Hydrogen Pre-charging Research Articles

Atomistic simulation of hydrogen-induced plastic zone compression during cyclic loading

To investigate the effect of hydrogen atoms on the size of the plastic deformation zone, molecular dynamics (MD) simulations were performed on a single crack model. The model uses pre-charged hydrogen to quantify the compression effect of hydrogen atoms on the plastic zone during cyclic loading. The results show that stress release at the crack tip occurs mainly in the form of plastic deformation, and the degree of compression in the plastic zone increases with increasing hydrogen concentration. A compression factor, which considers hydrogen concentration, is found with the help of the simulation results. A modified fatigue crack growth rate (FCGR) model, combined with the compression factor, was then used to predict the hydrogen-assisted fatigue crack growth rate. The proposed model shows excellent agreement with the experimental data for X100 steel, and it provides a new framework to describe hydrogen-assisted cracking.

Effect of Hydrogen Precharging on Mechanical and Electrochemical Properties of Pure Titanium

The electrolytic hydrogen precharging process is used to analyze the influence of hydrogen on the microstructure, corrosion resistance, and mechanical properties of pure titanium. The results demonstrate that two types of titanium hydrides (δ‐TiHx and ϵ‐TiH2) are detected by electron backscatter diffraction (EBSD), and the volume fraction of titanium hydrides, the local strain caused by hydrogen precipitation, and the thickness of hydrides layer increase with the increase in precharging time. It is noteworthy that the formation of hydrides layer has a protective effect on general corrosion, but the local strain caused by hydrides precipitation leads to the destruction of corrosion resistance, as a result the corrosion resistance of pure titanium increases first and then decreases with the increase in hydrogen precharging time. Moreover, with the precipitation of hydrides, the fracture mode changes from ductile failure to duplex‐ and multimode failure, resulting in the decrease in ultimate tensile strength (UTS) and elongation. Therefore, the mechanical and electrochemical properties of the precharged samples first increase and then decrease with the increase in precharging time, reaching their optimum values at 10 h, and the critical precharging time is determined to be 75 h when comparing those properties with pure titanium.

Effect of PTFE coating on enhancing hydrogen embrittlement resistance of stainless steel 304 for liquefied hydrogen storage system application

The phenomenon of hydrogen embrittlement phenomenon is known to be a major obstacle to proposed to overcome this phenomenon. In the present study, polytetrafluoroethylene (PTFE), which is known to be an effective hydrogen adsorption and desorption material, was coated on the surface of stainless steel 304 to improve its hydrogen embrittlement resistance. To make a hydrogen embrittlement environment, electrochemical hydrogen pre-charging was applied to the PTFE-coated stainless steel 304. To investigate the effects of PTFE coating on the hydrogen embrittlement resistance of stainless steel 304, the Charpy V-notch impact (CVN) test was performed under three different temperatures: 25, −83, and −196 °C. Additionally, hydrogen concentration, electron back scatter diffraction (EBSD), and scanning electron microscopy (SEM) evaluations were carried out to verify the results of the CVN impact test. The PTFE coating did not have a significant effect on the quantitative reduction of hydrogen concentration; however, we confirmed its excellent performance in terms of toughness reduction due to the increase in hydrogen loading time at room temperature.

Mechanical Properties and Hydrogen Embrittlement of Laser-Surface Melted AISI 430 Ferritic Stainless Steel

In the present study, the feasibility of laser surface melting (LSM) of AISI 430 ferritic stainless steel to minimize hydrogen embrittlement (HE) was investigated. LSM of AISI 430 steel was successfully achieved by a 2.3-kW high power diode laser (HPDL) with scanning speeds of 60 mm/s and 80 mm/s (the samples are designated as V60 and V80, respectively) at a power of 2 kW. To investigate the HE effect on the AISI 430 steel without and with LSM, hydrogen was introduced into specimens by cathodic charging in 0.1 M NaOH solution under galvanostatic conditions at a current density of 30 mA/cm2 and 25 °C. Detail microstructural analysis was performed and the correlation of microstructure with HE was evaluated. By electron backscatter diffraction (EBSD) analysis, the austenite contents for the laser-surface melted specimens V60 and V80 are found to be 0.6 and 1.9 wt%, respectively. The amount of retained austenite in LSM specimens was reduced with lower laser scanning speed. The surface microhardness of the laser-surface melted AISI 430 steel (~280 HV0.2) is found to be increased by 56% as compared with that of the substrate (~180 HV0.2) because of the presence of martensite. The degree of embrittlement caused by hydrogen for the charged and non-charged AISI 430 steel was obtained using slow-strain-rate tensile (SSRT) test in air at a strain rate of 3 × 10−5 s−1. After hydrogen pre-charging, the ductility of as-received AISI 430 steel was reduced from 0.44 to 0.25 while the laser-surface melted AISI 430 steel showed similar tensile properties as the as-received one. After LSM, the value of HE susceptibility Iδ decreases from 43.2% to 38.9% and 38.2% for V60 and V80, respectively, due to the presence of martensite.

Hydrogen embrittlement of X80 pipeline steel in H2S environment: Effect of hydrogen charging time, hydrogen-trapped state and hydrogen charging–releasing–recharging cycles

This study investigated the susceptibility of X80 pipeline steel to hydrogen embrittlement given different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles in H2S environment. The fracture strain of the steel samples decreased with increasing hydrogen pre-charging time; this steel degradation could almost be recovered after diffusible hydrogen was removed when the hydrogen pre-charging time was <8 d. However, unrecoverable degeneration occurred when the hydrogen pre-charging time extended to 16–30 d. Moreover, nanovoid formation meant that the hydrogen damage to the steel under intermittent hydrogen pre-charging–releasing–recharging conditions was more serious than that under continuous hydrogen pre-charging conditions. This study illustrated that the mechanical degradation of steel is inevitable in an H2S environment even if diffusible hydrogen is removed or visible hydrogen-induced cracking is neglected. Furthermore, the steel samples showed premature fractures and exhibited a hydrogen fatigue effect because the repeated entry and release of diffusible hydrogen promoted the formation of vacancies that aggregated into nanovoids. Our results provide valuable information on the mechanical degradation of steel in an H2S environment, regarding the change rules of steel mechanical properties under different hydrogen pre-charging times and hydrogen charging–releasing–recharging cycles.

Unravelling the effect of hydrogen on microstructure evolution under low-cycle fatigue in a high-manganese austenitic TWIP steel

We systematically investigated the effect of hydrogen on the low-cycle fatigue (LCF) behaviour of a Fe–28Mn-0.3C (wt.%) twinning-induced plasticity (TWIP) steel by comparing the fatigue microstructure of samples with and without hydrogen pre-charging using electron channelling contrast imaging (ECCI) and cross-correlation electron backscatter diffraction (CC-EBSD). The results reveal that a complex interplay of several hydrogen-microstructure interaction mechanisms is involved in the observed LCF behaviour of the studied TWIP steel with hydrogen pre-charging; first, hydrogen assists the nucleation of stacking faults (SFs) and deformation-induced ε-martensite in the studied TWIP steel by reduction of local stacking fault energy (Suzuki effect) and stabilization of the hexagonal close-packed (hcp) ε-phase. The evolution of fatigue dislocation pattern is strongly retarded in the presence of hydrogen. The rapid formation of ε-martensite leads to a stronger cyclic hardening. Meanwhile, the impingement of ε-martensite plates at GBs causes high local stress concentration, which plays a dominant role in the observed fatigue cracking. The crack opening angle indicates that the exact associated hydrogen embrittlement mechanism is the hydrogen-enhanced decohesion (HEDE) mechanism instead of the hydrogen-enhanced localized plasticity (HELP) mechanism. Among different GB types, annealing twin boundary shows the highest immunity to hydrogen-related fatigue cracking.

Shear and laser cutting effects on hydrogen embrittlement of a high-Mn TWIP steel

The influence of shear and laser cutting on hydrogen embrittlement of a hot-rolled Fe-18Mn-0.4C-1.2Al-1.8Cr TWIP (twinning induced plasticity) steel sheet in 3 mm thickness was investigated by hydrogen pre-charging and subsequent slow strain rate tests. Due to the severe plastic deformation during shearing, the ductility of the shear-cut specimen was significantly reduced in tensile test compared to the laser-cut specimen. After hydrogen pre-charging, the laser-cut specimen exhibited less susceptibility to hydrogen embrittlement than the shear-cut specimen. The fractographic analysis revealed that the shear-affected zone acted as crack initiation site during tensile test, which exhibited a flat fracture zone with high stress triaxiality. In hydrogen-charged condition, the flat fracture zone transited from large primary dimples to the mixed features with dimples and quasi-cleavage features.

Influence of tempering treatments on mechanical properties and hydrogen embrittlement of 13 wt% Cr martensitic stainless steel

The influence of tempering treatments on mechanical properties and hydrogen embrittlement (HE) of 13 wt% Cr martensitic stainless steel (MSS) have been investigated by Charpy impact tests and slow strain rate tensile (SSRT) tests followed by fracture surface examination. The austenitized and quenched specimens were tempered at 300, 550 and 700 °C for 2.5 h. The MSS tempered at 550 °C showed brittle intergranular (IG) fracture after impact tests indicating its susceptibility to temper embrittlement. The experimental results showed that the 13 wt% Cr MSS is sensitive to HE. The as-quenched condition showed cracking during hydrogen pre-charging itself. Hydrogen pre-charging duration increased the susceptibility to HE of tempered MSS. The maximum HE susceptibility was observed for specimen tempered at 550 °C with a drastic reduction in strength and strain to failure. Tempering at 300 and 550 °C showed brittle IG fracture with hydrogen pre-charging in SSRT tests whereas increased IG region with ductile dimples was observed for specimen tempered at 700 °C with increase of pre-charging duration. The reason for maximum susceptibility to HE of specimen tempered at 550 °C is due to synergistic interaction of hydrogen and impurities segregated at prior austenitic grain boundaries.

Interaction between hydrogen and cyclic stress and its role in fatigue damage mechanism

The interaction between hydrogen and cyclic stress were investigated on E690 steel by hydrogen pre-charging under cyclic loadings of various peak stresses, and hydrogen determination, tensile tests and observation on microstructures. Results show that hydrogen stays in E690 steel mainly in form of non-diffusion hydrogen (NDH). Peak stress exponentially increases the saturation NDH concentration through exponentially increasing the number of hydrogen trap. Different fatigue damage mechanisms were found after pre-charged for 12 h under different peak stresses (Smax) and various current densities (HCCD): (i) hydrogen-enhanced toughness (Smax ≤ 0.6 σp0.2, HCCD ≤ 25 mA∙cm−2), (ii) hydrogen embrittlement (Smax > 0.8 σp0.2, HCCD > 25 mA∙cm−2), (iii) hydrogen-enhanced localized plasticity (The balance).

Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials

Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching −135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.

The SCC initiation behavior of Alloy 600 during the transition of hydrogenated/oxygenated water condition at evaluated temperature

The present work investigated the stress corrosion cracking (SCC) initiation behavior of Alloy 600 during the transition of hydrogenated/oxygenated water condition at evaluated temperature. Slow strain rate test (SSRT) was applied and the test was divided into the pre-immersion period and load period. Hydrogenated/oxygenated water transition was implemented to realize the in-situ hydrogen charging. Pre-immersion in hydrogenated water chemistry can enhance SCC initiation happening in the following oxygenated water chemistry in terms of a higher crack density. The phenomena were attributed to the enhanced effect of the pre-charged hydrogen in metal on oxidation process and the consequent formation of less protective oxide film when the oxygenated condition is applied and the implications of the phenomena in the nuclear power plant were also given.

Influence of hydrogen on the propagation of intergranular corrosion defects in 2024 aluminium alloy

To study the influence of hydrogen on the intergranular corrosion mechanism of a 2024 aluminium alloy, samples were hydrogen precharged by cathodic polarisation and then exposed to a NaCl solution. EBSD analyses and SEM observations showed that hydrogen increased the number of corroded interfaces and led to the embrittlement of low-angle grain boundaries which were not susceptible to corrosion without hydrogen precharging. The increase of the reactivity of the 2024 aluminium alloy in the presence of hydrogen gave a new insight into the intergranular corrosion mechanism: corrosion-induced hydrogen promoted the intergranular corrosion propagation and partially controlled the corrosion defect morphology.

Investigation of hydrogen embrittlement in 12Cr2Mo1R(H) steel

Abstract

Effect of α′ Martensite Content Induced by Tensile Plastic Prestrain on Hydrogen Transport and Hydrogen Embrittlement of 304L Austenitic Stainless Steel

Effects of microstructural changes induced by prestraining on hydrogen transport and hydrogen embrittlement (HE) of austenitic stainless steels were studied by hydrogen precharging and tensile testing. Prestrains higher than 20% at 20 °C significantly enhance the HE of 304L steel, as they induce severe α′ martensite transformation, accelerating hydrogen transport and hydrogen entry during subsequent hydrogen exposure. In contrast, 304L steel prestrained at 50 and 80 °C and 316L steel prestrained at 20 °C exhibit less HE, due to less α′ after prestraining. The increase of dislocations after prestraining has a negligible influence on apparent hydrogen diffusivity compared with pre-existing α′. The deformation twins in heavily prestrained 304L steel can modify HE mechanism by assisting intergranular (IG) fracture. Regardless of temperature and prestrain level, HE and apparent diffusivity ( D app ) increase monotonously with α′ volume fraction ( f α ′ ). D app can be described as log D app = log ( D α ′ s α ′ / s γ ) + log [ f α ′ / ( 1 − f α ′ ) ] for 10 % &lt; f α ′ &lt; 90 % , with D α ′ is diffusivity in α′, s α ′ and s γ are solubility in α′ and austenite, respectively. The two equations can also be applied to these more typical duplex materials containing both BCC and FCC phases.

Investigation of the Influence of Pre-Charged Hydrogen on Fracture Toughness of As-Received 2.25Cr1Mo0.25V Steel and Weld.

Fracture failure caused by hydrogen embrittlement (HE) is a major concern for the system reliability and safety of hydrogen storage vessels, which are generally made of 2.25Cr1Mo0.25V steel. Thus, study of the influence of pre-charged hydrogen on fracture toughness of as-received 2.25Cr1Mo0.25V steel and weld is of significant importance. In the current work, the influence of hydrogen on fracture toughness of as-received 2.25Cr1Mo0.25V steel and weld was systematically studied. Base metal (BM) and weld metal (WM) specimens under both hydrogen-free and hydrogen-charged conditions were tested using three-point bending tests. Hydrogen was pre-charged inside specimens by the immersion charging method. The J-integral values were calculated for quantitatively evaluating the fracture toughness. In order to investigate the HE mechanisms, optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure of BM and WM specimens. The results revealed that the presence of pre-charged hydrogen caused a significant decrease of the fracture toughness for both BM and WM specimens. Moreover, the pre-charged hydrogen led to a remarkable transition of fracture mode from ductile to brittle pattern in 2.25Cr1Mo0.25V steel.

High-concentration carbon assists plasticity-driven hydrogen embrittlement in a Fe-high Mn steel with a relatively high stacking fault energy

We investigated the effects of electrochemical hydrogen charging on the mechanical properties of a Fe-33Mn-1.1C austenitic steel with high carbon concentration and relatively high stacking fault energy. Hydrogen pre-charging increased the yield strength and degraded the elongation and work-hardening capability. The increase in yield strength is a result of the solution hardening of hydrogen. A reduction in the cross-sectional area by subcrack formation is the primary factor causing reduction in work-hardening ability. Fracture modes were detected to be both intergranular and transgranular regionally. Neither intergranular nor transgranular cracking modes are related to deformation twinning or simple decohesion in contrast to conventional Fe-Mn-C twinning-induced plasticity steels. The hydrogen-assisted crack initiation and subsequent propagation are attributed to plasticity-dominated mechanisms associated with strain localization. The occurrence of dynamic strain aging by the high carbon content and ease of cross slip owing to the high stacking fault energy can cause strain/damage localization, which assists hydrogen embrittlement associated with the hydrogen-enhanced localized plasticity mechanism.

Influence of Slip and Lubrication Regime on the Formation of White Etching Cracks on a Two-Disc Test Rig

A common cause for maintenance and downtime in multiple fields of the mechanical transmission industries are premature rolling bearing failures due to white etching cracks (WEC). Within this work, WEC have been successfully recreated on a two-disc test rig under rolling contact loading without additional loading such as hydrogen pre-charging. This paper summarizes the state of the investigations regarding the influence of the slip type and the lubrication regime on the WEC formation on the two-disc test rig.

The influence of hydrogen on the very high cycle fatigue property and crack growth characteristics of bearing steels

In the present study, the growth rate of optically dark area (ODA) was investigated for two types of bearing steels using a fracture mechanics approach. Ultrasonic fatigue tests were performed for the specimens with hydrogen pre-charging followed by exposing in the atmosphere for a week in order to investigate the effect of hydrogen trapped by inclusions on the growth of ODA. Together with the constant amplitude loading test, two-step and repeated two-step variable amplitude loading tests were performed in order to determine the fatigue crack growth rate by beach marks. The results revealed that fatigue strengths of the both materials were reduced by the hydrogen pre-charging. The size of ODA increased with decreasing the stress amplitude at fracture so that the stress intensity factor range at the end of the ODA growth became a constant value ranging between 4.5 and 5.5 MPa √m independent of the materials and the fatigue lives. ODA formation behavior is enhanced by trapped hydrogen by inclusions, which reduces the fatigue strength in the very high cycle fatigue regime.

Localized Plasticity and Associated Cracking in Stable and Metastable High-Entropy Alloys Pre-Charged with Hydrogen

We investigated hydrogen embrittlement in Fe20Mn20Ni20Cr20Co and Fe30Mn10Cr10Co (at.%) alloys pre-charged with 100 MPa hydrogen gas by tensile testing at three initial strain rates of 10−4, 10−3, and 10−2 s−1 at ambient temperature. The alloys are classified as stable and metastable austenite-based high-entropy alloys (HEAs), respectively. Both HEAs showed the characteristic hydrogen-induced degradation of tensile ductility. Electron backscatter diffraction analysis indicated that the reduction in ductility by hydrogen pre-charging was associated with localized plasticity-assisted intergranular crack initiation. It should be noted as an important finding that hydrogen-assisted cracking of the metastable HEA occurred not through a brittle mechanism but through localized plastic deformation in both the austenite and ε-martensite phases.

Method of Evaluating Hydrogen Embrittlement Susceptibility of Tempered Martensitic Steel Showing Intergranular Fracture

A stress application method in delayed fracture susceptibility tests was investigated using 1450 MPa class tempered martensitic steel. Its fracture mode under hydrogen charging was mainly intergranular because of its relatively small Si content of 0.21 mass pct. The conditions for consistency in fracture strength between tensile tests and constant load tests (CLTs) were clarified: first, to conduct hydrogen precharging before stress application; and second, to choose a sufficiently low crosshead speed in tensile tests. When hydrogen precharging was not conducted before CLTs, the fracture strength was higher than the values in CLTs with hydrogen charging and in tensile tests. If the crosshead speed was too high, the fracture strength obtained was higher than the values in CLTs. The dependence of the fracture strength on crosshead speed was seen for both notched and smooth bar specimens. These results suggested that plastic deformation, i.e., dislocation motion, was related to intergranular fracture with a tear pattern as well as to quasi-cleavage fracture. In addition, cathodic electrolysis in an alkaline solution containing NaOH should be used as the hydrogen charging method to avoid the effects of corrosion.