Effects of Small Additions of Copper and Copper + Nickel on the Oxidation Behavior of Iron
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INTRODUCTION
LOW-CARBON steel production through scrap melting in an electric arc furnace is an economically[1] and environmentally[2] attractive process. However, many issues arise, due to the variation in chemistry that results from use of scrap steel as the input material. A particularly important residual impurity element is copper,[1] because of its nobility compared to iron and its common presence as a residual element in scrap steel. Since steel is exposed to oxidizing atmospheres (air, CO2, H2O) at high temperature for most of its processing prior to hot working, iron selectively oxidizes, enriching the area near the oxide/metal interface in copper. Considering the iron-copper binary phase diagram shown in Figure 1, copper has limited solubility in solid iron and the new phase that separates is both liquid and copper rich at temperatures greater than 1100 C. These temperatures occur during secondary cooling, reheating, and hot rolling. According to calculations by Po¨tschke,[5] when iron is oxidized in air at temperatures around 1100 C, the solubility limit of copper in iron is reached in less than one second. Therefore, rapid formation of a copper-rich layer appears to be unavoidable, given the current processing methods for low-carbon steel. During oxidation, iron dissolves out of the c-iron phase into the liquid and then it is rapidly transported through the liquid and oxidized at the liquid/oxide BRYAN WEBLER and LAN YIN, Graduate Students, and SEETHARAMAN SRIDHAR, Professor, are with the Department of Material Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] Manuscript submitted August 8, 2008. Article published online October 21, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B
interface. The remaining copper accumulates in the liquid layer and causes it to grow. A small amount of copper also diffuses back into the steel. Because the iron content of the c iron is high, iron is supplied to the oxide by dissolving more iron into the liquid (i.e., growing the liquid phase), eliminating the need for long-range diffusion to supply the iron. Therefore, processes in the solid and liquid-metal phases should not affect the oxidation rate. Once the copper-rich liquid forms, it can penetrate into the steel at austenite grain boundaries.[6] Where penetration has occurred, the boundaries are embrittled by the liquid copper. Intergranular cracking occurs because of this embrittlement, when the steel is subjected to stress during hot working.[6] This cracking phenomenon is known as surface hot shortness. Additions of nickel can alleviate hot shortness cracking.[6] The first beneficial effect of nickel is related to an increase in copper solubility in the c-iron phase.[6–8] Figure 2 shows a phase diagram for the iron-coppernickel system at 1150 C. Assuming that each component has equal diffusivities, enrichment of the alloy occurs along a straight line defined by the copper/nickel ratio. According to Figure 2, copper/nickel ratios greater than 1 will lead to the ap
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