Application of Different Extraction Methods for Investigation of Nonmetallic Inclusions and Clusters in Steels and Alloys

The characterization of nonmetallic inclusions is of importance for the production of clean steel in order to improve the mechanical properties. In this respect, a three-dimensional (3D) investigation is considered to be useful for an accurate evaluation of size, number, morphology of inclusions, and elementary distribution in each inclusion particle. In this study, the application of various extraction methods (chemical extraction/etching by acid or halogen-alcohol solutions, electrolysis, sputtering with glow discharge, and so on) for 3D estimation of nonmetallic Al2O3 inclusions and clusters in high-alloyed steels was examined and discussed using an Fe-10 mass% Ni alloy and an 18/8 stainless steel deoxidized with Al. Advantages and limitations of different extraction methods for 3D investigations of inclusions and clusters were discussed in comparison to conventional two-dimensional (2D) observations on a polished cross section of metal samples. 1. Introduction The characterization of nonmetallic inclusions (NMI) is of importance for the improvement of mechanical properties of steel products as well as for the production of clean steel. Some characteristics such as number, size and composition of inclusions are particularly important for understanding of the formation and growth mechanism of inclusions [1]. Previous studies [2, 3] have shown that three-dimensional (3D) investigations are more reliable regarding number and size of inclusion particles compared to conventional two-dimensional (2D) investigations on a polished cross section of metal samples. Moreover, the effect of matrix on composition analysis of small size inclusions is much higher with 2D compared to 3D investigations [4]. In this respect, a 3D investigation is significant for an accurate estimation of inclusion characteristics in steel. The 3D methods which have been used extensively for these purposes are so-called extraction methods. These include electrolytic extraction [5–8] as well as chemical extraction using halogen-alcohols and acids [5–7]. However, it is known that some nonmetallic inclusions in metal samples are dissolved during the chemical extractions. Inoue et al. [5] evaluated the possibility of dissolution of Al2O3 particles in acids (HCl-HNO3-H2O and H2SO4-HNO3-H2O), halogen-alcohols (bromine-methanol and iodine-methanol), and in a 10% AA electrolyte (10 v/v% acetylacetone-1 w/v% tetramethylammonium chloride-methanol). More specifically, the standard fine Al2O3 powders with a known size distribution (average sizes are 0.06 or 1？μm) were charged in the solutions