Characterization and simulation of the creep behavior of sandwich structures for cooling thermally highly loaded steam turbine components

Improvement of thermal efficiency in modern steam turbines can be achieved by increasing steam temperature and pressure. Since typical power-plant materials have presently reached their operation limit with higher steam temperature, the application of a new cooling system could reduce the material temperature to tolerable conditions. For this purpose, a new sandwich structure was developed comprising a woven wire mesh interlayer between two plane sheets. Cooling steam is fed into the interlayer where it can flow without severe losses. This sandwich structure is applied to the steam turbine casing as a wall cladding. Because during operation the structure is exposed to a combined creep–fatigue interaction in the steam turbine, the creep behavior was first investigated at 650 °C. To study the influence of different wire mesh geometries and constituent materials on the mechanical high temperature behavior, the sandwich structures were manufactured of the ferritic martensitic steel P92/NF616 and, for comparison, of the nickel-base alloy Nicrofer 6025HT. Three different wire diameters and mesh widths were considered. Additional creep tests were carried out with the constituent materials to determine constitutive equations used for a subsequent finite element simulation of the creep behavior of the structures.