Effects of Small Additions of Tin on High-Temperature Oxidation of Fe-Cu-Sn Alloys for Surface Hot Shortness
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INTRODUCTION
ALTHOUGH recycling scrap in electric arc furnaces (EAFs) saves energy, raw material, and cost compared with the conventional blast furnace/basic oxygen furnace (BF/BOF) route,[1] surface hot shortness currently limits steel manufacturing via EAF-based techniques.[2,3] The feed materials of EAFs are predominantly steel scrap and, thus, inevitably contain copper (Cu), usually between 0.25 wt pct and 0.45 wt pct,[4] which mostly originates from scraped cars and electronics parts. When the iron (Fe) in high-Cu residual containing steel oxidizes at temperatures above 1373 K (1100 C) during the secondary cooling and/or reheating processes, Cu enriches at the oxide–steel interface. Once the content exceeds the Cu solubility in the cFe phase, a Cu-rich liquid layer forms, and if it penetrates the cFe grain boundaries, then the steel surface will crack during rolling as a result of the weakened grain boundaries. Sometimes, Fe oxides grow on the austenite grain boundary near the oxide–cFe interface for nickel (Ni)and silicon (Si)-bearing steel during reheating, which are known as internal oxides.[5] Fe oxides also were found inside the cracks after deformation[6]; however, it is not known whether these oxides form during the reheating or the rolling processes. Studies found that a certain amount of tin (Sn) (£0.05 wt pct) and Ni (£0.30 wt pct) always is present in steel scraps along with Cu.[3,7] Tin usually comes from
cans and tin-plated steels, whereas nickel originates from alloy steels such as stainless steel.[3,8] While Ni is known to ameliorate Cu-induced hot shortness by increasing Cu solubility in the cFe phase,[9] promoting occlusion[10] and decreasing the oxidation rate,[11] Sn at contents as low as 0.04 wt pct exacerbates surface hot shortness.[12,13] Tin could lower the solubility of Cu in the cFe austenite phase[9] and lower the melting point of Cu-rich phase.[14] A relatively small amount of Sn will counteract the beneficial effect of Ni in the solubility point of view.[14,15] It also was found that Sn could promote the penetration of a Cu-rich phase into the grain boundaries,[16] but the detailed mechanism through which this occurs is not yet known. This article focuses on the effects of Sn on the microstructure near the oxide–cFe interface. Thermogravimetry (TG), scanning electron microscopy (SEM), and focused ion beam (FIB) serial-sectioning techniques were used to investigate oxidation behavior, interface morphology, and three-dimensional (3D) microstructure. A numerical model developed in a previous article[17] was applied to support the experimental results, which predicts the liquid–cFe interface concentrations and interface morphology. An insight on how Sn promotes grain-boundary penetration and cracking is also provided.
II. LAN YIN, Graduate Student, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. SEETHARAMAN SRIDHAR, POSCO Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, and
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