Primary Application in Ultra-Supercritical Boilers
07Cr25Ni21NbN, commercially known as Super304H, is an advanced austenitic stainless steel designed for the highest temperature sections of coal-fired power plants. Its primary application is in the manufacture of final-stage superheater and reheater tubes. In an ultra-supercritical boiler, steam temperatures reach 600 to 625 degrees C. At these extreme temperatures and pressures, conventional steels lack the necessary strength and corrosion resistance, and Super304H is deployed precisely in the sections where the steam is hottest.
Exceptional High-Temperature Strength
Through a sophisticated strengthening mechanism involving precipitation of nano-sized copper-rich particles, niobium carbonitrides, and M23C6 carbides, Super304H achieves a creep rupture strength approximately 30 percent higher than conventional TP304H and TP347H steels. This allows the design of thinner-walled tubes, which improves heat transfer and reduces thermal stress.
Superior Steam Oxidation Resistance
The steel forms a dense, adherent, protective chromium oxide scale on its inner surface, which significantly slows further oxidation when exposed to high-temperature steam. This extends the tube's service life and prevents the spalling of oxide scale, which can cause erosion damage in downstream turbine blades.
Structural Stability and Fabricability
The steel maintains its microstructural stability over long periods, more than 100,000 hours, under stress at high temperatures, ensuring consistent performance through the plant's design life. It possesses satisfactory weldability and formability, allowing the construction and repair of complex boiler components.
Development Prospects
Super304H has become a standard, well-understood material for the 600 to 620 degrees C steam temperature range and will continue to be the material of choice for existing and new-build ultra-supercritical plants. For life-extension programs of existing units, it is the key replacement material for critical superheater and reheater sections. Next-generation advanced ultra-supercritical plants targeting steam temperatures above 700 degrees C will require nickel-based alloys, but Super304H remains an enabling technology for today's high-efficiency plants.
Frequently Asked Questions
Q: What is Super304H?
A: Super304H, designated 07Cr25Ni21NbN in the Chinese system, is an advanced austenitic stainless steel for ultra-supercritical boiler superheater and reheater tubes.
Q: Why is Super304H stronger at high temperature?
A: Nano-sized copper-rich precipitates, niobium carbonitrides, and M23C6 carbides strengthen the matrix, giving about 30 percent higher creep rupture strength than TP304H.
Q: What temperature range is Super304H designed for?
A> It is designed for steam temperatures of 600 to 625 degrees C in ultra-supercritical boilers.
Q: How does Super304H resist steam oxidation?
A: It forms a dense chromium oxide scale on the inner surface that slows further oxidation and prevents scale spalling into the turbine.
Q: What is the carbon content of 07Cr25Ni21NbN?
A: The designation indicates about 0.07 percent carbon with 25 percent chromium and 21 percent nickel, stabilized with niobium and nitrogen.
Q: Can Super304H be welded?
A: Yes, it has satisfactory weldability and formability, with qualified welding procedures required for boiler pressure parts.
For Super304H tube quotations with size and testing requirements, contact our sales team.
