Yuhei Ogawa

National Institute for Materials Science | NIMS · Research Center for Structural Materials

Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogen...

The tensile mechanical properties of a Ni-based superalloy 718 uniformly precharged with ≈ 90 mass ppm hydrogen were investigated under a wide range of temperatures to shed light on the long-standing uncertainties surrounding the H-related embrittlement mechanisms of the material. The detrimental effect of H on ductility was found to be substantial...

After pre-charging at the gaseous phase with a concentration of ∼7,000 at. ppm, solute hydrogen was discovered to have an abnormal effect on both the strength and ductility enhancement of a commercially-available, Fe-24Cr-19Ni-based, stable austenitic stainless steel that had been subjected to tensile testing at various strain-rates. Specifically,...

For the wall-thickness reduction of the components destined for pressurized gaseous hydrogen, widespread use of high-strength martensitic steels has long been desired. However, their strong susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG) is still limiting their proactive applications. Here, we instead focused on pearlite as anothe...

Microstructural paths of hydrogen-assisted fatigue crack growth (HAFCG) in tempered martensitic steels were investigated relying on martensite boundary characteristics. Factors determining the HAFCG paths were tensile strength (TS)-dependent. HAFCG paths occurred preferentially along prior austenite grain boundaries with a brittle trend in 1025-MPa...

Mechanical properties of structural alloys, including Ni-based superalloy 718 (Alloy718), are degraded when hydrogen (H) is supplied: hydrogen embrittlement (HE). The presence of H notably deteriorates fatigue crack growth (FCG) property, which renders the growth rate much higher and shortens the lifetime of the components operating in the hydrogen...

Recent progress in realizing hydrogen society requires higher-strength steels having decent hydrogen embrittlement resistance via optimized microstructural controls. The present communication first reports the fatigue crack growth (FCG) property of pearlitic steel in a high-pressure, i.e., 90 MPa, hydrogen gas environment to probe this standard mic...

Fatigue tests of circumferentially notched Type 304 austenitic stainless steel (ASS) illuminated the unique dependences of fatigue limit on the stress concentration factor, Kt, and stress ratio, R. Fatigue limit under negative R was superior to that under positive R, a result was unexpected from the empirical behavior of smooth ASS. Moreover, switc...

Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth dista...

Tensile tests of five commercial Fe–Cr–Ni-based austenitic alloys were conducted after thermal hydrogen precharging in a pressurized gaseous environment. The divergence in Cr and Ni concentrations affected the hydrogen solubility significantly as well as the impacts of dissolved hydrogen on the mechanical performance of the alloy. Hydrogen solubili...

The effects of Ni concentration and dispersed conditions of vanadium carbide (VC) nano-particles on the hydrogen embrittlement (HE) behavior of precipitation-hardened high-Mn austenitic steels were investigated under the presence of thermally pre-charged hydrogen. Slow strain-rate tensile tests revealed that HE susceptibility decreased with an addi...

The impact of hydrogen on crack-tip plastic-zone development was revisited via a novel approach, utilizing the measurement of fatigue crack-growth retardation in a medium-strength martensitic steel after a single overloading in laboratory air and in 90-MPa-hydrogen gas. The plastic zone can be characterized according to the crack-propagation length...

Fatigue crack growth of two carbon steels with different pearlite volume fractions were studied in pressurized gaseous hydrogen environment. Notably, pearlite was found to mitigate hydrogen-assisted fatigue crack acceleration. This positive impact of pearlite was ascribed to ferrite/cementite lamellar aligned perpendicularly to the cracking directi...

Investigations were conducted on the interaction-effects of adjacent, circumferential V-notches on the fatigue limit of Alloy 718, with the aid of fully-reversed, tension-compression fatigue tests. Contrary to expectation, specimen with seven adjacent circumferential notches had lower fatigue limit than that with a single circumferential notch. Thi...

Toward a better understanding of the hydrogen embrittlement characteristics in nickel-based superalloy 718, tensile tests were performed under hydrogen pre-charged states (internal hydrogen) as well as in hydrogen gas environment (external hydrogen) at various temperatures ranging from −196 to 300°C. Under the internal hydrogen conditions, hydrogen...

Fatigue strength of Ni-based superalloy 718 was investigated relative to shear-mode crack-growth thresholds, to evaluate the influence of crack-opening/-closing stresses perpendicular to shear-mode crack-planes. The impact of precipitation-hardening was also examined. Using solution-treated and precipitation-hardened materials, three different type...

Four, hot-rolled, plain-carbon steels with varying carbon content were subjected to slow strain-rate tensile (SSRT) tests in a 95-MPa gaseous hydrogen environment at ambient temperature. The influence of pearlite volume fraction on the magnitude of hydrogen-induced degradation of the materials’ strength and ductility was thereby determined. Hydroge...

Hydrogen-assisted, fatigue crack-growth in pure iron within a low stress-intensity range was ascribed to the emergence of intergranular fracture, the significance of which was emphasized by increased hydrogen-gas pressure, while conversely mitigated by an elevation of test temperature. Based on conventional thermodynamic theory, a single parameter,...

Fatigue strength of precipitation-hardened materials is well known to be lower than that expected from their high yield and tensile strengths. To elucidate the reasons behind such anomaly, torsional fatigue tests, in addition to uniaxial fatigue tests, were conducted on samples of the Ni-based superalloy 718, subjected to two different heat treatme...

The sensitivity of the fatigue limit of an additively manufactured, Ni-based superalloy 718 sample to surface finishing conditions and solute hydrogen was investigated via a series of tension–compression fatigue tests. The results revealed that neither defects nor hydrogen diminished the fatigue limit of the sample. The high defect tolerance of thi...

Fatigue crack-growth (FCG) of Ni-based superalloy 718 was investigated under gaseous hydrogen environment (external hydrogen) and uniformly pre-charged state (internal hydrogen). Under external hydrogen, intergranular fracture predominated, whereas dislocation slip-band or twin boundary fracture were prevalent under internal hydrogen. This failure...

The fatigue limit of a circumferentially-notched austenitic stainless steel exhibited peculiar improvement under fully-reversed loading (R =−1), as compared with that under tension-tension loading (R = 0.1), behavior considered to be unconventional in smooth specimens of similar materials. This was attributed to the different cyclic stress-strain r...

Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompa...

In order to clarify the contribution of dislocation‒hydrogen interaction on the hydrogen embrittlement (HE) of pure Ni and of Cu‒55 wt% Ni binary alloy, slow strain rate tensile (SSRT) tests were conducted at room temperature (RT) and at 77 K on hydrogen-precharged specimens. Regarding the SSRT test at RT, hydrogen increased the flow stress and ind...

The role of hydrogen in tensile ductility loss and on the fracture behaviours of Ni-based superalloy 718 was investigated via tensile tests under hydrogen-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen), in combination with post-mortem analyses of fractured samples using electron microscopy techniques....

The influences of internal and external hydrogen on the tensile ductility loss and fracture behaviors of a precipitation-hardened Ni-based superalloy 718 were investigated via slow strain rate tensile (SSRT) testing under hydrogen pre-charged conditions (internal hydrogen) or in gaseous hydrogen environments (external hydrogen) . Severe degradation...

In order to elucidate the temperature dependence of hydrogen-assisted fatigue crack growth (HAFCG), the fatigue crack growth (FCG) test was performed on low-carbon steel JIS-SM490B according to ASTM E647 using compact tension (CT) specimen under 0.7 MPa (≈ 0.1 ksi) hydrogen-gas at room temperature (RT: 298 K (≈ 77 °F)) and 423 K (≈ 302 °F) at stres...

The effect of hydrogen on the deformation and fracture behavior in pure Cu, pure Ni and Cu–Ni alloy was studied via tensile tests of H-charged, smooth and circumferentially-notched specimens at room temperature (RT) and 77 K. Hydrogen-diffusion properties were determined by the desorption method. To obtain a uniform hydrogen concentration in the H-...

Elasto-plastic fracture toughness tests of a commercially pure iron were performed in air and in hydrogen gas at two different pressures. Some unique characteristics of hydrogen-enhanced cracking were exhibited at both the macroscopic and microscopic length scales, based on the observation of fracture surface, fracture plane, plasticity distributio...

Tension‐compression fatigue tests were performed on two types of Ni‐based superalloy 718 with different microstructures in which small artificial defects of various sizes and shapes were introduced. The susceptibility of the fatigue strength to the defects varied significantly with the microstructure morphology, ie, a smaller grain size made the al...

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 th...

To elucidate hydrogen embrittlement (HE) of pure Ni and Cu‒Ni alloy, slow strain rate tensile (SSRT) tests of hydrogen (H)-charged specimens were conducted at room temperature (RT) and 77 K. At RT, the tensile properties of both pure Ni and Cu‒Ni alloy were degraded, accompanied by the formation of intergranular (IG) facets (pure Ni) and flat fract...

The hydrogen embrittlement mechanism of a precipitation-hardened iron-based superalloy A286 was investigated by slow strain rate tensile (SSRT) test in combination with the analyses of surface slip step patterns and internal deformation substructures via optical microscopy and scanning electron microscopy (SEM) techniques. Hydrogen was introduced i...

Fatigue crack growth (FCG) tests were performed with two types of metastable austenitic stainless steels having different austenite phase stabilities under hydrogen-precharged conditions (internal hydrogen) and in gaseous hydrogen environments (external hydrogen). The materials showed a peculiarly slower FCG rate with internal hydrogen than with ex...

A brittle-like faceted morphology of a precipitation-strengthened Fe-Ni-Cr-based superalloy after charging via exposure to high-pressure hydrogen gas (100 MPa) at elevated temperature (543 K) was interpreted based on multiple electron microscopy observations: scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron c...

The fatigue limit properties of a carbon steel and a low-alloy Cr–Mo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the Cr–Mo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtai...

Hydrogen effect on fatigue performance of commercially pure BCC iron has been studied with a combination of various electron microscopy techniques. The fatigue crack growth (FCG) in gaseous hydrogen was found to consist of two regimes corresponding to a slightly accelerated regime at relatively low stress intensity factor range, ΔK, (Stage I) and t...

To develop safer and more cost-effective high-pressure hydrogen tanks used in fuel cell vehicles (FCVs), the metallic materials with the following three key properties, i.e. lightweight, high strength and excellent resistance to hydrogen embrittlement should be explored. In this study, the compatibility of high-strength, precipitation-hardened alum...

A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAF...

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subseque...

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to...

In order to study the influence of hydrogen on plastic deformation behavior in the vicinity of the fatigue crack-tip in a pure iron, a multi-scale observation technique was employed, comprising electron channeling contrast imaging (ECCI), electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The analyses successfull...

To investigate the effect of hydrogen on fatigue life characteristics and crack growth behaviors through the entire fatigue life of a carbon steel, tension-compression fatigue tests and elasto-plastic fracture toughness tests were conducted in a hydrogen gas environment under the pressures of 0.7 and 115 MPa. The fatigue tests revealed that the fat...

In order to develop more energy-efficient and safer, hydrogen pre-cooling systems destined for use in hydrogen refueling stations, a metallic material must first be researched and found to possess three excellent material properties: high strength, high thermal conductivity and low susceptibility to hydrogen embrittlement (HE). This study investiga...

In order to develop safer and more energy-efficient, hydrogen pre-cooling systems for use in hydrogen refueling stations, it is necessary to identify a high-strength metallic material with greater thermal conductivity and lower susceptibility to hydrogen embrittlement, as compared with ordinary, stable austenitic stainless steels. To accomplish thi...

For the storage and transportation of compressed gaseous hydrogen in forthcoming hydrogen energy-based society, hydrogen-assisted fatigue crack growth (HAFCG) in structural steels is one of the most considerable problems from the perspective of life-cycle assessment or safety use of the hydrogen-containing components. In this study, fatigue crack g...

In order to experimentally verify the fracture mechanism of a precipitation-hardened superalloy with internal hydrogen, we performed SEM observation of fracture surface and cross sectional area beneath the fracture surface of an A286 iron-based superalloy subjected to an internal hydrogen tensile testing, which was carried out in air after being ch...

Tension-compression fatigue tests using smooth specimens of low carbon steel JIS-SM490B were carried out in air and hydrogen gas environment under the pressure of 0.7 and 115 MPa at room temperature. In 0.7 MPa hydrogen gas, fatigue life curve was nearly equivalent to that in air. On the other hand, in 115 MPa hydrogen gas, fatigue life was signifi...

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...