Research on determination of the rare-earth content in metal phases of steel
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I.
INTRODUCTION
RARE-earthmetals are used widely in steel and iron. Rareearth elements in steel are found to exist in three forms. The thermodynamic calculation results show that rareearth elements added to steel form oxides and sulfides easily, because the free energy of formation of rare-earth oxides and sulfides have comparatively great negative values (Table I). Part of the balance of rare-earth added will be dissolved in a-Fe (solid solution) and form intermetallic phases with iron. The sum of the rare-earth content in the solid solution and that in the intermetallic phases is called the rare-earth content in the metal phases. Many previous studies have studied the reaction mechanism of rare-earth in steel. However, they have laid stress only on researching the relationship between the properties of steel and the change of inclusions due to rare-earth addition or the relation between the properties of steel and its total content of rare-earth. Research on the relation between the properties of steel and the rare-earth content in metal phases of steel has not been reported. To investigate this problem, first the rareearth inclusions in steel must be separated, and then the rare-earth content in the metal phases (the content that does not include the rare-earth inclusions) must be determined accurately. Kazou Kawamura et al. 2 have studied the separation of rare-earth inclusions in steel. They studied only the stability of rare-earth inclusions in the process of electrolytic separation. They did not study the behavior of rare-earth ions that enter into the electrolyte from the metal phase in steel, FANG KEMING and NI RUIMING are Lecturers with Beijing University of Iron and Steel Technology, Beijing, China. Manuscript submitted August 29, 1984. METALLURGICAl, TRANSACTIONS A
and did not study the behavior of the intermetallic compounds in the process of electrolytic separation. These items are very important to the quantitative separation of inclusions. In recent years, many research workers have attempted to get the rare-earth content in metal phases by subtracting the rare-earth content in inclusions from the total rare-earth content. The above cannot produce satisfactory results because the technique involves chemical analysis which is not sensitive enough, and because the distribution of inclusion in steel is not homogeneous. The experimental method in this paper has been examined extensively by means of the radiolabeling method, the electron probe, and X-ray analysis. After the experimental method had been demonstrated feasibly for application, the rare-earth content in metal phases of 16MnTiBRe and an electrical engineering steel was determined by using the electrolytic separation and radiolabeling method.
II.
EXPERIMENTAL METHODS
A. Separation of Rare-Earth Inclusions from the Metal Phases by Electrolysis with Nonaqueous Electrolyte 1. The choice of the electrolyte The key to quantitative separation of rare-earth inclusions is the choice of the electrolyte. The appropriate electrolyte must not dissolve
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