Papers by Keyword: Hydrogen Embrittlement

Abstract: Zirconium alloys, such as Zr-1Nb are widely used as cladding materials for nuclear fuel elements of light water reactors. Hydrogen embrittlement problem causes degradation of these parts of nuclear reactors. It is known, that hydrogen uptake causes changes in microstructure and defect structure of metals. The aim of this work is study of Zr-1Nb alloys’ defect structure after hydrogen saturation up to the concentration of 600 ppm. Saturation of hydrogen was carried out from the gas phase under high temperature and pressure. This study reveals the increase of average positron lifetime with the increase of hydrogen concentration. Value of the average positron lifetime achieves plateau when the concentration of hydrogen is about 600 ppm. Also the following effects were detected in the material after hydrogen uptake up to different concentrations: crystal lattice expansion, dislocations and vacancy-like defects formation, as well as the defect-hydrogen complexes formation.

Abstract: In this work, the mechanical behavior on AISI 4140 with boride coating and hardened quenching-tempering was studies subsequently to boriding. The boriding is conducted at temperatures 1203 and 1233 K for 7 h, after that all samples at a temperature of 1143 K for 1 h and tempering 473 K were exposed for 1 h 30 minutes. The microstructural characterization was examined by scanning electron microscopy with saw-tooth morphology of boride coatings type Fe₂B. Three-point bend test was used to study the effect of hydrogen cathodic charging on the mechanical behavior of AISI 4140 with boride coatings, the results show a reduction of ductility and 58.17% of indices of embrittlement () and show a crack on the substrate due to interaction of hydrogen atom. Also, sample boride at 1203 K with hydrogen show a low decrease of the bending stress with respectively a sample boride without hydrogen and a decrease of the parameter () with 35.32% due to the formation of boride coatings.

Abstract: The demand for new materials that provide excellent structural performance while reducing weight and being cost-effectively manufactured is increasing. For applications with high strength requirements, ultra-high strength steels (UHSS) have been widely used. However, with such a high strength level, UHSS are very sensitive to the hydrogen that could be ease by the tempering process. In this research, the correlation of hydrogen and tempering process on commercial UHSS 15B30 has been studied. Results show that the tensile strength (TS) of as-quenched 15B30 is about 1900MPa. After tempering treatment of the quenched 15B30, the TS decreases from 1600MPa to 1200MPa with tempering temperature increased from 200°C to 400°C. The 15B30 specimens, being subjected to hydrogen charge, exhibit the dramatic reduction of mechanical strengths.

Abstract: The 7000 series alloys have the highest strength in the aluminum alloys, but lower fatigue properties than 2000 series alloys. Thus, 7000 series alloys are not applied to a large proportion of the aircraft components. However, the mechanism for this has not been elucidated yet. In humid air, hydrogen embrittlement based on intergranular cracking has been known to occur in 7000 series alloys. To date, in order to explain the difference in the fatigue crack growth behavior in the two series alloys, the effect of the test environment on the fatigue crack growth of the two series alloys has been investigated in this study. A 7075-type as well as 2024-type alloy with relatively coarse equi-axed grains was T6-tempered and subjected to fatigue crack growth test in humid and dry environments. Crack growth rate at low ΔK level seemed to be larger in the 7075-type alloy than the 2024-type alloy in the humid air, when assessed by means of gradually decreasing K method. In order to clarify this result, crack growth rate of the two alloys was assessed by means of gradually increasing K method as well as decreasing K method. Crack growth rate of the 7075-type alloy in moist air was concluded to be the largest in consistent with the previous study. Thus, the large fatigue crack growth rate of the 7075-type alloy is attributable to hydrogen embrittlement.

Abstract: The effects of hydrogen during stress corrosion cracking mechanisms (SCC) have been highlighted for many years but hydrogen trapping mechanisms are not yet well understood for 7xxx aluminium alloys. The 7046-T4 Al-Zn-Mg alloy has been chosen for this study because its low corrosion susceptibility allows hydrogen embrittlement (HE) to be more easily distinguished during SCC tests. Tensile stress tests have been carried out at a strain rate of 10^-3 s^-1 on tensile samples after an exposure at their corrosion potential in a 0.6M chloride solution for 165 hours under an imposed loading of 80%Rp_0.2. The results were compared to those obtained for samples pre-corroded without mechanical loading applied and healthy specimens. A loss of mechanical properties was observed for the pre-corroded samples and presumably attributed to the absorption, the diffusion and the trapping of hydrogen which affects a volume under the surface of the alloy and modifies its mechanical properties. Scanning electron microscope (SEM) observations highlighted a strong effect of hydrogen on fracture modes. The ductile-intergranular initial fracture mode observed on the healthy samples was partially replaced for the pre-corroded samples by a combination of two main fracture modes, i.e. brittle intergranular and cleavage, in relation with the nature of the hydrogen trapping sites and local stress state.

Abstract: To develop modern CAE systems for durable and reliable structure components design, mathematical modeling of environmentally assisted metal cracking becomes very important. For structure components exploited in aggressive environment and under cyclic load, it is a problem of today. Besides, mathematical rule of damage accumulation of different causes (for instance, hydrogen media impact and cycling) is rarely used in CAE systems, but if such rule was used, crack propagation simulation considering several damage causes would be possible.Environmentally assisted metal cracking model (developed earlier by authors) is described in the paper. This model considers cyclic load and hydrogen embrittlement, the most important characteristics of which are hydrogen environmental concentration and load frequency respectively.The authors’ model successfully predicts effect, known from certain experimental data, that the greater is the frequency, the less hydrogen embrittlement affects fatigue cracks propagation and, vice-versa, that there are certain boundaries of cycling frequency for which embrittlement effect is comparably big.Such boundaries of frequency were numerically estimated by means of the presented model. Plots showing dependency of the component’s life on different defect and loading features are shown.

Abstract: The paper presents the study of the effect of hydrogenation on the mechanical properties of commercially pure titanium. It has been found that the localized deformation zones occurring in the plastically deforming Ti samples are stationary dissipative structures. The kinetics of dissipative structure evolution was studied. The hydrogenation treatment is found to enhance a tendency to strain localization in as-treated material, which affects significantly material strength properties.

Abstract: The effect of hydrogen embrittlement on the localized plastic deformation of aluminum alloy D1 was investigated. The studies were performed for the test samples of aluminum alloy subjected to electrolytic hydrogenation. It is found that the mechanical properties and localized plastic deformation parameters of aluminum alloy are affected adversely by hydrogen embrittlement. The hydrogenated counterpart of alloy has a lower degree of ductility relative to the original alloy; however, the plastic flow behavior of material remains virtually unaffected. The deformation diagrams were examined for the deformed samples of aluminum alloy. These are found to show all the plastic flow stages: the linear, parabolic and pre-failure stages would occur for the respective values of the exponent n from the Ludwik-Holomon equation. Using digital speckle image technique, the local strain patterns were being registered for the original alloy D1 and the counterpart subjected to electrolytic hydrogenation for 100 h.

Abstract: Residual stresses produced by cold drawing are an undesirable effect of the non-uniform plastic strain distribution generated during the conforming process used for obtaining prestressing steel wires. Among the diverse parameters of the process influencing the residual stress generation, one of the most relevant is the geometry of the drawing die and, in particular, the inlet die angle. Wires drawn with die angles as low as possible will exhibit a lower and more homogeneous plastic strain state and, therefore, a smaller and more uniform residual stress state. Thus, the hydrogen embrittlement (HE) susceptibility of such wires is also lower, thereby enlarging the life in service of these components. In this paper an innovative design of the drawing die is proposed using two consecutive angles (i.e., varying die angle) for reducing the residual stress-strain state in the cold drawn wires and, consequently, for improving the resistance to HE of prestressing steel wires.

Abstract: The effect of hydrogen embrittlement on the plastic flow of aluminum alloy D1 was investigated. The studies were performed for the test samples of aluminum alloy subjected to electrolytic hydrogenation in a three electrode electrochemical cell at a controlled constant cathode potential. It is found that the mechanical properties and plastic flow curves of aluminum alloy are affected adversely by hydrogen embrittlement. The hydrogenated counterpart of alloy has a lower degree of ductility relative to the original alloy. The deformation diagrams were examined for the deformed samples of aluminum alloy. These are found to show all the plastic flow stages: the linear, parabolic and pre-failure stages would occur for the respective values of the exponent n from the Ludwik-Holomon equation. Microhardness tests were performed for as-treated aluminum alloy D1. Using scanning electron microscopy method, the changes in the fracture surface were investigated.