Uncharged Specimens Research Articles

Effects of hydrogen charge on microscopic fatigue behaviour of annealed carbon steels

ABSTRACTThe effects of hydrogen charge on cyclic stress–strain properties, slip band morphology and crack behaviour of annealed medium carbon steels (JIS‐S45C) were studied. The total strain range of the stress–strain hysteresis loop in the hydrogen‐charged specimen was smaller than that in the uncharged specimen. Localized slip bands were observed in the hydrogen‐charged specimen, while the slip bands were widely and uniformly distributed in the uncharged specimen. It is presumed that the decrease in the total strain range of the hysteresis loop is due to the slip localization caused by the hydrogen charge and cyclic loading. The sites of fatigue crack initiation were mostly at grain boundaries in the uncharged specimen. The sites of crack initiation in the hydrogen‐charged specimen were not only at grain boundaries but also at slip bands inside ferrite grains. These results imply that hydrogen enhances dislocation mobility along slip bands and results in slip localization. These slip bands then attract hydrogen. This mechanism of hydrogen–slip band interaction may play an important role in the hydrogen‐influenced metal fatigue.

900 MPa 級低合金鋼 SCM435 の引張特性に及ぼす水素の影響

We investigated the effect of hydrogen on the tensile properties of a quench-tempered low-alloy steel, SCM435, with the tensile strength of 930 MPa used for hydrogen storage cylinders. Tensile specimens were machined from a cylinder with the inside and outside diameters of 245 and 315 mm. The specimens were immersed in a 20 mass% aqueous solution of ammonium thiocyanate (NH4SCN) at 313 K for 48 hours and then charged with hydrogen. Tensile tests were performed in the air at room temperature. The cross head speed was ranged from 0.01 to 100 mm/min. Hydrogen-charged specimens were hold in the air for a period of 1 and 300 hours. The 0.2% proof stress and tensile strength for the hydrogen-charged specimens were similar to those for the uncharged specimens, whereas the reduction of area was lower in the hydrogen-charged specimens than in the uncharged specimens. Thermal desorption spectroscopy showed that the residual hydrogen contents in the hydrogen-charged specimens fractured by tensile tests were between 0.14 and 0.93 mass ppm. The reduction of area of the hydrogen-charged specimens decreased linearly with increasing residual hydrogen content. Scanning electron microscopy showed that the cup-corn fracture occurred in the hydrogen-charged and the uncharged specimens and that the fracture surfaces were covered with dimples. The normal stress fracture area in the center of the hydrogen-charged and uncharged specimens was almost the same. The shear stress fracture area near the specimen surface was wider in the hydrogen-charged specimens than in the uncharged specimens. This means that hydrogen enhances slip deformation near the specimen surface and resulted in the lower reduction of area in the hydrogen-charged specimen. We therefore concluded that the hydrogen embrittlement behavior of the 900-MPa-class SCM435 steel was explained by the hydrogen enhanced localized plasticity model rather than by the lattice decohesion model.

The Effect of Hydrogen on Fatigue Crack Growth Behavior and Ductility Loss of Austenitic Stainless Steels

The effect of hydrogen on fatigue crack growth behavior and ductility loss after fatigue cycles of austenitic stainless steels, SUS 304 and SUS 316, was investigated. In the hydrogen-charged specimens, the crack growth rates were approximately twice higher than those of the uncharged specimens for crack growing from 100μm to 1000μm. The aspect ratio for the crack with 2a≥800 μm was smaller in the hydrogen-charged specimens than in the uncharged specimens due to higher hydrogen content in the surface layer than in the subsurface. The slip band density around the crack in the hydrogen-charged specimens was less than in the uncharged specimens. The ductility loss was measured by tensile test carried out by interrupting fatigue test. There was a critical crack length for a decrease in ductility. There were no definite differences in ductility loss between the hydrogen-charged specimens and the uncharged specimens within the hydrogen content and the test strain rate of this study. The ductility loss in hydrogen-charged specimens was dependent only on the length of the main crack produced during fatigue cycles. Although many small cracks were observed on the surface of hydrogen-charged specimens, they did not give more influence on the ductility loss than fatigue cracks. These small cracks were nucleated by the synergetic effect of the hydrogen contained in specimen, the applied tensile stress and the martensitic transformation which was detected after tensile test.

Effects of Hydrogen Charge on Fatigue Strength of Stainless Steels

The effect of hydrogen on fatigue strength of stainless steels was investigated. There were no definite differences in fatigue life of unnotched specimens between hydrogen-charged specimen (2.46.4 ppm) and uncharged specimen (2.2ppm) of SUS 304. High content of hydrogen (10100 ppm) was detected in a very thin surface layer, approximately at 100200 μm from specimen surface of SUS 304 specimens hydrogen-charged for 336672 hours. There were also no definite differences in fatigue life of unnotched specimens between hydrogen-charged specimen (1.85.8 ppm) and uncharged specimen (0.1ppm) of SUS 405. The uncharged specimens of 0.7 C13 Cr steel (0.2 ppm) failed from subsurface non-metallic inclusions in surface layer. ODAs (Optically Dark Area) were not observed due to the tensile residual stress in the surface layer although the specimens failed in the high cycle regime of 107 cycles. Increasing hydrogen content from 0.2 ppm to 2.42.7 ppm, the fatigue strength and fatigue life were markedly decreased in 0.7 C13 Cr steel.

Effect of Hydrogen on Fatigue Crack Growth and Martensitic Transformation of Stainless Steels

The effect of hydrogen on fatigue crack growth behavior of four stainless steels has been investigated from the viewpoint of martensitic transformation. The crack growth rates in hydrogen-charged SUS304 and SUS316 were accelerated. The crack growth rate in hydrogen-charged SUS316L was slightly higher than uncharged SUS316L. However, the crack growth rate in SUS405 hardly changed in comparison with uncharged specimens. The matensitic transformation on fatigue fracture surface was detected by X-ray diffraction both in hydrogen-charged and uncharged specimens of SUS304, SUS316 and even in SUS316L. However, the fracture surface of SUS316L, in which the crack growth rate was increased slightly by hydrogen, showed less martensitic transformation than that of SUS304 or SUS316. It is presumed that martensitic transformation in the vicinty of fatigue crack tip contributed to the effect of hydrogen on crack growth rate. Fatigue tests of SUS304 and SUS316L, which were pre-strained at -70°C to enhance a martensitic transformation, were carried out to study the influence of hydrogen and martensite on crack growth. Crack growth rate was remakably increased by hydroggen in not only pre-strained SUS304 but also in pre-strained SUS316L. The hydrogen content of pre-strained hydrogen-charged specimen was much higher than unstrained hydrogen-charged specimens due to the increase in martensite through which hydrogen diffuses much easier and faster than through austenite. The slip bands around crack tip in the hydrogen-charged specimens were less and more discrete than that in the uncharged specimens.

Effects of Test Frequency on Fatigue Behaviour in a Tempered Martensitic Steel with Hydrogen Charge

Effects of hydrogen charge and frequency on fatigue behaviour were studied in bearing steel (JIS-SUJ2) tempered at 823K. The crack growth rate of the hydrogen-charged specimen was faster than that of the uncharged specimen. The morphology of the crack observed on the surface of the hydrogen-charged specimen was thinner than that of the uncharged specimen. It is presumed that the slip deformation at the crack tip was restricted and localized by hydrogen. The definite dependency on test frequency in the crack growth rate was observed in the hydrogen-charged specimens. The lower the test frequency, the faster the crack growth rate in the test of the hydrogen-charged specimens at the frequency of 0.5∼15Hz. However, there was no clear difference in the fracture surface morphology between the uncharged specimens and the hydrogen-charged specimens at the frequency of 0.5∼15Hz. The transgranular fracture caused by fatigue was dominant and the intergranular fracture caused by so-called delayed fracture was little in all of the specimens. Therefore, it is presumed that the increase in the crack growth rate in the hydrogen-charged specimens at lower frequency was caused by enhancement of the fatigue mechanism related to hydrogen motion coupled with slip behaviour. Thus, it is presumed that the dependency on test frequency in the crack growth rate in the hydrogen-charged specimens was caused by the synergetic effect of the enhancement of hydrogen diffusion motion to the slip bands at the crack tip with time and easiness of slip motion by hydrogen.

3507 水素ステーション蓄圧器用SCM435鋼の低サイクル疲労特性におよぼす水素と周波数の影響(S20-2 水素チャージ材の強度特性,S20 材料強度特性に及ぼす水素の影響)

Low cycle fatigue tests on SCM435 steel tempered at 580℃ were carried out under the frequencies of 0.2, 2 and 20Hz with specimens containing a small artificial hole. The fatigue life of the hydrogen charged specimens decreased in comparison with the uncharged specimens. The acceleration of fatigue crack growth rate was observed at ΔK<40MPa√<minf=20Hz>, ΔK<60MPa√<minf=2Hz> and ΔK<100MPa√<minf=0.2Hz>. Facets were formed on the fracture surface of hydrogen charged specimens in the ΔK region where the crack growth rate was accelerated. Many slip bands were observed at the crack tip of the uncharged specimens. On the other hand, slip bands were not clear the hydrogen-charged specimens.

炭素鋼の繰返し応力―ひずみ特性および疲労挙動に及ぼす水素チャージの影響

Effects of hydrogen charge on fatigue behaviour of two carbon steels, JIS-S10C (SAE1010) and JIS-S45C (SAE1045) were investigated. There was no hydrogen effect in the cyclic stress-strain hysteresis loops of S10C hydrogen-charged with 0.2 ppm. On the other hand, the strain amplitude was decreased in S45C hydrogen-charged with 0.8 ppm. The delayed yielding and the decrease in the saturated value of the strain amplitude were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C under the constant stress amplitude tests. It is supposed that the degree of influence of hydrogen on cyclic stress-strain properties depends or material structure and/or hydrogen content. The effect of hydrogen charge (H : 0.5 ppm) on the fatigue life, the fatigue limit and the crack growth curves of S45C were not remarkable, while there was a distinct difference in the morphology of the slip bands between the hydrogen-charged and uncharged specimens. The localized slip bands were observed in the hydrogen-charged specimen of S45C. Therefore, it is presumed that the decrease in the strain amplitude in hysteresis loop by hydrogen charge is caused by the localization of slip bands. More crack initiations from ferrite grains were observed in the hydrogen-charged specimen (H : 0.5 ppm) of S45C. This phenomenon also corresponds to the localization and the formation of slip bands.

Photoelectrochemical Study of Hydrogen-Loaded Passive Film

The effects of hydrogen on the electronic structure of the passive film on iron were investigated by photoelectrochemical methods. It was found that hydrogen increases the magnitude of the photocurrents from the passive films formed at all investigated potentials, and decreases the bandgap energies of the passive films formed at potentials lower than 0.6 V, implying that hydrogen can affect the bond strength of the oxides comprising the passive films. Increasing the film formation time increases the magnitude of the photocurrent from the passive film on the charged specimens more significantly than that on the uncharged specimens. In addition, chloride ion shows a different influence on the magnitudes of the photocurrents from the passive films on the uncharged and charged specimens. The magnitude of the photocurrent from the passive film on the uncharged specimen is hardly influenced by chloride ions, whereas the magnitude of the photocurrent from the passive film on the charged specimen is decreased due to the presence of chloride ions in the solution. Measurements of photocurrent transient behavior show that as a result of the presence of hydrogen in the passive film, the peak value of the photocurrent becomes smaller than that of the cathodic dark current, which is contrary to the photocurrent transient behavior of the uncharged passive film. The change in photocurrent transient behavior of the passive film caused by hydrogen also suggests that hydrogen can influence the electronic structure of the passive film. © 2003 The Electrochemical Society. All rights reserved.

The Influence of Hydrogen and Tensile Stress on Passivity of Type 304 Stainless Steel

The effects of hydrogen and applied tensile stress on the passivity of type 304 stainless steel were investigated in borate buffer solutions containing chloride ions. Hydrogen was cathodically charged into specimens by applying a cathodic constant current density. Applied tensile stress is found to have no obvious effect on the breakdown potential of uncharged specimens unless the concentration of chloride ions in the borate buffer solution is high. Hydrogen significantly decreases the breakdown potential. For charged specimens at various tensile stress levels, breakdown potentials are well fitted as a function of charging current density using the second-order exponential decay equation. At a certain charging current density, the breakdown potential and applied tensile stress approximately follow a linear relationship. The slope of the straight line becomes more negative in the intermediate charging current density range than in both low and high current density ranges, indicating that tensile stress has a more obvious promoting effect on the breakdown of passive films formed on specimens charged in this current density range. It was observed that hydrogen increases the anodic current density in the passive range. Applied tensile stress affects the anodic current density in various degrees for the specimens charged at different charging current densities. © 2003 The Electrochemical Society. All rights reserved.

Investigation of Hydrogen-Promoted Pitting by the Electrochemical Noise Method and the Scanning Reference Electrode Technique

Hydrogen-promoted metastable pitting of iron and its development into stable pitting were first investigated by the combination of the electrochemical noise method and the scanning reference electrode technique (SRET). Time records of current noise show that hydrogen significantly increases the number of current fluctuations, inferring that hydrogen might increase the initiation of pitting, and hydrogen also increases the magnitude of current fluctuations, suggesting that hydrogen might promote the growth of metastable pits. In addition, the duration of the time period for initiation of metastable pitting on the charged specimen is longer than that on the uncharged specimen, supporting that the active sites which induce pitting on the charged specimen might be much more numerous than on the uncharged specimen. An analysis of the power spectrum density shows that the level of the frequency-independent plateau of the charged specimen is higher than that of the uncharged specimen and the roll-off frequency t...

Effects of hydrogen and tensile stress on the breakdown of passive films on type 304 stainless steel

The effects of hydrogen and applied tensile stress on the breakdown of passive films on Type 304 stainless steel have been investigated in chloride-containing solutions. Hydrogen was cathodically introduced into specimens by applying a constant current density. The current responses to the application of a passivation potential of 0.3 V versus a saturated calomel electrode (SCE) and additions of various concentrations of chloride ions were recorded. Hydrogen greatly decreases the critical chloride concentration for the breakdown of passive films, which indicates that hydrogen promotes the breakdown of passive films. Hydrogen is also found to hinder the repassivation process. With an increase in applied tensile stress, the critical chloride concentration for the breakdown of a passive film decreases. The critical chloride concentrations are always significantly lower for charged specimens than for uncharged specimens at the applied tensile stress range and the effect of stress on lowering the critical chloride ions is more significant for charged specimens than uncharged specimens.

Effects of Hydrogen on Semiconductivity of Passive Films and Corrosion Behavior of 310 Stainless Steel

The effects of hydrogen on the semiconductive properties and compositions of passive films on AISI 310 stainless steel (310 SS) and their corrosion behavior were investigated by Mott‐Schottky plot, polarization, noise resistance analyses, and secondary ion mass spectrometry (SIMS). The results indicate that the susceptibility of 310 SS to pitting is strongly influenced by hydrogen present in the specimens. The conductivity type of the passive film formed at 0.7 V vs. saturated calomel electrode on the specimens charged with hydrogen at the quantity higher than is p‐type while that formed on the uncharged specimen is n‐type, i.e., the presence of hydrogen in 310 SS causes an inversion of conductivity type of a surface film from p‐type to n‐type. When the concentration of ionized hydrogen in the passive film on the 310 SS, i.e., the donor density reaches the saturated value, the average content of the diffusible hydrogen released from a specimen measured by a U‐tube filled with mercury under the condition of mechanical vacuum is . The donor concentration evaluated from the Mott‐Schottky slope is about in the order of . The susceptibility of 310 stainless steel (SS) to pitting can be correlated to the electronic properties and chemical compositions of the passive film. The high susceptibility of the charged specimens to pitting corresponds to the n‐type conductivity of the passive film. While low susceptibility is connected to p‐type conductivity and high content of chromium and nickel oxides. Such a correlation can be explained with the band model of a passive film. © 1999 The Electrochemical Society. All rights reserved.

Gaseous hydrogen embrittlement of a hydrided zirconium alloy

ZIRCALOY-4 plate specimens were gaseously hydrided up to 340 ppm H and then tested in a hydrogen gas environment of various pressures up to 2020 kPa at 25 °C, 100 °C, and 200 °C. Notched tensile specimens were chosen to better understand the “ductile-brittle transition” associated with hydrogen content and hydrogen pressure. The purpose of the present investigation is to understand the synergistic effect of hydrogen gas and internal hydrides on the mechanical properties of ZIRCALOY-4. The results showed that for both uncharged and hydrided specimens, the notch tensile strength decreased with increasing hydrogen pressure as well as increasing temperature. Compared with uncharged specimens, the specimens with hydrides had lower values of notch tensile strength. A ductile-brittle transition was found on specimens tested at 25 °C and at hydrogen pressures between 0 and 1010 kPa. For the specimen containing 220 ppm H, the reduction of area (RA) at 25 °C and at hydrogen pressures of 1010 kPa and above was drastically reduced, resulting in almost completely brittle behavior. This hydrogen and hydride-induced cracking was found to be an autocatalytic process. From the fractographic finding, the ductile-brittle transition was closely related to the precipitation and distribution of brittle hydrides. The ductile-brittle transition disappeared as the temperature increased to 100 °C and above. This can be attributed to the improved ductility of the zirconium matrix with increasing temperature.

Hydrogen embrittlement of Ni-Cr-Fe alloys

The purpose of this work was to investigate the role of chromium on hydrogen embrittlement of Ni-Cr-Fe alloys and thus to develop a better understanding of the low-temperature stress corrosion cracking (SCC) phenomenon. The effect of chromium on hydrogen embrittlement was examined using tensile tests followed by material evaluation via scanning electron microscopy (SEM) and light optical microscopy. Four alloys were prepared with chromium contents ranging from 6 to 35 wt pct. In the uncharged condition, ductility, as measured by the percent elongation or reduction in area, increased as the alloy chromium content increased. Hydrogen appeared to have only minor effects on the mechanical properties of the low-chromium alloys. The addition of hydrogen had a marked effect on the ductility of the higher-chromium alloys. In the 26 pct chromium alloy, the elongation to failure was reduced from 53 to 14 pct, with a change in fracture mode from mixed ductile dimple and ductile intergranular failure to a brittle appearing intergranular failure. A maximum in embrittlement was observed in the 26 pct Cr alloy. The maximum in embrittlement coincided with the minimum in stacking-fault energy. It is proposed that the increased hydrogen embrittlement in the high-chromium alloys is due to increased slip planarity caused by the lower stacking-fault energy. Slip planarity did not appear to affect the fracture of the uncharged specimens.

Mechanical properties of a Ti [sbnd]15V [sbnd]3Cr [sbnd]3Sn [sbnd]3Al alloy affected by the impurity hydrogen

The effect of hydrogen on the tensile properties of a β titanium alloy (Ti 15V 3Cr 3Sn 3Al) has been studied. Cold-rolled specimens and specimens charged with hydrogen (annealed at 500°C for 1 h at atmospheric pressure of hydrogen gas) were solution-treated at 800°C for 1 h, water-quenched and subsequently aged at 510°C. Significant retardation in the age-hardening characteristics of the alloy occurred by the hydrogen charging. The effect of hydrogen charging on the ductility was not found except in the as-solution-treated condition, where slight decrease (from 65% to 60%) in the reduction in area by the charging was detected. However, when tensile tests were carried out using a unique testing machine equipped with an ultra high vacuum chamber and a quadrupole mass spectrometer, a large amount of hydrogen gas was evolved from the specimen at the moment of fracture. Furthermore, the ductility of the uncharged specimen was improved with a reduction in the hydrogen content. Thus it can be concluded that the impurity hydrogen even at the uncharged level has a strong correlation with the fracture and causes a deleterious effect on the ductility.

Hydrogen-assisted cracking in a precipitation-hardened stainless steel: effects of heat treatment and displacement rate

The hydrogen embrittlement susceptibility of PH 13-8 Mo stainless steel was evaluated using non-linear fracture mechanics methods. The initiation toughness, J i, and the resistance to stable crack growth, d J/d a, were measured using precracked compact specimens. Specimens were electrochemically charged with hydrogen prior to fracture testing in air. After charging, a monotonically increasing load-line displacement was applied to produce the J-integral curve for stable crack growth. Crack length was monitored by the direct-current electric potential method; current switching eliminated thermal voltage contributions. The fracture properties of PH 13-8 Mo stainless steel were severely degraded by hydrogen. J i for material in the hydrogen-charged condition was degraded by as much as 98% compared to the uncharged condition. The fracture mode exhibited dramatic transitions from microvoid coalescence in uncharged specimens to intergranular cracking in charged specimens tested at slow displacement rate. Toughness tests performed at higher displacement rates exhibited less susceptibility and failed by a mixed mode of transgranular facets and microvoid coalescence. The rate dependence is an indication that the higher displacement rates limit hydrogen transport to the crack-tip process zone. The contributions of hydrogen transport by diffusion and by dislocation transport to the crack-tip process zone are discussed. The displacement rate is related to a local controlling crack-tip parameter, or crack-tip strain rate.

Cracking of duplex stainless steel due to dissolved hydrogen

Ferallium 255 duplex stainless steel was cathodically precharged with hydrogen at 265 °C in a molten salt electrolyte. Sustained load tests were carried out in air at 0 °C, 25 °C and 50 °C with average hydrogen contents from 3 to 15 wt ppm. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position and allow calculation of crack velocity and stress intensity. The crack velocityvs stress intensity (K) curves generally rose gradually over a large range inK and had definite thresholds for subcritical crack growth. Second and third stages were not always clearly delineated. Threshold stress intensities decreased as hydrogen content increased. An identifiable stage II occurred most often for alloys containing about 10 wt ppm dissolved hydrogen. The crack growth velocities generally increased with increasing temperature or hydrogen content. As the dissolved hydrogen increased, the fracture mode changed from microvoid coalescence (MVC) to microcrack coalescence (MCC) with some tearing ridges. At high hydrogen content, both ferrite and austenite phases showed brittle morphology, which was identical to the fracture surface of the uncharged specimens tested in hydrogen gas at 108 kPa pressure. Comparing the embrittling effect of internal hydrogen with that of external hydrogen it is found that the threshold stress intensity in hydrogen gas at 1 atm is lower than that at the highest internal hydrogen concentration (15 wt ppm). In the case of external hydrogen, the hydrogen source is at the crack tip, whereas for the internal hydrogen case, outgassing reduces the hydrogen content in this region, even when the bulk hydrogen content is fairly high.

Internal hydrogen embrittlement of a ferritic stainless steel

AL 29-4-2 ferritic stainless steel was cathodically precharged with hydrogen at 230 ‡C in a molten salt electrolyte. Constant load crack growth tests were performed in air and in hydrogen gas at 108 kPa pressure at room temperature on both uncharged specimens and specimens containing 2 wt ppm hydrogen. The DC potential drop method was calibrated with optical measurements to continuously monitor the crack position, so that the crack velocity vs stress intensity relation could be calculated. From the test results, it was found that dissolved hydrogen does allow subcritical crack growth for tests in air, but its effect is masked by the external hydrogen for tests in hydrogen gas. The distribution of slip bands on the side surface indicates that the dissolved hydrogen causes localization of plasticity, with the strain concentrated in coarse slip bands. Comparing the embrittling effect of internal hydrogen with that of external hydrogen, it can be concluded that environmental hydrogen is far more damaging than internal hydrogen, at least in specimens containing 2 wt ppm hydrogen.

Hydrogen embrittlement in pre-deformed Ni 3Al alloy

Hydrogen embrittlement has been investigated in continuous cast sheet of a Ni 3Al alloy (Ni 77.82Al 21.73Zr 0.34B 0.1) after imparting various amounts of pre-deformation. The specimens (25.4 mm long, 4.4 mm wide and 1 mm thick) were deformed in tension and then taken out of the testing rig for charging hydrogen cathodically for 120 min with a current density of 50 mA cm −2. Upon testing again in tension, the specimens charged with hydrogen always failed after yielding, which occurred at the final pre-deformation stress. For instance, a specimen pre-deformed to 26% elongation and subsequently charged for 120 min failed (during tensile-testing at the strain rate of 5.8 × 10 −5 s −1) at a true stress of 1611 MPa, which is comparable to the true fracture stress (1753 MPa) of the uncharged specimen. However, the elongation at which the charged specimens failed decreased drastically from 5.4% to 1.3%, as the pre-deformation increased from zero to 2.0%. The fracture surface of the charged specimen consisted of regions of intergranular, quasi-cleavage and dimple fracture. The proportion of intergranular fracture decreased with the increasing pre-deformation, while that of dimple fracture increased. Our results suggest that grain boundaries in B-doped Ni 3Al were not weakened by hydrogen, rather ductility-related voids and cracks were involved in hydrogen embrittlement of B-doped Ni 3Al alloy.