Design of improved heat resistant materials by use of computational thermodynamics

Ferritic-martensitic steels are widely used for high temperature applications. Compared with austenitic materials they own a better thermal conductivity, lower thermal expansion coefficient, lower costs and a better resistance against stress corrosion cracking. In this contribution the modelling of the microstructure based on thermodynamic calculations has been used for the description and further alloy design for this steel group. The results are phase diagrams showing the precipitates to be expected as well as driving forces, phase compositions and diffusion coefficients as base for further modelling activities. A calculation model based on the determination of the (calculated) transformation temperatures Ae1 and Ae3 and the diffusion coefficients is proposed, which can help to predict the microstructural stability and the creep strength from the chemical composition. Based on this model, test melts for some new alloys has been produced. First creep tests of these alloys confirm the presented approach.