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TECHNICAL PAPERS: Gas Turbines: Microturbines and Small Turbomachinery

Stainless Steels With Improved Oxidation Resistance for Recuperators

[+] Author and Article Information
Bruce A. Pint

 Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6156pintba@ornl.gov

J. Eng. Gas Turbines Power 128(2), 370-376 (Mar 01, 2004) (7 pages) doi:10.1115/1.2056531 History: Received October 01, 2003; Revised March 01, 2004

New materials are being evaluated to replace type 347 stainless steel in microturbine recuperators operating at higher temperatures in order to increase the efficiency of the microturbine. Commercial alloys 120 and 625 are being tested along with potentially lower cost substitutes, such as Fe-20Cr-25Ni and Fe-20Cr-20Ni. Long-term testing of these materials at 650–700 °C shows excellent corrosion resistance to a simulated exhaust gas environment. Testing at 800 °C has been used to further differentiate the performance of the various materials. The depletion of Cr from foils of these materials is being used to evaluate the rate of attack. Although those alloys with the highest Ni and Cr contents have longer lives in this environment, lower alloyed steels may have sufficient protection at a lower cost.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of oxidation of type 347 stainless steel in dry air (a) and humid air (b,c). With minimal water vapor in the environment (a), a protective scale forms that thickens with time at temperature. With the addition of water vapor (b), there is an increase in the scale growth rate and increased evaporation of Cr2O3 as CrO2(OH)2. These result in increased Cr consumption and depletion of Cr in the substrate near the surface. With continued exposure, nodules of FeOx begin to form and grow with time (c). Eventually, the nodules grow together forming a duplex scale with an outer FeOx layer and an inner (Fe,Cr)Ox layer.

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Figure 2

Specimen mass gains for various foil (100μm thick) materials during 100 hr cycles in humid air at 650 °C. Several versions of type 347 stainless steel showed AA after <2000hr of exposure, whereas more highly alloyed materials have not shown AA after 8000 hr.

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Figure 3

Light microscopy of polished cross sections of 100μm foils after exposure in humid air: (a) commercial 347 foil after 2000 hr at 650 °C, (b) 20∕25∕Nb foil after 5000 hr at 650 °C, and (c) 20∕25∕Nb foil after 5000 hr at 700 °C

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Figure 4

Specimen mass gains for various foil (100μm thick) materials during 100 hr cycles in humid air at 700 °C

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Figure 5

Specimen mass changes for model Fe-Cr-Ni alloys (specified by their Cr∕Ni contents) during 100 hr cycles at 700 °C in humid air. Designations with +MS indicate Mn and Si additions.

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Figure 6

Specimen mass gains for various foil (100μm thick) materials during 100 hr cycles in humid air at 800 °C

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Figure 7

Light microscopy of polished cross sections of ORNL-rolled 100μm foils after exposure in humid air at 800 °C: (a) 20∕25∕Nb for 5000 hr (b,c) 20∕25∕Nb for 6000 hr, and (d) alloy 625 for 6000 hr

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Figure 8

Specimen mass gains for various model alloys (1.2 mm thick specimens) with different Cr∕Ni contents and additions of Mn and Si during 100 hr cycles in humid air at 800 °C. Adding 3.8% Al significantly improved the resistance to AA.

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Figure 9

Electron microprobe Cr profiles across 20∕25∕Nb foils after various exposures in humid and dry air at 800 °C. The addition of water vapor to the environment significantly increased the Cr consumption rate from the foils.

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Figure 10

Electron microprobe Cr profiles across 625 foils after various exposures at 800 °C. The addition of water vapor to the environment increased the Cr consumption rate from the foils.

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Figure 11

Electron microprobe Cr profiles into model alloys after 100 hr exposures in humid air at 650 °C. The different Ni contents did not significantly change the profiles.

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