Characterization of Microstructure, Hardness and Oxidation Behavior of Carbon Steels Hot Dipped in Al and Al-1 at% Si Molten Baths

Medium carbon steel was aluminized by hot dipping into molten Al or Al-1 at% Si baths. After hot-dipping in these baths, a thin Al-rich topcoat and a thick alloy layer rich in $Al_5Fe_2$ formed on the surface. A small amount of FeAl and $Al_3Fe$ was incorporated in the alloy layer. Silicon from the Al-1 at% Si bath was uniformly distributed throughout the entire coating. The hot dipping increased the microhardness of the steel by about 8 times. Heating at $700-1000^{\circ}C$, however, decreased the microhardness through interdiffusion between the coating and the substrate. The oxidation at $700-1000^{\circ}C$ in air formed a thin protective ${\alpha}-Al_2O_3$ layer, which provided good oxidation resistance. Silicon was oxidized to amorphous silica, exhibiting a glassy oxide surface.