Single phase ferritic, Laves phase strengthened, steels with a chromium content of 17 wt-% show potentially sufficient resistance to steam oxidation at high temperatures. This class of steel is supposed to be used in power plants. The required creep strength cannot be achieved based on the precipitation of carbides, nitrides and carbonitrides, because of the low solubility of carbon and nitrogen in ferritic steels. Alloying with tungsten, niobium and silicon enables reaching the desired combination of high oxidation resistance and creep strength by solid solution and precipitation strengthening effects. The alloying of these elements results in the precipitation of intermetallic (Fe,Si,Cr)2(Nb,W) - Laves phase particles, finely dispersed in the matrix. Design of this steel was accomplished by thermodynamic modeling (Thermo-Calc) with the main task of minimalization of the unwelcome brittle σ-phase and maximalization of the amount of strengthening Laves phase particles.
The present research deals with the development of a production process of high temperature resistant steels. The effect of a thermomechanical treatment on the Laves phase precipitation behavior is investigated. The focus is to create a fundamental understanding of the interaction of the alloy composition, deformation and precipitation microstructure with the mechanical properties. In this presentation, we address the influence of deformation parameters and tungsten content on the precipitation during the thermomechanical treatment. The precipitation behavior, amount and morphology of the Laves phase particles were analyzed by careful characterization of microstructure using scanning electron microscopy, SEM and energy-dispersive X-ray spectroscopy, EDX. The relationships between the chemical composition, the thermomechanical treatment and the precipitation of the strengthening Laves phase will be discussed.