Hydrogen-induced Ductility Loss Research Articles

Fracture analysis of hydrogen-embrittled API X52 pipes at low temperature

This paper quantifies the effect of temperature (ranging from room temperature (RT) to -90 °C) on fracture toughness for hydrogen-embrittled API X52 by combining small punch (SP) test data and finite element (FE) damage analysis. For the FE damage analysis, a multi-axial fracture strain damage model was used, and the parameters were determined by analyzing the tensile and SEN(T) test data in air at RT. The hydrogen-enhanced ductile fracture was considered using the hydrogen-embrittlement constant, which was determined by analyzing the SP test results in hydrogen. Due to the effect of the interaction between strength and hydrogen-induced ductility loss on fracture toughness, the predicted fracture toughness in terms of temperature does not show a monotonic decrease up to -90 °C; rather, it decreases up to -30 °C, then increases slightly before decreasing again. Fracture mechanics analysis of a hydrogen-embrittled pipe with an axial surface crack using the determined fracture toughness values showed that the maximum pressure decreased slightly (less than 9 %) with decreasing temperature up to -90 °C, suggesting that the effect of temperature on the maximum pressure would not be so significant for API X52.

Micro-Mechanical Characterisation of Hydrogen Embrittlement and Fatigue Crack Growth Behaviours in Metastable Austenitic Stainless Steels with Microstructure Refinement

This article reviews the microstructural evolution in ultrafine-grained and nanotwinned austenitic stainless steels that have been subjected to hydrogen embrittlement (HE) and fatigue cracking. It provides guidelines for the development of high-strength austenitic steels without sacrificing HE and fatigue performance. The author focuses on the hydrogen-induced ductility loss and short fatigue crack growth associated with deformation-induced martensitic transformation, using micro-tension and micro-fatigue testing technologies. In type 304 metastable austenitic stainless steel, the microstructure produced by high-pressure torsion depends strongly on the processing temperature. Nanocrystalline austenite with enhanced strength and moderate ductility can be obtained at a processing temperature of ~423–573 K, whereas dual-phase microstructures comprising austenite and martensite are formed by processing at room temperature. Introducing ultrafine grains and nanotwin bundles mitigates the hydrogen-induced ductility loss in metastable austenitic steel by controlling the dynamic martensitic transformation. The microstructure refinement also contributes to enhanced resistance to short fatigue crack growth by changing the route of the damage accumulation process via phase transformation and detwinning.

水素助長粒界破壊を伴う純ニッケルの延性低下における水素と硫黄の相乗的役割

Synergistic effect of hydrogen and sulfur on intergranular (IG) fracture and resultant ductility loss in pure nickel was quantitatively evaluated by means of Auger electron spectroscopy, thermal desorption analysis and slow strain rate tensile test. Both hydrogen and sulfur enhanced IG fracture, leading to the degradation of ductility. Increased sulfur concentration at grain boundaries (GBs) resulted in an appearance of second peak around 400°C in TDA profiles, indicating that the segregated GB sulfur trapped hydrogen. However, its trapped hydrogen hardly enhanced the hydrogen-induced ductility loss. Instead, hydrogen directly trapped at GBs was responsible for IG fracture and degradation of ductility. These results indicate hydrogen and sulfur independently induced the IG fracture and resultant ductility loss and therefore, the synergistic effect is negligible.

Interpretation of complex, tensile-fracture phenomena in precipitation-hardened, martensitic stainless steels, 17-4PH, in presence of hydrogen

Hydrogen-induced ductility loss in two precipitation-hardened, martensitic stainless steels was investigated via tensile testing with internal hydrogen, revealing it to be more significant with a higher hydrogen content or tensile strength. A lower hydrogen content and tensile strength resulted in quasi-cleavage fracture and considerable plastic strain, suggesting that hydrogen–dislocation interaction contributed to the loss in ductility. When either the hydrogen content or the tensile strength increased, the intergranular and cleavage surfaces were incorporated into the quasi-cleavage surface, a phenomenon attributed to the hydrogen-induced reduction of cohesive strength. Such a reduction was confirmed using impact tests at cryogenic temperatures, implying that some mechanisms were responsible for the loss in ductility.

Effect of sulfur on hydrogen induced ductility loss in pure Ni

Effect of sulfur on hydrogen induced ductility loss in pure Ni

Effects of finish rolling deformation on hydrogen-induced cracking and hydrogen-induced ductility loss of high-vanadium TMCP X80 pipeline steel

An optimum finish rolling deformation (FRD) of thermomechanical controlled processing (TMCP) is suggested to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel in this study. The results demonstrate that with increasing FRD the microstructure refines, the grain size of the steel decreases and the recrystallization degree deepens. The increase of FRD leads to the reduction of low angle grain boundaries (LAGBs) and the grains oriented with plane {100} parallel to normal direction ({100}//ND) fibres, which plays a significant role in improving the resistance of crack propagation. Besides, the differences of effective hydrogen diffusion coefficient and diffusible hydrogen concentration are negligible among four experimental steels with various FRD. However, the best hydrogen-induced ductility loss resistance is obtained in the steel with 40% FRD containing the most nano-scale precipitates acting as effective hydrogen traps.

Micro-mechanical characterisation of hydrogen embrittlement in nano-twinned metastable austenitic stainless steel

Micro-tensile tests were performed on single variants of nano-twinned 304 stainless steel with hard and soft orientations to determine the effect of nano-twins on hydrogen embrittlement in the metastable austenitic steel. Nano-twins were introduced into the 304 steel by deformation above the martensite-start temperature. The introduction of nano-twins increased the yield stress to 647 MPa for the hard-oriented crystals and 540 MPa for the soft-oriented crystals. For the soft-oriented crystals, the hydrogen charge induced quasi-cleavage although it did not change the uniform strain. For the hard-oriented crystals, the introduction of nano-twins retarded hydrogen-induced twin boundary separation. The relationship between the reduction of area and ultimate tensile strength revealed that the degree of strengthening and mitigation of hydrogen-induced ductility loss depended on the orientation of the nano-twins. These suggest that controlling the orientation and distribution of nano-twins allows for simultaneous strengthening and mitigation of hydrogen-induced ductility loss in metastable austenitic steel.

The effect of hydrogen on the crack initiation site of TRIP-assisted steels during in-situ hydrogen plasma micro-tensile testing: Leading to an improved ductility?

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.

Effect of Tempering Temperature after Thermo-Mechanical Control Process on Microstructure Characteristics and Hydrogen-Induced Ductility Loss in High-Vanadium X80 Pipeline Steel.

In this study, an optimum tempering temperature after a thermo-mechanical control process (TMCP) was proposed to improve the hydrogen-induced ductility loss of high-vanadium X80 pipeline steel. The results showed that with increasing tempering temperature from 450 to 650 °C, the size and quantity of granular bainite decreased but the spacing of deformed lath ferrite and the fraction of massive ferrite increased. The number of fine vanadium carbides increased as well. However, as the tempering temperature increased to 700 °C, the microstructure of T700 steel completely converted to massive ferrite and the grain size became larger. Additionally, the amount of nanoscale precipitates decreased again, and the mean size of precipitates evidently increased in T700 steel. The steel tempering at 650 °C, containing the most vanadium precipitates with a size less than 20 nm, had the lowest hydrogen diffusion coefficient and the best resistance to hydrogen-induced ductility loss.

Hydrogen Trapping Behavior in Vanadium Microalloyed TRIP-Assisted Annealed Martensitic Steel

Transformation induced plasticity (TRIP)-assisted annealed martensitic (TAM) steel combines higher tensile strength and elogangtion, and has been increasingly used but appears to bemore prone to hydrogen embrittlement (HE). In this paper, the hydrogen trapping behavior and HE of TRIP-assisted annealed martensitic steels with different vanadium additions had been investigated by means of hydrogen charging and slow strain rate tensile tests (SSRT), microstructral observartion, and thermal desorption mass spectroscope (TDS). Hydrogen charging test results indicates that apparent hydrogen diffusive index Da is 1.94 × 10−7/cm2·s−1 for 0.21wt.% vanadium steel, while the value is 8.05×10−7/cm2·s−1 for V-free steel. SSRT results show that the hydrogen induced ductility loss ID is 76.2% for 0.21wt.%V steel, compared with 86.5% for V-free steel. The trapping mechanism of the steel containing different V contents is analyzed by means of TDS and Transmission electron microscope (TEM) observations. It is found out that the steel containing 0.21wt.%V can create much more traps for hydrogen trapping compared with lower V steel, which is due to vanadium carbide (VC) precipitates acting as traps capturing hydrogen atoms.The relationship between hydrogen diffusion and hydrogentrapping mechanism is discussed in details.

The effect of graphite size on hydrogen absorption and tensile properties of ferritic ductile cast iron

Ductile cast iron (DCI) is one of prospective materials used for the hydrogen equipment because of low-cost, good workability and formability. The wide range of mechanical properties of DCI is obtained by controlling microstructural factors such as graphite size, volume fraction of graphite, matrix structure and so on. Therefore, it is important to find out an optimal microstructural condition that is less susceptible to hydrogen embrittlement. In this study, the effects of graphite size on the hydrogen absorption capability and the hydrogen-induced ductility loss of ferritic DCI were investigated.Several kinds of ferritic DCIs with a different graphite diameter of about 10 µm - 30 µm were used for the tensile test and the hydrogen content measurement. Hydrogen charging was performed prior to the tensile test by exposing a specimen to high-pressure hydrogen gas. Then, the tensile test was performed in air at room temperature. The hydrogen content of a specimen was measured by a thermal desorption analyzer.It was found that the amount of hydrogen stored in DCI was dependent on the graphite size. As the graphite diameter increased, the hydrogen content sharply increased at a certain graphite diameter, and then it became nearly constant irrespective of increase in graphite diameter. In other words, there was the critical graphite diameter that significantly changed the hydrogen absorption capability. The ductility was decreased by hydrogen, and the hydrogen-induced ductility loss was dependent on the hydrogen content. Therefore, the hydrogen embrittlement of DCI became remarkable when the graphite size was larger than the critical value.

Hydrogen-induced ductility loss of precipitation-strengthened Fe-Ni-Cr-based superalloy

A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron channeling contrast (ECC) imaging. The observation results revealed that the brittle-like facets were derived from intergranular cracking accompanied by hydrogen-assisted microvoid nucleation at the grain boundaries (GBs). Deformation twinning also played a crucial role in triggering the final grain boundary separation due to local stress concentration at its intersection with the GBs after severe strain hardening; such a process has not yet been considered to explain the hydrogen-induced ductility loss of this type of alloy.

Effect of vanadium content on hydrogen diffusion behaviors and hydrogen induced ductility loss of X80 pipeline steel

In this study, the number and size distribution characteristics of fine nanoscale precipitates and their effects on hydrogen diffusion and hydrogen induced ductility loss were investigated in X80 pipeline steel. The results showed that with the increase of vanadium content, more vanadium carbides precipitated in the tested steels and the proportion of precipitates with size less than 10 nm distinctly increased, which fixed a large amount of diffusible hydrogen atoms as effective traps. The hydrogen diffusion coefficient decreased from 5.12 × 10−6 cm2 s−1 in the vanadium-free V1 steel to 9.94 × 10−7 cm2 s−1 in the V4 steel with 0.13% V. In addition, for the tested steels containing vanadium, the hydrogen embrittlement gradually decreased with increasing nanoscale vanadium precipitates, which pinned the movable dislocations and weakened the hydrogen-enhanced localized plasticity. The V4 steel with 0.13% V had the best performance to resist to hydrogen induced ductility loss.

The detrimental effect of hydrogen at dislocations on the hydrogen embrittlement susceptibility of Fe-C-X alloys: An experimental proof of the HELP mechanism

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 induced mechanical degradation in tungsten alloyed steels

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.

Comprehensive Understanding of Ductility Loss Mechanisms in Various Steels with External and Internal Hydrogen

Hydrogen-induced ductility loss and related fracture morphologies are comprehensively discussed in consideration of the hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316, 316L), high-strength austenitic stainless steels (HP160, XM-19), precipitation-hardened iron-based super alloy (A286), low-alloy Cr-Mo steel (JIS-SCM435), and low-carbon steel (JIS-SM490B). External hydrogen is realized by a non-charged specimen tested in high-pressure gaseous hydrogen, and internal hydrogen is realized by a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile tests (SSRT) of the materials with external or internal hydrogen could be comprehensively categorized into five types: hydrogen-induced successive crack growth, ordinary void formation, small-sized void formation related to the void sheet, large-sized void formation, and facet formation. The mechanisms of hydrogen embrittlement are broadly classified into hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). In the HEDE model, hydrogen weakens interatomic bonds, whereas in the HELP model, hydrogen enhances localized slip deformations. Although various fracture morphologies are produced by external or internal hydrogen, these morphologies can be explained by the HELP model rather than by the HEDE model.

Effect of ultrafine grain refinement on hydrogen embrittlement of metastable austenitic stainless steel

Micro-tensile tests were performed on high-pressure-torsion-processed specimens of type 304 steel with grain sizes in the range of 0.1–0.5 μm to clarify the effect of ultrafine grain refinement on the hydrogen embrittlement (HE) of metastable austenitic steel. The ultrafine-grained (UFG) specimens with average grain sizes < ∼0.4 μm exhibited a limited uniform elongation followed by a steady-stress regime in the stress–strain curves, which was attributed to a martensitic transformation. A high yield stress and a moderate elongation to failure were attained for the UFG specimens with an average grain size of ∼0.5 μm in the uncharged state. Hall–Petch relationships well hold between the yield stress and the average grain size for each uncharged and hydrogen-charged specimen. Hydrogen charging increased the friction stress by 40% but did not change the Hall–Petch coefficient. Hydrogen-induced ductility loss was mitigated by ultrafine grain refinement. Ductility loss due to hydrogen charging manifested in the local deformation after a martensitic transformation. This indicates that hydrogen does not significantly affect the martensitic transformation, but shortens the subsequent local deformation process.

The effect of pearlite on the hydrogen-induced ductility loss in ductile cast irons

Hydrogen energy systems, such as a hydrogen fuel cell vehicle and a hydrogen station, are rapidly developing to solve global environmental problems and resource problems. The available structural materials used for hydrogen equipments have been limited to only a few relatively expensive metallic materials that are tolerant for hydrogen embrittlement. Therefore, for the realization of a hydrogen society, it is important to expand the range of materials available for hydrogen equipment and thereby to enable the use of inexpensive common materials. Therefore, ductile cast iron was, in this study, focused as a structural material that could contribute to cost reduction of hydrogen equipment, because it is a low-cost material having good mechanical property comparable to carbon steels in addition to good castability and machinability. The strength and ductility of common ductile cast irons with a ferritic-pearlitic matrix can be controlled by the volume fraction of pearlitic phase. In the case of carbon steels, the susceptibility to hydrogen embrittlement increases with increase in the pearlite fraction. Toward the development of ferritic-pearlitic ductile cast iron with reasonable strength for hydrogen equipment, it is necessary to figure out the effect of pearlite on the hydrogen embrittlement of this cast iron. In this study, the tensile tests were conducted using hydrogen-precharged specimens of three kinds of ferritic-pearlitic ductile cast irons, JIS-FCD400, JIS-FCD450 and JIS-FCD700. Based on the results, the role of pearlite in characterizing the hydrogen embrittlement of ductile cast iron was discussed.

Hydrogen-Induced Ductility Loss of Austenitic Stainless Steels for Slow Strain Rate Tensile Testing in High-Pressure Hydrogen Gas

For slow strain rate tensile (SSRT) test in hydrogen gas, the degradation in relative reduction in area (RRA) of 300-series austenitic stainless steels is mainly attributed to hydrogen-assisted surface crack growth (HASCG) accompanied by quasi-cleavages. To establish novel criteria for authorizing various austenitic stainless steels for use in high-pressure gaseous hydrogen, a mechanism of the HASCG should be elucidated. At first, this study performed SSRT tests on six types of austenitic stainless steels, Types 304, 316, 316L, 306(hi-Ni), 304N2 and 304(N), in high-pressure hydrogen gas and showed that the RRAs were successfully quantified in terms of a newly-proposed nickel-equivalent equation. Then, to elucidate the microscopic mechanism of the HASCG, elasto-plastic fracture toughness (JIC), fatigue crack growth (FCG) and fatigue life tests on Types 304, 316 and 316L were carried out in high-pressure hydrogen gas. The results demonstrated that the SSRT surface crack grew via the same mechanism as for the JIC and fatigue cracks, i.e., these cracks successively grow with a sharp shape under the loading process, due to local slip deformations near the crack tip by hydrogen. Detailed observations of SSRT surface cracks on Types 304 and 316L were also performed, exhibiting that the onset of the HASCG occurred at the true strain of 0.1 or larger in high-pressure hydrogen gas.

The impact of hydrogen on the ductility loss of bainitic Fe–C alloys

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.