ENHANCING THE CREEP LIFE OF STEELS BY SELF-HEALING
THROUGH DYNAMIC IN-SERVICE PRECIPITATION
G. Langelaan 1, S. Zhang 1, E. Brück 1, S. van der Zwaag 2 and N.H. van Dijk 1
1
Fundamental Aspects of Materials for Energy, Faculty of Applied Sciences, TU Delft, Mekelweg 15, 2629 JB Delft, the Netherlands – e-mail: G.Langelaan@TUDelft.nl; ShashaZhang@TUDelft.nl; N.H.vanDijk@TUDelft.nl; E.H.Bruck@TUDelft.nl
2
Delft Center for Materials, TU Delft, Kluyverweg 1, 2629 HS Delft, The Netherlands – e-mail: s.vanderzwaag@tudelft.nl
Keywords: Steel, grain boundary, precipitation, creep, cavities ABSTRACT
Increasing the operating conditions in steam turbines is desirable to improve the efficiency of electricity generating stations. However, the operating temperature and pressure are limited by the creep lifetime of the steels used for the boilers and piping. In recent experiments we have aimed to develop a creep resistant steel that self-heals deformation damage by the in-service formation of second phase precipitates. To achieve self-healing, the precipitates should form preferentially on or near the surfaces of creep cavities to prevent their further growth.
In the experiments, we used a custom made binary alloy of iron with 2 weight % vanadium. This composition is normally a solid solution but by introducing nitrogen, vanadium nitride (VN) precipitates form readily. We have performed a series of interrupted creep tests to determine whether adding nitrogen by gas nitriding would allow the preferential formation of VN on the grain boundaries and on the creep induced cavities. We will present the results of the tensile creep tests as well as microscopy, which shows the concentration of creep cavities throughout the creep test as well as the distribution of the VN particles produced during the nitriding steps. The self-healing capabilities of this system will be discussed in detail.
