Microstructure of hot dip coated Fe–Si steels

Hot dipping is a coating technique pre-eminently used in industry to galvanize machine parts or steel sheets for constructional applications. However, other hot dipping applications have been developed in order to have a positive effect on specific material properties. For instance, in Fe–Si electrical steels, a Si/Al rich top layer is applied and followed by diffusion annealing to increase the electrical resistivity of the material and consequently, lower the power losses. Hot dipped aluminised mild steels have been developed with increased corrosion resistance for high temperature applications by the development of a dense Al2O3 layer. Regardless of the type of steel coated and the intended application, after the interaction between the molten Al and the solid material, three constituents are formed: Fe2Al5, FeAl3 and an Al-rich alloy. The structural morphology, which can negatively affect the wear resistance and the thermal stability, also appears to be highly dependent on the chemical composition of the base material. To study thermo-mechanical and compositional effects on the coating behavior after hot dipping, cold rolling with different reductions was performed on different Fe–Si materials. It was demonstrated that hardness differences between the layers caused crack formation inside the Fe2Al5 layer during subsequent deformation. The present work reports the results obtained on materials that were hot dipped in a hypo-eutectic Al + 1 wt.% Si bath. The bath was used to coat Fe–Si steel substrates with variable silicon content with dipping times ranging from 1 to 20 s. Before dipping, the samples were heated to 700 °C and subsequently immersed in the liquid bath at temperatures of 710 °C, 720 °C and 740 °C. To further evaluate the interactions between Al, Si and Fe, a diffusion annealing treatment at 1000 °C was performed. The main diffusing elements during this treatment are Al and Fe, although small variations in Si content are also observed. At a certain distance from the surface, voids were observed, which most probably can be related to the Kirkendall effect. A characterization of the formed intermetallics was performed by Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-Ray Diffraction (XRD) and Electron Backscatter diffraction (EBSD).