Effect of controlled nitrogen potential on the phase composition and corrosion resistance of gas nitrided 45# steel

This study investigates the effect of nitrogen potential on the phase composition, microstructure and corrosion resistance of gas-nitrided 45# steel. Advanced characterization techniques, including XRD, Raman spectroscopy, EBSD, and TEM, were employed to analyze the microstructure of the compound layer. At a low nitrogen potential (0.786 atm^−1/2), the compound layer exhibited a mixed-phase structure dominated by γ′-Fe₄N, which resulted in high porosity (23.22 %) and poor corrosion resistance. As the nitrogen potential increased to 1.500 atm^−1/2, a dense, single-phase ε-Fe_2-3N layer was formed, leading to minimal porosity (1.22 %) and highly corrosion resistance. However, when the nitrogen potential was elevated to a high level (3.000 atm^−1/2), nitrogen supersaturation led to local denitriding, structural fragmentation, and increased porosity (27.94 %). Furthermore, during the cooling process, portion of the ε-Fe_2-3N was transformed to γ′-Fe₄N, which impairing corrosion resistance. These results demonstrate that nitrogen potential plays a critical role in determining both phase formation and densification of the compound layer. Proper control of the nitrogen potential enables the formation of a stable ε-Fe_2-3N layer with minimal porosity, providing significantly improved resistance against general and localized corrosion. This study offers valuable insights for optimizing nitrogen parameters to enhance surface durability of 45# steel components in chloride-containing environments.