Development of Aluminide Coated Steels with Gradually Changed Compositions by Lamination Cladding and 2step Heat Treatment

Aluminide coating layers with gradually changed compositions and microstructures on pure Fe plate substrates were developed by the process combining lamination cladding with 2step heat Treatment. The first heat treatment at 933 K for 1.8 ks was applied to initially form the coating layer consisting of Fe2Al5 by the reaction diffusion between the Fe substrate and the melted Al. The second heat treatments at temperatures ranging from 1173 K to 1473 K were subsequently applied to promote the gradual changes in the aluminum compositions and the microstructures, leading the diffusion reaction between the Fe substrate and the initial layer of Fe2Al5. By heating at 1173 K and 1273 K, the initial coating layers of Fe2Al5 exhibited gradual changes in microstructures, leading the formation of layers consisting of Fe2Al5, FeAl2, FeAl, alpha-Fe with Al. On the contrary, by heating at 1373 K for 180 s, the constituent phases of the graded coating layer changed into Fe2Al5, FeAl2, epsilon phase, FeAl, alpha-Fe with Al. epsilon phase existing at elevated temperatures more than about 1373 K changed into the FeAl2+FeAl lamellar structure by the eutectoid reaction during cooling, resulting in the formation of different microstructural morphology compared with those formed at 1173 K and 1273 K. By heating at 1373 K for 300 s, the constituent phases of the graded coating layer changed into epsilon phase, FeAl, alpha-Fe with Al, exhibiting the decomposition of Fe2Al5 and FeAl2. By heating at 1373 K for 3.6 ks, the dominant phases of the coating layer were found to be FeAl and alpha-Fe with Al, indicating low aluminum compositions below 50at%.