Investigations into the Fatigue Behaviour of Specific Steels Used in Power Plants at an Early Stage
Project Number 219
Different loading situations of plant components due to changes in boundary conditions such as
- increased steam parameter,
- larger units,
- changes in process engineering/technology (e. g. combined cycle power plant)
has to be considered. Research programmes are needed to provide knowledge about material properties such as (creep strength, fatigue properties) important to design and lifetime estimation.
Codes and guidelines, e. g. the TRD 301/508 are used for reference in determining the lifetime consumption because of material fatigue. When using non-destructive test methods for detecting fatigue cracking the problem arises that cracks initiating from the surface are usually filled with oxidation products which make it difficult to detect short cracks. Furthermore, incipient fatigue cracks in the order of 1 mm represent an end-of-life-state. Therefore means for an early recognition of fatigue cracking similar to those used in creep damage detection are not available. The objectives of the project are based on the following facts:
- To determine the lifetime consumption of power plant components because of fatigue load material characteristics are required. There is no systematic compilation of material resp. temperature specific fatigue crack initiation data. This data would complete or replace the appropriate curves in the codes also in view of modern steels so that lifetime consumption could be determined more realistically.
- The correlation between fatigue and micro-structural processes has to be investigated and quantitatively assessed in view of evaluating the numerical determination of lifetime consumption. Application of non-destructive early recognition of fatigue damages will be based on these findings.
- Possibilities of early recognition of fatigue crack initiation using non-destructive methods have to be determined. This helps in identifying methods for monitoring components.
Material and temperature dependent crack initiation curves especially for the more modern power plant steels are required to determine realistic exhaustion degrees. They should be in such a shape that they can be compared with the known reference curves for crack initiation of the general codes. Basic knowledge of the evaluation of micro-structural assessment is also required to monitor the component.
The aim of the project is to investigate the possibilities of quantifying the fatigue damage as follows:
- Representing the temperature dependent crack initiation curve, crack initiation versus number of cycles, as a reference curve for the code including a numerical description in relation to the damage phases ("Fatigue early recognition").
- Micro-structural description and assessment of damage processes by fatigue prior to crack initiation.
- Checking the possibilities of damage early recognition using non-destructive methods in principal and if need be quantifying the same.
In addition to theoretical work experiments are carried out on a typical power plant steel (9-12%Cr-steel). Exemplarily the development of micro-structural parameters such as precipitation behaviour, grain structure, grain size, slip line formation etc. in such a steel are investigated. Damage processes such as micro cracking and growth including changes in (micro)hardness during the fatigue load as a function of the number of cycles until fracture are determined. Different amplitudes in the typical strain range are applied. Specimen shall be removed after certain fractions of the number of cycles to crack initiation to perform metallographical investigations. The result shall be compared with the initial state. On the basis of the comparison of the development of the micro-structural parameter with the cyclic life time consumption a catalogue shall be put up including typical parameters for early recognition. This catalogue is intended to draw conclusions for the real exhaustion of materials during service. In view of the changes in the conductivity in the near surface area of the specimen caused by the fatigue processes supplementary measurements of conductivity are carried out (AC potential probe resp. multifrequency eddy current probe).
The project was funded by the German Federal Ministry for Economy and made available by Arbeitsgemeinschaft industrieller Forschungsvereinigungen e. V. (AiF). The project starting from November 2001 was running for 30 months at Staatliche Materialprüfungsanstalt (MPA), University of Stuttgart (Prof. Dr. Roos). From the technical point of view, the project was accompanied by the VGB Working Panel "Materials and Quality Supervision".