Modeling the Hydrogen Redistribution at the Grain Boundary of Misoriented Bicrystals in Austenite Stainless Steel

Hydrogen embrittlement, as one of the major concerns for austenitic stainless steel, is closely linked to the diffusion of hydrogen through the grain boundary of materials. The phenomenon is still not well understood yet, especially the full interaction between hydrogen diffusion and the misorientation of the grains. This work aimed at the development of a robust numerical strategy to model the full coupling of the hydrogen diffusion and the anisotropic behavior of crystals in 316 stainless steel. A constitutive model, which allows easy incorporation of crystal orientation, various loading conditions, and arbitrary model geometries, was established by using the finite element package ABAQUS. The study focuses on three different bicrystal models composed of misoriented crystals, and the results indicate that the redistribution of hydrogen is significant closely to the grain boundary, and the redistribution is driven by the hydrostatic pressure caused by the misorientation of two neighboring grains. A higher elastic modulus ratio along the tensile direction will lead to a higher hydrogen concentration difference in the two grains equidistant from the grain boundary. The hydrogen concentration shows a high value in the crystal along the direction with stiff elastic modulus. Moreover, there exists a large hydrogen concentration gradient in a narrow region very close to the grain boundary to balance the concentration difference of the neighboring grains.

Effect of Hydrogen, Hydride Orientation and Temperature on Low-Cycle Fatigue Resistance of Zr-1%Nb Fuel Rod Claddings

Low-cycle fatigue testing was conducted on annular samples with an outer diameter of 9.13 mm, a wall thickness of 0.68 mm and a width of 2.7 mm, namely: non-hydrogenated samples (cut out of standard Zr‑1%Nb cladding tubes); hydrogenated samples with a hydrogen concentration of 50 ... 400 ppm; samples cut out from hydrogenated dummy claddings after hydride reorientation tests performed according to various test modes. The tests were conducted at the temperatures of 25, 180, 350, 400 and 450 °С. The results obtained demonstrate that with increasing the hydrogen content in Zr-1%Nb alloy claddings the fatigue life increases.

Exploring the use of neutrons to detect hydrogen embrittlement in high strength steel

With the aim of exploring neutron techniques for the non-destructive detection of hydrogen in embrittled steel, three sets of steel samples were studied with neutron scattering: Ni coated, Cd coated, and Cr coated. Each set contained a non-embrittled or low-hydrogen concentration reference and one or two embrittled and high-hydrogen concentration samples. It is observed that the incoherent scattering, when normalized by the intensity of the Bragg peak, is significantly higher for high-hydrogen concentration or embrittled samples than in the reference. Although the difference is small, this represents a non-destructive technique of detecting hydrogen embrittlement. Neutron radiography, and inelastic or small-angle scattering could not distinguish between embrittled and reference samples.

Development of a fiber optic sensor for hydrogen monitoring in transformers

<p>Electric vehicles and photovoltaic power generation are two of factors that are increasing the demands on the electrical grid. To cope with these challenges and to improve grid stability, the development of a transformer monitoring system as a fundamental part of the smart grid is necessary. Dissolved hydrogen in the transformer oil can serve as a primary indicator of the transformer health. Depending on the hydrogen concentration and rate of increase the transformer can be diagnosed. The goal of this thesis was to develop a highly sensitive hydrogen sensor for online health monitoring of transformers. The developed sensors are based on palladium and fiber Bragg gratings (FBG). Palladium expands with hydrogen absorption and this expansion is measured with an FBG. Detailed guidance for optimizing the sensor design is given. First, the selection of the working temperature is discussed. Second, the influence of the palladium geometry on the sensitivity is elaborated: by varying the cross-sectional area ratio of palladium to fiber the sensitivity can be tuned. Two different options to attach palladium are discussed: vapour deposition of palladium and adhesive bonding of palladium foils. The sensitivity of the palladium foil sensors was improved by improved manufacturing processes. The foil sensor have a sensitivity of up to 295 pm/% hydrogen and a resolution of 0.006% hydrogen in gas atmosphere at 90◦C and 1060 mbar. To further increase the hydrogen sensitivity two concepts for amplification of the signal are presented. One relies on palladium silver foils, which have an increased hydrogen solubility and therefore expansion compared to pure palladium, which achieved an increase in sensitivity of a factor of 17. This leads to sensitivity of over 4500 pm/% hydrogen, which is the most sensitive hydrogen sensor reported so far. The other concept relies on a novel concept for strain concentration using a pre-strained palladium foil and FBG, which achieved an amplification of a factor of 2.5. The sensors were characterised in gas and oil environment in a newly developed setup which is stable in pressure, temperature and gas concentration. In gas the sensors were tested at 60, 75, 90, 105, and 120◦C and for a hydrogen concentration range of 0.01 (100 ppm) to 5%. In oil the sensor was tested at 90◦C and for a hydrogen concentration range of 5- 4000 ppm dissolved hydrogen. Furthermore, the influence of carbon monoxide (CO) on the hydrogen sensitivity was examined. A slowed response could be observed, but CO had no impact on the precision of the sensor. Finally, the hydrogen calibration of the sensor is discussed by investigating the strain transfer between expanding palladium and fiber. Three different methods are elaborated to determine the coefficient of strain transfer: (a) via hydrogen measurement, (b) via temperature measurement, and (c) via strain measurement. Methods (a) and (b) were applied directly on the hydrogen sensor, and gave similar results. Method (c) was applied on a reference structure and used to verify method (b). A hydrogen sensor suitable for transformer health monitoring has been developed and characterised, and is currently being implemented in a transformer in the New Zealand network.</p>

Development of a fiber optic sensor for hydrogen monitoring in transformers

<p>Electric vehicles and photovoltaic power generation are two of factors that are increasing the demands on the electrical grid. To cope with these challenges and to improve grid stability, the development of a transformer monitoring system as a fundamental part of the smart grid is necessary. Dissolved hydrogen in the transformer oil can serve as a primary indicator of the transformer health. Depending on the hydrogen concentration and rate of increase the transformer can be diagnosed. The goal of this thesis was to develop a highly sensitive hydrogen sensor for online health monitoring of transformers. The developed sensors are based on palladium and fiber Bragg gratings (FBG). Palladium expands with hydrogen absorption and this expansion is measured with an FBG. Detailed guidance for optimizing the sensor design is given. First, the selection of the working temperature is discussed. Second, the influence of the palladium geometry on the sensitivity is elaborated: by varying the cross-sectional area ratio of palladium to fiber the sensitivity can be tuned. Two different options to attach palladium are discussed: vapour deposition of palladium and adhesive bonding of palladium foils. The sensitivity of the palladium foil sensors was improved by improved manufacturing processes. The foil sensor have a sensitivity of up to 295 pm/% hydrogen and a resolution of 0.006% hydrogen in gas atmosphere at 90◦C and 1060 mbar. To further increase the hydrogen sensitivity two concepts for amplification of the signal are presented. One relies on palladium silver foils, which have an increased hydrogen solubility and therefore expansion compared to pure palladium, which achieved an increase in sensitivity of a factor of 17. This leads to sensitivity of over 4500 pm/% hydrogen, which is the most sensitive hydrogen sensor reported so far. The other concept relies on a novel concept for strain concentration using a pre-strained palladium foil and FBG, which achieved an amplification of a factor of 2.5. The sensors were characterised in gas and oil environment in a newly developed setup which is stable in pressure, temperature and gas concentration. In gas the sensors were tested at 60, 75, 90, 105, and 120◦C and for a hydrogen concentration range of 0.01 (100 ppm) to 5%. In oil the sensor was tested at 90◦C and for a hydrogen concentration range of 5- 4000 ppm dissolved hydrogen. Furthermore, the influence of carbon monoxide (CO) on the hydrogen sensitivity was examined. A slowed response could be observed, but CO had no impact on the precision of the sensor. Finally, the hydrogen calibration of the sensor is discussed by investigating the strain transfer between expanding palladium and fiber. Three different methods are elaborated to determine the coefficient of strain transfer: (a) via hydrogen measurement, (b) via temperature measurement, and (c) via strain measurement. Methods (a) and (b) were applied directly on the hydrogen sensor, and gave similar results. Method (c) was applied on a reference structure and used to verify method (b). A hydrogen sensor suitable for transformer health monitoring has been developed and characterised, and is currently being implemented in a transformer in the New Zealand network.</p>