ADVANCED FABRICATED 10 Cr ROTOR TECHNOLOGY FOR INCREASED EFFICIENCY Siemens Power Generation

It is well understood that higher steam temperatures enable increased operational efficiencies. For supercritical conditions, the forging best suited for elevated temperatures is X12CrMoWVNb10-1-1, or the advanced 10 Cr steel. This was developed by the European COST 501 programme for operations at 600°C. The 10 Cr is excellent for its high strength as well as its enhanced creep life and creep strength properties. For single cylinder machines it was realized that a composite rotor would be needed. In the areas of lower operating temperatures where the blades are larger and have increased diameters, there is a need for improved mechanical properties over the 10 Cr rotor forging. The optimum forging for these demanding conditions is the 3.5NiCrMoV which has superior high cycle fatigue strength and life, increased toughness, and higher yield strength. The challenges, however, are to weld and properly heat-treat a fabricated rotor where the two forgings have large dissimilarities in chemical composition and physical properties. A three-year research project investigated and solved the problems associated with all the phases of welding and heat treatment, including 2 scaled prototypes. Since the completion of the program, the welding procedure has been applied to a number of 10 Cr rotors with service issues in the lower temperature area. The stub ends of the 10 Cr rotors were removed and replaced with 3.5NiCrMoV forgings for more resistance to high cycle fatigue cracking. This paper will elaborate on the challenges as well as the benefits of composite fabrications, testing and mechanical property data. A case history will also be included for service repairs made to date to older 10 Cr rotors.