Evolution of the Laves Phase in Ferritic Heat-Resistant Steels During Long-term Annealing and its Influence on the High-Temperature Strength

Heat-resistant ferritic steels containing Laves phase precipitates were designed supported by thermodynamic modeling. High-temperature compression tests at 1173.15 K (900 °C) and a detailed characterization of the microstructural evolution during annealing at 1173.15 K (900 °C) were carried out to investigate the effect of Laves phase formation on the high-temperature strength. Due to the addition of W/Mo and/or Nb, the high-temperature strength of the newly designed alloys is significantly higher than that of the reference steels. However, the high-temperature strength of all investigated steels decreases slightly as the annealing time is increased up to 1440 hours. To determine the influence of Laves phase formation and coarsening on the high-temperature strength during long-term annealing, the precipitates were extracted from the ferritic matrix in different annealing states. The phases in the powder residue were determined by XRD, and the chemical composition of the Laves phase in dependence of the annealing time was analyzed by EDS measurements. During annealing, steel Fe18CrMoW forms Nb(C,N), Ti(C,N), Laves phase (Fe2Nb) and Fe3Nb3C, whereas alloy Fe19CrWAl forms Nb(C,N), Ti(C,N), and Laves phase (Fe2Nb). The Laves phase within the alloys Fe18CrMoW and Fe19CrWAl differs in its morphology as well as its chemical composition. The Laves phase in steel Fe18CrMoW attains its chemical equilibrium after 192 hours, whereas alloy Fe19CrWAl required 24 hours. Overall, the formation of the Laves phase prevents significant grain growth during high-temperature annealing, thus preserving the high-temperature strength over a long time period.