Study of solid solution rare earth content in steels

In the current study, solid solution rare earth ([RE]) content in quenched and hot-forged steel samples was systematically investigated, through three complementary methodologies: non-aqueous electrolytic extraction, inclusion morphological analysis, and thermodynamic calculation. Rare earth inclusions (RE inclusions) were extracted with a modified non-aqueous electrolysis method for their three-dimensional morphological characterization via scanning electron microscopy (SEM). Quantitative measurements of rare earth content (RE content) in both the solid solution phase and inclusion phases were conducted using inductively coupled plasma mass spectrometry (ICP-MS), where experimental results showed excellent consistency in most of steel samples. A mathematical model based on ASPEX particle statistical data was developed to calculate the [RE] content by subtracting the rare earth content fixed in inclusions (REin inclusions) from the total rare earth (T.RE.) content in steels, and this model exhibited optimal performance for steels containing spherical or ellipsoidal particles (aspect ratio≤1.5), as confirmed by experimental results. A systematic comparison between the above results and predicted results using FactSage software revealed some discrepancies, which were attributed to thermodynamic model limitations, the precipitation of RE inclusions during the solidification and cooling process, and measurement uncertainties. Furthermore, the effects of total rare earth (T.RE.), total oxygen (T.O.), total sulfur (T.S.), acid-soluble aluminum ([Al]s), and temperature on the [RE] content in steels were also discussed.