Aluminothermic Reduction Process Under Nitrogen Gas Pressure for Preparing High Nitrogen Austenitic Steels
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INTRODUCTION
AMONG the reducing agents, such as aluminum, calcium, magnesium, silicon, carbon, and hydrogen, that could be used in the pyrometallurgical production of metals and alloys from their oxides, aluminum has a number of advantages. In most cases, the aluminothermic process, having high reducing potential and producing great heat due to a redox reaction, proceeds in a self-sustaining mode and is a self-propagating high-temperature synthesis (SHS). According to the Ellingham diagrams,[1] aluminum can easily reduce the oxides of metals, such as Fe, Cr, Mn, and Ni, which are traditionally used as the basis of alloy steels. Strongly
GENNADY DOROFEEV, VLADISLAV KAREV, OLEG GONCHAROV, EUGENY KUZMINYKH, IRINA SAPEGINA, ALEXEY LUBNIN, MARINA MOKRUSHINA, and VLADIMIR LAD’YANOV are with the Udmurt Federal Research Centre, Ural Branch of the Russian Academy of Science, 34 Baramzinoy St., Izhevsk, 426067, Russia. Contact e-mail: [email protected] Manuscript submitted May 29, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
exothermic aluminothermic reactions are used to realize a high temperature for the preparation of bulk steel ingots. In this case, two molten phases will be produced, namely, a metallic melt and an Al2O3-based ceramic melt. For example, 304 and 316L chromium-nickel austenitic stainless steels have been successfully produced in References 2 through 5 using the aluminothermic casting SHS. The peculiarity of the approaches in these articles was to regulate the microstructure of steel in the process of aluminothermic synthesis and subsequent crystallization of the steel ingot. The dual nanoand microcrystalline austenite microstructure in the works by La et al.[2,3] and Li et al.[4] was obtained, providing an improvement in tensile properties. An increase in the hardness and wear resistance of steel was observed in Reference 5 due to the overstoichiometric Al content in the reagent mixture, which gave the composite microstructure of steel/Al2O3 in the final product. In the past few decades, obtaining high nitrogen austenitic steels (HNSs) has been of great interest. HNSs are widely used instead of chromium-nickel steels in the power, engineering, and medical industries because they are nonmagnetic and have high corrosion resistance,
strength, and durability.[6–10] Additionally, because nitrogen is a strong austenite-forming element, it is a good alternative to austenite-forming nickel and manganese. Replacement of nickel and manganese by nitrogen saves expensive alloying elements and has positive effects on the mechanical and corrosion properties of steel. Since nickel and manganese have been found to cause allergy problems in humans,[11] the production of Ni-free and Mn-free HNS has offered a new perspective for the development of more efficient metal biomaterials.[12–14] The existing methods for introducing superequilibrium nitrogen concentrations in steel are complicated to execute, are energy consuming, and require expensive equipment.[15–17] Therefore, the development of a simpler and economical me
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