Development of a new generation of creep resistant 12% chromium steels: Microstructure of Z-phase strengthened steels

Fossil-fuel fired steam power plants provide more than 60% of the electricity generated worldwide, and account for about one third of the global CO2 emissions. Increasing the steam temperature and pressure leads to a higher thermal efficiency of the power plants and thus lower emissions. The efficiency is limited by the long-term corrosion and creep resistance of economically viable materials used in the critical components of such a power plant, 9–12% Cr steels. Increasing the Cr content from 9% to 11–12% in the best commercially available steels provides sufficient corrosion resistance for an increase from the current maximum steam temperature of 620°C to 650°C for future power plants. However, after a few years of service, formation of coarse Z-phase (Cr(Nb, V)N) precipitates at the expense of a fine distribution of VN precipitates degrades the precipitation hardening and accordingly creep resistance of the steels.<br /><br />In this work a new family of 12% chromium steels is studied, where Ta or Nb is used instead of V to strengthen the steel by forming a dense distribution of Z-phase rather than VN. Z-phase does not nucleate directly as Z-phase, instead it forms through a gradual transformation of existing MX and M2N precipitates. The former leads to the formation of Z-phase with a blade-like morphology and the latter promotes large bulky Z-phase precipitates. As a result of the MX to Z-phase transformation, creep strength comparable to commercially available 9% Cr steels can be achieved. Investigation on the Z-phase precipitates based on Ta or Nb showed that Ta-based Z-phase benefits from a denser distribution and a slower coarsening rate, and thus is recommended for alloy design.<br /><br />Carbon is found to play the most critical role in the precipitation processes of Z-phase strengthened steels. An ultra-low C content and an optimal balance between Ta and N in a model alloy lead to the formation of a fine distribution of TaN in the as-tempered condition, which are transformed to blade-like Z-phase after short-term ageing. Such a low C content leads to very little formation of M23C6 at grain boundaries, which allows for the formation of a continuous film of Laves-phase there and a low impact toughness. Although, the addition of C results in precipitation of Ta(C, N), and hence a slower phase transformation to Z-phase, a low but not ultra-low carbon content is preferred in the new Z-phase strengthened steels