Interface reactions between steel 42CrMo4 and mullite

Reactions between steel 42CrMo4 and mullite were investigated using differential thermal analysis (DTA) up to 1650 °C and examined afterwards by X-ray diffraction (XRD), and scanning electron microscopy (SEM/EDX, SEM/EBSD). Several reactions were found and compared with thermodynamic calculations of pure steel, iron, and the mullite-steel interface at different partial pressures of oxygen. Two mullite phases with different composition were found after all experiments: composition of one mullite phase was in agreement with reported Al:Si ratio from literature and contained very low amount of metal impurities, whereas the second mullite had lower Al:Si ratio and relatively high amounts of dissolved manganese, magnesium, and iron.