Effect of Silicon on Hot Shortness in Fe-Cu-Ni-Sn-Si Alloys During Isothermal Oxidation in Air
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I.
INTRODUCTION
OWING to advantages such as decreased costs, energy, raw materials, and emitted pollutants,[1] the production of steel via recycling of scrap in electric arc furnaces has increased to approximately 61 pct of the U.S. steel production[2] compared with the 46 pct of production reported in 1999. However, the scrap used in EAF production contains residual elements such as Cu, Ni, and Sn from electrical wirings and other contaminants in the post-consumer scrap.[3] As the steel oxidizes during processing at temperatures above 1373 K (1100 C), the Cu and other nonoxidizing elements enrich at the metal oxide interface.[4] Once the concentration of Cu exceeds the back diffusion of Cu into the matrix and the solubility of Cu in austenite, a Cu-rich liquid forms at the interface[5], which can then penetrate into the grain boundaries resulting in the embrittlement of the grain boundaries and cracking during secondary cooling in the caster or later rolling stages.[3] This surfacecracking phenomenon is known as hot shortness. When the steel oxidizes, any residual elements that are nobler than Fe (Cu, Ni, Sn, etc.) will enrich at the scale/ metal interface. Sn has been shown to increase the tendency for hot shortness by decreasing the Cu solubility in austenite as well as the melting point of the liquid phase.[3,6] Ni increases Cu solubility and liquid-melting temperature as well as altering the interERICA SAMPSON, Doctoral Graduate Student, is with Carnegie Mellon University, Pittsburgh, PA. Contact e-mail: esampson@andrew. cmu.edu SEETHARAMAN SRIDHAR, formerly POSCO Professor of Steelmaking with Carnegie Mellon University, is now Professor with TATA Steel/Royal Academy of Engineering and Joint Chair for Research into Low C Materials Technology, International Digital Laboratory, Warwick Manufacturing Group, The University of Warwick, Coventry, U.K. Manuscript submitted December 5, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
face morphology to provide beneficial effects to counter hot shortness.[7,8] However, small amounts of Sn greatly increase the amount of Ni needed to produce this effect.[7] As large additions of Ni incur higher costs, it was of interest to determine if an oxidizing element added to smaller quantities of Ni could provide beneficial effects to reduce hot shortness by altering the oxidation behavior and potentially decreasing the amount of Cu-rich liquid being formed at the interface. Any elements that are added would need to be low in cost and not contribute to other detrimental effects at the levels in which they are added. Elements less noble than Fe, such as Si, Al, and Mn, will oxidize along with the Fe. Al and Mn have been shown not to alter hot shortness appreciably.[9] Additions of Si can alter the oxidation behavior of the Fe-alloys through the formation of Si-oxides such as fayalite (Fe2SiO4)[10–12] or silica (SiO2)[12–14] in the scale near the metal/oxide interface. Solid Si-oxides impede the transport of iron cations through the scale because of the slower diffusion of Fe cations and
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