrogen content is decreased compared to the as-Q condition due to the reduced dislocation density. The tempering time is reduced to 10 min to evaluate the size effect on the carbide trapping ability. As such, smaller W2C (

The present work considers the effect of hydrogen on the mechanical properties of generic Fe-C-W alloys. Thermal treatment aimed at a martensitic microstructure and two conditions are compared; i.e. an as-quenched state and a quenched and tempered state with lower dislocation density and with W based carbides formed during tempering. The hydrogen induced mechanical degradation is evaluated by in-situ tensile tests, while thermal desorption spectroscopy, hot/melt extraction and permeation experiments are performed to understand the observed hydrogen embrittlement degree. The hydrogen induced ductility loss increases with increasing carbon content due to the higher amount of hydrogen trapped by the denser martensitic lath boundaries. Furthermore, the quenched and tempered condition shows a lower susceptibility to hydrogen. This is correlated to the reduced hydrogen content when tempered due to a decreased dislocation density and the fact that the tempered induced W2C particles did not trap hydrogen. Moreover, corresponding observations to this interpretation are perceived by permeation experiments since, on the one hand, the diffusion coefficient decreases with carbon content due to the increasing trapping ability of alloy A ➔ B ➔ C, and, on the other hand, the diffusivity increases when tempering is applied, which is also linked to the decrease in dislocation density and the disability of the carbides to hinder hydrogen diffusion by efficient trapping.

The hydrogen trapping ability of 15 Fe-C-X alloys is compared in this work. Five types of carbides, i.e. Ti, Cr, Mo, W and V based carbides, and their effect on the hydrogen embrittlement susceptibility is considered while three carbon contents are prepared for each carbide former. Two conditions are compared for each alloy to evaluate the hydrogen/material interaction: an as quenched and quenched and tempered condition in which carbides are introduced. Next to the material characterization, also the interaction of hydrogen with the materials is completely elaborated. At first, in-situ tensile tests are done to determine the hydrogen induced ductility loss. To interpret the obtained degrees of hydrogen embrittlement, hot/melt extraction is done to determine the hydrogen content, whereas thermal desorption spectroscopy is performed to assess the hydrogen trapping capacity of the tempered induced precipitates and the different other potentially hydrogen trapping microstructural features. These measurements are done after hydrogen pre-charging till saturation. The tempered induced TiC and V4C3 are capable of trapping a significant amount of hydrogen, while the Mo2C and Cr23C6 particles only trap a limited amount of hydrogen. The W2C precipitates, however, are not able to trap hydrogen. The size and coherency of the carbides are considered to be the main factor determining their trapping ability. The degree of hydrogen embrittlement is correlated with the hydrogen present in the alloys. Three amounts of hydrogen were determined by the strength by which they were trapped by combining the different hydrogen characterization techniques, i.e. total, diffusible and mobile hydrogen. It was confirmed that hydrogen trapped by dislocations plays a determinant role. This further confirms the importance of an enhanced dislocation mobility in the presence of hydrogen, as described in the HELP mechanism.

Hydrogen characterization techniques like melt extraction and thermal desorption spectroscopy (TDS) are useful tools in order to evaluate and understand the interaction between hydrogen and metals. These two techniques are used here on cold deformed ultra-low carbon (ULC) steel with and without hydrogen induced damage. The material is charged electrochemically in order to induce varying amounts of hydrogen and variable degrees of hydrogen induced damage. The aim of this work is to evaluate to which extent the hydrogen induced damage would manifest itself in melt extraction and TDS measurements.

Hydrogen (H) presence in metals is detrimental as unpredictable failure might occur. Recent developments in material’s design indicated that microstructural features such as precipitates play an essential role in potentially increasing the resistance against H induced failure. This work evaluates the H trapping characteristics for TiC and V4C3 by thermal desorption spectroscopy and permeation experiments. Two microstructural conditions are compared: as quenched vs. quenched and tempered, in which the carbides are introduced. The tempered induced precipitates are able to deeply trap a significant amount of H, which decreases the H diffusivity in the materials and removes some of the detrimental H from the microstructure. For microstructural design purposes, it is important to know the position of H. Here, H is demonstrated to be trapped at the carbide/matrix interface by modifying the tempering treatment.

Using a light optical microscope (LOM), microstructural analysis is carried out on plain-carbon, DP600, and As-Quenched (As-Q) martensitic steels to identify the different phases interacting with hydrogen. Our recently developed electrochemical procedure, based on cyclic voltammetry (CV) and potentiostatic discharging method, is applied on these steel alloys having different phases to monitor H-uptake in the steels with respect to their microstructural features. The electrochemical method is capable of measuring diffusible H-concentration (including mobile hydrogen) for the steel alloys under H-charging condition, where hydrogen embrittlement phenomena can occur within in-service environments. The best practice in this procedure is to perform electrochemical H-measurements immediately after H-charging without interruption between steps to avoid spontaneous H-loss. Various charging times are investigated to estimate the time to near H-saturation for each steel alloy. To gain additional insights in our H-related findings, hot extraction measurements are performed to measure the diffusible H-concentration in the steels. A clear correlation between the results of hot extraction and electrochemical discharging methods is confirmed by a mathematical model, based on Fick’s Law, predicting diffusible H-loss due to the time lag. Thus, under the used charging conditions, As-Q martensitic steel has been found to contain the lowest amount of diffusible hydrogen, with its near H-saturation reached after 4 hours of H-charging. DP600 is H-saturated after one hour of charging, while near H-saturation for plain-carbon steel is attained after 30 minutes. The fraction of mobile-H in plain-carbon steel is relatively higher than in DP600 steel.

In-situ tensile testing of electrochemically hydrogen charged 304L stainless steel at different crosshead displacement rates results in a largely different elongation at fracture compared to the corresponding test in air. At slow engineering strain rates (below 1E-2 s−1), large ductility losses are observed and the alloy suffers from clear hydrogen embrittlement (HE). The HE increases with decreasing strain rate due to the increased time that is given for hydrogen to diffuse and accumulate. However, at higher engineering strain rates (above 1E-2 s−1), the ductility increases with hydrogen charging. Due to intense martensitic transformations triggered by a combined temperature and hydrogen effect, the strain hardening of the alloy improves and necking is postponed. The temperature effect is restricted by reference testing in solution showing that HE still prevails. The enhanced martensitic transformations with hydrogen open opportunities for the creation of hydrogen resistant materials where the balance between HE and enhanced martensitic transformations can be optimized for the required application. Furthermore, tensile pre-straining results in an increased HE susceptibility due to the presence of stress concentrations and α’-martensite.

Duplex stainless steels (DSS) have a two-phase microstructure of ferrite and austenite which results in a high strength combined with high ductility and good corrosion resistance. However, when DSS are heated to an inappropriate temperature range, e.g., during welding, the brittle sigma phase forms which deteriorates the mechanical properties. In the present work, a heat treatment is performed to intentionally create this deleterious phase. Hydrogen is introduced in this alloy to investigate the combined effect of embrittling phases (sigma phase) and hydrogen. Melt extraction analysis is performed to quantify the hydrogen uptake capacity in the steel. In-situ mechanical tests are used to assess the hydrogen embrittlement susceptibility. The uncharged DSS shows a low ductility and almost no hydrogen embrittlement is observed via an in-situ tensile test set-up due to its intrinsic brittle nature under tensile mode. Complementary in-situ bending tests which are more suitable for an intrinsically brittle material are done to further evaluate the role of hydrogen on the mechanical integrity. Hydrogen charging does indeed result in additional embrittlement in the in-situ bending set-up. The reason is thought to be the faster initiation, interconnection and propagation of cracks in the presence of hydrogen, as indicated by microstructural characterization.

The present work evaluates hydrogen induced cracking in a TRIP (transformation induced plasticity) assisted steel and pure iron. The goal of this work is to understand the effect of the macroscopic stress distribution in the material on the hydrogen induced cracking phenomenon. Additionally, the effect of a complex multiphase microstructure on the characteristics of hydrogen induced cracking was investigated by comparing results for TRIP-assisted steel and pure iron as reference material. Tensile tests on notched and unnotched samples combined with in-situ electrochemical hydrogen charging were conducted. Tests were performed until the tensile strength was reached and until fracture. The resulting hydrogen induced cracks were studied by optical microscopy and scanning electron microscopy (SEM). Hydrogen induced cracks showed a typical S-shape and crack propagation was mainly transgranular, independently of the presence of a notch or the material's microstructure. This was also the case for the V-shaped secondary crack network and resulting stepped crack morphology characteristic for hydrogen induced damage. These observations indicate that the stress state surrounding the crack tip has a very large impact on the hydrogen induced cracking characteristics. The use of a notch or the presence of a different microstructure did not influence the overall hydrogen induced cracking features, but did change the kinetics of the hydrogen induced cracking process.

The hydrogen trapping of TiC and its effect on hydrogen embrittlement is investigated for Fe-C-Ti alloys with variable carbon content. Two conditions are compared; an as-quenched and quenched and tempered state in which carbides are generated. The hydrogen impact is demonstrated by tensile tests on unstressed samples without and after in-situ hydrogen charging. Thermal desorption spectroscopy and hot/melt extraction are performed to understand the carbide trapping ability and interpret the observed tendencies. The results indicate a correlation between hydrogen trapped at dislocations and the embrittlement degree. Slightly modified thermal treatments allowed confirming the beneficial effect of TiC addition.

Duplex stainless steels (DSS) possess good mechanical properties of ferrite with the corrosion resistance of austenite. They are heavily employed in the petrochemical, paper and nuclear industries. However, they are susceptible to hydrogen embrittlement (H2). Samples of DSS 2205 type were received in the condition of homogenization annealing (1100C for 300 s and cooling in water). They were cold rolled and annealing at 1100C for 2 h and 850C for 24 h. They were then loaded with H2. Melt extraction analyses were applied to quantify the hydrogen in the steel. In situ tensile tests with simultaneous H2 loading were used to evaluate the embrittlement caused by this element. Completing the H2 analysis, thermal desorption spectra were constructed. The microstructure was characterized by scanning electron microscopy, back scatter electron diffraction (EBSD), X-ray diffraction and transmission electron microscopy. In the aged condition the sigma, chi and carbide phases were identified. The DSS presented a considerable reduction of ductility, reaching 5% of elongation already in the aged state. The hydrogen charging has not altered this condition. Fusion Extraction revealed hydrogen content in the microstructure of up to 50 wppm for the annealed condition, while for the aged sample was 10 wppm. Despite the high volume fraction of intermetallic phases and loading with H2, the fracture surfaces were characterized as microvoids coalescence and intergranular.

Hydrogen embrittlement of steels is known to have considerable impact in many engineering sectors. To be able to mitigate the hydrogen embrittlement problem, a profound comprehension of the interaction of hydrogen with the steel microstructure is required. Especially the interaction of hydrogen with dislocations and vacancies is very relevant as these defects are known to play an important role in hydrogen embrittlement. At present, thermal desorption spectroscopy is mostly used to study hydrogen-defect interactions. However, information obtained solely by this technique is insufficient to obtain a full understanding of the interaction of hydrogen with these defects in the steel microstructure. Herein, the use of internal friction, as a complementary technique to thermal desorption spectroscopy, to reveal the interaction of hydrogen with dislocations and vacancies, is reviewed based on the present understanding in the literature. Furthermore, the opportunities to use internal friction to characterize the interaction between hydrogen and these defects and to give more insight into the hydrogen embrittlement mechanism are discussed. It is demonstrated that internal friction has not yet been used to its full potential for this purpose, although it entails the opportunity to develop fundamental insights into the hydrogen embrittlement phenomenon.

The present work evaluates the acidic stress-corrosion cracking (SCC) of NiCrMoV steam turbine steel. Between a pH of 4.2 and 7, iron oxidises to Fe2O3, whereas iron dissolution (Fe2+) and hydrogen proton (H+) reduction are favoured at lower pH. The corrosion rate increases with the H+ concentration via a linear relationship as well as by a temperature increase. During in-situ constant extension rate testing, both the ductility and strength losses, caused by anodic dissolution and hydrogen embrittlement, increase with a higher acetic acid concentration. Hydrogen embrittlement causes larger embrittled zones with higher acetic acid concentrations as well as stress-corrosion cracking.

The impact of V 4 C 3 precipitates on the sensitivity to hydrogen embrittlement is evaluated in this work. Two main conditions are compared with each other, i.e. an as-quenched and a quenched and tempered condition in which carbides have precipitated. Tensile tests are performed on both uncharged and hydrogen charged specimen and a correlation with the amount of charged hydrogen is made by hot/melt extraction. The tempered induced carbides trap a significant amount of hydrogen which explains why the degree of hydrogen embrittlement increases for the quenched and tempered condition. To verify at which trapping sites hydrogen is located, thermal desorption spectroscopy is performed. A considerable amount of hydrogen is trapped in a rather irreversibly way, which still has an effect on the degree of hydrogen embrittlement. Additionally, a slightly modified temper treatment is applied to vary the carbide size and hence evaluate the impact of the hydrogen induced ductility loss.

The hydrogen induced ductility loss of three FeCMo alloys is evaluated in this work and correlated with the hydrogen interaction of the Mo2C precipitates. Two thermal treatments are applied to compare two main conditions; i.e. an as-quenched and a quenched and tempered state in which Mo2C particles have precipitated. The susceptibility to hydrogen embrittlement is determined by performing tensile tests on both uncharged and hydrogen charged tensile specimen. Tempering appears to reduce the resistance against hydrogen embrittlement. Hot/melt extraction and thermal desorption spectroscopy are subsequently performed to determine the role of the Mo2C precipitates. Tempered induced Mo2C particles trap a considerable amount of hydrogen and a correlation between the hydrogen embrittlement susceptibility and the hydrogen mobility is confirmed. A slightly modified thermal treatment and charging procedure allows evaluating these interpretations. Moreover, the crucial role of hydrogen diffusion is tested by lowering the tensile test speed on hydrogen pre-charged and non-pre-charged specimens.

This study evaluates the hydrogen trapping ability of Cr 23 C 6 precipitates and hence its effect on the hydrogen induced mechanical degradation in laboratory cast Fe-C-Cr materials. Two main conditions are compared: an as-quenched and a quenched and tempered condition containing carbides. Three materials were cast with a variable carbon content to study the role of the carbides in alloys with variable strength level. Mechanical tests are performed on both in-situ hydrogen charged and uncharged tensile samples. The presence of carbides decreases the susceptibility to hydrogen embrittlement. Thermal desorption spectroscopy and hot/melt extraction are performed to determine which trapping sites are active. A clear correlation between the degree of hydrogen induced embrittlement and the hydrogen mobility is confirmed by determining the hydrogen diffusivity and linking it to the available trapping sites. A slightly modified temper treatment is applied to vary the carbide size and evaluate its effect on the hydrogen embrittlement resistance and the trapping ability of the present carbides.

The present work investigates the influence of hydrogen on the mechanical properties of generic Fe-C alloys by tensile tests on notched samples. Different microstructures, such as pearlite, bainite and martensite are generated in a 0.2%C Fe-C alloy by an appropriate heat treatment. “Pure” iron is used as a reference material and a variation in the carbon content up to 0.4% is established for the bainitic grade. The effect of hydrogen is demonstrated by mechanical tests on both in-situ hydrogen charged and uncharged specimens. At high cross-head deformation speed (5 mm/min), the results indicate a considerable, though variable hydrogen effect for different microstructures. The bainitic and martensitic materials both show ductility drops of about 20%, whereas the pearlitic and ferritic grades display a higher sensitivity to hydrogen embrittlement (HE) with a ductility loss of approximately 50%. In order to evaluate the role of the diffusible hydrogen, tensile tests are performed at a lower cross-head deformation speed (0.05 mm/min) as well. Next to the correlation between the amount of diffusible hydrogen and HE, the distance over which hydrogen can diffuse during a tensile test, determined by hydrogen diffusion coefficient, seems to play a crucial role as well.

The present work evaluates hydrogen induced cracking by performing an elaborate EBSD (Electron BackScatter Diffraction) study in a steel with transformation induced plasticity (TRIP-assisted steel). This type of steel exhibits a multiphase microstructure which undergoes a deformation induced phase transformation. Additionally, each microstructural constituent displays a different behavior in the presence of hydrogen. The aim of this study is to obtain a better understanding on the mechanisms governing hydrogen induced crack initiation and propagation in the hydrogen saturated multiphase structure. Tensile tests on notched samples combined with in-situ electrochemical hydrogen charging were conducted. The tests were interrupted at stresses just after reaching the tensile strength, i.e. before macroscopic failure of the material. This allowed to study hydrogen induced crack initiation and propagation by SEM (Scanning Electron Microscopy) and EBSD. A correlation was found between the presence of martensite, which is known to be very susceptible to hydrogen embrittlement, and the initiation of hydrogen induced cracks. Initiation seems to occur mostly by martensite decohesion. High strain regions surrounding the hydrogen induced crack tips indicate that further crack propagation may have occurred by the HELP (hydrogen-enhanced localized plasticity) mechanism. Small hydrogen induced cracks located nearby the notch are typically S-shaped and crack propagation was dominantly transgranularly. The second stage of crack propagation consists of stepwise cracking by coalescence of small hydrogen induced cracks.

The influence of hydrogen on the mechanical properties of generic lab-cast Fe-C bainitic alloys is studied by tensile tests on notched samples. The bainitic microstructure is induced in a 0.2% C and 0.4% C Fe-C alloy by an appropriate heat treatment. The hydrogen embrittlement susceptibility is evaluated by mechanical tests on both in situ hydrogen pre-charged and uncharged specimens. The observed ductility loss of the materials is correlated with the present amount of hydrogen and the hydrogen diffusion coefficient. In addition to the correlation between the amount of hydrogen and the hydrogen-induced ductility loss, the hydrogen diffusion during the tensile test, quantified by the hydrogen diffusion distance during the test, appears to be of major importance as well.

The present work investigates the phenomenon of hydrogen embrittlement in dual phase steel by evaluating tensile tests performed on both hydrogen saturated and non-saturated samples together with a thorough investigation of the role of hydrogen diffusion. First, a hot/melt extraction was done to study the evolution of the hydrogen level up to saturation as a function of the electrochemical charging time. The results were correlated with in-situ hydrogen charged tensile tests performed after different pre-charging times. An increasing ductility loss was observed with longer pre-charging times up to a maximal 50% ductility loss for hydrogen saturated samples. Furthermore, changing the cross-head deformation speed allowed evaluating the effect of hydrogen diffusion. It was clearly demonstrated that the hydrogen sensitivity increased when lowering the test speed, since hydrogen could diffuse to critical regions ahead of the crack tip. Lastly, different cross-head deformation speeds were also applied on non-pre-charged samples. Here, hydrogen charging just started at the same moment as the actual tensile test. Fractography allowed to correlate the features of the fracture surface with the hydrogen diffusion distance during the test.

The present work evaluates hydrogen trapping and embrittlement (HE) for different laboratory cast Fe–C–X alloys with Ti, Mo and Cr (=X). Tempering generated X-based precipitates. The materials were examined under two conditions, as quenched and quenched and tempered. The hydrogen trapping capacity of the precipitates and matrix was investigated by thermal desorption spectroscopy (TDS), while hot and melt extraction, performed after cathodic charging, allowed to determine the diffusible and total hydrogen content, respectively. In-situ hydrogen pre-charged tensile specimens were tested to evaluate the hydrogen induced ductility loss. The different carbides exhibited a variable effect on the hydrogen embrittlement. The Fe–C–Ti material embrittled the most and tempering even increased its HE-susceptibility, whereas the opposite was observed for the Fe–C–Cr grade. Finally, the HE-resistance was best for the Fe–C–Mo alloys. Correlation between the mechanical behavior and the TDS and hot/melt extraction results was made as well.

The present work evaluates hydrogen-assisted cracking in 2205 duplex stainless steel by performing electron backscatter diffraction (EBSD) on in-situ hydrogen charged tensile tested specimens. The hydrogen embrittlement sensitivity increases with the electrochemical hydrogen charging time. The hydrogen presence results in deformation-induced martensite formation in the austenite during in-situ tensile straining, which starts immediately after yielding. Hydrogen-assisted cracking starts at a later stage in the tensile test and the cracks mainly initiate in austenite. The deformation-induced martensite is not vulnerable for crack initiation, neither is its interface with the austenite matrix. The presence of hydrogen-assisted cracks depends on the strain level reached in the specimen and is therefore most pronounced in the necked region. Crack propagation occurs through austenite, ferrite and their interface and is mainly governed by strain fields. Finally, propagation also occurs by coalescence of several smaller cracks.

The hydrogen induced damage and crack initiation in transformation induced plasticity (TRIP) steel is considered in the present work by micro tensile testing and subsequent microstructural assessment. Hydrogen characteristics are studied for two material conditions for this purpose, i.e. TRIP 0% and TRIP 15%. To increase, on the one hand, microstructural defects such as dislocations, and on the other hand, to provoke the strain induced phase transformation of retained austenite to fresh martensite, cold deformation of 15% is applied (TRIP 15%) and compared to the as-received material (TRIP 0%). Both electrochemical charging and hydrogen plasma charging is done compared to uncharged tensile specimens, as reference. A hydrogen induced ductility loss is found when the samples are hydrogen charged electrochemically, whereas plasma charging remarkably induces a ductility increase. A comprehensive hydrogen assisted crack analysis by environmental scanning electron microscopy reveals that the uncharged and electrochemically charged specimens show crack initiation in the martensitic regions, whereas plasma charging results into crack initiation in the soft, crack arresting ferritic matrix. Furthermore, the hydrogen effect is for both charging methods more pronounced after cold deformation, which is correlated to hydrogen trapping ability of the deformation induced defects, as verified by thermal desorption spectroscopy.

This work evaluates the mechanical degradation of Dual Phase steel due to acidic stress-corrosion cracking (SCC). The SCC susceptibility is studied by in-situ constant extension rate testing, followed by post-mortem fractographic evaluation, while the corrosion rate is determined by electrochemical measurements. Elevating the temperature results in a decrease in both strength and ductility. Organic acids addition further leads to a significant ductility loss, whereas oxygen removal yields a ductility gain. The corrosion rate shows a linear relationship with the H+ concentration. Between a pH of 4.2–7, anodic dissolution occurs, whereas hydrogen absorption takes place at lower pH, causing hydrogen-assisted stress-corrosion cracks.

Hydrogen embrittlement of modern high strength steels consists of different interacting time-dependent mechanisms. One of these mechanisms is hydrogen diffusion and trapping to accumulate hydrogen in critical areas with high mechanical loads. Therefore, understanding hydrogen diffusion and trapping behavior of carbides containing high strength steels is an essential part to effectively increase the hydrogen resistance. For that purpose, a microstructural based model was developed and parametrized to Fe–C–V and Fe–C–Ti alloys. Generalized analytical equations were derived to describe the evolution of different kinds of trap densities with the measured carbide mean radius, annealing temperature or dislocation density. Finally, the models support the idea of hydrogen trapping at carbon vacancies and coherent interface positions. In future, these models are well suited for finite element process simulations of industrial components to predict the local solubility and chemical diffusion as demonstrated in the last section of this work.

This work evaluates the mechanical degradation of Fe-C-X (X = Cr/Mo/V) steels due to stress-corrosion cracking (SCC) in acidic aqueous environment. Tensile testing of as-quenched and quenched-and-tempered Fe-C-X steels in corrosive environment shows a reduced ductility and yield strength. Secondary stress-corrosion cracks and embrittled regions are detected by scanning electron microscopy. Anodic dissolution and hydrogen embrittlement mechanisms are elaborated to explain the mechanical degradation. A linear correlation between the amount of SCC embrittlement and measured corrosion potentials is obtained. Tempering Fe-C-Cr introduces Cr7C3 and increases the SCC resistance, whereas introduction of Mo2C and V4C3 realizes an increased reactivity and SCC susceptibility.

The hydrogen induced blistering and internal cracking behavior of a TRIP-assisted (Transformation Induced Plasticity) high strength steel is investigated. TRIP-assisted steels are multiphase steels with a microstructure consisting out of ferrite, bainite, retained austenite, and some martensite. Each microstructural constituent demonstrates a different behavior in the presence of hydrogen. Additionally, such steels exhibit an austenite to martensite transformation upon deformation, which influences the hydrogen induced crack formation of the material. In order to assess this influence and to better understand the interaction of such a multiphase material with hydrogen, the hydrogen induced cracking effect is evaluated for a TRIP-assisted steel in the undeformed and cold deformed, i.e. 10% by tensile deformation, condition. The austenite/martensite phase ratio differs for both conditions, allowing one to evaluate the impact of the TRIP effect on the hydrogen sensitivity of the material. The materials are cathodically charged with hydrogen at various charging conditions, i.e. charging time and current density, in order to assess their sensitivity to hydrogen induced cracking. Subsequently, crack initiation and propagation is investigated by microstructural analysis using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Hydrogen induced crack initiation occurs preferably at martensitic islands, while crack propagation takes place along segregation lines, which also consist of martensite networks. Increased deformation leads to less internal crack initiation, but to a more outspoken crack propagation. On the one hand, deformation leads to strain hardening of the material, which leads to an increase of the critical hydrogen pressure necessary for internal crack initiation. On the other hand, deformation of TRIP-assisted steel results in the formation of martensite and provides a more favorable crack path for continuous crack propagation. Moreover, the synergetic effect of hydrogen induced and assisted cracking mechanisms is illustrated.

This work considers the effect of a constant load on the hydrogen diffusion characteristics studied by electrochemical permeation experiments. Dual phase steel and Armco iron are used for this purpose. Different degrees of load, both in the elastic and plastic regime, are applied on the sample during the permeation experiment when hydrogen is migrating through the material. The resulting transients show that applied elastic tensile stresses increase the hydrogen diffusion coefficient of both materials. Due to the elastic stress, the crystallographic lattice expands which increases hydrogen diffusivity. From an applied constant load of 100% of the yield strength and higher, the diffusivity decreases again. This is associated to the formation of lattice defects such as vacancies, vacancy clusters and dislocations. The strain state of the specimen in the constant loading device at different loading conditions is obtained by digital image correlation. This revealed that the in-situ instrumentation is key and confirmed that a homogeneous strain distribution is present in the zone of interest for the permeation experiments.