Diffusion driven columnar grain growth induced in an Al–Si-coated steel substrate

Abstract

An alternative method to increase the Al or Si content in steels for electrical applications consists of dipping the steel substrate in a molten Al–Si bath, followed by an annealing treatment to diffuse Al/Si into the substrate. Under specific conditions, a columnar grain structure develops during diffusion annealing. It is demonstrated that a diffusion driven mechanism is responsible for the development of this particular structure. During hot dipping, a 5μm thick layer of the substrate forms a solid solution of Al in Fe, with Al-concentrations up to the solubility limit. The nuclei for the columnar structure are grains of this 5μm thick layer, as this small zone does not transform into austenite during diffusion annealing. The intermetallics formed during hot dipping merely serve as an Al source during diffusion annealing.