Development of Low and High Chromium Alloyed Creep Resistance Steels for Power Plants and Related Applications

Abstract

The development of power plant technology towards larger and more efficient units is linked to the development of Cr alloyed creep resistant steels. These steels that are mainly used in power generating and petrochemical plants, have improved successively by introducing new alloying elements and new microstructures. Around the 1940s, molybdenum was added to low chromium steels such as 2.25 Cr 1Mo to improve their application temperature to about 560 oC. Niobium that is contained in all the latest high strength steels, which belongs to the group of 9-12% Cr steels, has been one of the most successful new elements. The use of these steels allows the design of power plants with steam temperatures up to 625 °C. Furthermore, ferritic steels are currently under development for steam temperatures of 650°C and more. Another recent development of the creep strength of creep resistant martensitic steels at higher temperatures was introduced by partial replacement of molybdenum by tungsten such as grade T23, NF616 (T92) and HCM12A (T122). This improvement in creep strength led to the raising of the operating temperatures of power plants using these new grades of steels, resulting in energy savings and reduction of the environmental impact. In addition to high creep strength other material properties are also important, e.g. hardenability, corrosion resistance, and weldability. All product forms of these steel grades such as large forgings and castings are used to build turbines, whereas tubes, pipes, plates and fittings are the typical products for application in pressure vessels, boilers and piping systems.