Abstract

Nucleation, growth and coarsening of three different precipitates (NbC, M23C6 and V(C,N)) in a novel 9% Cr heat resistant steel designed by the authors were investigated. The microstructure evolution after tempering (780 degrees C/2 h) and after creep (650 degrees C/100 MPa) was characterized using transmission electron microscopy in the scanning mode (STEM). Thermodynamic and kinetic modeling was carried out using the softwares Thermo-Calc, DICTRA and TC-PRISMA. The Thermo-Calc software predicted formation of NbC, V(C,N) and M23C6 carbides at the tempering temperature of 780 degrees C. STEM investigations revealed that M23C6 precipitated on prior austenite grain boundaries and lath or block boundaries whereas NbC and V(C,N) were located within sub-grains. Simulations by TC-PRISMA showed that M23C6, NbC and V(C,N) particles nucleation begins as soon as the tempering treatment starts and it is completed in a very short time, reaching the equilibrium volume fraction after 40 s for M23C6, 100 s for NbC and 80 s for V(C,N). Best agreement between simulations and experimental investigations was found for low interfacial energy values of 0.1 J m(-2). Both STEM measurements as well as DIURA simulations indicate very low coarsening rate for both kind of precipitates. Creep tests up to 4000-5000 h suggest that this special combination of NbC, V(C,N) and M23C6 may provide increased pinning of dislocations reducing boundary migration therefore enhancing creep strength. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.