Effect of carbon addition on the strength and creep resistance of FeAl alloys
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11/9/03
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Effect of Carbon Addition on the Strength and Creep Resistance of FeAl Alloys R.S. SUNDAR and S.C. DEEVI We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100 °C and 1300 °C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacancies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300 °C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300 °C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700 °C, and yield strength up to 800 °C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys.
I. INTRODUCTION
INTERMETALLIC alloys based on Fe3Al and FeAl are being considered as high-temperature structural materials due to low raw material cost, low density, high electrical resistivity, and excellent corrosion resistance in a variety of aggressive high-temperature gaseous environments.[1–4] In particular, FeAl alloys are highly resistant against oxidation, carburization, and sufidation even at high temperatures without the necessity of having a high chromium content as in the case of stainless steels and nickel-based superalloys. They can be used as heat-treatment fixtures, furnace components, resistive heating elements, porous gas-metal filters, automotive exhaust components, catalytic converters, and piston valves.[3,4] It is envisaged that some of the ferritic steels, austenitic steels, and iron-base and nickel-based superalloys will eventually be replaced by FeAl-based alloys due to their corrosion resistance. The commercial utilization of FeAl alloys are limited by the low ambient temperature ductility, rapid drop in strength above 500 °C, and poor high-temperature creep resistance. R.S. SUNDAR, Research Associate, and S.C. DEEVI, Fellow ASM Internati
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