Oxidation of Fe-7Cr-12Ni-(0-6)Al-(0-7)Si alloys
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I.
INTRODUCTION
THE designation
of Cr as a strategic material has spurred wide interest in finding substitutes for Cr in Fe-based, oxidation-, and corrosion-resistant alloys. Recently several authors have reported significant results in reducing or eliminating Cr in Fe-base alloys by A1 or Si substitution. ~.2 Small additions of A1 or Si to Fe-base alloys with Cr concentrations greater than 15 wt pct have been particularly effective in improving the oxidation resistance by decreasing the oxidation rate or extending the temperature range over that of commercial stainless steels. 3,4 Studies also have shown that the level of Cr can be reduced and comparable levels of oxidation protection can be achieved by additions of AI and/or Si. 3"5-8 Oxidation protection provided by A1 and Si alloying is achieved by two different mechanisms. Aluminumsubstituted Fe alloys form an outer layer of A1203 over the base metal and acts as a diffusion barrier, reducing further oxidation. 3'5'6'9'1~A1203 forms an outer oxide layer which temporarily protects the substrate metal and provides time for an inner layer of Cr203 to form. As the aluminum alloy concentration is increased, the oxidation protection increases. However, if the AI concentration is raised too high, the Al in the substrate metal begins to interact with any solid solution Ni to form intermetallics. The oxidation protection mechanism provided by alloying Si to Fe-base metals results from the formation of a thin SiO2 oxide sublayer between the metal and the C r 2 0 3 o x i d e . 4'11-14 While data exist that support the idea that this SiO2 oxide sublayer improves oxidation protection, no research has yet been reported that explains the exact mechanism of this protection or how the oxide sublayer is formed. Currently there is limited understanding of the oxidation protection mechanisms of Fe-based alloys that contain 5 to 10 pct Cr and to which A1 and Si have been added. Several of these alloys have oxidation resistance better than some Fe-alloy systems with greater than 15 pct Cr.15'16'17 In this study, a decreased-Cr, Fe-base alloy (Fe-7Cr12Ni) was alloyed with 0 to 6 pct A1 and/or 0 to 7 pct Si. The alloys were tested at oxide temperatures between 800 ~ and 1000 ~ for times up to 1000 hours. Oxidation samples and data were analyzed to determine whether any mechanism could explain the observed oxidation rates. J.C. RAWERS, Materials Scientist, and E . M . MATTLIN. Assistant Materials Scientist, are with the Bureau of Mines, Umted States Department of the Interior. Albany Research Center, Albany, OR 97321. Manuscript submitted November 10, 1986. METALLURGICAL TRANSACTIONS A
This research is part of an ongoing study by the United States Bureau of Mines to conserve strategic materials. One purpose of this work is to reduce United States dependence on chromium for high-temperature protection of iron alloys.
II.
E X P E R I M E N T AND DATA ANALYSIS
Fifteen alloys of fifteen different compositions were melted and fabricated. Samples were sectioned, oxidized, and examined
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