Structure and Composition of Nanometer-Sized Nitrides in a Creep-Resistant Cast Austenitic Alloy
- PDF / 762,667 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 17 Downloads / 167 Views
INTRODUCTION
A new cast austenitic stainless steel designated CF8C-Plus, ASTM HG10MNN, has improved hightemperature creep properties relative to its predecessor, alloy CF8C.[1–3] Cast CF8C, nominally an 18Cr-10Ni stainless steel precipitation hardened by NbC, is the equivalent of wrought 347H stainless steel. It has a relatively low nickel balance, which results in the as-cast microstructure containing ~15 pct d-ferrite.[3] However, d-ferrite can transform relatively rapidly to r phase,[4] which embrittles the alloy and reduces creep ductility. Therefore, modifications to the chemistry of CF8C were sought to improve high-temperature creep properties by making it fully austenitic while maintaining precipitation hardening mechanisms and preventing the formation of other embrittling phases. The alloy design methodology used to modify the CF8C steel is described in detail elsewhere.[1,2,5] The new alloy, which is designated CF8C-Plus, contains additions of N, Mn, and Ni relative to standard CF8C; Mn is a substitutional element for Ni and increases the solubility of N in austenite.[6] Nominal compositions of CF8C and CF8CPlus are given in Table I. This article investigates the NEAL D. EVANS, Research Assistant Professor, is with the Department of Materials Science & Engineering, University of Tennessee, Knoxville, TN 37996, and the Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Contact e-mail: [email protected] PHILIP J. MAZIASZ, Distinguished Research Scientist, is with the Materials Science & Technology Division, Oak Ridge National Laboratory. JOHN P. SHINGLEDECKER, Senior Project Manager, is with the Electric Power Research Institute, Charlotte, NC 28262. MICHAEL J. POLLARD, Engineering Specialist, is with the Caterpillar Technical Center, Peoria, IL 61629. Manuscript submitted December 8, 2009. Article published online August 11, 2010 3032—VOLUME 41A, DECEMBER 2010
microstructures observed in as-cast and creep-rupture tested alloys that are proposed to result in the improved high-temperature creep properties of the new alloy. Microstructures were investigated by detailed scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). II.
THERMODYNAMIC MODELING
Regions of equilibrium phase stability of CF8C-Plus were determined computationally using the thermodynamic program JMatPro (version 4.1; Sente Software, Guildford, United Kingdom).[7] The nominal alloy composition (weight percent) used for this calculation is given in Table I. Presented in Figure 1 is the calculated temperature-dependent relative amount of each equilibrium phase predicted to form in this alloy. The equilibrium phases predicted to solidify or precipitate are the cubic c parent phase, with precipitation that includes cubic M(C,N), M23C6, trigonal M2(C,N), and the tetragonal Z-phase and r phase. Additionally, the temperature-dependent compositions of these phases were computed; compositions at relevant temperatures are compiled in Table II.
Data Loading...
