Characterisation of the Property Profile of Newly Developed 600/620°C Steam Turbine Materials Based on Long-term Testing Data and the Correlation with Microstructure Analyses and Parameters

Project Number 314

The aim of the project is to determine the long-term properties of real production components made in new European 620°C steels which have been developed in COST522. Furthermore, the characterisation of the microstructure of long-term tested creep specimens of 600°C- and 620°C-materials will be made with the aim of installing correlations of strength to microstructure and proposals for residual life concepts.

The VGB projects 158, 217 and 259 already completed were performed to determine reliable material data of the newly developed 600°C steam turbine steels to validate the real long-term behaviour of highly stressed turbine rotors, turbine castings, and valve casings, and to enable a safe extrapolation for life times up to 200,000 h.

The new project 314 is now dealing with the next generation of steels containing Chromium, Boron and Cobalt (FB2, CB2 type) developed within the COST programs and resulting in new forging and casting alloys for application temperatures T > 600°C. First prototype components have already been made and qualified within the COST programs. The steels are now being introduced to new high temperature power plants under construction.

As in the new project real components made from the next generation of steels are available for long-term investigations, essential parameters for the long-term behaviour of the materials can be determined, i.e.

  • the given range in chemical composition caused by the supplier specifications,
  • the melting and manufacturing route,
  • the applied quality heat treatment,
  • and the testing locations in the component.

This information is required to enable a safe long-term operation and the subsequent residual life estimation in the power plant for deciding on revision intervals and life time prolongation measures.

The investigations on the new 600°C materials have shown that possible indications in the microstructure to be correlated with the life time consumption (e.g. pores detected in the component by replica technique), behave differently compared to low alloyed steels. For example, the number of pores, pore density, and the time to develop first pores, is very different. Therefore new and additional indicators are required to establish correlations to the life time consumption. Microstructure parameters which have to be measured very systematically by applying a defined testing and preparation technique are candidates for this approach.

Thus the new project VGB 314 is aiming for the following three topics:

  1. Topic: 620°C components
    Determination of the creep rupture strength scatter-band for samples from real components for 620°C application of the material type FB2 and CB2 (possibly also involving Japanese steels applied in European steam power plants); identification of the range of other mechanical properties (hot tensile test, LCF); detection of the long-term ageing effects on strength and toughness.
  2. Topic: 600°C components
    Long-term properties’ determination by continuing the running creep rupture tests of the 600°C materials up to t > 70,000 h to validate the scatter-band and enable follow-up microstructure analyses.
  3. Topic: Microstructure analyses and parameters
    Characterisation of the microstructure of the 600°C and 620°C materials in the as-received condition and after very long creep testing; investigation of melts which describe the upper and lower scatter-band limits.

The utilities involved will gain the following benefits from the project:

  • The systematic investigations of material from real power plant components with T ≥ 600°C now under construction will result in the determination of parameters influencing the long-term properties up to 50,000 h within project runtime. Therefore the safe long-term extrapolation to > 100,000 h can be founded on a wider basis to enable a high availability and long life times of the plants.
  • At the same time, results from the microstructure together with structural parameters will be generated which can support the later residual life time estimation of the main components of the USC SPP.
  • The microstructure characterization of long-term tested specimens from the 600°C materials of the former projects 158, 217 and 259 will help to identify parameters to correlate the components’ status and the remaining life time. Thus the revision interval plans of the 600°C USC SPP can be adapted to the real life time consumption, enabling the specific measures in maintenance and replacement to avoid unscheduled outages due to service issues.
  • The common generation of data ensures the direct knowledge share and transfer between the power plant producers and the utilities.

The project started in August 2008 is headed by Dr. Torsten-Ulf Kern, Siemens AG Energy Sector. The material investigations are carried out at the Darmstadt Technical University (Institut für Werkstoffkunde) and the Stuttgart University (Materialprüfungsanstalt).

The utilities involved,

  • Dong Energy Power A/S,
  • E.ON Kraftwerke GmbH,
  • EnBW Kraftwerke AG,
  • RWE Power AG, and
  • Vattenfall Europe Generation AG & Co. KG,

provide components from the newly built European power plants and jointly finance the long-term project (planned duration: 6 years).

In addition, Alstom Switzerland (Ltd.) is a member of the steering committee.