The lattice defects, especially vacancies, formed during tensile deformation in a hydrogen environment have been evaluated by positron annihilation lifetime spectroscopy (PALS). The results from several such evaluations in previous studies in hydrogen-charged iron, steels, and Ni-based alloys are reviewed in this study with reference to hydrogen embrittlement models. A strong tendency to increase the positron lifetime for the vacancy cluster component, that is, the larger the vacancy cluster size, the lower the fracture strains, was found in many PALS studies on tensile-deformed metals. This suggests that plastic strain localization, a characteristic feature of hydrogen embrittlement, is consistent with hydrogen-enhanced vacancy clustering during plastic deformation. Early studies suggested that hydrogen precharging would result in a significant increase in the vacancy density, as inferred from the hydrogen content obtained from thermal desorption analysis (TDA). However, recent PALS studies have been negative, as no significant increase in vacancy density were observed.
The electrochemical hydrogen penetration measurement technique, to which a sinusoidal perturbation method was applied, was modified using a signal containing multiple frequency components. The technique was successfully applied to measurement of the hydrogen diffusion coefficient in a ferric sheet specimen. A series of numerical calculations for the technique, in which the constituent frequencies of the signal were selected from the measurement result, also provided the same diffusion coefficient and verified the validity of the technique. The use of this technique enables rapid determination of the hydrogen diffusion coefficient in a specimen.
High strength steel sheets are increasingly being used due to the growing need to improve fuel efficiency by reducing vehicle weight. Steel sheets are usually used as automotive parts formed by pressing, etc., and complicated strain is generated in the steel sheet after forming. This strain is thought to affect the occurrence of hydrogen embrittlement. In this study, a new hydrogen embrittlement evaluation method using a forming limit diagram was devised. A forming limit diagram of a steel sheet with a strength level adjusted to 1470 MPa was prepared for uniaxial, plane strain and biaxial modes, and stress and hydrogen were applied to the specimens formed with respective strain modes to evaluate the occurrence of fracture. In order to examine the relationship between the hydrogen content and the cracking, evaluation of the hydrogen content by Thermal Desorption Analysis, visualization of hydrogen by Secondary Ion Mass Spectrometry and microstructure analysis by Electron BackScatter Diffraction were carried out. It was found that cracks are generated in the strain mode in which hydrogen accumulates locally in the steel even when the apparent hydrogen content is small, and it was clarified experimentally that evaluation of local hydrogen concentration is important for the evaluation of hydrogen embrittlement.
The research aimed to detect the rate of hydrogen absorption into Fe with rust layer during atmospheric corrosion in humidity-controlled air, and to realize the effect of relative humidity (RH) on hydrogen absorption rate. One side of an Fe plate specimen was covered by electrochemical Ni plate and the other side was covered with rust layer containing NaCl. The specimen was set between the double cells for electrochemical hydrogen permeation test. The cell for hydrogen detection was filled with 1 kmol·m−3 NaOH solution and the Ni side of the specimen was subjected to 0 VAg/AgCl in the solution. The cell for hydrogen absorption was filled with the air with a controlled RH to make the rust layer side corrode. During the corrosion, a hydrogen absorption current and an RH were continuously monitored. In the tests, the following results were obtained. In the region of RH between 42 and 74%, a hydrogen absorption rate increased with an increase in an RH. At an RH of 80%, a hydrogen absorption rate suddenly decreased. In the region of RH between 80 to 95%, a hydrogen absorption rate again increased with an increase in an RH. The pH in the rust layers during the corrosion under the tested RH range was estimated to be 4.2 and 4.3, slightly acidic.
Summary of relation between stable hydrogen absorption rate (iH) and an RH under atmospheric corrosion.
In order to clarify the effect of metal cations (Zn2+, Mg2+, Na+) in aqueous solution on hydrogen absorption into iron, the amount of hydrogen absorption from iron surface was measured by electrochemical tests with a laser ablation. Moreover, in order to obtain the basic mechanism of hydrogen absorption with adsorption of metal cation, we obtained the adsorption potential of the adsorbed atom and the electronic state around the adsorbed atom using first principles calculations. Peak value of permeation current and the time until the current reached the peak value decreased and elongated in the order of NaCl, MgCl2, and ZnCl2 solutions. Also, by first-principles calculations adsorption strength of each metal atom increased in the order of Na < Mg < Zn. It was suggested that dissolution of Fe is inhibited due to formation of dense metal layer in the solution including the metal cation which has large adsorption strength to Fe surface like Zn, and finally permeation current may have been reduced.
In order to clarify the mechanism of hydrogen embrittlement in high strength steel, it is necessary to evaluate local hydrogen concentration in stress and strain fields where hydrogen embrittlement is considered to occur. There are several ways to visualize hydrogen, but SIMS is advantageous in that it can detect hydrogen directly without reaction. In this study, visualization of hydrogen accumulated in stress and strain field was tried using SIMS. Hydrogen flux intensity in the bent portion of the U-bend specimen, where the stress gradient exists, changed according to the stress gradient, and the hydrogen flux intensity was higher on the outside of the bending than on the inside, with the center of the plate thickness at the boundary. On the shearing surface with strain field, the hydrogen flux intensity had a positive correlation with the stress. On the other hand, it remained constant with respect to the strain. From these observations, the hydrogen partitioning behavior in steel could be visualized semi-quantitatively by using the isotope labeling method with SIMS.
Appearance of the U-bend specimen (d = 10.0 mm) and hydrogen flux intensity (2D−/72FeO− image).
The influence of scratch size on the hydrogen permeation behavior of Zn coated steel during wet and dry cycle corrosion tests was investigated electrochemically. A size controlled scratch was formed on Zn coated steels with a pulsed YAG laser machining technique. The hydrogen permeation current was observed independent of formed scratch size. The periodic changes of the hydrogen permeation currents following the cyclic changes of the relative humidity were observed. After the tests, the red rust and zinc-hydroxychloride (simonkolleite) were observed at initially placement of NaCl solution. The total amount of permeated current did not proportionally increase with increasing the formed scratch area, indicating that the corrosion products played an important role in the hydrogen permeation behavior during wet and dry corrosion.
An evaluate method for hydrogen embrittlement property of high strength steel sheet has been proposed in this study. To take into consideration of the effect of plastic strain in addition to the effects of applied/residual stress and diffusible hydrogen, U-bend specimens have been adopted because steel sheets for automobiles are usually used after press forming into various parts. After U-shape bending, the specimen was loaded using a bolt. The proposed evaluation method is based on the measurement of critical hydrogen content or critical hydrogen charging condition for hydrogen embrittlement fracture at given stress and strain conditions. The hydrogen charging current density was increased in step-wise manner until cracking was observed, and cracking was detected by optical observation and by monitoring voltage between the sample and a counter electrode. The critical hydrogen contents for specimens with varied applied stress were obtained by means of thermal desorption spectroscopy. For the critical hydrogen content, both the hydrogen contents in strained portion of the specimen and no-strained portion were measured. The former is affected by introduced dislocations caused by straining and the latter is thought to be proportional to the hydrogen fugacity. Both critical hydrogen contents tended to be decreased slightly when the applied stress was relatively high.
Microelectrodes for hydrogen permeation measurements were fabricated by photolithography. Although the application of a positive-type photoresist coating was effective for the formation of a circle-shaped pattern with a diameter of several tens of micrometers on an iron surface, the coating had poor adhesion to the iron surface and poor durability in an H2SO4 solution. However, the addition of a silica coating derived from tetraethoxysilane (TEOS) on the iron surface as an inner layer resulted in improvement of durability as well as resistance of the coating. Furthermore, the introduction of a layer derived from a mixture of TEOS and glycidyl 3-trimethoxysilylpropyl ether (GPTMS) between the inner layer derived from TEOS and the photoresist coating resulted in long durability showing a large impedance of more than 109 Ω cm2 for 4×105 s in an H2SO4 solution. Cathodic polarization of the microelectrode on the iron surface revealed that the hydrogen evolution reaction (HER) rate is dependent on the plane orientation of the surface. HER rate on an SCM435 steel surface also strongly depended on the microstructure and hardness of the local surface.
In this study, the electrochemical hydrogen permeation technique was introduced to investigate the relationship between scratch shape and hydrogen permeation on of Zn coated steel with wet and dry cyclic corrosion. Six kinds of scratches with different aspect ratio (width/length) and area were fabricated by focused pulsed Nd: YAG laser irradiation. Hydrogen permeation currents with each scratches formed on Zn coated steels were investigated. Regardless of scratch shapes, the peaks were recorded in hydrogen permeation currents during wetting and Zn corrosion products were observed around and on the formed scratches. The total hydrogen permeation charges calculated from hydrogen permeation currents depended on aspect ratio and area of scratch. It is suggested that Zn corrosion products easily covered high aspect ratio scratch and prevented hydrogen entry. In detail change of hydrogen permeation currents during wet and dry corrosion tests was also explained based on coverage and composition of Zn corrosion products.
In this study, hydrogen permeation current from scratch formed on Zn coated steel during 1000 h wet and dry cyclic corrosion test was investigated. The scratch was fabricated by focused pulsed Nd: YAG laser irradiation. The obtained hydrogen permeation current has peaks during wetting, and the peak currents decrease with time. Formation of Zn corrosion products prevent hydrogen entry. After the test, there are Zn oxide (ZnO) on the substrate steel, hydrozincite (Zn5(OH)6(CO3)2) around the scratch boundary and simonkolleite (Zn5(OH)8Cl2) on Zn coated region. It is suggested that pH change with distance from the formed scratch is the reason for the composition of Zn corrosion products change with location.
Duplex stainless steels possess ferrite and austenite microstructures, which exhibit different mechanical properties. The strength level and hydrogen diffusion constant of the phases are different; therefore, it is expected that the microscopic stress and hydrogen concentration distribution are inhomogeneous. Assuming that hydrogen-induced cracking occurs at locally stress-concentrated and hydrogen-accumulated locations, it is important to consider the influence of the microstructure in the evaluation of hydrogen-induced cracking. In order to observe crack locations at the microstructural scale, a slow strain rate test of the hydrogen-charged specimen was performed and the cross-section of the specimen was observed following the test. Hydrogen-induced cracks were mainly observed in the ferrite phase. A numerical simulation was performed to determine the contribution of the stress and hydrogen concentration distribution to the initiation of hydrogen-induced cracks. A microstructure-based finite element model consisting of ferrite and austenite phases was designed based on the micrograph of the duplex stainless steel used. The stress–strain curves of the ferrite and austenite phases were used and macroscopic tension was applied to calculate the microscopic stress distribution. The microscopic distribution of hydrogen concentration was calculated by incorporating the stress distribution into the hydrogen diffusion simulation as one of the driving forces. From the simulation results, the stress concentration and hydrogen accumulation occurred at the ferrite phase or at the ferrite/austenite boundary. This tendency corresponds closely to the experimentally observed results; therefore, the above approach can be applied to the evaluation of hydrogen-induced cracking at the microstructural scale.
An Fe plate, whose one side was electro-polished and the other covered with the rust layer containing 25.7 g·m−2 MgCl2, was used as the specimen to investigate the effect of humidity on the hydrogen absorption of the plate. The specimen was subjected to an electrochemical hydrogen-absorption test during which the rusted surface was exposed to the air with controlled relative humidity (RH) and atmospheric corrosion occurred on it. When the rusted surface was subjected to dry (RH 0%)–wet (RH 27%) repeated cycle tests for 10.8 ks each, the anodic current density corresponding to the hydrogen-absorption rate was measured on the hydrogen detection surface. The maximum current density was almost independent of the cycle during the first 10 cycles, after which it decreased with an increase in the cycle, reaching almost a steady-state after about 40 cycles. After 55 cycles of the dry–wet repeated cycle test, the specimen was subjected to an electrochemical hydrogen-absorption test to obtain the relationship between the steady-state hydrogen-absorption rate and RH. Hydrogen absorption was observed at RH of about 15%, and the absorption rate increased rapidly with an increase in RH, reached a maximum at RH of about 30%, and then decreased rapidly. When RH increased beyond 40%, the absorption rate increased again, reached a maximum value at RH of 80%, and then decreased gradually. The specimen with the rust layer containing 39.8 g·m−2 MgCl2 also showed two peaks in the hydrogen-absorption rate versus RH plot.
Effect of RH during wet period on hydrogen absorption rate as a function of amount of MgCl2 in the rust.
The effect of ammonium thiocyanate (NH4SCN) on the behavior of hydrogen entry into low alloy steel under cathode charging was investigated using electrochemical hydrogen permeation technique. In this study, hydrogen entry sides were polarized galvanostatically to control the rate of hydrogen evolution reaction. The potential, hydrogen charging current density and hydrogen permeation current density were measured at pH of 3.0 in acetic buffer solution with and without 3 g·L−1 NH4SCN. From the Tafel slope of the cathode reaction and the dependence of hydrogen concentration on hydrogen charging current density, it was confirmed that the hydrogen evolution reaction proceeds under Volmer-Tafel mechanism in this study. NH4SCN drastically increased hydrogen entry into steel. To analyze the results of this study, the efficiency of hydrogen entry was calculated from the relationship among hydrogen charging current density, hydrogen permeation current density and hydrogen overpotential. It was found that the hydrogen entry efficiency was drastically higher in NH4SCN environment than that in NH4SCN free environment. However, the coverage of adsorbed hydrogen atoms on hydrogen entry side decreased in NH4SCN environment. To discuss the mechanism that NH4SCN increases hydrogen entry efficiency, the activation energies of hydrogen adsorption and hydrogen absorption were estimated by temperature dependence of the hydrogen charging current density and the hydrogen permeation current density. It is suggested that NH4SCN increased the activation energy of hydrogen adsorption although it decreased that of hydrogen absorption.
The effect of shot peening and subsequent low-temperature annealing (SP treatment) on hydrogen embrittlement in tempered martensitic steel was investigated comparing the typical hydrogen charging methods, constant current controlled cathodic charging test and combined cyclic corrosion test (CCT). The hydrogen entry behavior was evaluated by hydrogen permeation technique and hydrogen visualization using secondary ion mass spectrometry. The SP treatment improved hydrogen embrittlement resistance in both hydrogen charging methods. On the other hand, the effect of SP treatment on the hydrogen entry was different depending on hydrogen charging method. The hydrogen entry was suppressed by SP treatment in the cathodic charging test because the compressive residual stress reduced hydrogen concentration in the surface layer and the potential increased by the increase of surface roughness and the formation of a surface film. In CCT, although the surface hydrogen concentration decreased due to compressive residual stress, the total hydrogen content did not decrease by SP treatment since the surface film disappeared by corrosion and an increase of surface roughness led to an increase in hydrogen entry sites. The improved hydrogen embrittlement resistance by the SP treatment in the cathodic charging test is a result of hydrogen entry suppression in addition to the effect of compressive residual stress. In CCT, the hydrogen embrittlement resistance was improved by SP treatment due to effect of compressive residual stress, i.e., the suppression of stress concentration as well as the stress-induced hydrogen diffusion in stress concentration area and the reduction of hydrogen concentration in the surface layer.
The effects of residual stress on the hydrogen embrittlement behavior of a tempered martensitic steel sheet with 1-GPa-class tensile strength stretch-formed by a hemisphere punch simulating press-formed automotive structural parts were investigated. Cracking on the stretch-formed specimen induced by potentiostatic hydrogen charging was initiated in the foot of the impression of the specimen and propagated to the radial direction both toward the hillside and the plain. The mixture of quasi cleavage and intergranular fractures were observed whole through the fracture surface. Residual stress in the stretch-formed specimens was analyzed by using energy-dispersive X-ray diffraction method utilizing the synchrotron X-ray radiation at SPring-8. In addition, stress and plastic strain distributions in the specimen were analyzed by using Finite Element Method (FEM). These analyses depicted that the high tensile stress in the circumferential direction was in the foot of the impression, corresponding to the direction of the crack growth. The FEM analysis revealed that the high triaxial stress was in the foot suggesting accumulation of hydrogen. It was considered that the preferential crack initiation at the foot was promoted by the high residual stress in the circumferential direction and the hydrogen accumulation due to stress-induced diffusion.
Fe plates with rust layers containing various MgCl2 amount were prepared as specimens. Each specimen was subjected to dry/wet repeat test beyond 50 cycles, and then subjected to electrochemical hydrogen absorption test under atmospheric corrosion in the air with controlled relative humidity (RH). For an MgCl2 amount of 39.8 g·m−2, a hydrogen absorption rate (iH) started to increase from an RH around 15%, steeply increased with an increase in RH up to about 30%, steeply decreased up to about 35%, gradually increased up to 65% and gradually decreased up to about 92%. A decrease in MgCl2 amount in the range between 0.514 and 39.8 g·m−2 induced a decrease in iH in wide RH range. The maximum iH at an RH around 30% increased with an increase in MgCl2 amount in the rust layer. Besides, the RH where the maximum iH was obtained beyond an RH of 40% increased with a decrease in MgCl2 amount. From theoretical relationship between RH and thickness of MgCl2 solution film on the Fe plate without rust layer, it is found that the solution film thicknesses at the RHs were about 0.18 mm, almost independent of MgCl2 amount. In addition, thicknesses of the rust layers containing 25.7 and 39.8 g·m−2 MgCl2 were measured to be almost 0.18 mm each other. The trends of iH depending on MgCl2 amount were tried to be explained using nature of deliquescence for MgCl2.
The effect of iron rust on hydrogen uptake into steel during corrosion under an aqueous NaCl droplet was investigated. Pre-rusted steel was obtained by exposing a steel coupon to natural environmental conditions for 1 month at the Choshi site of the Japan Weathering Test Center. The iron rust that formed on the coupon was partly removed, and model rust/steel samples differing in the area ratios of rusted and bare steel were prepared. The hydrogen permeation current and the corrosion potential were simultaneously measured by Devanathan-Stachurski (DS) method and the Kelvin probe technique, respectively. As the applied droplet of aqueous NaCl dried, the corrosion potential shifted in the negative direction and the hydrogen permeation current slightly increased in all model samples. However, the corrosion potential and hydrogen permeation current did not differ substantially among the model samples once the rusted area of the model sample exceeded 50%. These results indicate that iron rust cause a positive shift in the corrosion potential, and hydrogen uptake was significantly suppressed due to the inhibition of the hydrogen evolution reaction. The hydrogen uptake behavior of the model sample is discussed with consideration of the cathodic reduction reaction of iron rust.
Wet/dry cyclic corrosion tests were conducted to investigate the effect of relative humidity (RH) at dry periods on the hydrogen permeation behavior of steel. Steel specimens were exposed to a temperature and humidity-controlled environment with an initial placement of NaCl droplet. The RH at the dry period was controlled at 20%, 55%, 60%, and 65%, which was lower than the deliquescence RH of NaCl (75%). Hydrogen permeation current was detected using electrochemical methods. The corrosion process on the steel was observed using optical techniques. The results show that the amount of permeated hydrogen through the steel increases with increasing RH at the dry period from 20% to 65%. The hydrogen evolution reaction is inhibited with the accumulation of corrosion products on the steel surface, resulting in a decrease in the amount of permeated hydrogen. Based on the results, the state of NaCl inside corrosion products is supposed to be a solid phase at 20% RH, a liquid-solid phase at 55% RH, and a liquid-rich phase above 60% RH. The increasing volume of the liquid phase contributes to the rise in the amount of permeated hydrogen at the dry period.
The objective in this study is to improve the responsivity and the sensitivity of the hydrogen mapping technique using the WO3 thin film by the optimization of the Pd intermediate layer. Especially, the effect of the thickness of Pd on the responsivity and the sensitivity of the hydrogen detection during hydrogen charging was investigated. Pd and WO3 thin films were coated on the detection side of the pure iron sheet by a magnetron sputtering system. No color change was observed on the hydrogen detection side of the specimens with the Pd intermediate layer more than 25 nm in thickness during the hydrogen detection test for 7.2 ks. The responsivity for the hydrogen mapping technique using WO3 was essentially improved by decreasing the Pd thickness. In terms of the onset time of the average color change, the earliest response was obtained when the Pd thickness was 4 nm because of the uneven distribution of the color change in the case of the Pd thickness of 2.5 nm. The sensitivity for the hydrogen mapping technique using WO3 was improved by decreasing the Pd thickness. Taking into account the responsivity, the sensitivity, and the spatial resolution comprehensively, the best thickness of the Pd intermediate layer seems to be 4 nm in this study.
Surface appearance of the specimen with the Pd intermediate layer 4 nm in thickness on (a, b) the hydrogen entry side and (c, d) the hydrogen detection side. The images were taken (a, c) before and (b, d) after hydrogen charging for 6 ks. The images in the hydrogen entry side (a, b) was horizontally-flipped. The inset in Fig. 2(b) is an enlarged view around the hydrogen charging area. (Online version in color.)
To evaluate the susceptibility of steels to hydrogen embrittlement, it must be hydrogen charged and have its hydrogen content controlled before any mechanical testing. In this study, hydrogen permeation tests of an iron sheet were performed during potentiostatic hydrogen charging in various solutions containing ammonium thiocyanate to obtain a guideline for efficient hydrogen charging for a wide range of hydrogen contents. As the polarization potential shifted in the negative direction, the hydrogen permeation current increased before becoming almost constant. In all cases, besides when an aqueous sodium hydroxide solution was employed, the hydrogen permeation current increased due to the addition of ammonium thiocyanate. The effect of adding ammonium thiocyanate was enhanced as the aqueous solution pH was decreased. The hydrogen permeation current under various hydrogen charging conditions obtained in this study can be used as a reference for hydrogen charging of steels.
The surface potential measurement has been applied for elucidation the hydrogen entry behavior of pure iron under wet-dry cyclic corrosive environment. The hydrogen detection side was electroplated with Ni to prevent any changes of the surface due to a long-time experiment. Distilled water was used in the corrosion test after adding the droplet of NaCl solution in the first cycle. Although the first cycle gave the surface potential change immediately after the surface dried, after 2 cycles the change occurred while the surface was still wet. The potential change area corresponded to the area that showed metallic luster. The surface potential further decreased in the drying process, and the change area also was expanded. In the third cycle, the surface potential changed even on the back side of the surface covered with the corrosion product. It is possible to two-dimensionally detect and visualize the change in permeated hydrogen with time under atmospheric corrosive environment by the surface potential measurement.
In this study, a Kelvin probe (KP) technique combined with the Devanathan-Stachurski electrochemical hydrogen permeation method was applied for simultaneous measurements of polarization resistance and hydrogen permeation current for iron to clarify the hydrogen uptake mechanism during drying of an NaCl droplet. The reciprocal of the polarization resistance, which is an index of the corrosion rate, the hydrogen permeation current, and the corrosion potential under the droplet were successfully measured. The corrosion potential decreased, and the hydrogen permeation current increased, after the NaCl droplet had been applied to the iron surface. The reciprocal of the corrosion resistance increased gradually during the drying of the droplet with increasing the corroded areas on the iron. The hydrogen permeation current decayed with the shift in the corrosion potential toward the noble side during the drying stage, before the droplet dried up completely. The hydrogen permeation current mainly followed the change in the corrosion potential. The hydrogen uptake mechanism of iron during corrosion is discussed in detail based on the corrosion potential, corrosion rate and hydrogen permeation behavior.
The effects of temperature and chloride deposition on hydrogen absorption into steel were evaluated during wet/dry cyclic corrosion using a temperature-compensated hydrogen absorption monitoring system based on the electrochemical hydrogen permeation method. Hydrogen absorption into steel was detected through the measurement of hydrogen permeation currents during the wet periods under the wet/dry cyclic corrosion. The enhancement of hydrogen absorption was mainly caused by the changes in the solution chemistry during the wetting and drying periods, with a decrease in pH due to the hydrolysis reaction of Fe3+ at high Cl− concentration. Hydrogen absorption into steel increased with increasing temperature and chloride deposition. The reasons for the increment of hydrogen absorption are considered that enhancement of the hydrogen evolution reaction with temperature and that the corrosion potential shifted to less noble by increase in the electrolyte thickness with increasing chloride deposition. Based on these results, the amount of absorbed hydrogen map effected by these factors under atmospheric corrosion environment was described.
Duplex stainless steels and their deposited weld metal have ferrite and austenite microstructures with different material properties. In addition, the microstructure of the base metal and weld metal clearly differs, affecting hydrogen diffusion and accumulation, and hydrogen-induced cracking behavior at the microstructural scale. In this study, the influence of microstructure on hydrogen-induced cracking behavior of the duplex stainless-steel weld metal was investigated. Duplex stainless-steel weld metal specimens were prepared and slow strain rate tensile test was performed after hydrogen charging. Cracks were observed at the ferrite/austenite boundaries. A microstructure-based finite element simulation was performed to clarify the concentration distribution at the microstructural scale. A finite element model based on the cross-section of the microstructure was designed to calculate the stress and hydrogen concentration distribution. The simulation result showed that hydrogen accumulation occurred at the ferrite/austenite boundaries, which corresponded to the locations where cracks were observed. On the other hand, the hydrogen concentration at the accumulation site in the weld metal was lower than that in the base metal. Therefore, the influence of the phase fraction and stress–strain curves of the ferrite and austenite phases on the hydrogen concentration was investigated by the proposed numerical simulation. Both phase fraction and stress–strain curves significantly influenced the microscopic distribution of hydrogen concentration.
Weld cold cracking is a kind of hydrogen embrittlement and is a serious problem when high strength steel is concerned. Cold cracking usually occurs at the area where hydrogen locally accumulates, and hence hydrogen diffusion modelling is important to understand the hydrogen distribution in a welded joint. In this paper, we conducted the permeation tests to measure the apparent diffusion coefficients of hydrogen with varying the hardness and the plastic strain of the steel. The experimental results were used to develop the empirical equation to predict the diffusion coefficient. The empirical equation is described as a function of temperature, hardness and plastic strain. Next, we measured the evolution curves of hydrogen released from the rectangular specimens. We considered the boundary condition of the surface exposed to the atmosphere, and the experimental results were used to determine the boundary condition. The empirical equation and the boundary condition obtained in this paper will be used for the numerical calculations of hydrogen diffusion in a weld cold cracking test in the companion paper.
The purpose of this work is to compare the numerical calculation results of the hydrogen diffusion with the experimental results. We firstly conducted a y groove weld cracking test using 980 MPa grade steel and 780 MPa grade welding consumable to observe HAZ cracking. Next, we showed the present diffusion equation and the procedure to determine the physical properties for the calculations such as stress-strain curve, and using the apparent diffusion coefficient and the boundary condition obtained in the companion paper, we conducted the numerical calculations for the two cases, that is, the under-match case (980 MPa grade steel and 780 MPa grade welding consumable) and the even-match case (780 MPa grade steel and 780 MPa grade welding consumable). The calculation results of the under-match case show that hydrogen tends to accumulate in the stress concentration area of both the HAZ and the weld metal, which indicates crack may occur in both the HAZ and the weld metal. The results of the even-match case show not only the hydrogen accumulation around the stress concentration area but also high hydrogen concentration in the weld metal, which indicates crack may occur in the weld metal. Calculation results for both of the cases fairly agree with the experiments.
Hydrogen absorption behavior and microstructural change of carburized JIS SCr420 steels containing different amounts of retained austenite in rolling contact fatigue were investigated. The thermal desorption analysis confirmed hydrogen desorption at the second-peak between 423 and 623 K after rolling contact fatigue. The hydrogen concentration at the second-peak increased with number of cycles in the rolling contact. This increment was larger when using the steel with higher amount of retained austenite before the fatigue test. It was still large even when the amount of martensitic transformation from retained austenite under cyclic stress to introduce dislocation with trapping capacity was small. The activation energies of desorption for the second-peak hydrogen were calculated to be 50.6 kJ·mol−1 for the steel with 10.4% retained austenite and 55.8 kJ·mol−1 for the steel with 4.9% retained austenite. The activation energies of cathodically charged 0.8%C steels with 10.9% and 6.0% retained austenite, simulating carburized layer before the test, were 36.2 and 42.2 kJ·mol−1, respectively. This means that the activation energy of hydrogen desorption increased during rolling contact. The absorbed hydrogen during the rolling contact fatigue was likely trapped in more stable trapping sites related to the retained austenite which were formed under cyclic stress.
To provide reliable relationship between hydrogen embrittlement (HE) and hydrogen distribution, a duplex stainless steel (DSS: JIS SUS329J4L) annealed and electrolytically hydrogen-charged was investigated by means of hydrogen microprint technique (HMPT), where distribution of hydrogen was examined on the opposite side of the charged surface. Quantitative analysis was made by classifying the site of detected hydrogen into three categories: ferrite matrix, austenite grain and phase boundary. The HMPT was performed on the 1.5 h and 24 h charged specimens with two holding times in the ambient air for 0.5 (as quick as possible) and 300 h. In the 1.5 h charged and 0.5 h held specimen, hydrogen atoms were mostly detected on the phase boundary. When charging time was increased to 24 h, relative fraction of hydrogen desorbed in the austenite phase against the ferrite matrix and phase boundary increased. The relative fraction of hydrogen atoms in the austenite phase was also increased by increasing holding time to 300 h irrespective of the charging time. During the holding, hydrogen atoms inside the ferrite matrix were presumed to preferentially diffuse out from the specimen or transferred to the phase boundary, while hydrogen atoms already trapped at the phase boundary will move into the interior of the austenite phase. The results obtained in the present study where experimental conditions are systematically selected can be rationally interpreted only with the higher solubility and smaller diffusivity of hydrogen in the austenite phase, rather than considering the binding energy of hydrogen with phase boundary.
Area fraction of the silver grains in the HMPT images, corresponding to the conditions in Fig. 3. Viewing area and hydrogen-charging time: (a) 1318 μm2 and 1.5 h, (b) 2164 μm2 and 24 h, respectively.
Hydrogen-assisted crack growth of pre-strained twinning-induced plasticity (TWIP) steel was investigated using artificial defects (micro-drilled holes), which acted as artificial crack initiation sites. Hydrogen was introduced into the specimens by electrochemical hydrogen charging during slow strain rate tensile test. The quasi-cleavage crack propagation observed was due to repeated crack initiation near the crack tip and subsequent coalescence. Crack initiation near the crack tip occurred after plastic deformation of the crack tip, and pre-straining facilitated plasticity-driven crack initiation. The early stage of plasticity-driven crack growth was sensitive to the crack length and remote stress level. Accordingly, the crack growth rate in the early stage increased with the increase in the initial defect size. In the following stage of the crack growth, the crack growth rate exhibited a complicated trend with respect to the crack length, which is possibly due to the plastic-wake-altered stress field around the crack tip, which depends on the initial defect size.
We investigated the effects of substitutional alloying elements on the microstructure, hydrogen diffusivity, and tensile properties of Fe–X binary ferritic alloys (X = Si, Al, Mn, Cu, Ni, Co, Cr, Mo, V, W, and Ti) in air and under hydrogen charging. We find using X-ray diffraction that these elements, except for Si and Co, cause ferrite lattice expansion. The hydrogen diffusion coefficient D (measured via hydrogen-permeation tests under cathodic charging at 24°C) reduces as a function of the added alloy concentration. The D-value reduction is enhanced more for Ti, Mn and Cr than other elements. This D variation cannot be simply explained based on the lattice expansion effect, which means that D depends on both hydrogen trapping at the expanded internal lattice spaces adjacent to substitutional solute atoms and hydrogen-solute-atoms chemical interactions. As regards the tensile properties obtained based on slow strain rate tests in air and under hydrogen charging, we find that the all elements, except for Al and Co, afford alloy strengthening in air. Under hydrogen charging, Ti, Mn, and Cr addition reduces the fracture elongation, thereby indicating that these elements increase alloy susceptibility to hydrogen embrittlement. The elongation loss due to hydrogen does not depend on the strengthening effects; however, it exhibits good correlation with the observed D-value reduction and the increment in surface hydrogen concentration C0, which is inversely proportional to D. This correlation indicates that substitutional alloying elements act as reversible hydrogen-trapping sites, which supply hydrogen to potential and developing cracks.
To predict hydrogen embrittlement in steels and clarify its mechanism, it is necessary to understand the time variation of hydrogen distribution. Since the quantum mechanical effect is remarkably observed in hydrogen diffusion, even at room temperature, there is a need to take into account it for analyses. In this study, we evaluated the diffusion coefficients of hydrogen in body-centered cubic (bcc) iron via density functional theory and small-polaron theory calculations. The analyses were carried out under various magnitudes of volumetric strains to investigate its effect on the diffusion coefficient. The temperature dependence of the diffusion coefficient was found to change at about 400 K. This is attributed to the fact that the tunneling between ground states dominantly contributes to the diffusion at lower temperatures, whereas at high temperatures, that between low excited states contributes dominantly. The diffusion coefficient was also found to increase with compressive volumetric strain and decrease with tensile volumetric strain. The volumetric strain dependence of the diffusion coefficient was clarified by the volumetric strain dependence of the tunneling matrix elements and that of the activation enthalpy.
Diffusion coefficients of hydrogen in bcc iron under volumetric strain. (Online version in color.)
In this study, the decarburization behavior, and its effect on mechanical properties of 2000 MPa class martensitic steel were investigated with the purpose of collecting the basic data to realize the further strengthening of structural members for automobiles. Concerning the decarburization behavior, the maximum of the decarburization rate of 2000 MPa class martensitic steel with a basic chemical composition of 0.35%C-1.2%Mn was found around 750°C and the thickness of the decarburized zone grew proportional to the square root of time. The addition of Nb reduced the decarburization rate.
Concerning the effect of decarburization on mechanical properties, the decarburization significantly improved bendability even though the thickness of the decarburized layer is relatively thin. An observation of the crack behavior revealed that the initiation and propagation of the crack were suppressed by the decarburization layer. The resistance to delayed fracture was improved by decarburization. This improvement is presumed to be based on a similar mechanism for the improvement of bendability.
From the aspect of crack/void initiation and growth characteristics, the effects of pre-strain on the hydrogen embrittlement resistance of a transformation-induced plasticity-aided bainitic ferrite steel were examined. The hydrogen uptake in the specimens without pre-strain caused degradation of crack growth resistance, but the crack initiation probability did not change significantly. It is noteworthy that the degree of degradation was independent of the hydrogen content in the present hydrogen charging condition. Pre-straining to 3% and 6% improved the crack growth resistance of the hydrogen-charged specimens because of a reduction in the probability of austenite presence at the crack tip. Furthermore, a high level of pre-strain provided high hydrogen concentration and resulted in strain-age-hardening, which caused an acceleration of quasi-cleavage fracture, an increase in yield strength, and a stress/strain concentration associated with Lüders deformation. These factors diminished the crack initiation resistance and crack growth resistance.
In order to suppress global warming, it is necessary to reduce the weight of autobodies, whereas improvement of the collision safety of automobiles is required which results in a weight increase. Research and development of high-strength steel sheets for autobodies are being carried out to achieve both two objectives. Among various high-strength steel sheets, low alloy steels utilizing transformation-induced plasticity of retained austenite (TRIP-aided steels) have attracted attention as body frame and automotive parts materials because of their high strength and ductility. However, there is concern about hydrogen embrittlement in low alloy TRIP-aided steels with a tensile strength exceeding 1000 MPa, as in conventional high-strength steels. In this study, in order to reveal the role of retained austenite for hydrogen embrittlement on TRIP-aided bainitic ferrite steel (TBF steel), (i) several TBF steel sheets with different volume fraction and carbon concentration of retained austenite were prepared by austempering the original sheet for different durations, and (ii) slow strain rate technique tensile tests were carried out on hydrogen-charged and uncharged test pieces of the sheets together with microstructure observations and X-ray diffractions. It was revealed that hydrogen embrittlement of TBF steel sheet became suppressed with increasing austempering time, which was attributable to the increase of surface area of lath and/or filmy metastable retained austenite acting as a trapping site for hydrogen.
The effects of stress and plastic strain distributions on the hydrogen embrittlement fracture of the U-bent martensitic steel sheet specimen were investigated. The hydrogen embrittlement testing of the U-bent specimen was performed. Fracture morphology mainly consisting of intergranular fracture was found inside the hydrogen charged U-bent specimen, which indicated that the crack initiation took place in the interior, and shear lips were found near both surfaces of the U-bent sheet. The synchrotron X-ray diffraction measurement and the finite element simulation were utilized to analyze the stress and plastic strain distributions in the thickness direction of the U-bent specimen. The elastic strain distributions obtained by the measurement showed a good agreement with the simulation. The crack initiation site of the hydrogen-charged U-bent specimen was considered to be correspondent with the region where the tensile stress was the highest, suggesting that the maximum tensile stress predominantly determine the crack initiation.
A light-addressable potentiometric sensor (LAPS) is a semiconductor-based chemical sensor, in which the measurement area is defined by a probe light illuminating the sensor plate. In this study, a novel method to detect hydrogen permeation through metal was proposed using a measurement system, in which a LAPS was combined with a micro-volume measurement cell. The sensing surface of the LAPS was placed in the vicinity of the hydrogen exit surface of an iron sheet to form the micro-volume cell. The cell was filled with a Na2HPO4 solution, and then the pH change of the solution due to production of protons during the electrochemical hydrogen permeation was detected. After loading hydrogen atoms from the hydrogen entry side into the iron sheet, the photocurrent – bias voltage characteristics of the LAPS showed a shift indicating production of protons. The result suggested the possibility of applying the LAPS to visualization of hydrogen permeation sites on the metal surface.
Components for hydrogen vessels and accessories in fuel cell vehicles (FCVs) are exposed to high pressure hydrogen gas environments. A limited range of materials that do not exhibit hydrogen embrittlement in such environments have been permitted for use as component materials. However, expansion of the range of usable materials is required for the widespread commercialization of FCVs. National projects have thus been promoted in the automobile field to expand the usable materials for high-pressure hydrogen environments. In these projects, the establishment of test methods to accurately evaluate the hydrogen compatibility of materials was one of the primary aims. The second aim was to standardize materials selection methods for high pressure hydrogen gas services. Two different test methods are proposed to evaluate the hydrogen compatibility of materials for FCVs. One is a methodology based on the notched specimen fatigue life test and the other is based on the slow strain rate technique (SSRT) and smooth specimen fatigue life test. These test methods are now under deliberation to be included in the Global Technical Regulation (HFCV-gtr) phase 2.