Abstract
Abstarct —In this paper, we present data from studies of the iron boride synthesis upon induction heating to 1000°C for 5 min of samples of steel 20 coated with a charge containing Fe–H3BO3. The content of boric acid in the charge composition varied from 25 to 75 wt %. The charge in the experiments can be diluted with a solution of liquid glass in water with the addition of a small amount of ammonium hydroxide and coal. The microhardness study of the surface layer showed that a macroscopically extensive diffusion zone with a size of 900–1000 μm, in which the boride content gradually decreases when moving deeper into the matrix, is formed during the saturation of the surface layer of carbon steel 20 with boron. Such a size of the diffusion zone indicates an anomalously high mass transfer during boriding of steel 20. Indeed, the calculated diffusion coefficient during boriding under induction conditions (about 1.35 × 10–9 m2/s) is by two orders of magnitude higher than the diffusion coefficient in the classical version of boriding. X-ray studies showed that, under the considered conditions, Fe2B and FeB borides are synthesized, and a solid solution of boron in α-iron is also formed. An analysis of the phase composition of the structural components of the diffusion zone indicates that, from the surface to the matrix, the formation of boride phases occurs in the sequence of FeB → Fe2B → (α-phase + B) → base metal. The microstructure of the diffusion zone consists of more or less pronounced layers consisting of FeB and Fe2B boride phases. In general, especially deep-lying regions of the diffusion zone are a composite material consisting of a plastic α-phase and iron boride crystals. Crystals FeB and Fe2B in the layer are oriented mainly perpendicular to the diffusion front. This is due perhaps to the rapid predominant growth of the boride phase under conditions of high diffusion mobility of boron atoms in one direction and hindered in others.