1. Introduction
The production of energy with the lowest impact on the environment has
become of topical concern. Even though the participation of electricity
generation such as renewable energy resources increases, the power
plants do still represent a dominant contribution regarding electricity
production all over the world. A new generation of thermal power plants,
Advanced ultra-supercritical (A-USC) power plants working at
temperatures up to 700 °C have been designed to assure enhanced
efficiency and to achieve the reduction of emissions as well. The
materials utilized in boiler construction have to be chosen according to
both service and economy criteria. To minimalize the expenses concerning
superheat and reheat tubing, highly alloyed advanced austenitic
stainless steels such as NF709, H3RC and Sanicro 25 should be used1. Sanicro 25 exhibits very good resistance to steam
oxidation, high temperature corrosion and has high creep rupture
strength, higher than the other austenitic stainless steels available
today 2, 3.
The original interest was devoted to the creep resistance2, 4-7 Basic study of the low cycle fatigue at ambient
and high temperature of Sanicro 25 8 and the study of
its cyclic stress-strain response using statistical theory of the
hysteresis loop 9 started a number of more thorough
studies of this material. Transmission electron microscopy (TEM) was
used to study the dislocation arrangement 10, the
sources of its extraordinary cyclic hardening 11 and
damage mechanisms at room and at elevated temperatures12-19. Further study was devoted to the damage
mechanisms in thermomechanical fatigue 20-25.
Previous papers on thermomechanical fatigue were devoted to in-phase and
to out-of-phase cycling between temperature 200°C and 700°C with
constant strain rates (IP-TMF and OP-TMF)20 and to
in-phase cycling with dwell time in maximum tension
(IPD-TMF)22, 25. In this contribution the effect of
the dwell period in a cycle applied in maximum compression (OPD-TMF) is
studied. Cyclic stress-strain response, the internal structure
evolution, the production and analysis of nanoprecipitates and the
damage mechanisms are documented and discussed. The behaviour of the
material in cycling with dwells in maximum compression is compared with
previous results without dwell and with dwell in maximum tension.
Electron microscopy investigation of the dislocation structure and
precipitate distribution in specimens cycled with all four types of
thermomechanical cycling is exposed. Also cyclic stress-strain curves
and fatigue life curves for all four types of cycling are presented and
discussed in terms of deformation and damaging mechanisms.