Microstructure evolution and corrosion behavior of Nb-alloyed cast heat-resistant steel during different aging treatments

The microstructure evolution of heat-resistant cast stainless steels aged at 600, 700, 800 and 900 °C for 2 h was investigated by scanning electron microscopy and X-ray diffraction. Potentiodynamic polarization curves were applied to study the effects of Nb addition and aging temperature on corrosion resistance. The results demonstrated that eutectoid decomposition of the ferritic phase (δ → σ + γ 2 ) was observed at 700–900 °C. The content of σ-phase first increased and then decreased in the steels, where the maximum content was obtained at 800 °C, indicating that the highest hardness occurred at 800 °C accordingly. The hardness of Nb-containing steels was significantly higher than that of Nb-free steels. The corrosion resistance of heat-resistant cast steels in various aging temperatures was different due to the formation of σ-phase. Both Cr-rich carbides and σ-phase were harmful to the corrosion resistance, while Cr-rich carbides were the main factor. Nb-containing heat-resistant cast steels exhibited superior corrosion resistance, as Cr-rich carbides were reduced and the corrosion products of Nb-rich slowed down the formation of steady-state pits.