In industrial applications, refractory materials are subjected to extreme mechanical, thermal or chemical loads. These loads can in simple cases, e.g., for a component of furnace insulation, be a stationary temperature gradient which leads to thermal stresses. Cyclical thermal loads are often added by rapid heating and cooling cycles. In addition, refractory materials often carry mechanical loads - e.g., in the case of firing auxiliaries, the material to be heated. When dealing with molten materials, for example in glass or steel production, the refractory materials are subjected to massive thermal shocks by sudden heating on contact with the melt. Due to contact with melts, slags or aggressive gases, the refractory materials are attacked by corrosive processes. All of the loads mentioned require a regular replacement of the refractory components in order to avoid damage to the material and the furnace systems or to ensure the safety of working with liquid melts. From an economic perspective, the replacement should be made as late as possible, but without increased damage risk. The Fraunhofer-Center HTL offers various analyses and methods to estimate the service life of shaped refractory products more precisely.
In order to determine the thermal shock resistance, refractory bricks are tested at the HTL by means of thermal shock experiments according to the standard (DIN EN 993-11). In addition, the relative degree of damage can be measured at the HTL by measuring the residual sound velocity with a mobile ultrasonic measuring device after each thermal shock cycle. The limitation of the service life by crack formation in the refractory brick is thus determined more precisely. The initial and residual strengths of the refractory material are determined after a distinct number of cycles and then correlated with the sound velocities. Since this correlation is known for a refractory material, the on-site measurement of the sound velocity can - during regular maintenance - be used as a criterion for the necessary exchange of refractory bricks. In connection with failure probability models, an estimation of the remaining useful life can also be made.
Much more flexible thermal shock and temperature change tests are possible at the HTL using the specially developed ThermoOptical measurement system TOM_wave. In TOM_wave, disk-shaped samples (diameter up to 35 mm and thickness up to 20 mm) are preheated to an initial temperature of up to 1750 °C and then subjected to almost arbitrary user-defined temperature rises by single or double-sided irradiation with a powerful (600W) CO2 laser. Through this type of thermal cycling, realistic thermal stresses of the refractory materials can be simulated much better than in standard thermal shock tests. The damage (cracks) occurring during the temperature load can be detected acoustically in-situ with the TOM_wave measuring system. Furthermore, there is the option of characterizing the progressive degradation of the material between the individual thermal cycles by means of acoustic resonances. For this purpose, vibrations in the sample are excited with short pulses of the CO2 laser. These vibrations are detected acoustically. As a follow-up, accumulated damage can also be characterized by non-destructive testing and imaging methods at the HTL. With the aid of finite element (FE) simulations, the thermal stresses occurring in the components during the real temperature cycles are calculated. From this, temperature cycles for the TOM_wave are determined, which lead to comparable loads in the samples to be examined. These loading scenarios are then performed in a time-lapse manner in order to determine the service life. In addition, the maximum stationary temperature gradients or heating and cooling rates of refractories being possible without fracture risk can be determined.
For quantitative predictions on the probability of failure of a refractory component under defined loading conditions, a multi-stage process is used at the HTL. Experimental basis of the method are fracture experiments carried out under different conditions (stress rate, temperature, sample geometry). From a statistical evaluation of the Weibull data, WeibPar software is used to determine so-called service life parameters. These parameters are transferred to the software CARES, developed by NASA, together with a stress distribution calculated by a FE simulation of the component under load. CARES calculates the temporal development of the failure probability of the component under load. If a sufficiently good data basis and an experimental validation for a special loading case are available for a refractory material, reliable forecasts on the service life of refractory components can be produced at the HTL.
In addition to the thermomechanical stresses, the chemical modification of refractory materials also limits their service life in the presence of corrosive media. In order to prevent or at least reduce the corrosion of the affected refractory components, the HTL offers a series of methods for the analysis of corrosion processes. Firstly, a wide range of analytical methods such as X-ray fluorescence analysis, X-ray diffraction, electron microscopy, etc. are used to elucidate the chemical reactions that contribute to the corrosion of the refractory material. By experiments in the measuring system TOM_chem, in which various conditions of temperature, gas composition and flow velocity can be adjusted, corrosive mechanisms are identified, and the course of the corrosive degradation is determined. From these analyses, specific concepts against corrosive problems are developed by the HTL, such as, for example, exchange of the refractory material, application of protective layers or altered process conditions. Their effectiveness can, then, also be tested in measuring furnaces such as TOM_chem. Finally, estimates for the corrosion-resistant life of the refractory materials can be derived.