7 - Steels and alloys for turbine blades in ultra-supercritical power plants

Abstract

Blades are among the most important components of a steam turbine. Their design critically affects the machine working and its thermodynamic efficiency. Moreover, blades are subjected to heavy cyclic loads and their working environment is considered aggressive for the material. In addition to these two factors, ultra-supercritical (USC) turbine blades must withstand high temperatures and pressures, which makes the choice of material even more critical, especially for high-pressure (HP) section blades. While for intermediate-pressure and low-pressure section blades, the current trend is the use of traditional martensitic stainless steels and precipitation-hardenable steels due to their high mechanical properties and good corrosion and oxidation resistance; for HP section blades, new steels were developed with elemental additions focused on improving the material resistance to creep at exercise temperatures maintaining good fatigue properties. For A-USC HP turbine blades, working at 700°C and more, steels seem to have insufficient creep resistance. Therefore, nickel-based superalloys have been tested to the scope. Nimonic 105, Haynes 282, and Waspaloy are currently the most suitable alloys, presenting the best properties trade-off for A-USC exercise conditions. One typical production route for steam turbine blades is through closed-dye forging, which results in higher mechanical reliability of the material. Blades are in fact typically produced via closed-dye forging to comply with very stringent mechanical properties requirements. Thermal barrier coatings are also under study to lower the cost of the base material of the blades.