Development of ferritic steels for steam turbine rotors with the aid of a molecular orbital method

Ferritic steels for steam turbine rotors were designed using a new method based on the DV-X alpha molecular orbital calculations. In this method two theoretical parameters were utilized; one is the d-orbital energy level of alloying transition metals, Md, and the other is the bond order between atoms, Bo. The appearance of the delta ferrite in the steels was predictable quantitatively with the Md parameter. Also, there was a relationship between the allowable tensile stress at 600 degrees C and the Bo parameter. The five alloys (TK1 similar to TK5) were designed employing these two theoretical parameters and their properties were examined in a series of experiments. As a result, it was found that no delta ferrite was observed in the designed alloys after the normalisation treatment. All the designed alloys were superior in both the tensile strength and the creep rupture life to TMK2. In particular, two alloys, TK3 and TK5 both containing Re, were superior even to HR1200. The FATT temperatures of the designed alloys were as high as 60 similar to 100 degrees C, but still they were in the allowable temperature range where any serious problems will not occur in the practical use. It was concluded that new high performance ferritic steels for steam turbine rotors were developed successfully with the aid of electron theory. Many trial-and-error experiments were no longer needed for the development of ferritic steels.