Effect of Si on Mechanical Properties and Oxide Film Formation of AFA Alloy at Low Oxygen Pressure

The Cr2O3 film on the outer surface of traditional cracking furnace tubes is prone to spalling, which shortens the tube life. Fe-Ni-Cr-based austenitic stainless steel (AFA alloy) with added Al has attracted attention because it can form a more stable Al2O3 film on the surface. However, the alloy’s mechanical performance and the stability and oxidation resistance of the oxide film need to be improved simultaneously. This investigation examined silicon concentration variations (0–1.5 wt.%) on AFA alloy’s ambient-temperature tensile performance and oxidation response under reduced oxygen partial pressures (10−18–10−16 bar). The findings demonstrate that the alloy was composed of the FCC, B2-NiAl, and M23C6 phases. With Si addition, the B2-NiAl phase volume fraction increased. Mechanical testing demonstrated progressive elevation in tensile strength and hardness coupled with reduced elongation, attributable to combined solid-solution hardening and B2-NiAl precipitation strengthening. At low oxygen pressure, a continuous multi-layer oxide film developed on the alloy’s surface: the outermost layer was composed of a continuous Cr2O3 layer, with a fraction of MnCr2O4 spinel phase enriched on the outer surface. The middle layer was SiO2, which evolved from a particulate to a continuous layer with increasing Si content. The innermost layer was composed of Al2O3. Accelerated manganese diffusion through Cr2O3 facilitated MnCr2O4 spinel layer formation.