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.