Al-Si-Fe Intermetallics on Fe-Substrates during Hot-Dipping

Abstract

Steels alloyed with Si and Al are used as core material in flux carrying machines, they are commonly called electrical steels, divided into grain oriented and non-oriented when a material without magnetic anisotropy or not is desired and used in transformer and electrical motors, respectively. The appearance of brittle ordered structures when Si+Al content in steel is above 4 m.-% does not always make its industrial production easy. Therefore hot dipping in a Al-Si bath followed by a diffusion annealing was found to be a productive way of steels with high Si and/or Al concentration and to overcome the creation of fragile structures during deformation processes, such as rolling. The formation of different layered Al-(Si)-Fe intermetallics on the steel substrate depends on diverse processing parameters such as bath temperature and composition, immersion time, preheating of the steel substrate and its composition and cooling down to room temperature. This contribution reports the diffusion kinetics of Fe2Al5 products obtained during the hot dipping process in an Al iron saturated and a hypoeutectic Al – 5 m.-% Si baths of ultra low carbon steel and Fe-substrates with 3 m.-% Si, annealed and cold rolled to different thicknesses. The preheating of the samples and bath temperatures were varied between 670 to750°C. Dipping times between 1 to 600 sec. were applied. The different layers and compounds formed were characterized by Scanning Electron Microscopy (SEM), using Back Scattered Electron (BSE) detector and Energy Dispersive Spectroscopy (EDS). The influence of the substrate and bath chemical composition on the growth kinetics of the Fe2Al5 intermetallics was investigated assuming a parabolic law. Si addition retards the growth kinetics and, as result, raises the activation energy from 71.3 to 159.8 kJmol-1, the obtained results are in agreement with the literature.