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TECHNOLOGY REVIEW

Overview of Creep Strength and Oxidation of Heat-Resistant Alloy Sheets and Foils for Compact Heat Exchangers

[+] Author and Article Information
Philip J. Maziasz

Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6115maziaszpj@ornl.gov

John P. Shingledecker, Bruce A. Pint, Neal D. Evans, Yukinori Yamamoto, Karren More, Edgar Lara-Curzio

Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6115

J. Turbomach 128(4), 814-819 (Feb 01, 2005) (6 pages) doi:10.1115/1.2187525 History: Received October 01, 2004; Revised February 01, 2005

Abstract

The Oak Ridge National Laboratory (ORNL) has been involved in research and development related to improved performance of recuperators for industrial gas turbines since about 1996, and in improving recuperators for advanced microturbines since 2000. Recuperators are compact, high efficiency heat-exchangers that improve the efficiency of smaller gas turbines and microturbines. Recuperators were traditionally made from 347 stainless steel and operated below or close to $650°C$, but today are being designed for reliable operation above $700°C$. The Department of Energy (DOE) sponsored programs at ORNL have helped defined the failure mechanisms in stainless steel foils, including creep due to fine grain size, accelerated oxidation due to moisture in the hot exhaust gas, and loss of ductility due to aging. ORNL has also been involved in selecting and characterizing commercial heat-resistant stainless alloys, like HR120 or the new $AL20-25+Nb$, that should offer dramatically improved recuperator capability and performance at a reasonable cost. This paper summarizes research on sheets and foils of such alloys over the last few years, and suggests the next likely stages for manufacturing recuperators with upgraded performance for the next generation of larger $200–250kW$ advanced microturbines.

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Figures

Figure 1

Photograph of two PSR aircells from a Honeywell recuperator made from 347 stainless steel foil, after relatively short term microturbine exposure. The darker right-hand portion of the air cell has seen the maximum temperature and oxidation, whereas the lighter golden left-hand side has seen a lower temperature and has developed a slight heat tint; this indicates the temperature gradient across the air cell that such components typically have due to the counterflows of cool air and hot exhaust. This component showed no evident of AA yet.

Figure 2

(a) Scanning electron microscope (SEM) back-scattered electron (BSE) imaging of 347 stainless steel air-cell foil cross section from a recuperator with significant service in a microturbine. It shows (a) the formation of heavy, Fe-rich oxide nodules at the exhaust-side surface (higher % of H2O) that are characteristic of the onset of AA due to oxidation in the presence of water vapor. Higher magnification SEM x-ray mapping images using Kα peaks of the elements indicated show the complex nature of the surface oxides for the elements (b) Cr, (c) Fe, and (d) Ni. The thick outer oxide scale is Fe-rich (c), with a thin Cr-rich oxide (b) and a Ni-depleted region in the sub-surface metal beneath (d).

Figure 3

Creep data for commercial sheets and foils tested at 750°C and 100MPa (except for the PM2000 ODS alloy, tested at 120MPa) in air. The 3–4 mil (0.003–0.004in. thick) foils crept at a high rate with little secondary creep regime, but both the HR120 and HR230 alloy foils exhibited almost 10 times longer rupture life than typical 347 steel foil. By contrast, both alloy 625 and the PM2000 (ODS ferritic alloy) exhibited a prolonged secondary creep regime, with a very low creep rate, and lasted more than 100 times longer than 347 steel (still in test).

Figure 4

Creep tests of fine-grained foils and coarser-grained tubing of HR230, illustrating the fine-grain size effect, which dramatically reduced or eliminated the secondary creep regime, when that grain size was below the critical grain size for creep resistance. Such data are the basis for the premise that foil behavior cannot be predicted from creep data on tube or plate of the same alloy.

Figure 5

Creep testing of foil and bar of alloy HR 120 shows roughly parallel creep curves, despite significant differences in processing and grain size between these specimens

Figure 6

Larson-Miller Parameter (LMP) plot of creep data for commercial foils of several different alloys, tested at 704–750°C and 100–150MPa in air. Creep-rupture stress is plotted against the LMP, calculated from both the test temperature and rupture-time to unify the data from different test conditions. The foils include 347 and T347CR (AL347HP) austenitic stainless steels, the new AL20-25+Nb and HR120 austenitic stainless alloys, and HR230 Ni-based superalloy. Clearly the AL20-25+Nb has creep strength as good as or better than HR120 and HR230, and much better than standard 347 stainless steel.

Figure 7

SEM micrographs showed (a) formation of Fe-Crσ-phase at grain boundaries in 347 steel during creep at 704°C and 152MPa (rupture after only 51.4h), whereas (b) a relatively stable dispersion of Si-Mo-Cr-NiM6C phase developed and remained stable along grain boundaries of alloy 625 during creep at 750°C and 100MPa (rupture after 4510h)

Figure 8

SEM micrographs show the precipitation that occured along grain boundaries in foils of (a) alloy NF709 (rupture after 5015h), and (b) alloy HR120 (rupture after 3319h), both creep tested at 750°C and 100MPa. The NF709 alloy has mainly the Si-Mo-Cr-NiM6C particles with some Cr-rich M23C6 particles, while the HR120 alloy has only M23C6 particles dispersed along the grain boundaries and in the matrix.

Figure 9

TEM micrographs show the fine NbC precipitation within the grains in foils of (a) alloy NF709 (rupture after 5015h), and (b) alloy HR120 (rupture after 3319h), both creep tested at 750°C and 100MPa

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