TECHNICAL PAPERS: Gas Turbines: Microturbines and Small Turbomachinery

Creep Strength and Microstructure of AL20-25+Nb Alloy Sheets and Foils for Advanced Microturbine Recuperators

[+] Author and Article Information
P. J. Maziasz

 Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115maziaszpj@ornl.gov

J. P. Shingledecker, N. D. Evans, Y. Yamamoto, K. L. More, R. Trejo, E. Lara-Curzio

 Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115

J. Eng. Gas Turbines Power 129(3), 798-805 (Oct 20, 2006) (8 pages) doi:10.1115/1.2718569 History: Received October 06, 2006; Revised October 20, 2006

The Oak Ridge National Laboratory (ORNL) and ATI Allegheny Ludlum worked together on a collaborative program for about two years to produce a wide range of commercial sheets and foils of the new AL20-25+Nb (AL20–25+Nb) stainless alloy for advanced microturbine recuperator applications. There is a need for cost-effective sheets/foils with more performance and reliability at 650–750°C than 347 stainless steel, particularly for larger 200–250 kW microturbines. Phase 1 of this collaborative program produced the sheets and foils needed for manufacturing brazed plated-fin air cells, while Phase 2 provided foils for primary surface air cells, and did experiments on modified processing designed to change the microstructure of sheets and foils for improved creep-resistance. Phase 1 sheets and foils of AL20-25+Nb have much more creep-resistance than 347 steel at 700–750°C, and those foils are slightly stronger than HR120 and HR230. Results for Phase 2 showed nearly double the creep-rupture life of sheets at 750°C/100 MPa, and similar improvements in foils. Creep data show that Phase 2 foils of AL20-25+Nb alloy have creep resistance approaching that of alloy 625 foils. Testing at about 750°C in flowing turbine exhaust gas for 500 h in the ORNL Recuperator Test Facility shows that foils of AL20–25+Nb alloy have oxidation-resistance similar to HR120 alloy, and much better than 347 steel.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 6

Optical metallographic microstructures of cross-section specimens of AL20-25+Nb sheets (0.010in. or 0.254mm thick) from (a) Phase 1 processing, and (b) Phase 2 processing. Superimposed on the left hand portions of (a) and (b) are redrawn schematic diagrams of the actual grain structure that were used for quantitative grain size distribution analysis. Histographs of the grain size distribution obtained from analysis of those images are plotted in (c) Phase 1 and (d) Phase 2. Dotted lines on these histrographs indicate the average grain size, and the smaller inset histographs are analyses of the smallest grains using finer grain-size steps.

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Figure 7

Optical metallographic micrographs showing the transverse cross-sections sheet specimens of sheets of AL20-25+Nb after creep-rupture testing at 750°C and 100MPa in air. The top sheet specimen is 15mil Phase 1 alloy, which ruptured after about 400h, while the bottom sheet is 10mil Phase 2 alloy, which ruptured after over 800h. The creep-rupture curves corresponding to these specimens are shown in Fig. 4.

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Figure 8

Scanning electron microscopy back-scattered (SEM/BS) images of typical grain boundary regions showing the differences in precipitate phases developed during creep-rupture testing at 750°C and 100MPa in 3.2mil foils of (a) AL20-25+Nb stainless alloy, Phase 2 processing, with rupture after about 1700h, and (b) HR120, with rupture after about 530h. Both alloys contain mixtures of Cr-rich M23C6 and Mo-, Si-, and Ni-rich M6C phase along the grain boundaries and within the grains, and both pictures are at the same magnification. Creep curves corresponding to both of these specimens are shown in Fig. 5.

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Figure 9

Sample holders for the ORNL Recuperator Testing Facility were made by welding foils (0.089mm or 0.0035in. thick) to the various positions, and then pressurizing them with air inside. Holders made with standard 347 steel, with HR120 alloy, and with AL20-25+Nb alloy were inserted into the recuperator of a modified Capstone 60kW microturbine, and exposed to flowing exhaust gas for 500h. Scanning electron microscope (SEM) analysis of foil cross-section specimens from the hottest position are shown in (A)–(C) for each of the alloys, while the macroscopic exposed foil surfaces are shown in (D)–(F). The 347 steel experiences very severe moisture-enhanced oxidation attack in the exhaust gas at about 750°C (A), and microscopically exhibits bulging (indicative of creep) and oxide spallation (D). By contrast, both the HR120 and AL20-25+Nb alloys show good oxidation-resistance at this point, with thin, protective Cr-rich oxide scales (B) and (C), and clean looking surfaces with no evidence of bulging.

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Figure 1

Creep-rupture strain versus time data for various commercial heat-resistant alloy sheets and foils tested at 750°C and 100MPa (except for the PM2000 ODS alloy, tested at 120MPa) in air. Both alloy 625 and the PM2000 (ODS ferritic alloy) are still in-test.

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Figure 2

Creep data on commercial sheets and foils tested at 704°C and 152MPa in air. The sheet of alloy 625 has shown little creep at these conditions so far, and is still in-test.

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Figure 3

A plot of creep-rupture stress versus Larson-Miller parameter (LMP) for the various foils of commercial heat-resistant stainless steels, stainless alloys, and Ni-based superalloys tested at 650–750°C in air at ORNL

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Figure 4

Comparison of creep-strain versus time plots for creep-rupture testing of 10–15mil sheets of standard 347 stainless steel, and of AL20-25+Nb stainless alloy at 750°C and 100MPa in air. Differences in processing parameters produced coarser uniform grain size in the Phase 2 material compared to the Phase 1 material.

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Figure 5

Comparison of creep-strain versus time plots for creep-rupture testing of 3–5mil foils of standard 347 stainless steel, and of HR120 and AL20-25+Nb stainless alloys at 750°C and 100MPa in air. Differences in processing parameters of the AL20-25+Nb alloy produced a coarser uniform grain size in the Phase 2 material compared to the Phase 1 material.



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