Papers by Author: Kim Verbeken

Abstract: In order to increase the sustainability of metals, a more detailed understanding of the corrosion phenomenon is of crucial importance. In current literature, corrosion is often considered as a purely chemical interaction with nearly exclusive dependence on compositional effects, whilst ignoring the microstructural features of the metal surface. In the present work, results are presented which illustrate both the role of grain orientation and grain boundaries in the corrosion process. To evaluate the grain orientation dependent electrochemical behavior, polycrystalline Cu, was brought into contact with a corrosive electrolyte. Subsequently, the attack was evaluated by measuring the surface with both Atomic Force Microscopy (AFM) and Electron Backscatter Diffraction (EBSD). It was demonstrated that the grain orientation itself did not significantly influence the corrosion kinetics, but, alternatively, that the orientation of the neighboring grains seemed to play a decisive role in the grain dissolution rate. To increase understanding on the role of grain boundaries, a method was developed based on the electrochemical (galvanic) displacement of gold, which is deposited from an aqueous solution on a pure copper substrate. This technique demonstrated its sensitivity to the grain boundary characteristics as far less gold was deposited on special boundaries, such as coincidence site lattice boundaries, as compared to the random high angle grain boundaries.

Abstract: When applying an aluminium coating on a steel substrate, the corrosion resistance of aluminium is combined with the mechanical properties of steel. There are different ways to coat steel among which hot dipping is the most applied because it is technically feasible in large volumes and economically the most beneficial one. Although aluminized steel accounts for about 5% of the total world production of all hot dip coated steels, the interactions between the molten aluminium and steel substrate are not yet completely understood. During hot dipping, several intermetallic layers are formed between the outer nearly pure aluminium layer and the steel substrate. These layers mostly comprise of a thin FeAl3 and a thicker Fe2Al5 layer. The thickness of these intermetallic layers must be kept minimal since they embrittle the coating. Thickness reduction of the intermetallics is mainly realized by silicon addition to the aluminium bath. In this work, the formation of intermetallic layers was investigated for a deep drawing steel grade which was hot dipped in aluminium baths with six different Si contents ranging from 1 to 10 wt%Si. The Si-addition was demonstrated to lead to a decrease in the thickness of the intermetallic layers and a flattening of the intermetallic-steel interface. An increase in Si also led to the formation of other intermetallic compounds, which are investigated by a systematic analysis by scanning electron microscopy (SEM), Energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) on multiple samples and discussed in terms of the Si-effect on a possible formation mechanism for the intermetallics.

Abstract: The present work evaluates hydrogen induced cracking in a high strength TRIP steel with a complex multiphase microstructure, containing ferrite, bainite, retained austenite, and some martensite. Each structural constituent demonstrates a different behavior in the presence of hydrogen and when deformed, the retained austenite transforms to martensite. The goal of this work is to understand the response of the hydrogen saturated multiphase structure to a mechanical load. A tensile test on notched samples combined with in-situ electrochemical hydrogen charging was carried out. The test was interrupted at certain specific points, before the macroscopic failure of the material. Hydrogen induced crack initiation and propagation were examined by studying several intermediate elongations. The microstructure of the samples was characterized by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The EBSD measurements allowed both microstructural and crystallographic characterization of the hydrogen induced crack surroundings. A correlation was found between the occurrence of martensite, which is known to be very susceptible to hydrogen embrittlement, and the initiation of hydrogen induced cracks. These cracks were located at the surface in specific high stressed regions. Finite element simulations indicated that these regions were induced due to the presence of the notch.

Abstract: The present work evaluates the hydrogen trapping behavior of different laboratory cast generic Fe-C-Ti martensitic alloys. Titanium carbides were precipitated in the materials by well-designed heat treatments. A quenched and tempered martensitic matrix with final strength above 1000 MPa was aimed for and verified by means of hardness measurements. Tempering allowed generating precipitates with different characteristics in terms of coherency, size and distribution due to the secondary hardening effect, as was evaluated by transmission electron microscopy. The hydrogen trapping capacity of the TiC precipitates was investigated by thermal desorption spectroscopy, while melt extraction was performed to determine the amount of hydrogen present after cathodic hydrogen charging. Generally, it could be concluded that the incoherent particles in the quenched material were not able to trap hydrogen, whereas the quenched and tempered material trapped hydrogen at the interface of small probably coherent TiC.

Abstract: Hot dipping is a coating technique used in industry for galvanizing machine elements and steel profiles for construction or automotive applications. However, an alternative use of this process might be to improve specific properties. For instance, in order to improve the magnetic properties of electrical steels, it may be desirable to increase the Si and/or Al content. A possible and alternative route to realize this is by the application of an Al-Si-rich coating on the steel substrate using a hot dipping process, followed by a diffusion annealing treatment in order to distribute the Al/Si more evenly in the steel. The obtained distribution depends on the annealing parameters and can be both beneficial and detrimental for the magnetic properties. In the present work, Fe-Si substrates were hot dipped in different Al-Si baths. Subsequently, the samples were annealed at 1100°C during 20 minutes and concentration profiles were measured with scanning electron microscope energy dispersive spectroscopy line scans. The experimental results were analyzed using a specifically designed simulation model in order to determine the Al and Si diffusion coefficients. This model uses an inverse algorithm to determine interdiffusion coefficients that arise in a macro ternary diffusion system.

Abstract: There currently is a strong interest in using ‘as-hot rolled’ steels instead of ‘heat treated’ steels within the field of microalloyed steels with superior strength and improved ductility. However, mechanical and microstructural characterization of these steels is less elaborated. The present work is an effort in that direction, focusing on the evaluation of the impact of a variable Nb and C content on the microstructure and mechanical properties for seven hot rolled microalloyed steels and deriving an empirical relation between the mechanical properties and the Nb/C ratio.

Abstract: A homogenous intensity distribution along the cube texture fibre is important to achieve an easy magnetization in non-oriented electrical steels. Several alternatives have been discussed in literature to achieve this goal namely, tertiary recrystallization (surface energy controlled), decarburization annealing, two step cold rolling (strain induced boundary migration), twin-roll thin strip casting (directional solidification), phase transformation (surface energy anisotropy) and columnar grains formation (selective grain growth). In the present study, a hypoeutectic Al-Si alloy was deposited on the surface of cold rolled Fe-Si steels with a hot dipping simulator and subsequently annealed at 1000°C for different times. This procedure was developed previously in order to enrich the substrate with Al and/or Si and consequently improve their resistivity. Of specific interest was the formation of columnar grains in the low Fe-Si steel after annealing. These columnar grains were found to grow from the surface towards the centre of the substrate. The microstructure and texture in the columnar grains were significantly different than those in the middle of the material. Therefore, the evolution of these features during processing was studied in detail in this work.

Abstract: Thermal desorption spectroscopy (TDS) is a very important tool in hydrogen embrittlement (HE) related research and has been applied on many different materials over the last decades in order to improve knowledge on the HE phenomenon. TDS provides the opportunity to distinguish between different types of hydrogen traps based on the analysis of a spectrum with different peak temperatures each corresponding to hydrogen desorption from a specific trap. These peak temperatures, and consequently the different traps in a material, arise from the various microstructural characteristics of the material. However, TDS results are also influenced by many other parameters, such as the sample surface preparation, the electrolytes used for hydrogen charging, sample geometry, charging time, current density, charging temperature. Even though the use of thermal desorption to evaluate hydrogen-metal interactions has increased over the past years, a careful evaluation of the effect of these other parameters was not yet performed. In this work, the impact of some of the above mentioned parameters was studied. It was demonstrated that the sample geometry, the surface roughness, and the initial total pressure of the TDS chamber influenced significantly the obtained TDS spectrum.

Abstract: Thermal desorption spectroscopy (TDS) is a very important tool in hydrogen related research. It allows to distinguish between the different types of microstructural hydrogen traps based on the analysis of the different temperatures at which hydrogen desorbs from the material during heating. These peak temperatures depend on the metallurgical and microstructural characteristics of the steel under investigation and provide important information on the possible mechanisms for hydrogen embrittlement (HE). In the present work, multiple TDS experiments and an in-depth study of the microstructure were performed on a TRIP steel (TRIP700) that was previously cold deformed in order to make a correlation between the microstructural features of this material, e.g. grain boundaries, dislocations, martensite formation and the peaks that became visible during TDS. The results obtained for the TRIP grade were compared with those obtained for electrolytic pure iron, which only contained a limited amount of possible trap sites such as grain boundaries and an increasing amount of dislocations due to previous application of cold deformation. Significant differences between both materials and a significant impact of the degree of cold deformation for TRIP steels were observed.

Abstract: An accurate isotropic and kinematic hardening model and description of the strain rate dependent material behaviour is necessary for simulation of fast forming processes. Consequently, the material model parameter identification requires experiments where large strains, high strain rates and strain path changes can be attained. Usually, quasi-static tension-compression Bauschinger tests are used to assess the materials kinematic hardening. Hereby it’s important to have the same specimen geometry and boundary conditions in the forward and reverse loading step which is not easily achieved in high strain rate testing techniques. In this work, high strain rate split Hopkinson bar torsion experiments on Ti6Al4V are carried out to study the constitutive material behaviour at large plastic strain and strain rate. In torsion experiments, due to the absence of cross sectional area reduction, higher strains than in tensile tests can be obtained. In addition, a modified torsional split Hopkinson bar setup is developed to perform dynamic Bauschinger tests. A shear reversed-shear load is applied instead of the classical tension-compression load cycle. The test results are analysed to find out if the technique can be used for characterisation of the kinematic material behaviour. Digital image correlation and finite element simulations are used to improve the interpretation of the experimental results.