Influence of Delta Ferrite Concentration on the Properties of 9-12 % Chromium Steels

Project Number 318

The new 9-12 % chromium steels, such as P 91 and P 92, are used at component temperatures of about 600 °C in the current power plant new builds.

Their properties, especially the creep strengths, are influenced by the chemical composition, but to a significant extent also by the respective forming process, heat treatment and the resulting material structure. This concerns tempered martensite with finely dispersed carbides, nitrides and carbonitrides.

Possibly, so-called delta ferrite may also be formed, which occurs in some 9-12 % chromium steels, such as HCM12, even as a normal, specified structural constituent. Significant amounts of delta ferrite were found in some current deliveries of thick-walled components for steam pipes of P 92 material, although the structure was supposed to be purely martensitic without any delta ferrite according to the order specifications.

However, it is not yet entirely clear what impact delta ferrite has on the material properties of 9-12 % chromium steels.

Since the correlation between the delta ferrite concentration and the expected long-term properties is not entirely clear, it is currently difficult to define clear limits for orders. Moreover, it is necessary to specify to what extent and by which inspection methods agreed limits can and should be checked as part of acceptance.

The present research project is designed to give answers to these questions on the basis of a bibliographical reference study by way of evaluating a suitable pool of data obtained from available creep tests. The delta-ferrite concentration is to be calculated by means of the Schaeffler diagram and to be compared with empirical data (chemical composition and actual delta-ferrite concentration) to verify the validity of the calculation. Suitable sample material will be used to check the possibilities for the experimental determination of the delta-ferrite concentration.

The project will be carried out from October 2008 to April 2009 and will be managed by Prof. John Hald of the Technical University of Denmark (Department of Mechanical Engineering, Material Science and Technology). The research is supported by the Technical Committee Materials and Quality Supervision .