Distinguishing MgO·Al2O3 Spinel Inclusions from Alumina or Magnesia Inclusions in Aluminum-killed Stainless Steel Using Cathodoluminescence Imaging

Removal of Al2O3 and MgO·Al2O3 spinel inclusions is a critical issue in the production of stainless steel. Understanding the formation mechanism of these inclusions is important for facilitating their removal. Electron probe microanalysis (EPMA) has been widely used to understand their formation mechanism. However, using EPMA to identify inclusions with agglomerated MgO·Al2O3 spinel and Al2O3 inclusions or agglomerated MgO·Al2O3 spinel and MgO inclusions is difficult because of the wide composition range of MgO·Al2O3 spinel. This study presents a novel analytical method to distinguish among MgO·Al2O3 spinel, MgO, and Al2O3 in these agglomerates based on the differences in their emission colors in cathodoluminescence (CL) imaging. Al rich and stoichiometric MgO·Al2O3 spinel inclusions were distinguishable by detecting the green luminescence in CL images in the wavelength regions of 350–1000 nm and 395–575 nm, respectively. In contrast, MgO and Al2O3 inclusions emitted red luminescence in the wavelength region of 350–1000 nm but no luminescence in the wavelength region of 395–575 nm. In addition, MgO inclusions with small particle sizes, which were agglomerated with MgO·Al2O3 spinel inclusions, were detected based on broadening around 750 nm corresponding to the CL peak at 690 nm. The proposed method could lead to a better understanding of the formation mechanism of Al2O3 and MgO·Al2O3 spinel inclusions in stainless steels.