Nanosized MX Precipitates in Ultra-Low-Carbon Ferritic/Martensitic Heat-Resistant Steels
- PDF / 821,622 Bytes
- 8 Pages / 593.972 x 792 pts Page_size
- 49 Downloads / 215 Views
INTRODUCTION
REDUCTION of CO2 emission is an urgent task for power industries, especially for fossil power plants. The amount of CO2 emission decreases with increasing operation temperature of the plants. Plant operation at higher temperature requires heat-resistant materials with higher creep strength. 9-12Cr ferritic/martensitic heat-resistant steels have been widely used in fossil power plants, because those steels have high thermal conductivity and low thermal expansion coefficient, and they also possess less susceptibility to the thermal fatigue than austenitic stainless steel.[1–7] In order to increase creep strength, these steels are strengthened by various strengthening mechanisms such as solution hardening by Mo or W, particle hardening by MX within subgrains, particle hardening due to particles (M23C6 and Fe2W) on sub-boundaries, as well as dislocation hardening.[1] The precipitation strengthening is the most effective mechanism for conventional 9-12Cr steels. In particular, MX-type carbonitrides, which are composed of vanadium, niobium, and so on, and precipitate finely and densely in the matrix of the steels, increase creep strength significantly.[4,7] Hasegawa et al.[6] analyzed the precipitation behavior and morphology of nanosized precipitates in ASME Gr. FENG-SHI YIN, Professor, is with the School of Mechanical Engineering, Shandong University of Technology, Zibo 255049, P.R. China. Contact e-mail: [email protected] WOO-SANG JUNG, Principal Researcher, is with the Division of Materials Science and Technology, Korea Institute of Science and Technology, Cheongryang, Seoul 130-650, Korea. Manuscript submitted March 3, 2008. Article published online December 17, 2008 302—VOLUME 40A, FEBRUARY 2009
92 (9 pct Cr-1.8 pct W-MoNbVNB) by transmission electron microscope (TEM) observation and characterization of precipitates. Granular or tabular MX precipitates and composite MX precipitates called V-wing were found in ASME Gr. 92 steel. The average particle size of the MX precipitates as measured with an energyfiltered TEM is, reportedly, 20 to 50 nm. Taneike et al.[7] reported that extremely low-carbon 9Cr ferritic steel (9Cr-3W-0.2V-0.06Nb-3Co-0.05N-B) exhibits much higher creep strength than 9Cr-0.5Mo-1.8WVNbN (ASME-P92) steel does. By reducing the carbon content of the ferritic steel, not the M23C6 carbides but only fine MX nitrides precipitate along lath, block, packet, and prior austenite grain boundaries as well as in the matrix during tempering. The fine MX nitride can pin the boundaries during long-term creep deformation, because the growth rate of the MX nitride is much lower than that of the M23C6 carbide. In our previous investigation, nanosized MX precipitates were found to distribute densely and homogeneously in the matrix within martensitic lath after normalizing-and-tempering heat treatment in an ultra-low-carbon ferritic/martensitic heat-resistant steel with 9.37Cr, 3.2W, 4.0Co, 0.22V, 0.09Nb, 0.068N, and 0.007Ti (in mass pct).[8] The time to rupture at 650 °C under high stress for the ultra-lowcarbon
Data Loading...
