Ingenieurwissenschaften und zugeordnete Tätigkeiten
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- Creep-resistant Steel (2) (entfernen)
In the field of modelling hydrogen assisted cracking (HAC) phenomenon, hydrogen diffusivity is an important input parameter for numerical simulation. In terms of hydrogen diffusion coefficients, they have great impact on realistic assessment of the evolution of possible crack critical hydrogen concentrations. In addition, the chemical compositions of steels can have a strong effect on hydrogen diffusion. Unfortunately, literature provides a wide range of available hydrogen diffusion coefficients even for similar microstructures and equal temperatures. The scattering of the data can lead to significant deviations in the results of simulating the evolving hydrogen concentrations due to hydrogen uptake (by fabrication or service). Thus, the application of such data to crack-models or for component life tie predictions can be realized up to the present only by considering envelope curves of such value, corresponding to a work or bench case scenario, respectively. For improved reliability of numerical simulaitons, it is necessary to minimize the mentioned deviation of these data. Hence, this work focuses on the validation of hydrogen diffusion coefficients obtained from permeation experiments at room temperature. Two baintic steels with different alloying concepts were investigated, the creep-resistant 7CrMoVTiB10-10 and the reactor pressure vessel grade 20MnMoNi4-5. A numerical model is presented for simulation of the corresponding hydrogen diffusion during permeation experiments using the finite element software ANSYS. Three different diffusion coefficients (obtained from different common calculation methods) are considered and compared to numerical results. The vases of thes calculation methods are permeation transients which are a direct measure for hydrogen. The results of the simulated hydrogen diffusion coefficients show that only one procedure for calculation of diffusion coefficitnes is suitable in comparision to the experimental values. Thus, it is suggested to use this method for analysis of experimental results in case of hydrogen diffusion during permeation experiments. Furthermore, this work supplies validated values for the hydrogen diffusion coefficients of both steel grades.
Low-alloyed CrMoV steels, such as T24, are widely used for welded components in fossil power stations due to their excellent creep-strength. Spectacular failure cases in the recent years exhibited severe cracking in T24 welds. The results showed that hydrogen-assisted cracking (HAC) occurring up to 200 degree Celsius cannot be excluded. Hence, a basic understanding is necessary on how hydrogen affects the material properties of welded microstructures. In this regard, each weld microstructure (HAZ and weld metal) has influence on the HAC susceptibility and respective hydrogen diffusion. Thus, the present contribution summarizes different results obtained from experiments with grades T24 (CrMoV alloy) and T22 (CrMo) and thermally simulated HAZ. Tensile tests were conducted with hydrogen charged specimens and compared to hydrogen-assisted stress corrosion cracking results obtained from slow strain rate tests (SSRT) up to 200 degree Celsius. Electrochemical permeation and degassing experiments were performed to identify a particular weld microstructure influence on hydrogen diffusion and trapping (especially in the HAZ). The results showed that T24 base material has improved resistance to hydrogen-assisted degradation/cracking. In contrast, the as-welded HAZ had remarkably increased susceptibility (tesnile tests at hydrogen concentration of 1 to 2 ppm). SSRT experiments confirmed this at elevated temperatures for both the T24 and the T22. Hence, the evaluation of a particular degradation of the mechanical properties should be performed independently for each weld microstructure. In addition, the HAZ showed decreased diffusion coefficients (at room temperature) of approximately one magnitude compared to the base materials. Trapped hydrogen was determined in the T24 at temperatures up to 120 degree Celsius compared to 75 degree Celsius in the T22. This has to be considered in case of changing operational temperatures, e.g. in the case of start-up and shutdown processes of boiler components.