Special formation of alumina inclusions
Thermite welding of railways rails

In an oxidizing steel bath such as an aluminum-quenched steel ladle, the inclusions formed are pure alumina inclusions, in alpha form, with a basaltic dendritic structure, and found in rails, fragmented into numerous particles of 80 to 100 μm.

In a reducing steel bath, as in aluminothermics, the inclusions formed are of a different structure (figures 5 and 6 ); they are not made up of pure alumina of basaltic structure, but of particles of a structure whose genesis is described below.