Future thermal energy conversion systems (e.g. concentrating solar power, energy thermal storage power plants etc.) will be characterized by highly dynamic changes in operating conditions. Therefore, in contrast to advanced ferritic-martensitic (AFM) steels, which were primarily developed for high creep rupture strength, the materials used in such plants must have increased fatigue resistance.
The stainless High performance Ferritic (HiperFer) steels, developed at Forschungszentrum Jülich, Germany, combine excellent creep rupture strength and increased resistance to steam oxidation up to temperatures of 650 °C and beyond. The relevance of creep strength nevertheless remained high in development, because it results in low component wall thickness and thus helps diminishing thermal stresses during operation. Furthermore a unique concept for fatigue strengthening was implemented in these steels to withstand increased thermomechanical fatigue loading of flexible power plant operation. The crack initiation (thermomechanical fatigue) is actively obstructed by thermomechanically induced particle precipitation; the crack propagation (fatigue crack growth) is additionally obstructed by subgrain formation and resulting crack deflection. A further advantage of HiperFer steels is its grain structure being stable even under severe cyclic loading, while highly energetic martensitic lath structures of AFM steels is instable and suffers polygonisation. In comparison to grade 92, the residual life time of HiperFer is up to a factor of 10 longer. Further development of HiperFer aims at increased particle density and optimized heat treatment to reach more effective retardation of crack nucleation and lower crack propagation rate and thus a further increase of cyclic operation life time.