What’s Super 304H steel?

With the development of ultra-supercritical units, the high-temperature strength of traditional 18-8 Austenitic stainless steels (such as TP304H steel) has been unable to meet their needs with steam parameters of 600℃. For this reason, Japan Sumitomo Metal Corporation has developed new materials for the boiler heating surface pipeline of the unit, like TP347HFG steel, SUPER304H steel and HR3C steel. Super 304H steel is a new type of 18-8 steel, mainly used in the manufacture of superheater and reheater of ultra-supercritical boilers whose metal wall temperature does not exceed 700 ℃. At present, Shasqida Mannesmann (formerly DMV Company) in Germany also produces similar steel tubes, with a grade DMV 304HCU.

Super304H steel is the steel by reducing the content of Mn, Si, Cr and Ni-based on TP304H steel, which adding 2.5% ~ 3.5% Cu and 0.30%~0.60% of Nb and 0.05%~0.12% of N, so that to produce the diffusion precipitation phase and copper-rich strengthened phase in service, occurs precipitation strengthening with NbC(N), NbCrN and M23C6, which greatly increases the allowable stress at service temperature, and the allowable stress at 600 ~ 650℃ is 30% higher than that of TP347H steel. The steam oxidation resistance of the steel is comparable to that of TP347HFG steel and significantly better than that of TP321H steel. It has been listed in ASME Code Case 2328-1, ASTM A-213 Standard, the number is S30432.


The Chemical Composition of Super 304H

C Si Mn P S Cr Ni N Al B Nb Cu V Mo
0.08 0.21 0.79 0.03 0.001 18.42 8.66 0.11 0.007 0.004 0.5 2.77 0.04 0.35


The Mechanical Property of Super 304H

Yield strength, Mpa Tensile strength, Mpa Elongation, %
360/350 640/645 58/60


Due to the high steam parameters of ultra-supercritical units, the oxidation resistance of steel used in high-temperature pressure parts of power plants becomes very important. Generally, the inner wall of the super 304H steel pipe is shot blasting to improve the anti-steam oxidation performance. A 30μm thickness shot blast layer was formed on the inner surface of the steel tube and its microstructure was refined compared with that of the non-shot peening steel tube. After the steam oxidation test at 650℃ and 600h, the oxide layer thickness of the steel tube treated by the shot blast is thinner and denser, and the steam oxidation resistance of the steel tube is improved. Currently, several leading steel mills in China have produced a similar grade 10CrL8Ni9NbCu3Bn, specified in GB 5310-2008, which is currently used in several ultra-supercritical unit projects in China.