Nonmetallic Inclusions in Rails Made of Electro-Steel Alloyed with Chromium

Resumo

Based on metallographic (with an “OLYMPUS GX-51” microscope) and spectral (using spectrometer “ARL iSpark” method “Spark-DAT”) analyses, the relative concentration and size of most non-metallic inclusions for rail elements (head, web) from electro-steel of E79KhF and E90KhАF grades were determined. It was found that the highest relative concentration of manganese sulfides (MnS) is 30.8–43.4 ppm. At the same time, 60–100% of these inclusion types have a small size (less than 4 μm), and cannot be detected using standard metallographic analysis with 100-fold magnification. The revealed high relative-concentration of sulfide inclusions directly correlates with the established positive sulfur liquation in considered rail elements, which reaches up to 40%. Despite the high MnS concentration, their influence on the rail quality can be considered not dangerous, considering their high ductility during hot deformation and the prevalence of these inclusion types with a small size (less than 4 μm). Among silicate-type inclusions, SiO₂ inclusions (3.4–14.9 ppm) have a significant concentration. All detected inclusions of this type have a size not exceeding 4 μm. It was found that the concentration of complex inclusions containing alumina (Al₂O₃–CaO–MgO, Al₂O₃–CaO–MgO–CaS, Al₂O₃–CaO, Al₂O₃–MgO) is insignificant: in total, it does not exceed 3.1 ppm and 1.6 ppm for individual types. The concentration of corundum (Al₂O₃) is also insignificant and does not exceed 0.3 ppm. In this case, small-sized alumina inclusions (less than 6 μm) prevail. Due to the low contamination (considering the relative concentration and size of inclusions) with non-plastic silicate and alumina non-metallic inclusions, their influence on the rail quality was not significant. It is confirmed by the absence of defects detected during ultrasonic testing.