No abstract

To cite this article: Fujio Abe (2008) Precipitate design for creep strengthening of 9% Cr tempered martensitic steel for ultra-supercritical power plants, Science Abstract It is crucial for the carbon concentration of 9% Cr steel to be reduced to a very low level, so as to promote the formation of MX nitrides rich in vanadium as very fine and thermally stable particles to enable prolonged periods of exposure at elevated temperatures and also to eliminate Cr-rich carbides M 23 C 6 . Sub-boundary hardening, which is inversely proportional to the width of laths and blocks, is shown to be the most important strengthening mechanism for creep and is enhanced by the fine dispersion of precipitates along boundaries. The suppression of particle coarsening during creep and the maintenance of a homogeneous distribution of M 23 C 6 carbides near prior austenite grain boundaries, which precipitate during tempering and are less fine, are effective for preventing the long-term degradation of creep strength and for improving long-term creep strength. This can be achieved by the addition of boron. The steels considered in this paper exhibit higher creep strength at 650• C than existing high-strength steels used for thick section boiler components.

Creep is a time-dependent mechanism of plastic deformation, which takes place in a range of materials under low stress-that is, under stresses lower than the yield stress. Metals and alloys can be designed to withstand creep at high temperatures, usually by a process called dispersion strengthening, in which fine particles are evenly distributed throughout the matrix. For example, high-temperature creep-resistant ferritic steels achieve optimal creep strength (at 923 K) through the dispersion of yttrium oxide nanoparticles. However, the oxide particles are introduced by complicated mechanical alloying techniques and, as a result, the production of large-scale industrial components is economically unfeasible. Here we report the production of a 9 per cent Cr martensitic steel dispersed with nanometre-scale carbonitride particles using conventional processing techniques. At 923 K, our dispersion-strengthened material exhibits a time-to-rupture that is increased by two orders of magnitude relative to the current strongest creep-resistant steels. This improvement in creep resistance is attributed to a mechanism of boundary pinning by the thermally stable carbonitride precipitates. The material also demonstrates enough fracture toughness. Our results should lead to improved grades of creep-resistant steels and to the economical manufacture of large-scale steel components for high-temperature applications.

The effect of fine precipitation and subsequent coarsening of Fe 2 W Laves phase on the creep deformation behavior was investigated for simple 9Cr-W steels containing 0, 1, 2, and 4 wt pct W. After tempering, the specimens were subjected to creep tests at 823, 873, and 923 K for up to 15,000 hours. The precipitation of Fe 2 W Laves phase takes place during creep at boundaries from the supersaturated solid solution of the high-W steels, the 9Cr-2W and 9Cr-4W steels, but not in the low-W steels, the 9Cr-0W and 9Cr-1W steels. The fine precipitation of Fe 2 W Laves phase decreases the creep rate in the primary or transient creep region, while the subsequent large coarsening of Fe 2 W Laves phase reduces the precipitation strengthening and promotes the acceleration of creep rate in the tertiary or acceleration creep region after reaching a minimum creep rate. The change in shape of creep rate curves with stress and temperature is explained by taking fine precipitation and subsequent coarsening of Fe 2 W Laves phase into account.

No abstract

The distributions and precipitated amounts of M 23 C 6 carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heatresistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M 23 C 6 carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M 23 C 6 carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M 23 C 6 carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.

Hyogo 660-0891 JapanFor the construction of ultra super critical (USC) power plant, 9Cr-3W base ferritic heat-resistant steels with relatively high B and no N have been investigated. Authors have been revealed in the previous report that the addition of i39ppm B significantly improves creep strength of the steels, whereas most of added B forms unidentified borides, which are deemed almost ineffective to creep strength. The effect of improved heat treatment on creep strength and distribution of B in precipitates is investigated to effectively utilize and decrease added B As a result of the analysis of the extracted residue and characterization of precipitates using field emission Auger eleotron spectroscopy (FE-AES), most of added B still forms borides in the 92ppm B added steel. These composites are almost dissolved and the B content in M23Ce carbides is significantly increased by normalizing at 142SK. It is also found by FE-AES analysis that B content in M23CG oarbides near prior-austenite grain boundaries is relatively higher than that inside grains. Creep strength at 923K for the 92ppm B added steel normalized at 1423K is not improved ~lt short times, but it is remarkably improved to almost the same level as the 139ppm B added steel at long times. This ex~Ilent creep strength is achieved resulting in improving microstructural stability through the effective utilization of added B by high-temperature normalizing.KEY WORDS: ferritic heat-resistant steel; creep; boron; FE-AES M23C6 oarblde mlcrostructure USC power plant.

No abstract