Contributions of different factors to the improvement of the creep rupture strength of creep resistant martensitic steels

Boilers, pipes, and turbine components of fossil power generating plants are expected to be exposed to steam at increased temperature and pressure to improve power generating efficiency. This trend requires high Cr creep resistant martensitic steels having higher creep rupture strength (CRS) than Modified 9Cr-1Mo steel. The aim of this paper is to overview different factors that improve the creep rupture strength of these required new steels. The addition of W, B, Co, Cu and other elements to the 9-12 % Cr steels which has been studied by many researches to improve CRS is explained. Improvement of the method of heat treatment that has been investigated by other investigators is introduced as well. It has been reported that CRS can be raised by optimizing the normalizing temperature, controlling the cooling rate from normalization temperature before tempering, or by applying the thermo-mechanical control process (TMCP). By the application of TMCP process, a great increase in the CRS can be obtained