Hydrogen Trapping Sites Research Articles

Effects of Nb and Mo alloying on resistance to hydrogen embrittlement in 1.9 GPa-grade hot-stamping steels

Ultra-high-strength (over 1.8 GPa) of hot-stamping steels can be achieved by increasing C content of conventional hot stamping steel (1.5 GPa) or controlling precipitation behavior, but the enhanced strength often leads to the deteriorated resistance to hydrogen embrittlement. The complex alloying of Nb and Mo improves the resistance; however, the underlying microstructural evolutions and synergistic mechanisms are not understood yet. In this study, Nb- or (Nb + Mo)-alloyed 1.9 GPa-grade hot-stamping steels were fabricated in the laboratory, and their resistance to hydrogen embrittlement was evaluated by slow strain-rate tensile (SSRT) tests without and after hydrogen charging. Most of Nb consumed to form Nb carbides, hindering the migration of prior austenite grain boundaries mainly by a common Zener's pinning effect. The reduced grain size resulted in the decreased amount of diffusible hydrogen per unit grain boundary area. In addition, coherent or semi-coherent precipitates provided stable hydrogen trapping sites, leading to the low diffusivity and the consequently high resistance to hydrogen embrittlement. In the (Nb + Mo)-alloyed steel, Mo carbides preferred to nucleate at the surface of the pre-existing Nb carbides or to form needle-shaped isolated Mo2C carbide, but a considerably large amount of Mo remained inside the matrix as a solute. The prior austenite grain size was thus further reduced mainly by an additional solute drag effect. The solute Mo also enhanced the grain boundary cohesion, thereby leading to the absence of intergranular fracture and the sufficient post elongation even after the hydrogen charging.

Effect of shot peening coverage on hydrogen embrittlement of a ferrite-pearlite steel

Effect of a wide range of shot peening (SP) coverage on hydrogen diffusion and hydrogen embrittlement (HE) of a steel consisting of ferrite and pearlite was investigated. Conventional SP (CSP) reduces diffusivity and HE because of increase of dislocation trapping sites. Although severe SP (SSP), which uses higher coverages than CSP, increases the area of ferrite grain boundaries as a result of grain refinement, it cannot suppress further the diffusion and HE, as it cannot induce a further evident increase in strong hydrogen trapping sites such as dislocations. However, when coverage is increased to so high that causes the breaking and refinement of pearlite phase, diffusion and HE is suppressed further, because the dispersed distribution of ultrafine pearlite particles within ferrite matrix provides more strong interface trapping sites. SP-induced change of trapping sites plays a more critical role in HE and diffusion, in comparison with induced residual compressive stresses.

Hydrogen diffusion and trapping in X70 pipeline steel

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

Effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and hydrogen embrittlement susceptibility of hot-stamped boron steel

Multi-microalloying technique has been widely used to improve strength and ductility in steels, however, the effect of multi-microalloying on the hydrogen diffusion and hydrogen embrittlement (HE) behavior of steels is still not completely understood. This study evaluates the effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and HE of hot-stamped steel by a combination of hydrogen permeation test, slow strain rate tensile (SSRT) test and quantitative analysis. The microstructural examination and hydrogen permeation tests showed that with increasing Nb + Ti content, more nano-sized (Nb, Ti) C precipitates were formed, and the martensite grain boundaries areas increased, thus increasing the irreversible and reversible hydrogen trap sites and decreasing the hydrogen diffusion coefficient in the hot-stamped steel. A quantitative analysis of the hydrogen traps demonstrated that the number of hydrogen traps induced by the (Nb, Ti) C precipitates was larger than that of the grain boundaries and dislocations in the Nb–Ti bearing steels. In addition, the SSRT tests showed that the HE susceptibility of the test steel decreased with increasing Nb + Ti content. This was because the additional hydrogen traps generated by the Nb–Ti addition hindered localized hydrogen accumulation at prior austenite grain boundaries, the cracking resistance was improved by a lower Σ3 boundary fraction, and the pinning role of the (Nb, Ti) C precipitates on the movable dislocation could inhibit H-dislocation interaction.

Hydrogen distribution in metallic polycrystals with deformation

To understand hydrogen embrittlement and predict probable damage sites based on high hydrogen segregation, it is important to identify hydrogen distribution mechanisms in metallic microstructure with deformation. Hydrogen segregation in metals is affected by various factors such as grain-size, the character of grain boundaries (GBs), loading direction and strain-rate. To this end, a computational framework consisting of non-local dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is presented to study the role of (i) grain-size, (ii) loading direction, (iii) strain-rate and (iv) GB character on hydrogen distribution and segregation in the pre-charged metallic microstructure. The computational framework is capable of accounting for the change in local hydrogen concentration due to prevailing hydrostatic pressure, trapping by dislocations, GB energetics and local slip-rates in the metallic microstructure. The difficulty in dislocation motion at the inter-granular regions of polycrystal due to high cross-hardening offered by geometrically necessary dislocations (GNDs) is incorporated in the model as additional isotropic hardening, whereas the back stress due to the GND pileups is included as a kinematic term in the flow rule. In addition, GNDs act as hydrogen trap sites leading to increased hydrogen concentration in the inter-granular regions. The strain-rate factor initially provided by Krom & Bakker (1999) is modified in the present work to make it compatible with the dislocation density-based coupled framework, which is able to calculate trapped hydrogen in dislocations along the slip systems. Plastically deformed polycrystals of various grain-sizes, along different directions, with varying strain rates and containing Σ3[11¯0](111) and Σ5[001](210) GBs show that the gradients of hydrostatic pressure and GB character are major factors controlling hydrogen segregation in the microstructure.

Hydrogen-induced embrittlement of nickel-chromium-molybdenum containing HSLA steels

ABSTRACTSeveral advanced nickel-chromium-molybdenum high strength lowalloy steels newly developed by our research team exhibit excellent mechanical strength, toughness and hardenability. However, the phenomenon of hydrogen-induced embrittlement will easily occur for these high strength steels. In this research, the hydrogeninduced embrittlement of 8625-Modified steel (8625M steel) was studied. Experimental results show that the dominant hydrogen trapping site of the 8625M steel is dislocation, of which trapping energy is about 20 kJ/mol, indicating that the hydrogens trapped in the dislocations are diffusible. The as-quenched 8625M steel has the highest dislocation density and accordingly the highesthydrogen content after hydrogen charging. This makes the asquenched 8625M steel exhibit severe hydrogen embrittlement. After tempering at 200°C and 300°C, the dislocation density drops, and hence these tempered specimens have lower ultimate tensile strength loss. After 400°C tempering, the hydrogen embrittlement phenomenon becomes serious again, being ascribed to the formation of needlelike and film-like cementite which will weaken the strength of martensite. After 500°C tempering, the 8625M steel has the lowest dislocation density, and the inter-lath cementite become discontinuous and spheroidal, making the 500°C tempered specimen have the lowest ultimate tensile strength loss and the highest elongation after hydrogen charging in this study.

Identification of Hydrogen Trapping Sites in a Strained Ferritic-Martensitic Dual-Phase Steel

Identification of Hydrogen Trapping Sites in a Strained Ferritic-Martensitic Dual-Phase Steel

Improved hydrogen embrittlement resistance after quenching–tempering treatment for a Cr-Mo-V high strength steel

The effect of quenching-tempering (QT) treatment on the hydrogen embrittlement (HE) resistance of a reactor pressure vessel steel was studied. Decomposition of M3C/VC carbides and precipitation of M7C3 carbides were confirmed by transmission electron microscopy and atom probe tomography observations. Tensile tests showed that HE sensitivity decreased to a negligible level after QT treatment. The improvement of HE resistance was mainly attributed to the decreased number of M3C carbides which act as the reversible trapping sites for hydrogen. This was supported by the decreased concentration of reversible hydrogen as measured by thermal desorption spectroscopy. The amount of irreversible hydrogen (probably trapped at VC carbides) also decreased, which is however not considered responsible for the HE improvement.

Effects of Molybdenum on HIC Susceptibility in Normalized Pressure Vessel Steels for Sour Service Applications

This paper discusses a molybdenum-added alloy design for normalized pressure vessel steels, to reduce hydrogen induced cracking (HIC) and inhibit crack propagation. The change in microstructure produced by the modified alloy composition was analyzed to determine its effect on HIC characteristics. The microstructural change was observed by optical microscopy, hardness tests, scanning electron microscopy, and electron backscattered diffraction. The crack length ratio and crack thickness ratio were evaluated using the NACE TM 0284 standard, and ultrasonic testing was used for HIC analysis. The formation of polygonal ferrite and pearlite during the processing of the alloy creates localized areas of high stress concentration at the polygonal ferrite/pearlite interface, due to the expansion/contraction of various structures during the transformation. This results in the generation of potential hydrogen-trapping sites, subsequent HIC, and crack propagation. The addition of molybdenum leads to a decrease in the volume fraction of the pearlite structure in the steel in favor of a more beneficial bainitic ferrite microstructure, which is generated during the normalizing process. This bainitic transformation creates a more favorable expansion/contraction compatibility and reduces/breaks up the ferrite/pearlite banding. The combination of these two characteristics can result in an overall lower stress-intensity state, which can minimize hydrogen-trapping and crack propagation. This study demonstrates that the resistance of normalized pressure vessel steels to HIC can be significantly improved by incorporating molybdenum in the alloy design. Key words: sour service, molybdenum, hydrogen induced crack (HIC), normalizing steel, pressure vessel steel

Assessment of hydrogen embrittlement via image-based techniques in Al–Zn–Mg–Cu aluminum alloys

Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate.

Hydrogen partitioning behavior and related hydrogen embrittlement in Al-Zn-Mg alloys

To develop high strength Al-Zn-Mg alloys, suppression of hydrogen embrittlement is indispensable. The hydrogen embrittlement behavior of different prepared Al-10.1-1.2Mg alloys with various hydrogen trap sites was observed using in situ synchrotron X-ray tomography in this study. Furthermore, we quantified the hydrogen partitioning based on hydrogen occupancies and hydrogen trap site densities in the prepared alloys. The combined analysis of hydrogen embrittlement and hydrogen partitioning showed that initial trapped hydrogen content in grain boundaries, vacancies, and dislocations before deformation was not crucial for inducing both intergranular fracture and quasi-cleavage fracture. However, hydrogen accumulates at grain boundaries and precipitate interfaces during deformation, inducing intergranular and quasi-cleavage fracture, respectively. Due to hydrogen accumulation, intergranular and quasi-cleavage fracture initiate when the hydrogen content at the grain boundary enriches 103–104 times the initial content and hydrogen content at the precipitate interface enriches 3.9 × 102 times the initial content, respectively. The change in hydrogen trap sites by processing and heat treatments did not suppress the intergranular and quasi-cleavage fracture. We conclude that generating new hydrogen trap sites (e.g., the interior of the intermetallic particle) in which hydrogen trap site density and binding energy are higher than the precipitate interface (>33.87 kJ/mol) is beneficial to suppress hydrogen embrittlement.

Hydrogen isotope permeation and retention behavior in the CoCrFeMnNi high-entropy alloy

Owing to its excellent properties, high-entropy alloys (HEAs) have been regarded as a kind of promising candidate structural material for nuclear devices. For the safety concern, it is vital to figure out the compatibility of hydrogen isotopes with the HEAs. In this work, the hydrogen isotope permeation and retention behavior of a representative HEA, CoCrFeMnNi, has been mainly checked by a gas-driven permeation device and a thermal desorption spectra device. The hydrogen isotope permeation parameters have been obtained, which are quite similar to those of the 316L austenitic steels. The hydrogen isotope retention behavior has been compared with that for a reduced activation ferritic/martensitic steel, and the relatively high hydrogen isotope inventory in HEA has been explained by crystalline structure and high density of hydrogen trapping site.

Hydrogen Permeation of Multi-Layered-Coatings

Using a substrate of AISI 316L austenitic stainless steel, which is used for components in high-pressure hydrogen systems, the hydrogen barrier properties of samples with single-layer coatings of TiC, TiN, and TiAlN as well as a multi-layered coating of TiAlN and TiMoN were evaluated. The ion plating method was used, and coating thicknesses of 2.0–2.6 μm were obtained. Hydrogen permeation tests were carried out under a differential hydrogen pressure of 400 kPa and at a temperature between 573 and 773 K, and the quantities of hydrogen that permeated the samples were measured. This study aimed at elucidating the relationship between the microstructures of the coatings and the hydrogen permeation properties. Coatings of TiC, TiN, TiAlN, and TiAlN/TiMoN facilitated reductions of the hydrogen permeabilities to 1/100 or less of that of the uncoated substrate. The samples coated with TiN and TiC that developed columnar crystals vertical to the substrate exhibited higher hydrogen permeabilities. The experiment confirmed that the coatings composed of fine crystal grains were highly effective as hydrogen barriers, and that this barrier property became even more efficient if multiple layers of the coatings were applied. The crystal grain boundaries of the coating and interfaces of each film in a multi-layered coating may serve as hydrogen trapping sites. We speculate that fine crystal structures with multiple crystal grain boundaries and multi-layered coating interfaces will contribute to the development of hydrogen barriers.

Response of Hydrogen Desorption and Hydrogen Embrittlement to Precipitation of Nanometer-Sized Copper in Tempered Martensitic Low-Carbon Steel

This work is aimed at studying the hydrogen–copper precipitate interaction in a martensitic steel. Analysis of hydrogen thermal desorption revealed that precipitation of copper particles enhances the hydrogen trapping capability of tempered copper-containing martensitic steel. Moreover, precipitation of copper could make hydrogen retain longer in the steel, indicating a retarded diffusion of hydrogen. Copper precipitates as a hydrogen trapping site were observed to preserve an activation energy of 35.6 kJ mol−1 by Choo-and-Lee method after release for 4 h at room temperature. This value is higher than the activation energy of dislocation. Moreover, tempered steel with copper particles displayed better resistance to hydrogen embrittlement in notched, slow-strain-rate tensile tests.

A Study on Sulfide Stress Cracking Susceptibility of GMA Girth Welds in X80 Grade Pipes

The present work evaluates the SSC susceptibility of linepipe steel weld metals produced with various microstructures consisting of different ratios of intragranular (acicular) ferrite and grain boundary ferrite. It is shown that weld metal with high fractions of intragranular ferrite and low grain boundary ferrite passed SSC tests even though their hardness exceeded 250 HV, the widely accepted guideline to prevent SSC fracture initiation. Using a novel combination of hydrogen microprinting combined with SEM and TEM electron microscopy analysis techniques, the intragranular ferrite grain boundaries are shown to provide key hydrogen trapping sites which consist of fine grains and finely dispersed nano-scale carbide precipitates. The presence of a high fraction of trapping sites is suggested to account for the good SSC resistance coupled with high hardness, while increased grain boundary ferrite led to rejection during SSC testing due to inferior fracture toughness associated with coarser grains.

Cohesive zone modeling of hydrogen-induced delayed intergranular fracture in high strength steels

A sequentially coupled hydrogen diffusion-cohesive zone modeling approach was applied to the simulation of hydrogen-induced delayed intergranular (IG) fracture in high-strength low-alloy steels. The effects of multiple hydrogen trap sites and mechanical deformation on the diffusion and cohesive strength of grain boundaries (GB) were taken account, in order to reveal that the hydrogen trapped at GB play a dominant role in the degradation processes of hydrogen of high-strength low-alloy steels, which leads to the IG fracture. The approach was implemented by Abaqus software in the form of a two-steps procedure including the coupled elastoplastic-transient hydrogen diffusion analysis and cohesive stress analysis. To validate the approach, the constant load tests of hydrogen pre-charged AISI 4135 high-strength low-alloy steel notched bars in literature were analyzed. Good agreement is observed between the simulation and experimental data of time to failure. The results confirm that hydrogen-induced IG fracture of high strength low-alloy steels can be related to the hydrogen concentration trapped at GB. The critical hydrogen concentration at GB for crack initiation is independent of the initial hydrogen concentration but depends strongly on the local stress level and stress triaxiality. The critical hydrogen concentration linearly decreases with increasing normalized peak maximal principal stress normalized by the critical cohesive strength in absence of hydrogen.

Strong resistance to hydrogen embrittlement of high-entropy alloy

The resistance to hydrogen embrittlement (HE) of CrMnFeCoNi high-entropy alloy (HEA) at both room and cryogenic temperatures was examined through tensile experiments on specimens hydrogenated via cathodic electrochemical charging method. Two representative steels, i.e. 316L stainless steel (SS) and X80 pipeline steel (PS), were chosen for comparison due to their similar main constituent elements to CrMnFeCoNi HEA. Results show that the hydrogen pre-charged CrMnFeCoNi HEA has the smallest loss of ductility among the three materials at room temperature, while displays no reduction of elongation at 77 K, compared with the uncharged one. Fracture surfaces at both room and cryogenic temperatures of hydrogen pre-charged CrMnFeCoNi HEA are mainly composed of dimples, indicating ductile fractures, while brittle characteristics occur in pre-charged 316L SS and X80 PS. Typical deformation microstructure of the hydrogen pre-charged CrMnFeCoNi HEA at room temperature is tangled dislocations instead of highly dense dislocation walls (HDDWs) found in the pre-charged 316L SS. At 77 K, more deformation twins are formed in the both materials. Reasons for a higher resistance to HE of CrMnFeCoNi HEA at room temperature are attributed to the formation of less hydrogen trapping sites, thus a lower degree of hydrogen enrichment than 316L SS. While at 77 K, the atomic hydrogen is not able to promptly accumulate near these trapping sites due to its slow diffusion rate, which leads to strong HE resistance.

Application of atom probe tomography to fundamental issues of steel materials

Although steel research has a very long history, there are still various questions and uncertainties on the fundamental issues of steel materials. Our aim is to clarify the issues by atomic‐scale characterization using atom probe tomography (APT). This paper introduced our developed techniques in application of APT into steel materials. The first one is the specimen tip preparation technique from the site‐specific regions of interest in steel, which significantly expanded the application field of steel analysis. The second one is the observation technique of trapped hydrogen in steel using our developed “deuterium charge cell,” which accomplished direct observation of the hydrogen trapping site in alloy carbide precipitation strengthened steels. Such atomic‐scale analyses bring many new findings in steel materials science.

Designing steel to resist hydrogen embrittlement: Part 1 – trapping capacity

ABSTRACTA novel steel has been designed for use in the oil and gas industry, displaying properties comparable with the currently available F22 grade and possessing the additional quality of excellent hydrogen trapping capacity. Its high strength is derived from a martensitic microstructure containing a dispersion of fine vanadium-molybdenum carbides that evolve during thermal treatments. If the tempering cycle is controlled such that the precipitates maintain a degree of coherency with the matrix, then they act as hydrogen trapping sites, due to the associated strain fields, thus mitigating the problem of diffusible hydrogen. Using material modelling programmes and small-scale sample alloys this work describes the process of the new steel design and demonstrates its superior trapping capacity through thermal desorption analysis.

Influence of hydrogen on strain localization and fracture behavior in Al[sbnd]Zn[sbnd]Mg[sbnd]Cu aluminum alloys

Hydrogen-induced dislocation motion is characterized in terms of the microscopic strain distribution in AlZnMgCu aluminum alloys. Hydrogen-induced strain localization was visualized in 3D using X-ray tomography and related microstructural tracking techniques. The strain localization was observed as a form of obliquely aligned shear bands. The strain localization becomes more intense with an increase in holding time at each loading step, indicating that more internal hydrogen is partitioned to the strain localization regions with holding time. In addition, the concentration of hydrostatic strain is observed in the strain localization region. Numerous nano voids were generated after deformation and were determined from the precise interpretation of the measured hydrostatic tension. Direct observation of the nano voids was then successfully performed by employing high-angle annular dark-field (i.e., HAADF) scanning transmission electron microscopy imaging and imaging-type computed tomography (CT) techniques. It is assumed that nano voids can serve a dual role as a fracture origin site and a hydrogen trap site. However, no evidence for hydrogen embrittlement originating from nano voids was observed. Instead, it can be assumed that the most hydrogen was partitioned to nano voids in strain localization regions during deformation due to its high density. A hydrogen embrittlement model was proposed based on these findings, where in-situ hydrogen repartitioning, which is necessary for hydrogen embrittlement to occur, is considered.