In the test loop HWT II (High Temperature Materials Test Loop) installed in the fossil power plant Grosskraftwerk (GKM) Mannheim in Germany, thick-walled components made of nickel base alloys were operated up to temperature of 725 °C. The operation mode chosen (creep-fatigue) was to simulate a large number of start-ups and shutdowns with high gradients as expected for future high efficient and flexible power plants and to investigate the damage due to thermal fatigue of the used nickel base alloys. In this paper the damage evolution of a header made of the nickel base alloys Alloy 617 B and Alloy C263, which was a part of HWT II test rig, were investigated using nondestructive and destructive techniques. Furthermore, the damage has been considered and evaluated by using numerical methods. In addition, different lifetime assessment methods of standards and recommendations with focus on creep-fatigue damage were used and evaluated. The different lifetime models are applied to the header and the results were compared to the results of metallographic investigations and damage observations.

In the later half of the last century great progress in alloy development for power applications was seen to improve thermal efficiency with increasing steam temperature. Meanwhile, many material-related troubles have been experienced due to rising temperature and uncertainty in the properties of fabricated metal. For further improvement in the thermal efficiency of fossil-fired power plants with ultra supercritical steam parameter conditions aiming at temperatures above 700°C, alloy development concepts and material issues with increasing steam temperature must be reviewed and discussed. In this paper new findings in the areas of alloy developments, creep failure in base metal and weldments, thermal fatigue failure and steam oxidation/hot corrosion are presented and discussed, as well as the economical aspect of material development, which is essential to realize unprecedented ultra supercritical steam conditions.