Alexander Schramm

In the present study, the chemical processes occurring during high‐temperature testing are investigated for two commercial MgO‐C brick grades. One grade consists exclusively of virgin raw materials, while the other contains 47.5 wt.% MgO‐C recyclate. High‐temperature testing is conducted under argon atmosphere at ambient pressure using induction heating. To evaluate the effect of MgO‐C recyclate incorporation on thermo‐chemical processes, X‐ray diffraction (XRD), electrothermal vaporization (ETV), differential thermal and thermogravimetric analysis (DTA/TG) coupled with mass spectrometry (MS) are applied. Scanning electron microscopy (SEM) combined with energy‐dispersive X‐ray spectroscopy (EDS) and electron backscatter diffraction (EBSD) provide phase characterization after exposure at 1300 °C and above. The results show that mechanisms typically occurring during the service of refractory materials such as the carbothermal reduction of magnesia, the incorporation of impurity phases into the newly formed MgO surface layer, and the deposition of calcium‐rich phases and whisker‐like structures also occur when MgO‐C is exposed to the applied gas atmosphere. This confirms the relevance of the observed thermo‐chemical processes and surface phase formation for real contact with molten steel and slag. Under the present test conditions, however, no significant influence of MgO‐C recyclate incorporation on the thermo‐chemical behavior of the coked MgO‐C materials is detected.

Given the growing emphasis on refractory recycling, the performance characteristics of commercial MgO‐C bricks incorporating MgO‐C recyclates are of considerable economic and environmental relevance. In the present study, four‐point bending tests are conducted in an argon atmosphere on three commercial MgO‐C brick grades over a temperature range from room temperature to 1300 °C. One MgO‐C brick grade is produced exclusively from virgin raw materials, whereas the other two grades incorporate MgO‐C recyclate at a high mass fraction of 47.5 wt%. Due to the different sources of the recyclates, their carbon contents vary, resulting in different total carbon contents in the two MgO‐C brick grades containing recyclate. Post‐mortem SEM investigations show predominantly transgranular fracture through the MgO‐C recyclates. The evaluation of the four‐point bending tests reveals no detrimental effect of the MgO‐C recyclates on bending strength. Instead, a decisive influence of the total carbon content is observed, with lower total carbon contents correlating with higher bending strength values. Furthermore, the heterogeneous structure of the MgO‐C recyclates contributes to enhanced stress dissipation caused by anelastic deformation processes in the respective MgO‐C brick grades, despite the accompanying decrease in virgin flake graphite content.

A duplex treatment consisting of plasma nitriding (PN) and physical vapor deposition (PVD) significantly improves the thermal, tribological, and corrosion resistance of forming tools, and especially if they are intended for applications subject to high mechanical loads. This study investigates the influence of nitriding on the properties of a conventionally heat‐treated AISI D2 tool steel, coated with a Cr‐Al‐Ti‐B‐N layer, while the effect of the presence or absence of a compound layer is discussed. PN is performed at 510–520°C using different N₂‐H₂ gas mixtures. The Cr‐Al‐Ti‐B‐N layers are deposited at 480°C using a combination of cathodic arc evaporation and magnetron sputtering. The samples are characterized using electron probe microanalysis with wavelength‐dispersive X‐ray spectroscopy, optical and scanning electron microscopy, glancing‐angle X‐ray diffraction, surface hardness measurements, profilometry, Rockwell indentation, and scratch tests. These techniques reveal relationships between the depth gradient of the chemical composition and microstructure of the nitrided interlayer, the adhesion of the PVD coating, and the hardness of the tool steel. Although the PVD process induces a structural transformation in the compound layer, this transition does not have a negative influence on the adhesion of the PVD coating.