Effect of homogenizing heat treatment on the microstructure of new low-chromium alumina-forming austenitic steels

The formation of the Cr-rich sigma (σ) phase negatively affects the performance of alumina-forming austenitic (AFA) steel by reducing ductility, toughness, and oxidation resistance. Therefore, optimizing the chemical composition of alumina-forming austenitic (AFA) steels is essential to minimize the formation of the σ phase during heat treatments and ensure a single-phase austenitic microstructure. This research investigates the microstructural evolution of a new low-chromium (5 wt %) alumina-forming austenitic (AFA) steels. For this purpose, samples with 0, 4, 6, and 8 wt % aluminum were cast and homogenized at 1200 °C for 4 h. The alloys were analyzed using optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), dilatometry analyses, and hardness tests in both as-cast and homogenized conditions. The results show that adding up to 4 wt % aluminum leads to a fully austenitic microstructure, while increasing the aluminum content to 8 wt % results in a dual-phase ferritic-austenitic microstructure. During the homogenization process, the delta-ferrite phases dissolve within the matrix, leading to a more uniform microstructure. Additionally, no Cr-rich sigma (σ) phase was formed after the homogenizing heat treatment. Furthermore, adding aluminum decreases the coefficient of thermal expansion (CTE) and increases the austenite lattice parameter due to the diffusion of aluminum and nickel from delta-ferrite to austenite. As the aluminum content increases from 0 to 8 wt %, the hardness of the alloys rises from 93 to 258 HBW for as-cast alloys and from 112 to 234 HBW for homogenized alloys.