Creep Behaviour and Life Assessment of Modified 12 % Chromium Steels

Project Number 233

The project aims at the development of microstructure models and creep models for martensitic creep resistant 9-12%Cr steels based on advanced microstructure investigations.

The martensitic creep resistant 9-12%Cr steels like the X20CrMoV121 have been widely used in thermal steam power plants in the last 35 years for steam lines, boiler piping, superheaters and steam turbines. In the recent two decades a number of new improved modified 9CrMo steels like the P91 (X10CrMoVNb91) and the tungsten and boron alloyed 9Cr steel P92 (NF616) or E911 have been developed. These improved steels have up to 100 % better creep rupture strength than the X20CrMoV121 at 600 °C, and they have formed the basis for the recent development of modern steam power plants with advanced steam parameters and improved efficiencies.

Practical experiences with the conventional as well as the new advanced martensitic 9-12%Cr steels have shown one major disadvantage: The conventional methods to assess the state of the microstructure of creep-loaded components based on surface replication and light microscopy yield very little useful information for these steels. The additional information on the state of the microstructure, which can be obtained from microstructure and creep models developed in the project, is therefore necessary to make reliable quality control or life assessments of the martensitic 9-12%Cr steels. Practical examples of the usefulness of such investigations include pipe bends in steel X20CrMoV121 with premature creep damage due to improper heat treatment, premature creep failures of flat end plates on steam headers made of X20CrMoV121 and P91 as well as quality control of the P92 and E911 steels.

Further, prior to the introduction of new steels like the P92 and E911 to practical application in power plants all uncertainties related to long term effects of new alloying concepts like the addition of tungsten and boron must be eliminated. Experiences so far have clearly demonstrated that this can only be achieved by advanced microstructure investigations combined with microstructure and creep modelling as performed in the present project. The project will therefore support a much faster introduction of new steels like the P92 and E911 into practical application in power plants.

Finally, the potential for further developments of new martensitic 9-12%Cr steels to even higher creep strength does not seem to be exhausted. The improved understanding of strengthening mechanisms and microstructure stability obtained in the project will support this development. The transfer of knowledge in this respect is secured through the close interaction with the European COST 522 project.

In summary the project supports scientifically based developments of microstructure and creep models, which have a high degree of feedback to practical material problems experienced in power plant operation, and to the development and practical application of new steels for future advanced power plants.

The project is carried out from January 2002 to December 2004 by the Chalmers University of Technology, Göteborg (Prof. Hans-Olof Andrén), and the Royal Institute of Technology, Stockholm (Prof. John Ågren). The project is accompanied by a working group of the Technical Committee "Materials and Quality Supervision".