Effects of heating atmosphere and steel chemistry on delamination of the surface layer of aluminized steel

The effects of the heating atmosphere and steel chemistry on the delamination of the surface layer of aluminized steel were studied. When aluminized steel is heated to 950 °C or higher, the delamination of its surface layer becomes possible. Delamination depends on the heating atmosphere and the steel grade of the substrate material. Heating the material in a nitrogen2-rich atmosphere leads to delamination. When an aluminized layer is fully alloyed with the substrate iron by heating to a high temperature, voids form at the boundary between the FeAl and bcc phases in the alloyed layer. Then, nitrogen enters the voids and reacts with Al on the void surfaces to form AlN, which spoils the adhesion at the boundary. The steel substrate composition also affects delamination because it changes the density and shape of the voids. When the steel substrate contains Cr and Mo, fewer voids form and delamination is suppressed. From the elemental distribution in the FeAl and bcc phases in the coating layers of different steel substrate grades, the interdiffusion coefficients in these phases were calculated. The results indicate that the interdiffusion coefficients are different, depending on the substrate steel grade; the interdiffusion coefficient is more stable when the steel substrate contains Cr and Mo.