Ceramics Research Centre
 

X-Ray diffraction
High-Temperature X-ray diffraction (HT-XRD) is an important analytical tool to study chemical reactions, phase transformations and structural variations at elevated temperatures. Results are directly available within a small amount of time. The equipment at CRC allows the simulation of processes close to industrial applications.
 
It is often difficult or even impossible to understand processes occurring at high temperatures. Conventional experimental approaches, quenching the sample from run temperature and investigating the sample at room temperatures often lead to inconsistent and hardly interpretable results. Moreover, these are extremely time consuming experimental techniques. Therefore, an X-ray diffraction system was built up (Fig .1) which allows recording X-ray patterns at temperatures up to temperatures of 1900°C under different atmospheric conditions (vacuum, inert gases, air). We can study processes which are relevant for different industrial applications, such as crystallization of slag or oxidation of metals during hot rolling.
 
Oxidation During Hot Rolling
In order to investigate the cooling step during the hot rolling of steel, a strip (1 x 10 cm) of the chosen steel quality is heated up under an inert atmosphere to the finishing temperature e.g. 888°C. On the way to the coil the strip material is cooled with enormous amounts of water in the plant. In the experiment, humid air is blown into the XRD HT-chamber while rapidly cooling the strip to the coiling temperature. After this step the first XRD pattern has been taken, and in fact massive growth of the iron oxide wuestite (FeO) can be detected (Fig. 2). Within the coil the steel strip is no longer exposed to the atmosphere and the cooling rate is relatively low. A slow cooling of the strip under nitrogen atmosphere simulates this. The almost constant intensities of the two wuestite reflections clearly indicate that wuestite is the stable iron oxide under these conditions.
 

Fig 2: The oxidation of steel monitored with High-T XRD (click to enlarge)
 
At a certain temperature < 400°C, however, magnetite Fe3O4 formed at the expense of wuestite. As a result, the intensities of the wuestite reflections decrease whereas those of the magnetite reflections increase. This holds until the whole amount of wuestite has disappeared. Thus, we can expect that inside of a coil with such a cooling history only magnetite will be found on the surface of the steel strip.
 
Using different cooling paths and atmospheric conditions, the cooling step of the hot rolling process may easily be simulated at different points of a coil (inside, outside). This can be done on the same time scale as in the plant and the results can be monitored in-situ.
 
Crystallization of Molten Slag
The physical properties and the usability of slag are controlled by its mineralogical composition. Controlling factors of these compositions are both the bulk chemistry as well as the cooling history. Therefore, it is extremely important to know which phases crystallize at which temperature. Of course, in an experiment slag can be heated up to a certain temperature, subsequently quenched and finally, the cold slag can be studied. However, some phases are not quenchable and other ones change their composition during quenching. A solution is to X-ray the molten slag at the temperature of interest. Following this experimental approach, a molybdenum strip was prepared as the heating media with small deepening, used as a container for powdered slag. To avoid the vaporization and oxidation of the slag, the experiments were done under helium.
 

Fig 3: XRD-patterns of cooling and crystallizing slag (click to enlarge)
 
The slag samples were heated up rapidly to 1600°C and then cooled down in steps of 50°C to 1000°C. Figure 3 shows the spectra taken during the cooling. The present reflections indicate the crystallization of Mg-wuestite followed by Ca-silicates and chemically complex (Ca,Fe) molybdenates. The crystallization sequence is clearly visible and allows proving thermo dynamical calculations. Based on this knowledge, an optimal cooling path for slag can be derived.