Hydrogen-charged Specimens Research Articles

Combined impact of elastic stress, prestrain and electrochemical charging on the hydrogen-induced cracking of high-strength steel

The elastic stress, prestrain and electrochemical hydrogen charging were controlled separately using a stress ring to investigate their roles in the initiation of hydrogen-induced cracks. The brittle features of hydrogen charging-induced damages, i.e., a mixture of quasi-cleavage and intergranular cracks, on the fracture surface were confirmed for a high-strength steel, made possible by applying degassing and tension-to-fracture to the hydrogen-charged specimens. The hydrogen charging-induced cracks eliminated the ductility of material, leading to premature fracture before the yield point in subsequent tensile tests. The strong dependence of hydrogen-induced cracking sensitivity on hydrogen concentration and hydrogen charging time was observed. X-ray microtomography and tensile tests were also utilized to investigate the effect of inclusions on crack formation. This study contributes to the understanding of the combined effects of residual stress and hydrogen on the cracking of deformed steel plates.

Temperature-Dependent Hydrogen Embrittlement of Austenitic Stainless Steel on Phase Transformation

A critical issue that needs to be addressed for wider utilization of hydrogen as fuel is protection against hydrogen embrittlement during cryogenic storage as it weakens the microstructure bonding force of metals through hydrogen penetration. Austenitic stainless steel, which is usually used in cryogenic vessels and is well known for its high hydrogen resistance at room temperature, has also been reported to be vulnerable to hydrogen embrittlement under cryogenic temperatures. In addition, because large storage vessels are operated over a wide range of temperatures, material behavior at various temperature conditions should also be considered. Therefore, in the present study, hydrogen charging of austenitic stainless steel was performed under various temperature conditions for carrying out prestrain and tensile tests. A decrease in the tensile strength and elongation and an increase in the yield strength were observed in all cases. In particular, the case of 20% prestrain at cryogenic temperature followed by tensile test at room temperature after hydrogen charging showed fracture in the elastic region. The hydrogen index was evaluated from the perspective of elongation and reduction in area, which are factors that indicate the degree of ductility. The aforementioned case showed the most severe results, while non-prestraining followed by tensile tests at room temperature was the least effected by hydrogen. In addition, the effect of strain-induced martensite on hydrogen embrittlement was analyzed using electron backscattered diffraction (EBSD). It was observed that the higher is the prestrain at cryogenic temperatures, the greater is the volume fraction of α’ martensite, which leads to hydrogen embrittlement. The edges and center of the fracture surface were analyzed using scanning electron microscopy (SEM). The hydrogen-charged specimens exhibited brittle fractures at the edges and ductile fractures at the center. The more severe the embrittlement, the more were the number of intergranular fractures and microdimples observed at the edges.

The Effect of Strain Rate on the Hydrogen Embrittlement Susceptibility of Aluminum 7075

Abstract The effects of changing the strain rate regime from quasi-static to medium on hydrogen susceptibility of aluminum (Al) 7075 were investigated using tensile tests. Strain rates were selected as 1 s−1 and 10−3 s−1 and tensile tests were conducted on both hydrogen uncharged and hydrogen charged specimens at room temperature. Electrochemical hydrogen charging method was utilized and the diffusion length of hydrogen inside Al 7075 was modeled. Material characterizations were carried out by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) and microstructural observations of hydrogen uncharged and hydrogen charged specimens were performed by scanning electron microscope (SEM). As opposed to earlier studies, hydrogen embrittlement (HE) was more pronounced at high strain rate cases. Moreover, hydrogen enhanced localized plasticity (HELP) was the more dominant hydrogen embrittlement mechanism at slower strain rate but coexistence of hydrogen enhanced localized plasticity and hydrogen enhanced decohesion was observed at a medium strain rate. Overall, the current findings shed light on the complicated hydrogen embrittlement behavior of Al 7075 and constitute an efficient guideline for the usage of Al 7075 that can be subject to different strain rate loadings in service.

Metallic Material Evaluation of Liquid Hydrogen Storage Tank for Marine Application Using a Tensile Cryostat for 20 K and Electrochemical Cell

A series of material tests were performed on cryogenic metallic materials meant for liquid hydrogen storage tanks using a 20 K tensile cryostat and an electrochemical hydrogen-charging apparatus. Mechanical evaluation of the electrochemically hydrogen-charged specimens was performed in a tensile cryostat using helium gas at ambient temperature and cryogenic temperature (20 K). The tensile cryostat was equipped with a vacuum jacket and a G-M cryocooler with gaseous helium. Furthermore, the cathodic electrolysis cell used for charging the specimens was adopted for internal hydrogen conditions with a reflux condenser and heating mantle to increase hydrogen diffusivity. The target materials were austenite stainless steel and aluminum alloy, which are suitable for liquefied natural gas and gaseous hydrogen environments. No significant change in the yield strength and flow stress of the hydrogen-charged specimen up to 20% strain was observed. However, changes in tensile strength and elongation were observed thereafter. Electrochemical hydrogen charging of stainless steel leads to a high concentration of hydrogen on the surface of the specimen. The resulting surface cracks reduced the flow stress. The 20 K tensile test showed discontinuous yielding in the austenitic stainless steel with an abrupt increase in temperature. The mechanical performance of the aluminum alloys improved in terms of strength and elongation. Changes in the mechanical performance and relative area reduction were observed for all the metallic materials at 300 K and 20 K.

Improvement of resistance against hydrogen embrittlement by controlling carbon segregation at prior austenite grain boundary in 3Mn-0.2C martensitic steels

This study challenged to improve the resistance against hydrogen embrittlement by increasing the concentration of carbon segregated at prior austenite grain boundary (PAGB), XPAGB, in low-carbon martensitic steels. The specimens with/without carbon segregation treatment (Non-seg and Seg specimens, respectively) had almost the same microstructure, other than higher XPAGB in the Seg specimen. While the uncharged Non-seg and Seg specimens exhibited similar mechanical properties, the maximum stress of the hydrogen-charged specimen was much higher in the Seg specimen than that in the Non-seg specimen even when diffusible hydrogen contents were almost the same. In addition, the fraction of intergranular fracture surface was much smaller in the Seg specimen. Based on these results, we conclude that the segregated carbon suppressed the accumulation of hydrogen around PAGB by site competition and increased cohesive energy of PAGB, leading to the significantly improved resistance against hydrogen-related intergranular fracture.

Effect of hydrogen on low-cycle fatigue behavior of HRB400 steel under asymmetric cyclic loading

This paper focuses on the study of ductility changes, ratcheting, and low-cycle fatigue behavior of materials after hydrogen charging. Hot-rolled ribbed bar 400 (HRB400) steel was electrochemically charged with H2SO4 solution, and the samples subjected to different hydrogen charging times were subjected to quasi-static tensile and low-cycle fatigue tests under asymmetric cyclic loading. The tests indicated that hydrogen charging changed the initial yield stress of the material, but the ultimate strength remained almost unchanged; hydrogen charging significantly changed the cyclic hysteresis behavior of the material, which increased the elastic range of the stable hysteresis curve; hydrogen charging changed the pattern of ratcheting deformation. The pattern of ratcheting deformation in the later stage was very different from that of the sample without hydrogen charging; hydrogen charging reduced the ductility of the material and resulted in a significant decrease in fatigue life. The fracturing of the hydrogen-charged samples exhibited apparent quasi-cleavage and intergranular fracture characteristics. The non-hydrogen-charged samples had two failure modes (fatigue failure and increased ratcheting deformation resulting in necking instability), whereas the hydrogen-charged samples had only one failure mode of fatigue failure. Based on the experimental analysis, a fatigue life equation that simultaneously considers the effect of hydrogen embrittlement index, mean stress, and stress amplitude is proposed. The equation was used to predict the low-cycle fatigue life of hydrogen-charged specimens under asymmetric cyclic loading, and the error between the results and the measured values was in the two-factor band.

Hydrogen Embrittlement Behavior of Plastically Pre-Strained and Cathodically Hydrogen-Charged 316H Grade Austenitic Stainless Steel

In this work, the effects of electrochemical hydrogen charging of 316H grade austenitic stainless steel were investigated in order to characterize its hydrogen embrittlement (HE) resistance. The as-received 316H material was in a fully recrystallized (solution-annealed) material condition. The susceptibility to HE of the studied material was evaluated by determination of the embrittlement index from the results of conventional uniaxial tensile tests of nonhydrogenated and hydrogen-charged test specimens. The study was focused on the effects of two selected plastic pre-strain levels of tensile specimens on their resulting HE resistance. The selected pre-strains corresponded to the tensile stress conditions within the “yield stress–ultimate tensile strength” (YS–UTS) range and directly at the UTS point. The obtained embrittlement indices for the presently used pre-straining and hydrogen charging conditions indicated that the HE of the studied material states was small. However, it was revealed that the observed degradation of deformation properties of plastically pre-strained and hydrogen-charged materials was mainly caused by gradual plasticity exhaustion due to tensile straining, which well correlated with the observed effects indicated by electron backscatter diffraction analyses and indentation hardness measurements.

Hydrogen embrittlement susceptibility of dual-phase DP980 steel and its prevention with the help of coatings

ABSTRACTAHSS DP980 is widely used in automobiles and is susceptible to hydrogen embrittlement (HE), which negatively impacts the mechanical properties of the material. The aim of this work was to prevent the DP980 steel against the HE with the help of various surface coatings. Hydrogen charging of bare and various coated samples was accomplished by electrochemical reactions at a current density of 40 mA/cm2 for 24 hours. After charging, SSRT testing was performed at a strain rate of 10−5/s. Coating characterisation was completed by Coating Porosity Test, EDX and Microhardness test. Test results stated that the Ni-Cr and Gr ACS coating effectively reduces the hydrogen diffusion inside the material with improved mechanical properties, and the HE ratio decreases from 43.3% for the bare sample to 9.6% and 7.3% for those coated samples, respectively. The microhardness test also gave a high microhardness value for these coatings. Fracture surface examination shows that the bare hydrogen charged specimen failed in a mixed-mode manner, whereas all regions of Ni-Cr and Gr ACS-coated specimen fracture surface comprise dimple rupture.

The Mechanism of the High Resistance to Hydrogen-Induced Strength Loss in Ultra-High Strength High-Entropy Alloy

The resistance of the Al0.5Cr0.9FeNi2.5V0.2 high-entropy alloy (HEA) to hydrogen embrittlement (HE) was investigated by a slow strain rate test (SSRT), and the fracture surface was examined through a scanning electron microscope. Compared with other high-strength steels, Al0.5Cr0.9FeNi2.5V0.2 showed insignificant strength loss after hydrogen charging. The fracture surface of the hydrogen-charged specimens mainly consisted of dimples, and no intergranular morphology was observed. The coupling effect of the dispersed nano-structured precipitates and high-density dislocations in Al0.5Cr0.9FeNi2.5V0.2 improves the resistance to hydrogen-induced strength loss.

Study on hydrogen embrittlement and reversibility of hot-stamped aluminized 22MnB5 steel

This study investigated the hydrogen embrittlement behavior of ultrahigh strength hot-stamped aluminized 22MnB5 steel. Hydrogen was introduced through electrochemical charging under different hydrogen conditions to assess their sensitivity to hydrogen-induced cracking. Then, the hydrogen-charged specimens were subjected to the slow strain-rate tensile tests. The fracture morphologies revealed that the charging current density altered the fracture mechanism translated from ductile (uncharged) to quasi-cleavage (0.001A-2h) and a mixture of intergranular and quasi-cleavage fracture (0.1A-2h). Furthermore, the reversibility of hydrogen embrittlement of aluminized specimen was investigated. At low-current density, the coatings were corroded by electrolyte, resulting in additional hydrogen which facilitated the formation of hydrogen-induced cracking and inhibited the restoring of mechanical properties. However, with high-current density, effective cathodic protection ensured the integrity of the surface coating, effectively prohibiting the hydrogen intrusion and promoting the reversibility of hydrogen embrittlement.

Effects of hydrogen on the crevice corrosion behaviors of duplex stainless steel 2205

Crevice corrosion behaviors of duplex stainless steel 2205 after hydrogen charging are investigated. The absorbed hydrogen is introduced into the specimens through cathodic galvanostatic method. It has been found the absorbed hydrogen decreases the critical crevice corrosion conditions (breakdown potential, temperature, and chloride ions content), the stable propagation potential, and the repassivation potential. The results reveal that hydrogen can promote the initiation and propagation stages and inhibit the repassivation process of crevice corrosion. Microstructure observation and element distribution analysis find that, owing to the promotion of hydrogen on the precipitation of insoluble molybdenum oxides and molybdate at the bottom of corrosion positions, crevice corrosion of the hydrogen charged specimens exhibits wider but shallower morphologies compared with the uncharged specimens.

Synergistic effects of hydrogen and deformation temperature on mechanical properties of TRIP-aided bainitic ferrite steel

Mechanical properties of TRIP-aided steel with bainitic ferrite matrix in the presence of hydrogen were investigated at the deformation temperature range of −100 to 100 °C. In hydrogen-uncharged condition, the yield strength (YS) first increased and then remained almost unchanged, while the highest uniform elongation (UEl) was observed at a deformation temperature of 21 °C. Hydrogen charging slightly increased the YS and deteriorated the UEl at all the temperature ranges. In the hydrogen-uncharged and hydrogen-charged specimens, the transformation ratio of retained austenite increased with a decrease in the deformation temperature; however, the hydrogen charging decreased the transformation ratio. The fracture behavior drastically changed when hydrogen was introduced, owing to which deformation-induced martensitic transformation occurred and hydrogen played dual roles in accelerating brittle-like cracking.

Hydrogen absorption due to wear between Ni–Ti superelastic alloys in water

Hydrogen absorption behavior has been investigated during wear between Ni–Ti superelastic alloys in purified water at room temperature by a hydrogen thermal desorption analysis. For the present wear test under a rotation speed of 100 rpm and an applied load of 2.0 N, the amounts of mass and thickness losses of a specimen increase without marked corrosion with increasing wear time. Even after 0.5 h, there is sufficient hydrogen absorption (approximately 12 mass ppm) to be detected. The amount of desorbed hydrogen (absorbed hydrogen) increases with increasing wear time. The hydrogen desorption behavior of wear specimens is not necessarily the same as those of cathodically hydrogen-charged specimens and specimens that absorbed hydrogen in acid solutions, as reported previously. The residual martensite phase and hydride are observed after the wear test. The present results indicate that Ni–Ti superelastic alloy absorbs a substantial amount of hydrogen, which causes hydrogen embrittlement, during wear in water within a short time.

Failure analysis in API X70 pipeline steel in sour environment with an emphasis on fracture surfaces and crack propagation

The aim of this study is to investigate the fracture surfaces and crack propagation in an API X70 pipeline steel in acidic environment. To this purpose, as-received and electrochemically hydrogen-charged tensile specimens are subjected to tensile testing. Moreover, EBSD measurements were carried out on the cross section (RD-ND plane) in as-received and hydrogen-charged specimens. The results showed that hydrogen charging and strain rate of tensile testing were considered as two effective parameters in determining the type of fracture. Moreover, the fracture of the uncharged specimens was quite ductile and the strain rate had little effect on the fracture type. A direct correlation was found between type of fracture and the strain rate in hydrogen-charged specimens. With increasing strain rate, the fracture in hydrogen-charged specimens did not have enough time to initiate from inclusions and precipitates and connect to the main fracture surface. <001>∥ND oriented grains have the highest hydrogen atoms due to the largest interatomic space and the distortion energy accumulated at grain boundaries due to the lack of sufficient activated slip system. The variation of KAM distribution to higher values of hydrogen-charged specimen could be effectively associated to the formation of dislocation walls and dislocation tangles especially in the vicinity of bcc grain boundaries. The higher volume fraction of the grains with high Taylor factor data formed in the hydrogen-charged specimen were highly susceptible to plastic instability, enhancing the crack propagation.

Effect of the inner liner on the hydrogen distribution of zircaloy-2 nuclear fuel claddings

Hydrides precipitation has been induced by slow cooling of hydrogen-charged specimens of three different Zircaloy-2 claddings commonly used in boiling water reactors. The resulting hydrides distribution has been analysed by optical and scanning electron microscopy and neutron radiography. With the latter technique, it is possible to quantify radially the hydrogen concentration with a resolution of ±10 wppm and a spatial resolution of 10 µm. Hydrogenated samples were subjected to a temperature of 400°C and afterwards a cooling rate of 0.3°C/h and 10°C/h showed that practically a complete migration of the hydrogen towards the liner-cladding interface took place, leading to very high local hydrogen concentrations exceeding 2000 wppm.

Differences between internal and external hydrogen effects on slow strain rate tensile test of iron-based superalloy A286

To investigate the evaluation method of hydrogen compatibility of A286 superalloy in high pressure hydrogen gas, SSRT tests of hydrogen-charged specimens were conducted at ambient temperature at various strain rates. The relative reduction in area (RRA), one of the ductility parameters, was determined. The hydrogen content in the hydrogen-charged specimen was the same as the equilibrium hydrogen content on the specimen surface at 150 °C in 70 MPa hydrogen gas. The strain rate dependence of RRA was smaller than that of RRA obtained in 70 MPa hydrogen gas at 150 °C. All the hydrogen-charged specimens showed slip-plane fractures in the grains in their cores. However, the specimens in 70 MPa hydrogen gas at 150 °C showed fracture surfaces morphology ranging from dimples to quasi-cleavages and intergranular fractures with decreasing strain rate. These dissimilarities are expected to arise from differences in the hydrogen concentration behaviors of the specimens during the deformation process.

Relationship between mechanical response and microscopic crack propagation behavior of hydrogen-related intergranular fracture in as-quenched martensitic steel

The present study investigated the relationship between the mechanical response and microscopic crack propagation behavior of hydrogen-related intergranular fractures in high-strength martensitic steel. In contrast to cracks in the uncharged specimen, the cracks in the hydrogen-charged specimen propagated under a small crack opening displacement. Crack tip bluntings were frequently observed in the uncharged specimen, whereas no obvious blunting of the tip was observed in the hydrogen-related intergranular cracks. In addition, a high strain was localized around the hydrogen-related intergranular crack tip. The results indicate that strain localization can induce the formation of new quasi-cleavage cracks inside prior austenite grains ahead of the existing crack tip. The hydrogen-enhanced decohesion at prior austenite grain boundaries and the quasi-cleavage crack formation/propagation ahead of the arrested intergranular crack result in hydrogen-related crack propagation with a small crack opening displacement.

Characteristics of hydrogen-related fatigue fracture in 2Mn-0.1C martensitic steel

This study investigated the hydrogen-related fatigue fracture in 2Mn-0.1C steel having a lath martensite microstructure. The presence of hydrogen significantly reduced the fatigue life. The transgranular surface was a main component in each stress range of the uncharged specimen, while the intergranular surface was frequently observed in the hydrogen-charged specimen. The crystallographic orientation analysis by electron backscattering diffraction revealed that the cracks mainly propagated along {011} planes regardless of the presence of hydrogen. Compared with the uncharged specimen, however, plastic deformation was localized near the fatigue crack in the hydrogen-charged specimen. According to the reconstructed fracture process by fracture surface topography analysis, the hydrogen-related fatigue fracture was discontinuous and composed of isolated nucleation of intergranular cracks and quasi-cleavage crack propagation initiated at the pre-existing intergranular cracks.

A comparative fracture analysis on as-received and electrochemically hydrogen charged API X60 and API X60SS pipeline steels subjected to tensile testing

In this study, a comparative analysis of fracture surfaces in API X60 and API X60SS pipeline steels is carried out after hydrogen charging followed by tensile testing. The JIS test setup is also utilized in order to measure the amount of discharged hydrogen through the microstructure of both steels. EBSD measurements are also done on as-received and hydrogen charged specimens in order to observed the HIC propagation. The results show that a considerable amount of hydrogen is able to diffuse through the microstructure of both steels. The tensile testing after hydrogen charging proves that hydrogen embrittlement occurs significantly in both steels. At the center of cross section in both steels, the fracture occurs by micro-voids formation, their growth and coalescence around the different types of inclusions. In charged specimens, dimples are shallower in both specimens compared with the dimples in the fracture surfaces of as-received specimens. Comparatively, hydrogen embrittlement in API X60 steel is more noticeable and its surface is more flat than the API X60SS steel. In hydrogen-charged API X60 steel, EBSD map shows a crack stepwise jumping with 45° from one crack traction line to another, with both intergranular and transgranular crack propagation mechanisms. Moreover, the crack propagates intergranularly along gains with mismatch Taylor values, while transgranular propagation mostly observed within the hard grains with high amount of dislocation density and internal energy associated with KAM map. In the case of API X60SS steel, no evidence of HIC crack was observed.

Tensile behavior of pre-stress corroded and post hydrogen embrittled spring steel

Stress Corrosion Cracking (SCC) and Hydrogen Embrittlement (HE) are two major degradation phenomena experienced by spring steel. The consequences of these phenomena have an impact both on the mechanic response, sufficiency, and life of the mechanical system. The susceptibility to premature failure is never the same and is unique for different environment combinations. There will be a combined effect of these phenomena. Here in this experimental investigation, an attempt is made to understand the tensile behavior of combined treatment. The pre-stressed specimens were corroded and post-treated with the HE process. Traditional tensile testing was used to investigate the combined effect of corrosion and post-treated HE. From the test results, it can interfere that the post hydrogen charged specimen exhibits ductility loss, but recover in the ultimate strength, and increase in yield strength. The dominance of recovery was also shown by an increase in fracture strength and toughness. The scanning electron microscope was used to assess the mechanism and to characterize it.