Characterisation and simulation of the creep behaviour of Nicrofer 6025HT wire material at 650°C

Abstract

Thermal efficiency of combined cycle power plants can be improved by increasing temperature and pressure in the steam turbine . 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. Due to the combination of constant overpressure of cooling steam and high temperature exposure of hot steam, the structures are stressed parallel and perpendicular to the intermediate layer primarily by creep loads. To simulate the creep behaviour via the finite element method the exact knowledge of the creep behaviour of the constituents, the wire and the sheet, is essential. Therefore, creep tests at 650 °C on wire material, manufactured from the nickel base alloy Nicrofer 6025HT, were carried out to determine constitutive equations. The creep behaviour was simulated on the basis of the concept of the internal backstress, which was implemented in an adequate user subroutine of the commercial FEM-software Abaqus .