Ferromagnetism is of increasing importance in the growing field of electromobility and data storage. In stable austenitic steels, the occurrence of ferromagnetism is not expected and would also interfere with many applications. However, ferromagnetism in austenitic stainless steels after low-temperature nitriding has already been shown in the past. Herein, the presence of ferromagnetism in austenitic steels is discovered after low-temperature carburization (Kolsterizing), which represents a novel and unique finding. A zone of expanded austenite is established on various austenitic stainless steels by low-temperature carburization and the respective ferromagnetism is investigated in relation to the alloy composition. The ferromagnetism occurring is determined by means of a commercial magnetoinductive sensor (Feritscope). Ferromagnetic domains are visualized by magnetic force microscopy and a ferrofluid. X-ray diffraction measurements indicate a clear difference in the lattice expansion of the different alloys. Furthermore, a different appearance of the magnetizable microstructure regions (magnetic domain structure) is detected depending on the grain orientation determined by electron backscatter diffraction (EBSD). Strongly pronounced magnetic domains show no linear lattice defects, whereas in small magnetizable areas linear lattice defects are detected by electron channeling contrast imaging and EBSD.
Investigation of magnetic effects on austenitic stainless steels after low temperature carburization
(2018)
This work aims at investigating the magnetic effects of austenitc stainless steels which can occur after a low temperature carburisation depending on the alloy. Samples were prepared of different alloys and subjected to a multiple low temperature carburisation to obtain different treatment conditions for each alloy. The layer characterisation was carried out by light microscope and also by hardening profiles and shows that the layer develops with each additional treatment cycle. A lattice expansion could be detected in all treated samples by X-ray diffraction. Magnetisability was measured using Feritscope and SQUID measurements. Not all alloys showed magnetisability after treatment. In addition to MFM measurements, experiments with Ferrofluid were also used to visualize the magnetic areas. These studies show that only about half of the formed layer becomes magnetisable and has a domain-like structure.