Creep and Oxidation Behavior of Modified CF8C-Plus with W, Cu, Ni, and Cr
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TRODUCTION
CF8C-PLUS (ASTM HG10MNN) is a cast austenitic stainless steel developed by ORNL and Caterpillar in 2002 from its precursor cast strengthened by NbC CF8C (18Cr-10Ni) stainless steel. It has excellent high-temperature creep properties due to a fully austenitic matrix. This austenitic matrix is free of the d-ferrite frequently observed in the base CF8C alloy, and the tendency to form ferrite is overcome by additions of Mn, N, and Ni, and by the presence of nanoscale Nb-rich carbonitrides.[1–3] The creep resistance of this alloy was shown to be significantly superior in comparison with CF8C steel (the cast version of wrought alloy 347), Ni resist, and SiMo cast-irons. The absence of d-ferrite, which can transform to r phase during aging or creep, is responsible for the alloy’s resistance to embrittlement and maintaining good creep ductility properties,[4] both of which improve creep resistance of the alloy.[5] CF8C-Plus has been deployed in exhaust components in on-highway diesel engines and has been demonstrated in a turbine casing.[2] The microstructure of as-cast CF8C-Plus consists of primary austenite and a cubic, Nb-rich, MX-type interdendritic phase (M = Nb; X = C,N). After exposure to high
KINGA A. UNOCIC, SEBASTIEN DRYEPONDT, YUKINORI YAMAMOTO, and PHILIP J. MAZIASZ, Research Staff, are with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mail: [email protected] U.S. Government Work. Not Protected by U.S. Copyright. Manuscript submitted July 20, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature [1023 K (750 C)], face-centered cubic (fcc) M23C6, Nb(C,N), and the fine-scale tetragonal Z-phase (Nb2Cr2N2: a0 = 0.3037 nm, c0 = 0.7391 nm) can form.[1] In addition, based on thermodynamic calculations, r phase (FeCr) and trigonal M2(C,N) phase (i.e., Cr2N) can also form.[1,5] It has been shown that small additions of Cu (~3 wt pct) to ferrite and Fe-austenitic alloys can improve the mechanical properties and enhance the balance between strength and ductility of the alloy[6,7] due to precipitation hardening. During the initial stages of aging, Cu clusters form, followed by the Cu precipitation stages of body-centered cubic (bcc) structures, which transition to 9R and then to fcc during the aging.[8,9] The size and number density of the nanoscale Cu precipitates are the main microstructural factors affecting mechanical behavior of precipitate-hardened alloys.[10–15] Thus, a new version of CF8C-Plus was developed with the addition of ~3 wt pct Cu and 1 wt pct W for solution strengthening. The W and Cu additions resulted in improved creep resistance at temperatures below 1073 K (800 C).[1,15,17] However, one limitation for the use of both versions of CF8C-Plus is the high-temperature corrosion resistance. Exposure of these alloys at 1073 K (800 C) in an environment containing H2O resulted in significant oxide scale spallation and specimen mass losses, similar to what was observed for alloys such as CF8C or 347HFG.[18] All these alloys contain
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