Hydrogen embrittlement (HE) is one of the main limitations in the use of advanced high-strength steels in the automotive industry. To have a better understanding of the interaction between hydrogen ...(H) and a complex phase steel, an in-situ method with plasma charging was applied in order to provide continuous H supply during mechanical testing in order to avoid H outgassing. For such fast-H diffusion materials, only direct observation during in-situ charging allows for addressing H effects on materials. Different plasma charging conditions were analysed, yet there was not a pronounced effect on the mechanical properties. The H concentration was calculated while using a simple analytical model as well as a simulation approach, resulting in consistent low H values, below the critical concentration to produce embrittlement. However, the dimple size decreased in the presence of H and, with increasing charging time, the crack propagation rate increased. The rate dependence of flow properties of the material was also investigated, proving that the material has no strain rate sensitivity, which confirmed that the crack propagation rate increased due to H effects. Even though the H concentration was low in the experiments that are presented here, different technological alternatives can be implemented in order to increase the maximum solute concentration.
A model-based evaluation procedure by a trap-diffusion integrated finite element model (FEM) is used to interpret the experimentally measured thermal desorption spectra (TDS) of Fe-C-Ti alloys. Two ...material conditions are compared, i.e. as-quenched vs. quenched and tempered, in which TiC have precipitated. The discrepancy between measurement and simulation is found to be very small for all material conditions. The model shows high trapping energies of around 58 kJ/mol to 110 kJ/mol (0.61 eV–1.14 eV), when the materials are tempered. These traps could be related to carbon vacancies in TiC precipitates.
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•A model-based evaluation procedure is successfully applied to measured TDS of the Fe-C-Ti alloy.•A spectrum of five trapping energies is related to corresponding microstructure.•TiC carbides trap hydrogen mainly at the interface.•The appropriateness of Oriani's trapping model is verified.
Multiple types of hydrogen trapping sites in advanced high-strength steels (AHSS) are often experimentally characterized by means of thermal desorption spectroscopy (TDS). The evaluation is regularly ...based on the peak deconvolution procedure combined with Kissinger's theory, which provides distinctive desorption energies of hydrogen trapping sites at microstructural defects. However, the desorption energies published in literature are often non-conclusive and from time to time contradictive in nature. Therefore, it is of utmost importance to verify the evaluation procedures according to Kissinger's theory for multiple types of hydrogen trapping sites. For that purpose, theoretical TDS spectra were simulated using a bulk diffusion model according to Oriani's theory. Binding energies and trap densities were chosen for providing TDS spectra with clearly separated as well as overlapping TDS peaks. Finally, the desorption energies according to Kissinger's theory were compared with the theoretical trapping energies used in the models. Based on this theoretical work, it is strongly recommended to apply the Kissinger theory only for the evaluation of single or well separated TDS peaks. If peaks overlap, complementary microstructural variation and characterization are a perquisite to correctly evaluate the TDS spectra.
•Verification of the Kissinger theory and peak deconvolution procedure.•Desorption energy fulfils the Kirchheim criterium based on bulk hydrogen diffusion.•The R2-values do not necessarily mean trustworthy desorption energies.•The TDS evaluation requires complete comprehension of the metal microstructure.
Knowing the hydrogen distribution c(x,t) and local hydrogen concentration gradients grad(c) in ferritic steel components is crucial with respect to hydrogen embrittlement. Basically, hydrogen is ...absorbed from corrosive or gaseous environments via the surface and diffuses through interstitial lattice sites into bulk. Although, the lattice diffusion coefficient DL∼0.01mm2/s is in the order of magnitude of those for well-annealed pure iron, trapping sites in the microstructure retard the long-range chemical diffusion jL=−Dchem(c)grad(c), causing local hydrogen accumulation in near surface regions in limited time. Considering pure ferritic crystals without trapping sites in the microstructure, the limited characteristic diffusion depth xc∼Defft is proportional to the square root of the effective diffusion coefficient Deff and of time t. Effective diffusion coefficients are measured independently for hydrogen using the electrochemical permeation technique. For pure crystals, the effective diffusion coefficient is constant at given temperature and allows accurate calculations of the diffusion depths. However, with trapping sites in the microstructure the effective diffusion coefficient is not a material property anymore and becomes dependent on the hydrogen charging conditions. In the present work, the theory of hydrogen bulk diffusion is used to verify the concept of effective diffusion. For that purpose, the generalized bulk diffusion equation was solved numerically by using the finite difference method (FDM). The implementation was checked using analytical solutions and a comprehensive convergence study was done to avoid mesh and time dependency of the results. It is shown that effective diffusion coefficients can vary by magnitudes depending on the sub-surface lattice concentration. This limits the application of the effective diffusion concept and also the calculation of the characteristic diffusion depth.
•FDM is presented to simulate permeation and diffusion depth profiles.•The effective diffusion coefficient strongly depends on the charging conditions.•The characteristic diffusion depth correlates with t, but not with Deff.•The basic assumption of the effective diffusion concept is violated.
Metals that are exposed to high pressure hydrogen gas may undergo detrimental failure by embrittlement. Understanding the mechanisms and driving forces of hydrogen absorption on the surface of metals ...is crucial for avoiding hydrogen embrittlement. In this study, the effect of stress-enhanced gaseous hydrogen uptake in bulk metals is investigated in detail. For that purpose, a generalized form of Sievert's law is derived from thermodynamic potentials considering the effect of microstructural trapping sites and multiaxial stresses. This new equation is parametrized and verified using experimental data for carbon steels, which were charged under gaseous hydrogen atmosphere at pressures up to 1000 bar. The role of microstructural trapping sites on the parameter identification is critically discussed. Finally, the parametrized equation is applied to calculate the stress-enhanced hydrogen solubility of thin-walled pipelines and thick-walled pressure vessels during service.
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•Improvement of Sievert's law to consider trapping sites and stress states.•Calculation and validation of gaseous hydrogen solubility under extreme conditions.•Hydrogen uptake measured for L450 steel at 200 and 1000 bar.•Calculation of the hydrogen solubility in pipes and pressure vessels.•Comprehensive literature survey of gaseous hydrogen solubility.
Hydrogen embrittlement of modern high strength steels consists of different interacting time-dependent mechanisms. One of these mechanisms is hydrogen diffusion and trapping to accumulate hydrogen in ...critical areas with high mechanical loads. Therefore, understanding hydrogen diffusion and trapping behavior of carbides containing high strength steels is an essential part to effectively increase the hydrogen resistance. For that purpose, a microstructural based model was developed and parametrized to Fe–C–V and Fe–C–Ti alloys. Generalized analytical equations were derived to describe the evolution of different kinds of trap densities with the measured carbide mean radius, annealing temperature or dislocation density. Finally, the models support the idea of hydrogen trapping at carbon vacancies and coherent interface positions. In future, these models are well suited for finite element process simulations of industrial components to predict the local solubility and chemical diffusion as demonstrated in the last section of this work.
•Parametrized hydrogen diffusion model for Fe–C–V alloys.•Verification of the deep trapped hydrogen concentrations.•Microstructural based evolution models for trap densities.•Interpretation of the main trapping sites related to carbides on a lower scale.•Simulation of the diffusion depth on the component level.
A microstructural based creep model applied to alloy 718 Drexler, Andreas; Fischersworring-Bunk, Andreas; Oberwinkler, Bernd ...
International journal of plasticity,
June 2018, 2018-06-00, 20180601, Letnik:
105
Journal Article
Recenzirano
In this work a creep model based on the microstructural evolution of precipitation strengthening metals is presented. The model describes the influence of precipitates on the threshold stress as well ...as the power-law and the exponential creep rates depending on the aging condition. To show the predictive capabilities of this model, it was applied to the nickel based superalloy Alloy 718. This alloy is precipitation strengthened by the γ′ and γ’’ phases. Thermo-kinetic simulations based on a calibrated MatCalc routine were performed to determine the evolution of the volume fraction and mean radius of the precipitates. In addition, sequential creep and tensile tests were performed to characterize the mechanical material behavior. The simulated microstructural evolution and the corresponding measured mechanical properties were used to parametrize the creep model. Finally, the fully parametrized model was applied to simulate deformation mechanism diagrams for different aging conditions. From these diagrams the purely elastic, the power-law and the exponential deformation regimes can be estimated depending on the precipitate volume fraction and mean precipitate radius.
•A microstructurally based creep model accounting for the precipitation strengthening is proposed.•The creep model is applied to the widely used nickel-based superalloy Alloy 718.•Deformation mechanism diagrams for the half-aged and aged material were presented.
Hydrogen embrittlement (HE) of advanced high-strength steels is a crucial problem in the automotive industry, which may cause time-delayed failure of car body components. Practical approaches for ...evaluating the HE risk are often partially and contradictive in nature, because of hydrogen desorption during testing and inhomogenous hydrogen distributions in, e.g., notched samples. Therefore, the present work aims to provide fully parametrized and validated bulk diffusion models for three dual phase steels to simulate long-range chemical diffusion, trapping and hydrogen desorption from the surface. With one constant set of parameters, the models are able to predict the temperature dependency of measured Choo-Lee plots as well as the concentration dependency of measured effective diffusion coefficients. Finally, the parametrized and validated bulk diffusion models are applied for studying the role of the current density on the permeation time and the role of coatings as effective diffusion barriers.
Graphical abstract
Multiphase ultra-high strength steels (UHSS) containing retained austenite (RA) appear to be among the most interesting steels for the automotive industry. Developments in the last decades have ...allowed obtaining a very good combination of mechanical strength and ductility. Quenching and partitioning (Q&P) steels have been proposed as third-generation UHSS, reaching ultimate tensile strength up to 1300 MPa along with excellent fracture elongations of more than 15%. However, the use of Q&P steels is mainly limited by their susceptibility to hydrogen embrittlement (HE). The present work investigates the influence of the Q&P heat treatment parameters on the mechanical properties and on the HE resistivity of 20Mn-Si wire rod steel. The HE resistivity was measured using incremental step load testing with in situ electrochemical hydrogen charging according to ASTM F1624-12 standard. A comprehensive microstructure characterization was performed to examine volume fraction, nucleation sites and morphologies of RA. Although the mechanical properties were similar after Q&P heat treatment, an increase in the partitioning time revealed a significant increase in the HE threshold stress of more than 200 MPa.
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