Life Assessment of 12 % Cr Steels

Project Number 200

VGB is participating in a joint research project of Swedish and Danish VGB member companies within the scope of the Co-ordinated Research Programme "New Materials for Power Plants".

The project was started on 1st April 1999 and will be finished by 31st December 2001 at the Chalmers Technical University Göteborg by Prof. Andrén and at the Royal Institute of Technology, Stockholm by Prof. Agren. The VGB Technical Committee "Materials and Quality Supervision" is supporting and pursuing the project aiming at the modelling of creep in modified 12%-Cr steels in order to be able to establish reliable future predictions about creeping and residual service life.

The project is based on a model for simultaneous nucleation, growth and coarsening of the phases present in modified 12% chromium steels. The model is being finalised and partly experimentally verified. A computational model, based on the model developed in Erlangen by Prof. Blum, for creep rates as function of stress and time (strain) is being elaborated. In the current form the model requires the input of experimental observations of the microstructure development. By now the experimental microstructure can be replaced by calculations. Consequently, the complete model will be based on fundamental thermodynamics and kinetics. The complete model also comprises part modelling for time variation of dislocated density and subgrain size. Besides it has to be considered up to what extend creeping is influencing phase transformation. These partial models are simultaneously developed with the entire model for creep rate. Further co-operation with the working party of Prof. Cerjak in Graz/Austria is planned as part of the COST522-programme.

After termination of the modelling activities the model will be verified by comparison with microscopic data of the Chalmers university, creep data of ELSAM/ELKRAFT, ABB STAL and VOLVO AERO, with field test data of VATTENFALL, microstructure and creep data of the COST501 and 522-programmes and further data from Japanese test programmes.

The final result, a programme for modelling complete processes as a function of composition as well as temperature and stress exposure, will be an important break-through for the practical application of the knowledge about the microstructure of materials that has been accumulated in recent years.