Effects of Heat Treatment Conditions on Formation of Fe–Al Alloy Layer during High Temperature Aluminizing

Abstract

Diffusion couples of Fe₂Al₅ and various substrates with different carbon content, pure iron, 0.27 mass% C steel and 0.45 mass% C steel were processed, and the formation of an Fe–Al alloy layer in the temperature range from 750 to 1000°C was investigated. FeAl₂, FeAl, and aluminum solid solution (αFe(Al)) were confirmed in the alloy layer formed by heating the diffusion couples. Voids were generated inside the Fe₂Al₅ layer and near the interface between the FeAl layer and αFe(Al) layer. The layer growth of FeAl and αFe(Al) obeyed the parabolic law of the diffusion time, t^1/2. However, the thickness of FeAl saturated over a diffusion time longer than 3.6 ks at 950°C because of the concentration of voids at the FeAl layer/αFe(Al) layer interface and the disappearance of the Fe₂Al₅ layer. The activation energy for the formation of the FeAl layer in all kinds of the substrates was approximately 200 kJ/mol in the whole temperature range and the substrate. On the other hand, the activation energies for the αFe(Al) layer in the 0.27 mass% C steel and the 0.45 mass% C steel were larger than that in pure iron at 750–800°C.