Nanoindentation of alumina and multiphase inclusions in 42CrMo4 steel

Cleaning filter systems for metal melt filtration combine reactive filtration by functionalized filter surfaces for chemical reaction with the steel melt and active (mechanical) filtration. In the present work, a combined cleaning filter system was applied to 42CrMo4 steel melt containing nonmetallic inclusions, which were generated artificially before filtration. The nonmetallic inclusions in the solidified steel were investigated by an automated scanning electron microscope (ASPEX), nanoindentation, atomic force microscopy (AFM), focused ion beam technique (FIB) and confocal laser scanning microscopy (CLSM). The combined filter system was found to reduce the amount of aluminum in the steel and to form novel multiphase inclusions. These multiphase inclusions consist of TiO2-x and Al2O3 in a low-melting spessartine matrix and were characterized by nanoindentation. Compared to pure Al2O3 inclusions, the spessartine inclusions are softer and have a lower Young's modulus, fitting much better to the mechanical properties of the steel. The area below the indents was investigated by focused ion beam technique and transmission scanning electron microscopy revealing cracks and dislocations. Furthermore, the low-melting character of the spessartine matrix was proven by means of a confocal laser scanning microscope. Accordingly, the low-melting spessartine inclusions appear as liquid inclusions during steel melt processing and could contribute to clogging prevention in continuous steel casting.