Magnetic properties represent essential information for engineering applications such as the design of efficient electric engines and induction heating processes in the steel industry. Modern electromagnetic simulation techniques can reduce development times for industrial processes and components, but these rely on proper material data, such as B-H hystereses that are unaffected by eddy currents, so that these B-H hystereses are not distorted. This work summarizes the possibilities of a newly developed electromagnetic measurement setup to determine these magnetic properties and investigates the influencing factors during such measurements, i.e. microstructure, eddy currents and temperature.
Since the described measurement setup represents a novel approach, the development of both setup and test method posed the greatest challenges of this work. In literature, the influences of eddy currents, microstructure and temperature on the magnetic properties are described either incompletely or in a simplified manner, and the magnetic properties of induction hardened products are poorly recorded. These challenges are dealt with in this work to be able to properly analyse the relationship between microstructure and magnetic properties of these products.
An indirect yoke-based measurement setup was supplemented to an induction heat treatment facility to be able to measure the magnetic properties at room and elevated temperatures, realized by induction heating. Additionally, finite element, analytical and semi-empirical models were developed and applied to determine the field distribution of the setup, the sample specific magnetic properties and to correct the effect of the eddy currents. To demonstrate the functionality of the models and the measurement method, experiments were performed on Ø22 x 300 mm pure iron and 50CrMo4 steel rod samples in a normalized initial microstructure state.
For the analysis of the relationship between the microstructure and the mechanical properties, induction heat treatments using different austenitizing and tempering temperatures and heating rates were performed. The goal was to adjust specific microstructures of 50CrMo4 for having meaningful comparisons and explanations of the relationships. Furthermore, synchrotron radiation techniques were used to analyse microstructural changes in terms of residual stresses and retained austenite phase content in relation to the hardness distribution of a locally quenched and tempered 50CrMo4 sample. These studies allowed for setting the right induction heating parameters for the magnetic measurements performed at locally heated samples to understand the temperature related microstructural changes and their effect on the magnetic properties.
Measurement of the temperature dependent magnetic properties up to Curie temperature and even higher was successfully established, and the results are in line with acknowledged measurement methods. The main advantage of the novel method is the capability to measure indirectly with the sensor not being thermally influenced at elevated temperatures. The application of the models allows the determination of intrinsic magnetic properties of the sample material with minimized eddy current influences. Based on correlations between magnetic and mechanical properties, the measurement setup can also be used for non-destructive determination of hardness, tensile and yield strength and precipitation size within a 3 to 20% standard deviation. The achieved accuracy depends on various parameters such as yoke-type (bulk or sheet-like), sample geometry (diameter / length ratio), excitation strength and frequency (non-saturated and below 1 Hz) as well as the selected analysis method (distortion, permeability, hysteresis).
The main result of this work is the new indirect measurement method for measuring magnetic properties and for non-destructive determination of the aforementioned mechanical properties and microstructure states having the potential for additional analysis methods in the future using only one setup. The current research results set the basis for future advanced sensorial applications such as in-line process monitoring and process control in the steel processing industry and for smart induction heat treatment processes.