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This study investigated the effect of the hydrogen charging time on the hydrogen embrittlement of hot-rolled and accelerated-cooling treated API X70 steel by using the electrochemical hydrogen charging method. Tensile and V-notch Charpy impact test were completed for the specimens of hydrogen charged for 0, 2, 4, and 10 h, and then microstructural observation were performed. The tensile and yield strength tended to decrease slightly and the total elongation started to decrease rapidly up to 2 h charging time and then these gradually decreased with the hydrogen charging time. The post-uniform elongation increased slightly after the holding time of 2 h due to the occurrence of two-step stress reduction phenomenon and its mechanism has suggested. The effect of the hydrogen embrittlement on impact absorption energy according to the hydrogen charging time were also investigated. The fracture surface of the hydrogen charged ones are generally composed of cleavage fracture surfaces. It was demonstrated that the external cracks were formed on the outside of tensile tested specimen and the external shape of the fracture portion was fractured without large elongation at hydrogen charged specimens.

We introduce a volumetric analysis technique to characterize the solubility of hydrogen dissolved in polymers and its diffusion coefficient by electrical capacitance measurement electrodes to determine the water level in graduated cylinders. This new and simple technique measures the volume of hydrogen released from rubber inside a graduated cylinder after a sample is exposed to high-pressure hydrogen and subsequent decompression. A diffusion analysis program is utilized to determine the total uptake (C0) and diffusivity (D) of hydrogen, which are used to calculate the solubility (S) and permeability (P). This method is applied to spherical rubber samples of nitrile butadiene rubber (NBR), ethylene propylene diene monomer (EPDM), and fluoroelastomer (FKM), which are potential sealing materials for H2 energy infrastructures. C0 follows Henry's law for all samples. No sample size and pressure dependence is observed for S, whereas an appreciable size dependence for the three rubbers is detected for D. The uncertainty is evaluated by considering uncertainty factors that affect the measurement. The correlation between D and density of rubber is found. The developed method can be utilized as a standard test for the transport properties versus pressure of various polymer membranes regardless of the sample shapes and size.

A series of fracture toughness and fatigue crack growth rate tests were performed on the X70 pipeline steel base and weld metals under 10 MPa of a natural/hydrogen gas mixture with 1% H2 blends. It is observed that the 10 MPa gas mixture with 1% H2 causes a significant reduction in the fracture toughness in both metals. The fatigue crack growth rates are markedly accelerated under the gas mixtures with 1 % hydrogen blend compared with the tests performed in ambient air. The obtained fracture parameters serve as inputs for fatigue life assessment analyses under the effect of a hydrogen-containing environment. The observed design fatigue life depends strongly only on the environmental conditions. The design fatigue strength of the structural pipeline exposed to hydrogen is much shorter than that under ambient air owing to the decrease in fracture toughness properties and acceleration of the fatigue crack growth rate.

Nickel-based superalloys have been widely used for various high-temperature and high-pressure applications such as gas-turbines, power plants, and boiler housings. In this study, we report new experimental results for thermophysical properties of René N5 alloy in a temperature range of room temperature to 1000 °C. Especially, specimens of René N5 alloy were studied in directions [001] and [111] from the same batch of commercial alloy bar. Thermal diffusivity, specific heat capacity, thermal conductivity, and coefficient of thermal expansion (CTE) with correction data of density were evaluated for each measurement method, and detailed data are provided in tables. Thermal conductivity of the [001] alloy had a higher trend than that of the [111] alloy, with relative deviation of 0.7% to 4.8%. Coefficient of thermal expansion values showed good agreement (within 5.3%) and had a curve similar to that of the specific heat capacity. All thermophysical property results were described and compared with those of single crystal alloy of CMSX-4, René N5 and conventional cast René 80 of reference data. The microstructures of alloys [001] and [111] of René N5 were observed by SEM and found to have phase of γ/γ′, which affects the thermophysical properties.

It has been reported that a multiphase microstructure with bainite, martensite, and retained austenite obtained by austempering, or quenching and tempering of spring steel containing Si, Mn, and Cr exhibits high strength and ductility. However, little research has been conducted on the bainite formation and impact fracture behavior of next-generation spring steel from the perspective of engineering and industrial applications. The microstructural transformation characteristics of bainite and martensite related to the heat treatment cycle on the maker side were quantitatively analyzed using electron backscatter diffraction (EBSD) and scanning electron microscopy (SEM) analyses. Moreover, the effects and mechanical properties of bainite and martensite formation in response to changes in lath length and width were studied and analyzed. That is, to obtain the mechanical properties of spring steel with the highest quality, tensile and impact specimens, whose microstructure and notch shape change according to the heat-treatment cycle, were prepared and studied.

The present study investigates the effects of a low partial pressure of hydrogen in a mixture with methane gas on the mechanical properties of welded API X70 pipeline steel using a series of smooth tensile and notched tensile, testing approaches. The microstructures and mechanical property performances of the weld metal are compared to the results obtained for the base metal. The weld metal was found to be more vulnerable to hydrogen-induced crack initiation than the base metal. A significant loss of ductility occurred in the weld metal, even when tested under 10 MPa of a gas mixture with 1% H2 using the smooth tensile specimen. The notched reduction in the area (RA) decreased markedly under 10 MPa gas mixtures with 1% H2. The microstructural inhomogeneity in the metal matrix was associated with a more severe hydrogen-assisted fracture in the weld metal. The hydrogen-induced crack initiation primarily occurred at the interface between the dendritic pro-eutectoid ferrite and bainitic ferrite and propagated with the dendritic ferrite in which the diffusion of hydrogen was promoted by the stress/strain incompatibility and localized deformations.

In this study, the effect of a low partial hydrogen in a mixture with natural gas on the tensile, notched tensile properties, and fracture toughness of pipeline steel X70 is investigated. An artificial HE aging is simulated by exposing the tested sample to the mixture gas condition for 720 h. In addition, a series of tests is conducted in ambient air and 10 MPa of 100% He and H2. Overall, 10 MPa of 100% H2 significantly degrades the mechanical properties of an X70 pipeline steel. However, it is observed that the 10 MPa gas mixture with 1% H2 does not affect the mechanical properties when tested with a smooth tensile specimen. In the notched tensile test, a significant reduction in loss in the area is observed when tested with a notched specimen with a notch radius of 0.083 mm. It is also confirmed that a 10-MPa gas mixture with 1% H2 causes a remarkable reduction in the toughness. The influence of the exposure time to 1% hydrogen in a mixture with natural gas was found to be minor.

An investigation of the effects of hydrogen on tensile flow, strain-hardening and fracture characteristics of low nickel-type 316L austenitic stainless steel was quantified by means of slow strain rate tensile (SSRT) tests. Both thermal precharged and gaseous hydrogen conditions will be considered. The strain-hardening parameters were derived for an idealized true-stress/true-strain curve expressed by Hollomon’s equation. It was found that hydrogen affects the strain-hardening rate and reduces the overall strain-hardening exponents with a similar tendency for diminution of reduction area and fracture strain. The severity of hydrogen embrittlement was observed to be greater in a gaseous hydrogen environment than when tested in the hydrogen pre-charged specimen. Moreover, the fracture mechanism caused by hydrogen pre-charged and gaseous hydrogen environments were different. Hydrogen-induced cracks in hydrogen were found to appear at the interior of the grain boundary, in which there was a strain-induced martensite transformation phase, and crack formation at the grain boundary was predominant gaseous hydrogen specimens.

We have investigated the volume effects on hydrogen diffusion properties in both sorption and desorption processes by employing a volumetric analysis technique. The total uptake (𝐶∞), total desorbed content (𝐶0), sorption diffusion coefficient (Ds), desorption diffusion coefficient (Dd), sorption and desorption equilibrium time of hydrogen in two rubbery polymers are determined relative to the diameter and thickness of the cylindrical sample in the two processes. 𝐶∞ and 𝐶0 do not demonstrate the appreciable volume dependence for all rubbers. The identical values in 𝐶∞ and 𝐶0 indicate the reversibility between sorption and desorption, which is interpreted by the occurrence of physisorption rather than chemisorption by introducing hydrogen molecules. The larger Dd values in the desorption process than Ds may be attributed to increased amorphous phase and volume swelling caused by increased hydrogen voids and polymer chain scission after decompression. The time to reach equilibrium in both sorption and desorption processes was found to be linearly proportional to the square of thickness above an aspect ratio of 3.7, which is consistent with the numerical simulations based on the solution of Fick’s law. This finding could be used to predict the equilibrium adsorption time depending on the sample size in the polymer.

Bearing steel is essential for the driving parts of the rotating body, but fisheye crack of inclusions such as TiN, Al2O3 and Cr matrix is formed in the very-high-cycle fatigue (VHCF) regime. Therefore, research on the development of a clean bearing steel that reduces the size of inclusion such as TiN or that does not contain inclusion has been made recently. In this study, the results of long-term attempts by the maker to improve the performance and quality of bearing steel for very long life were quantitatively analyzed from the viewpoint of fracture mechanics. In other words, the phase transformation properties, VHCF characteristics with ultrasonic nanocrystal surface modification (UNSM) treatment and fracture mechanics analysis of this clean bearing are compared quantitatively with those of other common bearing steels from the viewpoint of engineering and industrial applications. The quenching and tempering (QT) specimens of the clean bearing showed fisheye cracks and duplex [Formula: see text]–[Formula: see text] curves in the VHCF regime. The fisheye crack of the clean bearing steel was an internal fatigue crack caused by a small inclusion mainly composed of Al2O3 in the VHCF region. However, fisheye cracking due to TiN and Cr matrix, which was observed in other common bearing steels, did not occur in the clean bearing steel due to the efforts of the developers of new clean bearing.

A simple screening technique of hydrogen embrittlement (HE) of steels for hydrogen energy facilities using an in-situ small-punch (SP) test method under external hydrogen condition has been established. Using this method, we investigated the HE behaviors of API X70 steel welds under test conditions mimicking pipeline operating environments. The HE sensitivity at each region of the base metal, heat-affected zone, and weld metal of the steel weld was examined qualitatively and quantitatively and compared to one another. Under 8 MPa H2 gas, the welds showed pronounced HE: the HE behaviors induced were similar across all tested regions. This shows that the in-situ SP test is effective for screening HE susceptibility of pipeline steel welds in high-pressure hydrogen environments.

We report volumetric analysis techniques to analyze the transport properties of hydrogen dissolved in cylindrical-shaped polymers. The techniques utilize the volume measurement of the released hydrogen from rubber by gas collection in a graduated cylinder after charging sample with high-pressure hydrogen and subsequent decompression. We further improve the graduated cylinder with some modifications, such as reading the electrical capacitance of the water level using electrodes and changing the sample loading position. From the measurement results, the uptake (C∞), diffusion coefficient (D), and solubility (S) of hydrogen are quantified with an upgraded diffusion analysis program. These methods are applied to three cylindrical rubbers. Dual adsorption behaviors with increasing pressure are observed for all the samples. C∞ follows Henry's law up to ~15 MPa, whereas Langmuir model applies up to 90 MPa. D shows Knudsen and bulk diffusion behavior below and above pressure, respectively. A COMSOL simulation is compared with experimental observations.

The susceptibility of low nickel content type 316L austenitic stainless steel to hydrogen was quantified using low strain rate tensile tests and strain-controlled low-cycle fatigue life measurements. Both tests were performed under air condition after charging with high-pressure 10-MPa hydrogen gas at 300 °C for eight days. No significant influence of hydrogen was recognized in 0.2% proof stress, but the strain at fracture and reduction area was decreased significantly in both hydrogen pre-charged and in gaseous hydrogen conditions compared to companion tests conducted in air. The decrease of fatigue life in the high strain amplitude region was related to a significant decrease in the plastic component while the effect of hydrogen on the elastic component was negligible. Highly localized deformation and a pronounced martensite transformation occurred near the site of the fracture surface in the high strain amplitude regime, resulting in the early formation of abundant micro-surface cracks in this regime of the hydrogen pre-charged samples.

Continuous fibers produced from carbon nanotube (CNT) rugs through direct spinning find wide applications owing to their excellent mechanical and physical characteristics. However, the prac...

The hydrogen embrittlement (HE) behaviors of structural materials used to handle hydrogen must be tested at their use environments. In this study, an in-situ small punch (SP) test method was established to characterize HE behaviors of SA372 and STS304 steels under hydrogen gas environments at high pressures and low temperatures. In addition, a new influencing factor, relative reduction of thickness (RRT) was proposed to quantify the HE sensitivity of structural steels. Under 10 MPa H2 gas environment, load-displacement curves obtained and fractographic morphologies of the recovered specimens were analyzed to investigate the HE behaviors based on punch velocity and test temperature. When the HE sensitivity was evaluated by RRT, STS304 steel appeared to be vulnerable to HE as compared to SA372 steel across the tested temperature range. As a result, it was found that in-situ SP test and RRT can be used to quantify the HE sensitivity for structural materials screening regardless of test environments.

Low-temperature mechanical properties of a high-manganese austenitic steel were evaluated with and without hydrogen pre-charging to examine the applicability of the alloy as a material for hydrogen infrastructure. The high-manganese steel, along with the conventional 304 and 316 L austenitic steels, was examined for hydrogen-related properties including hydrogen content after gas-phase pre-charging, tensile properties, and Charpy impact toughness at different temperatures ranging from room temperature to −80 and −196 °C, respectively, and the resultant fracture surfaces. Under hydrogen-charged conditions, the high-manganese steel showed low-temperature mechanical properties comparable to those of conventional austenitic steels, suggesting the potential of the alloy for structural applications in hydrogen environment.

Recently, a simple screening technique based on the quantitative evaluation of the hydrogen embrittlement (HE) sensitivity of the metallic materials using an in-situ small-punch (SP) test method was developed by the author group. The in-situ SP test can be easily carried out even under a high-pressure hydrogen gas environment. It makes possible to investigate the HE behaviors of metallic materials quantitatively adopting as a characterizing performance factor of the relative reduction of thickness (RRT) measured at the fractured parts of specimen after SP tests. In this paper, the application of the newly established in-situ SP test method for the hydrogen compatibility screening of austenitic stainless steels was performed at room and low temperatures. The influence of punch velocity on RRT of the HE sensitivity was examined for various austenitic stainless steels. Their HE sensitivities were evaluated quantitatively using RRT and checked by comparing to a factor, the relative reduction of area (RRA) obtained by SSRT tests.

The hydrogen-trap states in TiC and MoC that have coherent interfaces with ferrite were investigated using first-principles calculation. The trapping sites of TiC were the interfaces and interstitial sites of ferrite. On the other hand, the trapping sites of MoC were ferrite interstitial sites; the interface had a negative binding energy with H. Thermal desorption analysis confirms that the amounts of diffusible hydrogen were significantly reduced by addition of Mo in Ti-bearing steel.

In this study, carbon fiber was exposed to a pressure of 7 MPa for 24 h in high pressure chamber. The tensile test for carbon fiber was conducted to estimate the effect on the high pressure hydrogen in the atmosphere. To determine the tensile strength and Weibull modulus, approximately thirty carbon fiber samples were measured in all cases, and carbon fiber exposed to high pressure argon was evaluated to verify only the effect of hydrogen. Additionally, carbon fiber samples were annealed at 1950 °C for 1 h for a comparison with normal carbon fiber and then tested under identical conditions. The results showed that the tensile strength scatter of normal carbon fiber exposed to hydrogen was relatively wider and the Weibull modulus was decreased. Moreover, the tensile strength of the annealed carbon fiber exposed to hydrogen was increased, and these samples indicated a complex Weibull modulus because the hydrogen stored in the carbon fiber influenced the mechanical characteristic.

The influence of hydrogen embrittlement on the fatigue behaviors of AISI 304 stainless steel is investigated. The fatigue endurance limits of the untreated and hydrogen-embrittled materials were almost the same at 400 MPa, and hydrogen embrittlement had little influence even though the sample contained about 8.1 times more hydrogen. Thus, the sensitivity of hydrogen gas in this material is very low. A surface crack initiation, growth, coalescence, and micro ridge model is proposed in this study. Slip line formation ⇒ microcrack formation ⇒ increases in the crack width, and blunting of the crack tip as it grows ⇒ formation of many slip lines because of deformation in the shear direction ⇒ growth of the crack in the shear direction, forming micro ridges, coalescence with adjacent cracks ⇒ continuous initiation, growth, coalescence, and ridge formation of surface cracks and specimen breakage.

본 연구는 실온에서 미처리재와 UNSM처리재, 수소취화재(100 bar, 300℃에서 120 h) 및 이 수소취화재의 UNSM처리재의 스테인리스강 316L 시험편의 재료특성파악을 위한 실험을 실시하였다. 여기서 수소취화재는 기존의 미처리재의 S-N곡선보다 피로강도가 약간 감소하는 경향을 나타내었으며, 수소취화효과는 적게 나타났다. 미처리재에 비해서 UNSM처리재의 피로한도는 약 43.8 % 상승하였고, 수소취화재의 UNSM처리재는 수소취화재보다 약 57.1 % 상승하는 경향을 나타내었다. EBSD에 의한 IQ, IPF 및 KAM 맵에서 UNSM처리 후, 이 처리의 영향을 받는 나노 입자 표면층의 두께는 약 152 μm이다. 그러나 세 가지 맵에서 수소취화의 깊이 영향을 정량적으로 평가할 수 없었다. 시험편상에서 발생하는 표면균열은 수소취화의 영향으로 평균입경(35 μm)보다 작은 균열의 비율이 약 90 %를 차지하였다.

This study investigates the effect of a hydrogen/natural gas mixture on the mechanical properties of API X42 pipeline steel using small punch (SP) test. The hydrogen-assisted fracture behavior demonstrated a transition from ductile to brittle failure, similar to the temperature-assisted brittle fracture mode. The preferential sites for hydrogen-assisted crack initiation in the X42 pipeline steel were located at the ferrite/pearlite interface, where hydrogen diffusion is enhanced by localized stress/strain conditions. The feasibility of the hydrogen environment embrittlement susceptibility obtained in the SP test and the correlation to the conventional test method is also discussed.

This paper reports simulation and evaluation of the magnetic flux density (MFD) of a rotating Halbach magnet (RHM) composed of a rotating inner Halbach cylinder (HC) and a fixed outer HC. We first chose the dimension of a single HC, then determined the dimensions of an inner and outer HC to get MFD = 2 T. Simulation results were used to guide fabrication of an RHM; its magnetic field in the central axis direction agreed well with simulated values.