Degree Of Hydrogen Embrittlement Research Articles

Local strain evolution and microstructural characterisation of hydrogen-induced damage at different strain rates in dual phase (DP 780) steel

Effect of strain rate on the damage behaviour of hydrogen (H)-charged dual phase (DP 780) steel via the in-situ digital image correlation (DIC) technique is investigated in this work. Since stress concentration sites like notches are common in engineering practice, two types of uniaxial tensile tests have been carried out using smooth and notch tensile specimens for detailed analysis. The study reveals the significance of hydrogen embrittlement in DP 780 steel, as no strain rate effect is observed on the mechanical property in the case of an uncharged smooth tensile specimen. However, a significant effect of strain rate is detected after the H-charging. Hydrogen showed a lesser ability to aid the failure process when the applied strain rate is raised, as it could diffuse over a limited distance during the tensile test. The local axial and width strains, along with necking and fracture strains, are quantified for each specimen to understand the strain rate effect better. A centre-line crack is observed in every H-charged specimen's fracture surface owing to the presence of MnS inclusion in DP steel along the central line and its interaction with the atomic hydrogen. Moreover, the degree of hydrogen embrittlement is substantially higher in the notch tensile specimens than in the smooth ones.

Hydrogen-induced intergranular cracking of pure nickel under various strain rates and temperatures in gaseous hydrogen environment

Industrial-grade pure nickel, NI-201, was tensile tested in high-pressure hydrogen gas at various testing temperatures and strain rates. The degree of hydrogen embrittlement (HE) was strongly affected by the testing temperature and strain rate, and elevating the strain rate and lowering the temperature result in suppressed HE. While hydrogen trapped at GBs via local equilibrium dominated the HE degree in hydrogen-charged cases, hydrogen supply to GBs during the tests was required in gaseous hydrogen cases. Although hydrogen–dislocation interaction was one of candidates for the hydrogen supply, the lattice hydrogen, which can interact with dislocations, hardly existed and related behaviors such as increase in flow stress and serrated yielding were not observed in the gaseous hydrogen cases; therefore, the contribution of the hydrogen–dislocation interaction was speculated to be minor. Alternatively, we proposed an HE model considering hydrogen diffusion along grain boundaries (GBs) from the crack tip, which successfully demonstrated the experimental results.

Influence of microstructure on the mechanical properties and hydrogen embrittlement characteristics of 1800 MPa grade hot-stamped 22MnB5 steel

The influence of the microstructural characteristics on the mechanical and hydrogen embrittlement properties of 1800 MPa grade hot-stamped 22MnB5 steel was experimentally investigated using samples processed under various hot-stamping conditions, i.e., different heating temperatures and strain levels. The tensile strength increased with increasing hot-stamping temperature up to approximately 920 °C and subsequently decreased owing to the increasing sizes of the lath martensite and prior austenite phases. Some degree of internal strain was introduced into the 22MnB5 steel specimen during hot stamping at 920 °C, which led to a slightly higher hardness although no clear microstructural change was observed. The severity of hydrogen embrittlement of the hot-stamped 22MnB5 steel samples was investigated after immersion in a NH4SCN solution, and the degree of hydrogen embrittlement was found to be directly associated with the amount of hydrogen that penetrated into the grain boundary and lath martensite boundary. The high-strength 22MnB5 steel with a very small lath martensite phase exhibited severe hydrogen embrittlement due to the large amount of hydrogen in the sample, and the high internal strain (or high dislocation density) could lead to accelerated hydrogen embrittlement. Severe hydrogen embrittlement occurred upon charging with more than approximately 0.8 ppm hydrogen. Based on the obtained results, models are proposed for the hydrogen embrittlement characteristics of hot-stamped 22MnB5 steel.

Atomistic simulations of effect of hydrogen atoms on mechanical behaviour of an α-Fe with symmetric tilt grain boundaries

The effects of the hydrogen concentration, crystal orientation and grain size on the mechanical properties of an α-Fe bicrystal with symmetric tilt grain boundaries under tensile loading are investigated by molecular dynamics simulation. The results indicate that regardless of crystal orientation, the yield strength of bicrystal α-Fe decreases with the increase of hydrogen concentration. Hydrogen atoms have no influence on the primary dislocation (or twin) nucleation mechanism, but rather influence their multiplication process. The results also show that the degree of hydrogen embrittlement is obviously dependent on the misorientation angle, but it is almost independent of the grain size.

Corrosion fatigue crack growth rates of X70 base metal and heat affected zone in H2S corrosive environment

Fatigue crack growth rates (FCGRs) of X70 pipeline steel base metal (BM) and heat affected zone (HAZ) were studied in the hydrogen sulphide environment. X-ray diffraction method was used to measure the residual stress of BM and HAZ. Modified wedge open loaded specimen was selected for the test in the simulated actual working conditions. In the saturated H2S environment, the crack growth experiments were carried out under low frequency (f = 0·01 Hz) and three stress ratios (R = 0·7, 0·8 and 0·9). Results showed that there was obvious residual tensile stress in HAZ. The presence of H2S greatly accelerated the FCGR. With rising stress ratio, crack closure effect weakened, and then the increasing degree of hydrogen embrittlement of the crack tip leads to FCGR acceleration. Microstructure defects, inclusions and high residual tensile stress of HAZ made the FCGR of this area higher than the FCGR of BM. Scanning electron microscopy scans of the fracture suggested that the fracture mode of the corrosion fatigue of the X70 material belonged to brittle fracture. Cleavage fracture was the dominant form. The differences between corrosion fatigue fracture morphologies of BM and HAZ were not very large, and the relief of the fracture of HAZ was larger and the cracks longer.

Study of the Resistance of the Near-Weld Section of the Heat-Affected Zone to Hydrogen Embrittlement

Hydrogen embrittlement is one of the most common reasons for the premature failure of welded structures used in media that contain hydrogen. Studies have found that a high susceptibility to hydrogen embrittlement (HE) is shown by the metal of the near-weld section of the heat-affected zone ( HAZ NWS) [1, 2]. In light of this, we evaluated the effect of possibly controlling the resistance of HAZ NWS metal to embrittlement by changing the phase components and dispersity of the structure. We used normalized low-alloy steel 09G2S. Its chemical composition (%): 0.13 C; 0.74 Si; 1.61 Mn; 0.012 P; 0.11 Cr; 0.069 Ni; 0.17 Cu; 0.005 V; 0.03 Al; 0.005 Ti; 0.01 Mo. The mechanical properties: σ Y = 570 MPa; σ B = 650 MPa; δ = 21%. An HAZ NWS was simulated by using a high-frequency-current unit designed for use in a tube-rolling shop [3]. The maximum heating temperature was 1350°C and the rate of cooling w 8‐5 of the given section was varied from 3 to 30°C in order form different types of structures (Table 1) on the NWS ‐ from a coarse ferrite-pearlite structure to a bainite-marten site structure. The resistance of the different types of HAZ NWS structures to hydrogen embrittlement was evaluated on the basis of the amount of ductility lost during tests in which the method described in [4, 5] was used to subject previously hydrogenated and unhydrogenated specimens to delayed tension. We also performed such tests on specimens of the base metal ‐ steel 09G2S. Half of the flat 3-mm-thick specimens prepared in accordance with GOST 1497‐87 were hydrogenated electrolytically in a 0.1 N solution of sulfuric acid with a thiourea addition of 1.5 g/liter. Cathodic current density was 10 mA/cm 2 . After being hydrogenated, the specimens were subjected to delayed tension at room temperature in accordance with GOST 1497‐87. The testing was done on universal machine FPz-100/1. Active clamp speed was 0.3 mm/min and the load was 20 kN. The test results were compared with data obtained for similar specimens in the initial state. The degree of hydrogen embrittlement was evaluated on the basis of the decrease in the reduction of area ψ and elongation δ 5 , which were determined from the formulas

電気防食におけるステンレス鋼の防食限界電位の検討

The appropriate cathodic protecting potential of the two-phase stainless steel (SUS 329J4L) was investigated in artificial sea water by the constant load method. In the constant load method, the rupturing time was used as an estimation parameter. And then, the constant tensile stress test was able to be carried out at a short time before rupturing by using the estimation parameters of mechanical characteristics. It was found that the hydrogen embrittlement of steel occurred at less noble than potential -0.85V (vs. Ag/AgCl) by the decreasing of elongation and the contraction percentage of area. After the test for 400h test, 0.2% proof strength and tensile strength of steel were increased by embrittling hardening at less noble potential than -0.85V. At the potential occuring hydrogen embrittlement, the fractured surface differed from that at the potential unoccuring embrittlement the observation using scanning electron microscope (SEM), and total length of fracture was correlated with the degree of hydrogen embrittlement.

Embrittlement of amorphous Fe40Ni38Mo4B18 alloy by electrolytic hydrogen

The effects of hydrogen on the tensile properties and fracture processes at room temperature were investigated. Specimens were tested at various strain rates in air or under different cathodic charging-current densities. The slopes of the stress-strain curves were essentially identical for all the specimens, except that the fracture points varied under different test conditions. Macroscopically, hydrogen only affected the elastic deformation behavior, but microscopically, the embrittlement was caused by the heterogeneous nucleation of localized plastic deformation. The degree of hydrogen embrittlement increased as the charging current increased or as the strain rate decreased. With the same charging current and time, longer dynamic charging resulted in more severe embrittlement. Before fracture took place, the strength of the alloy could be completely restored if hydrogen had been removed. Hydrogen diffusivity and solubility were used to draw the time-dependent hydrogen concentration profiles for the specimens under different charging conditions. The difference in the mechanical properties was correlated with the hydrogen concentration within the specimen.

Hydrogen Embrittlement of Cast Al-AL2O3 Particulate Composites

The effect of hydrogen on the mechanical properties of cast Al-Al 2 O 3 composites, containing 2, 4 and 8 wt.% Al 2 O 3 , prepared by vortex casting process was assessed by tensile testing hydrogen pre-charged specimens. Hydrogen charging reduced both ductility and ultimate tensile strength of the composites. The degree of hydrogen embrittlement was lower in composites compared with the base metal. The hydrogen embrittlement behaviour is explained by considering the reversible and irreversible trap sites in the composites

Hydrogen induced fracture in a RSP AlLi alloy

Rapid solidification processing (RSP) is an important trend in manufacturing Al-Li alloys, which has many advantages over the ingot metallurgy (IM) method. But, the RSP introduces an important problem with aluminum alloys, namely that of oxide contamination. The extremely large surface areas associated with rapidly quenched particulates, coupled with the reactivity of aluminium and lithium, make it impossible to suppress oxidation of rapidly solidified particulates completely. Therefore, an oxide film is always formed on the powder surface of Al-Li alloys. The oxide film usually contains significant amounts of physically adsorbed water, oxygen and hydrogen. Although a vacuum hot degassing has been carried out before the powders of Al-Li alloys are consolidated, the hydrogen levels in RSP Al-Li alloys are usually high. The embrittlement of aluminum alloys by cathodically charged hydrogen has been extensively investigated, and has been observed to depend on temperature, strain rate and microstructure. It is reported that slip character controls the extent of dislocation transport of hydrogen and thereby may control and dominate the distribution of hydrogen in material and the degree of hydrogen embrittlement. There also have been many studies on the hydrogen embrittlement in IM Al-Li alloys. Here, the influence of hydrogen on the fracturemore » behavior of RSP Al-Li alloys is studied.« less

Drift diffusion model of hydrogen atoms for hydrogen embrittlement in polycrystalline Ni 3Al

It is well known that single crystals of Ni[sub 3]Al are ductile, whereas polycrystalline Ni[sub 3]Al has very low ductility at room temperature due to the intrinsic brittleness of its grain boundaries, which results in premature intergranular fracture. Since Aoki and Izumi found that adding a small amount of boron to polycrystalline Ni[sub 3]Al could suppress intergranular fracture and improve its ambient temperature ductility dramatically, many studies have been carried out on the effects of boron on the structure and chemical composition of grain boundaries in polycrystalline Ni[sub 3]Al. At present, there have been several explanations for the beneficial effect of boron, but its origin has not yet been clearly established. Recently, environmental embrittlement of grain boundaries has been observed in polycrystalline Ni[sub 3]Al with and without boron in air involving moisture and in hydrogen gas. The initiation of intergranular crack in polycrystalline Ni[sub 3]Al relates to the local stress concentration induced by a piled up of edge super dislocation [111] at grain boundary, while hydrogen atoms in the Ni[sub 3]Al superlattice would diffuse toward the end of the edge super dislocation pile-up under the action of the local stress concentration. Considering the relationships of intergranular crack initiation withmore » both of the edge super dislocation pile-up at the grain boundary and the drift diffusion of hydrogen atoms in the superlattice toward the grain boundary, a drift diffusion model of hydrogen atoms for hydrogen embrittlement in polycrystalline Ni[sub 3]Al is proposed to explain the effects of boron content, grain size in polycrystalline Ni[sub 3]Al and test environment, tensile rate as well as test temperature on the degree of hydrogen embrittlement in polycrystalline Ni[sub 3]Al.« less

Microstructural effects on hydrogen embrittlement in a high purity 7075 aluminum alloy

A high-purity 7075 alloy was heat treated to produce a variety of precipitate sizes and populations in grain interiors and at grain boundaries. Specimens were then evaluated for slip character and hydrogen embrittlement susceptibility following cathodic precharging or by simultaneous cathodic charging and straining. Embrittlement susceptibility was found to correlate very well with the size and type of matrix precipitates, consistent with the rationale that slip character controls the extent of dislocation transport of hydrogen and thereby may control and dominate the distribution of hydrogen in the material and the degree of hydrogen embrittlement. The results also indicate that a smaller role is played by grain boundary precipitates in controlling hydrogen embrittlement susceptibility.

Hydrogen embrittlement of amorphous alloys based on iron and nickel

Hydrogen embrittlement of amorphous alloys based on iron and nickel was examined in constant extension rate tests during cathodic polarization. Tests were carried out at a current density of 50 A·m −2 in 0.1 M H 2SO 4 with and without addition of NaAsO 2 and NaCl. Hydrogen permeation and polarization curves were measured. Differences in the degree of hydrogen embrittlement were ascribed mainly to differences in the entry of hydrogen into the alloys. It was suggested that a low degree of embrittlement resulted largely from a hindrance of hydrogen entry. For alloys containing phosphorus, this might be associated with phosphorus-containing oxyanions and salts. For the alloy with higher silicon content it may be associated with the formation of oxides of silicon.

Character of adsorptive weakening of steel by hydrogen-containing gases

Under normal conditions, the hydrogen-containing gases reduced the initial value of 7 (8"102 J/m 2) of the steel by 25% (H2), 50% (H2S), 25% (HCI), and 20% (CH~). In helium, A7 = 0. The reduction of Y in all cases occurred immediately after admission of the gas into the working chamber containing the specimen, subsequently remaining nearly constant over time. Holding specimens in the gas for i00 h with subsequent testing in the vacuum did not change the initial value of y. Both the above results and the temperature dependence of Ay obtained earlier [5] in gaseous hydrogen (Fig. i) can be explained only by the specific features of the adsorptive interaction of hydrogen and steel in the investigated temperature range. This presumption is also supported by the following considerations: i. An increase in test temperature from 120 to 300~ undoubtedly intensifies the diffusion of hydrogen in the steel, and the greater embrittlement of the metal with increasing temperature (Fig. I) maybe explained by the large amount of hydrogen having penetrated the metal. However, a further increase in temperature should have further embrittled the metal, but the test results show the opposite effect. Thus, the hydrogen absorbed by the metal cannot be considered responsible for the reduction in 7 for the metal. 2. The decrease in A7 with an increase in temperature to 420~ (Fig. i) could be due to some additional tempering of the metal during its testing. As is known [6], the increase in the ductility of the metal that occurs during tempering makes it less sensitive to the effects of the medium. We therefore conducted tests to obtain the dependence of A 7 for the steel on the preliminary tempering temperature. The resultant data (Fig. 2a) (7 was determined at room temperature) shows that additional tempering, even at 500~ has almost no effect on the degree of hydrogen embrittlement of the metal. Thus, we explain the reduction in the strength of the steel by the action of gaseous hydrogen in the investigated temperature range first of all in terms of its chemisorptive effect on the metal at the tip of a developed crack (Rebinder effect). However, the character of the adsorptive process facilitating embrittlement of the metal remains unclear.

Hydrogen transport and embrittlement in structural metals

The close relationship between hydrogen transport and embrittlement is indicated by evidence of hydrogen absorption preceding degradation of mechanical properties. Concentration of hydrogen at a crack or flaw by diffusion or by transport with moving dislocations is probably necessary also. Experimental studies show that hydrogen permeation is significantly influenced by surface conditions, particularly oxide films, and internal defects and impurities that trap diffusing hydrogen. The usual thermodynamic and diffusion relations, therefore, do not predict accurately the final distribution of hydrogen and the kinetics of the processes. Investigation of the effects of hydrogen on the mechanical properties of approximately fifty structural alloys at ambient temperature and pressures up to 69 MPa indicates that all alloys show evidence of susceptibility to hydrogen embrittlement. The degree of hydrogen embrittlement appears to be related to the amount of hydrogen absorbed and its distribution within the metal lattice. Surface condition, defect structure, hydrogen purity, and hydrogen pressure influence embrittlement. Of major importance is the transport of hydrogen with moving dislocations, a mechanism for concentration and redistribution of hydrogen that is operative at temperatures lower than for diffusion.