Non-charged Specimens Research Articles

Alterable fracture toughness of amorphous silica by injection and removal of electrostatic charge

In this study, we demonstrate that the fracture toughness of amorphous silica, an electrical insulator, can be dramatically increased and restored via injecting and removing the electrical charge content. Micropillar specimens of amorphous silica were fabricated using focused ion beam machining. The specimens were charged by electron-beam irradiation (charged specimens), and the charge was removed from the specimens by exposure to atmospheric conditions and annealing (charge-removed specimens). Fracture toughness testing was conducted on non-charged, charged, and charge-removed micropillar specimens. The fracture toughness of the charged specimen was 2.4 times higher than that of the non-charged specimens. Furthermore, the fracture toughness of the charge-removed specimens was restored to a level similar to that of the non-charged specimens, but not completely restored. These results indicate that the fracture toughness of amorphous silica can be controlled by injecting and removing electrostatic charges.

Hydrogen embrittlement susceptibility of additively manufactured 316L stainless steel: Influence of post-processing, printing direction, temperature and pre-straining

The influence of post-build processing on the hydrogen embrittlement behavior of additively manufactured (AM) 316L stainless steel fabricated using laser powder bed fusion was assessed at both room temperature and −50 °C via uniaxial tensile experiments. In the absence of hydrogen at ambient temperature, all four evaluated AM conditions (as-built (AB), annealed (ANN), hot isostatic pressed (HIP), and HIP plus cold worked (CW) to 30%) exhibit notably reduced ductility relative to conventionally manufactured (CM) 316L stainless steel. The AM material exhibits sensitivity to the build direction, both in the presence and absence of hydrogen, with a notable increase in yield strength in the X direction and enhanced ductility in the Z direction. Conversely, testing of non-charged specimens at −50 °C revealed similar ductility between the CM, AB, ANN, and HIP conditions. Upon hydrogen charging, the ductility of all four AM conditions was found to be similar to that of CM 316L at ambient temperature, with the HIP condition actually exceeding the CM material. Critically, testing of hydrogen-charged samples at −50 °C revealed that the ductility of the HIP AM 316L condition was nearly double that observed in the CM 316L. This improved performance persisted even after cold working, as the CW AM 316L exhibited comparable ductility to CM 316L at −50 °C after hydrogen charging, despite having a 2-fold higher yield strength. Feritscope measurements suggest this increased performance is related to the reduced propensity for AM 316L to form strain-induced martensite during deformation, even after significant post-processing treatments. These results demonstrate that AM 316L can be post-processed using typical procedures to exhibit similar to or even improved resistance to hydrogen embrittlement relative to CM 316L.

The influence of hydrogen on the low cycle fatigue behavior of strain-hardened 316L stainless steel

The present study focuses on the low cycle fatigue (LCF) behavior of strain-hardened 316L stainless steel under plastic strain control, aiming to understand the influence of internal hydrogen on fatigue performance and cyclic deformation behavior for energy-related technologies. The strain-hardened 316L specimens tested at plastic strain amplitudes between 0.1% and 0.7% with and without hydrogen displayed continuous cyclic softening. Internal hydrogen increased the cyclic strength of this steel, with a greater difference in strength at low plastic strain amplitudes. A Bauschinger analysis revealed that effective stresses represented the major contribution to the flow stress for all material conditions and amplitudes. At low plastic strain amplitudes, effective stresses were responsible for differences in cyclic strength between hydrogen precharged and non-charged specimens. In contrast, back stresses became more significant at high amplitudes, and more similar deformation structures were observed. Although internal hydrogen reduced the total LCF lifetime at all amplitudes, this degradation was more significant in the low amplitude regime. This result is attributed to high effective stresses that led to the onset of multiple slip at lower cumulative plastic strains in the hydrogen precharged condition compared to the non-charged condition at low amplitudes. In addition, the evolution of the apparent elastic response of the material suggests that the primary crack(s) nucleated at a smaller percentage of the total lifetime and propagated in fewer number of cycles in the hydrogen precharged than in the non-charged condition. Fracture surface and gage section observations revealed a transgranular crack path and planar slip traces in both material conditions, with internal hydrogen promoting multiple slip at lower values of cumulative plastic strain than in the non-charged condition.

The combined effects of hydrogen and aging condition on the deformation and fracture behavior of a precipitation-hardened nickel-base superalloy

The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.

Effect of defects and hydrogen on the fatigue limit of Ni-based superalloy 718

Tension-compression fatigue tests were performed on two types of Ni-based superalloy 718 with different microstructures, to which small artificial defects of various shapes and sizes were introduced. Similar tests were also conducted on hydrogen-charged specimens with defects, with a solute hydrogen content ranging from 26.3 to 91.0 mass ppm. In the non-charged specimens in particular, the fatigue strength susceptibility to defects varied significantly according to the type of microstructural morphology, i.e., a smaller grain size made the alloy more vulnerable to defects. The fatigue limit as a small-crack threshold was successfully predicted using the √area parameter model. Depending on the size of defects, the fatigue limit was calculated in relation to three phases: (i) harmless-defect regime, (ii) small-crack regime and (iii) large-crack regime. Such a classification enabled comprehensive fatigue limit evaluation in a wide array of defects, taking into consideration (a) the defect size over a range of small crack and large crack and (b) the characteristics of the matrix represented by grain size and hardness. In addition, the effect of defects and hydrogen on fatigue strength will be comprehensively discussed, based on a series of experimental results.

A frequency dependent embrittling effect of high pressure hydrogen in a 17-4 PH martensitic stainless steel

The effects of hydrogen on tensile and fatigue-life properties of 17-4PH H1150 steel have been investigated by using a smooth, round-bar specimen for tensile tests and circumferentially-notched specimen for fatigue-life tests. The specimens were precharged by an exposure to 35-100 MPa hydrogen gas at 270°C for 200 h. For the 100 MPa hydrogen exposure, the steel showed a significant degradation in ductility loss, translated by a relative reduction in area, RRA, of 0.31. The fatigue-life test of the present notch specimen (stress concentration factor of 6.6) reflects the fatigue crack growth (FCG) for long cracks. The fatigue limit of the non-charged and H-charged notched specimens, defined by the threshold of non-propagation for long cracks, was not affected by hydrogen. At a higher stress amplitude, the H-charged specimen showed a significant FCG acceleration ratio compared to the non-charged specimen. Although, an upper bound of the FCG acceleration seemed to exist, this ratio was approximately 100. The fracture surface of the H-charged specimen was covered with quasi-cleavage (QC) at a lower stress amplitude and with a mixture of QC and intergranular (IG) facets at higher stress amplitudes. It has been suggested that a cycle-dependent crack growth accompanied by QC occurs at a lower stress amplitude, whereas a mixture of cycle-dependent crack growth (accompanied by QC) and time-dependent crack growth (accompanied by IG) occurs otherwise. This mixture justifies the 100 times FCG acceleration ratio in spite of the existence of the upper bound.

Comprehensive Understanding of Ductility Loss Mechanisms in Various Steels with External and Internal Hydrogen

Hydrogen-induced ductility loss and related fracture morphologies are comprehensively discussed in consideration of the hydrogen distribution in a specimen with external and internal hydrogen by using 300-series austenitic stainless steels (Types 304, 316, 316L), high-strength austenitic stainless steels (HP160, XM-19), precipitation-hardened iron-based super alloy (A286), low-alloy Cr-Mo steel (JIS-SCM435), and low-carbon steel (JIS-SM490B). External hydrogen is realized by a non-charged specimen tested in high-pressure gaseous hydrogen, and internal hydrogen is realized by a hydrogen-charged specimen tested in air or inert gas. Fracture morphologies obtained by slow-strain-rate tensile tests (SSRT) of the materials with external or internal hydrogen could be comprehensively categorized into five types: hydrogen-induced successive crack growth, ordinary void formation, small-sized void formation related to the void sheet, large-sized void formation, and facet formation. The mechanisms of hydrogen embrittlement are broadly classified into hydrogen-enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). In the HEDE model, hydrogen weakens interatomic bonds, whereas in the HELP model, hydrogen enhances localized slip deformations. Although various fracture morphologies are produced by external or internal hydrogen, these morphologies can be explained by the HELP model rather than by the HEDE model.

Study of the effect of hydrogen charging on the tensile properties and microstructure of four variant heat treatments of nickel alloy 718

This study examined the hydrogen embrittlement sensitivity of nickel alloy 718 given four different heat treatments to obtain various microstructural states. The four heat treatments examined are the oil and gas 718 heat treatment, the aerospace 718 heat treatment, and two variant two-step heat treatments, with a difference in aging heat treatment named 718 low band and high band. Each heat treatment leads to differences in the precipitate morphologies of γ′, γ′′ and δ phases.Material characterisation and fractography of the examined heat treatments were performed using a high resolution FEG SEM. Three specimens of each condition were pre-charged with hydrogen and tensile properties were compared with those of non-charged specimens. It was observed that hydrogen embrittlement was associated with intergranular and transgranular microcrack formation, leading to an intergranular brittle fracture. δ phase may assist the intergranular crack propagation, and this was shown to be particularly true when this phase is coarse enough to produce crack initiation, but this is not the only factor determining embrittlement. Other microstructural features play a role, as does the strength of the material. The aerospace heat treatment, which gives the highest strength and ductility in the uncharged state, shows the greatest reduction in properties with hydrogen charging.

Hydrogen effect on compression mechanical properties of TiNb alloys at elevated temperatures

The study of this work was focused on the hydrogen effect on hot deformation behavior of hydrogen charged TiNb based alloys at three temperatures in comparison with non-charged specimens. The Ti24Nb and Ti26Nb (at.%) alloys were heat treated by three step regime in argon or hydrogen atmospheres. The hot compression tests were performed on a Gleeble 3800 machine at 800, 750 and 700°C with compression strain of 5×10-3 s-1 and deformation degree of 50 %. The microstructure resulting from heat treatment as well as from isothermal compression test was analyzed using optical and scanning electron microscopies. Measurement of microhardness revealed that higher microhardness values for Ti24Nb after thermo-hydrogen treatment corresponded to fine grained microstructure and strengthening by grain boundaries. Hydrogen contents determined using LECO RH600 analyzer showed that Ti26Nb contained higher amount of hydrogen due to the stabilization of beta phase by higher Nb concentration. Based on the hot compression test results, the plasticity at elevated temperatures was evaluated. The evolution of uniaxial compressive test curves showed the stabilization of flow stress due to the hydrogen for high deformations at lower temperatures. The differences in hot compression behavior for both niobium contents on the one hand and for hydrogen charged and non-charged specimens on the other hand were observed.

The Effect of Hydrogen on the Corrosion and Anodic Behavior of 13Cr Stainless Steel in Chloride Solution

Hydrogen evolution is one of the common cathodic reactions involved in corrosion of metals and alloys in aqueous solutions. The reduction of hydrogen ions is a source of atomic hydrogen on the surfaces of metals. Corrosion reactions, the application of cathodic protection and electroplating, etc., can introduce hydrogen into metals via diffusion. The presence of hydrogen in metals can have a significant influence on the mechanical properties and electrochemical behavior of metals depending on the environments. Stainless steels generally corrode at very low rates in high temperature aqueous environments because of their tendency to form a protective oxide film. Hydrogen is incorporated into the structural alloys during production, fabrication, processing, from in service conditions, and from the electrochemical reaction during corrosion. The transport of hydrogen through passive films is involved in many corrosion processes such as pitting, stress corrosion cracking and corrosion fatigue. The nature of passive films on metals and alloys is a decisive factor that controls their corrosion behavior. Mmuch work has been devoted to the study of the electronic properties and chemical composites of passive films on metals and alloys. Based on the fact that the potential-dependent transpassive dissolution varies with the electronic properties of the passive film, Sato proposed a breakdown mechanism of the passive film on metals, the electrochemical stability of a passive film strongly depends on the electron energy band structure in the film. Bianchiet al. concluded that the high susceptibility of stainless steel to pitting nucleation was connected to n-type conductivity of the oxide film. Cr-bearing stainless steels have been widely used in power generation systems and other industrial systems. The effects of hydrogen on the surface film and electrochemical behavior of 13Cr stainless steel in a chloride solution were investigated in the present work. Hydrogen was introduced into the samples by cathodic charging under galvanostatic condition at 25 oC in sulfuric acid solution with thiourea. The specimens were charged at a current density 2 mA/cm2 for 2 h. Electrochemical measurements were carried out on both uncharged and hydrogen charged specimens for evaluating their corrosion resistance. Potentiodynamic anodic polarization curves were carried out in 200 ppm wt.% NaCl solution at 25 oC, as shown in Figure. 1. Pre-charged hydrogen specimen showed a significantly lower open-circuit potential of about -0.67 V(SCE), and the non-charged specimen showed OCP of about -0.11V (SCE). Anodic current density in the anodic polarization curves of the hydrogen-charged specimen was significantly higher than that of the non-charged specimen. These results indicate that the corrosion resistance of 13Cr steel was significantly reduced by charged-hydrogen. Figure 2 show the electrochemical impedance spectra (EIS) for hydrogen-charged and non-charged specimens after different immersion periods in 200ppm wt.% NaCl solution at 25 oC. EIS diagrams of hydrogen-charged and non-charged specimens exhibited two parts: a capacitive loop in the high-frequency region and a capacitive loop in the low-frequency region. The second capacitive arc radius for the hydrogen-charged specimen was significantly less than that of the non-charged specimen. The scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used for the element and surface morphology analysis of the specimens after immersion tests in 200ppm wt.% NaCl solution at 25 oC for 68 h. Little corrosion product could be found on the non-charged specimen surface while more corrosion product appeared on the hydrogen-charged specimen surface, as shown in Fig. 3. Cr content in the film formed on hydrogen-charged specimen was lower than that on non-charged specimen, indicating the effect of hydrogen on the chemical composition of surface film. Figure 1

Rate Dependency During Relaxation of Superelastic Orthodontic NiTi Alloys After Hydrogen Charging

The relaxation behavior under tensile loading of a superelastic NiTi alloy was investigated after hydrogen charging with respect to aging from one to 77 days in air at room temperature. The specimens were immersed for 3 h in a 0.9 % NaCl aqueous solution and then relaxed with an imposed strain of 4.8 %—which results in half of the martensite transformation—for different strain rates of 10−4, 10−3, and 5 × 10−3 s−1. For the non-charged specimens, the relaxed stress at the beginning exhibited a temporary dependence on the strain rates and then reached the same equilibrium stress after 2.5 h. After hydrogen charging, this equilibrium stress did not vary for the as-charged specimen. Nevertheless, the greater the aging period is the greater the equilibrium stress is. This behavior can be attributed to the diffusion of hydrogen into the entire specimen, which hinders the relaxation mechanism of the martensite bands.

ステンレス鋼 SUS304 の微小疲労き裂の進展下限界に及ぼす水素の影響

For the safety use of hydrogen utilization systems, it is necessary to understand properly the effect of hydrogen on the fatigue properties of structural materials. In the present study, fatigue life tests of 304 stainless steel with small artificial defects were carried out under the presence of hydrogen; the tests in hydrogen gas environment and the tests using hydrogen-charged specimens, and the effect of hydrogen on the fatigue limit of the material was investigated by paying attention to the behavior of short fatigue cracks. As a result, the fatigue limit was not degraded in hydrogen gas environment, whereas there was slight degradation of fatigue life in short life regime (i.e. Nf < 105 ). On the other hand, fatigue limit of hydrogen-charged specimens was increased compared with that of non-charged specimens. It is noteworthy that non-propagating cracks existed in the unbroken specimens in all environmental conditions. As is well known, fatigue limit of steels with low or moderate strength is determined by the non-propagation limit of small fatigue cracks. Consequently, it was revealed that the fatigue limit of the specimens was determined by the arresting limit of fatigue cracks, which was not degraded in the tests in hydrogen gas as well as in the tests using hydrogen-charged specimen.

高強度マルテンサイト系ステンレス鋼JIS-SUS630 (H1150)の引張特性に及ぼす水素チャージの影響

Tensile properties of high-strength martensitic stainless steel, JIS-SUS630 (H1150) were investigated with hydrogen-charged specimens. The reduction in area of the steel was more degraded with a higher hydrogen content. The non- and hydrogen-charged specimens failed by a cup-and-corn fracture; however, the shear stress fracture region of the hydrogen-charged specimen was larger than that of the non-charged specimen. The normal stress fracture surface of the hydrogen-charged specimen was covered with quasi-cleavage intergranular-like surface and fine dimple, different from that of the non-charged specimen. In the hydrogen-charged specimen, voids grew in the direction perpendicular to the loading direction and caused the quasi-cleavage and intergranular-like fracture. A series of experimental results inferred that the degradation in tensile ductility and unique fracture surface morphologies of the steel were attributed to localized slip deformations by hydrogen.

TENSILE DEFORMATION BEHAVIOR OF HYDROGEN CHARGED ULTRAHIGH STRENGTH STEEL STUDIED BY IN SITU NEUTRON DIFFRACTION

The tensile deformation behavior and the axial lattice strain response of 1250 MPa ultrahigh strength steels with and without hydrogen charging were comparably investigated using time-of-flight neutron diffraction together with the fracture morphology and microstructure observation. Before tensile loading, the axial (110) lattice plane spacing of hydrogen charged specimen was found larger than that of noncharged specimen while the axial (200) lattice plane spacing of the former was smaller than that of the latter, suggesting that the hydrogen atoms occupied the tetrahedral site promoted the increment of axial (110) lattice plane spacing while the balanced internal stress resulted in the proper decrement of axial (200) lattice plane spacing. The necking and ductile fracture after approaching the 1250 MPa tensile strength occurred in the non-charged specimen, while the brittle *收到初稿日期: 2014-09-30,收到修改稿日期: 2015-07-18 作者简介:徐平光,男, 1973年生,研究副主干,博士 DOI: 10.11900/0412.1961.2014.00541 第1297-1305页 pp.1297-1305

Factors affecting hydrogen-assisted cracking in a commercial tempered martensitic steel: Mn segregation, MnS, and the stress state around abnormal cracks

The purpose of this paper is to reveal the dominant factors affecting tensile fracture under a hydrogen gas atmosphere. Tensile tests were conducted in hydrogen gas with circumferentially-notched specimens of a commercial tempered martensitic steel. Two specimens were exposed to hydrogen gas for 48h before tensile testing; the other two specimens were not pre-charged. Longitudinal cracks along the loading direction and a transverse crack perpendicular to the loading direction were observed on a cross section of the non-charged specimen, but there was only one small crack on a cross section of the pre-charged specimen. Electron back scatter diffraction, energy dispersive X-ray spectrometry and finite element method analyses were applied to clarify the relationships among the longitudinal crack, Mn segregation, microstructures of martensitic steel and hydrogen. As a result, it has been demonstrated that Mn segregation and MnS promote hydrogen-assisted cracking in the tempered martensitic steel, causing the longitudinal cracking which is a mechanically non-preferential direction in homogeneous situations. More specifically, we have shown that the role of the Mn segregation is to promote the hydrogen-enhanced decohesion effect (HEDE), which is particularly important for crack propagation in the present case. These considerations indicate that the presence of Mn is crucially important for hydrogen-assisted cracking associated with hydrogen-enhanced localized plasticity (HELP) as well as HEDE.

Inhibition of localized corrosion of Ni–Ti superelastic alloy in NaCl solution by hydrogen charging

Inhibition of the localized corrosion of Ni–Ti superelastic alloy in 0.9% NaCl solution has been attempted by charging with a small amount of hydrogen, which causes negligible hydrogen embrittlement. Upon a small amount of hydrogen charging, no pitting potential is observed in anodic polarization curves. From scanning electron microscope observations, localized corrosion is inhibited on the entire side surface of charged specimens. With increasing amount of charged hydrogen, the corrosion potential shifts in the less noble direction and the current density increases under anodic applied potential. When the hydrogen charged specimens are aged in the atmosphere at room temperature, the corrosion potential becomes almost the same as that of the non-charged specimen, but the inhibition of localized corrosion remains. The present study indicates that a small amount of hydrogen charging is effective for inhibiting the localized corrosion of the alloy in NaCl solution.

Ductility Loss in Ductile Cast Iron with Internal Hydrogen

Hydrogen-induced ductility loss in ductile cast iron (DCI) was studied by conducting a series of tensile tests with three different crosshead speeds. By utilizing the thermal desorption spectroscopy and the hydrogen microprint technique, it was found that most of the solute hydrogen was diffusive and mainly segregated at the graphite, graphite/matrix interface zone, and the cementite of pearlite in the matrix. The fracture process of the non-charged specimen was dominated by the ductile dimple fracture, whereas that of the hydrogen-charged specimen became less ductile because of the accompanying interconnecting cracks between the adjacent graphite nodules. Inside the hydrogen-charged specimen, the interspaces generated by the interfacial debonding between graphite and matrix are filled with hydrogen gas in the early stage of the fracture process. In the subsequent fracture process, such a local hydrogen gas atmosphere coupled with a stress-induced diffusion attracts hydrogen to the crack tip, which results in a time-dependent ductility loss.

Hydrogen-Induced Ductility Loss in Cast Irons

Effect of hydrogen-charging was investigated with respect to the tensile properties of three types of cast irons: JIS FCD400, FCD450 and FCD700. In this study, hydrogen charging led to a marked ductility loss in all the cast irons. The thermal desorption spectroscopy and the hydrogen microprint technique revealed that, in the hydrogen-charged specimens, most of solute hydrogen was diffusive and mainly segregated at graphite, graphite/matrix interface zone and pearlite. In the fracture process of non-charged specimen, neighboring graphites were interconnected with each other mainly by ductile dimple fracture. On the other hand, in the fracture process of hydrogen-charged specimen, the graphites were interconnected by cracks. The difference in the fracture morphology between the non-charged and the hydrogen-charged specimens is attributed to the presence of diffusive hydrogen in graphite and graphite/matrix interface. During early stage of fracture process in hydrogen-charged specimen, the interspace between graphite and matrix is filled with hydrogen gas, which leads to the ductility loss of matrix in the vicinity of graphite. Even after the initiation of crack from graphite, hydrogen is continuously outgassed from graphite and supplied to the crack tip. Therefore, concerning the hydrogen effect on the strength of cast irons, a role of subsurface graphite as a “local hydrogen supplier” should be taken into consideration.

Visualization of Hydrogen Diffusion in a Hydrogen-Enhanced Fatigue Crack Growth in Type 304 Stainless Steel

To study the influence of hydrogen on the fatigue strength of AISI type 304 metastable austenitic stainless steel, specimens were cathodically charged with hydrogen. Using tension-compression fatigue tests, the behavior of fatigue crack growth from a small drill hole in the hydrogen-charged specimen was compared with that of noncharged specimen. Hydrogen charging led to a marked increase in the crack growth rate. Typical characteristics of hydrogen effect were observed in the slip band morphology and fatigue striation. To elucidate the behavior of hydrogen diffusion microscopically in the fatigue process, the hydrogen emission from the specimens was visualized using the hydrogen microprint technique (HMT). In the hydrogen-charged specimen, hydrogen emissions were mainly observed in the vicinity of the fatigue crack. Comparison between the HMT image and the etched microstructure image revealed that the slip bands worked as a pathway for hydrogen to move preferentially. Hydrogen-charging resulted in a significant change in the phase transformation behavior in the fatigue process. In the noncharged specimen, a massive type α′ martensite was observed in the vicinity of the fatigue crack. On the other hand, in the hydrogen-charged specimen, large amounts of e martensite and a smaller amount of α′ martensite were observed along the slip bands. The results indicated that solute hydrogen facilitated the e martensitic transformation in the fatigue process. Comparison between the results of HMT and EBSD inferred that martensitic transformations as well as plastic deformation itself can enhance the mobility of hydrogen.

低炭素オーステナイト系ステンレス鋼の大気酸化挙動に及ぼす鋼中水素の影響

The influence of hydrogen content in steel on atmospheric oxidation behavior of low carbon austenitic stainless steel SUSF316L was investigated for clarifying the mechanism of SCC initiation in a boiling water reactor environment, which is closely associated with the localized oxidation and its acceleration. The stainless steel was charged with hydrogen by the means of cathodic electrolysis followed by subjecting the hydrogen-charged steels to the atmospheric oxidation test at 300°C for 10 h. The oxide films that formed on the non-charged and hydrogen-charged specimens were analyzed by optical microscopy, scanning electron microscopy, laser Raman spectroscopy and glow discharge optical emission spectroscopy. Experimental results revealed that relatively thick oxide films were formed on the hydrogen-charged specimen resulting in the hydrogen accelerated oxidation (HAO), when the hydrogen content was 35 mass ppm. However, there was almost no difference between the non-charged and the hydrogen-charged from the viewpoint of oxidation behavior when the hydrogen content was 24 mass ppm. The spinel-type oxides such as Fe3O4 and FeCr2O4 were identified by laser Raman spectroscopy only on the specimen surface where the HAO occurred. In addition, the Cr concentration in the inner oxide layer of the oxidation-accelerated specimen was around 20% and was lower than that of the non-charged (30-35%). The HAO seemed to be closely associated with this decrease in Cr concentration in the inner oxide layer.