Creep and Creep-Fatigue interaction for rotor material made of MarBN (HOWEFLEX)

The MarBN material family (Martensitic steels alloyed with Boron and Nitrogen) is known for years as new development of high strength creep resistant martensitic steels for application in pipes. But there are activities running to cover further component classes for power plants like castings and forgings with their special requirements in manufacturing and quality assurance. The German funded project HOWEFLEX performed by a consortium of two turbine manufacturers, a forge master, and two research organizations, has made an upscaling of trial melt results of MarBN family alloys to a large rotor forging with diameter of up to 1,200mm and typical weight representative for e.g. intermediate pressure steam turbine rotor forgings. The manufacturability is confirmed, and different, service relevant material properties required for design purposes have been determined. Long-term creep and creep-fatigue interaction are important features to allow meeting the requirements of future energy systems working on the base of rotating equipment, media like water, and thermodynamic processes. The paper will discuss creep and creep rupture results comparing it with other MarBN alloys, and findings for creep-fatigue interaction by testing via LCF w/o and with dwell time, thermo-mechanical fatigue, and service relevant cyclic loading. Furthermore, results of modelling tools will be presented which are able to support long-term extrapolation, installation of creep curves for lower stresses, and describing the findings of creep-fatigue testing.