Untersuchungen der Strukturstabilität von Ni-(Fe) - JUWEL ...

ABSTRACT
Investigations of the Microstructural Stability of Wrought Ni-(Fe)-Based Superalloys
for Steam Turbine Rotor Applications beyond 700 °C
by Tomáš Seliga
There is a continuous trend to improve the efficiency of modern power plants with steam
turbine require operating steam temperature from 700 to 720°C. For substantial parts like
turbine rotors and discs this means increased requirements on the high temperature resistance,
which can not longer be fulfilled by the presently used steel. As new materials for the
components, which are thermo-mechanically loaded, only Ni-based superalloys are suitable
for their fabrication, structure stability and thermo-mechanical characteristics.
With view on creep and creep crack growth resistance as suitable candidates in the context a
DFG research project wrought Ni-based superalloy Waspaloy and Ni-Fe-based alloy Inconel
706 were selected, which exhibited different hardening mechanisms. Waspaloy is a
γ’-hardened material with small portion of carbides on the grain boundaries. Inconel 706 is a
particle hardened alloy with a very complex microstructure, it consists of γ’, γ’’-particle,
η-phase und carbides. Concerning to their castability, forgeability, creep and creep crack
growth and microstructural stability these candidate materials have been investigated and
modelled.
The knowledge about their long time stability of the microstructure, castability, forgeability
and mechanical properties leads to the two new modification (Waspaloy→DT750 and Inconel
706→DT706). The modification of the Waspaloy to DT 750 served better castability
(elimination of the Freckle formation). An improvement of the long-term stability of the
microstructure was not necessary, by a changed heat treatment was reached a homogeneous,
monomodale γ’-particle distribution. The modification of Inconel 706 to DT 706 served to
stabilize the γ’-phase and to reduce of the γ’’ phase without suppressing the cellular η-phase
colonies on the grain boundaries.
This work presents the test results for the structural long-term stability of the mentioned
materials and their modifications. In this paper influences of heat treatments and thermal
exposure on the microstructure and consequences for the mechanical behaviour will be
discussed.