With the object of surface hardening the basic experiments were performed on the formation of a Fe-Fe
2B eutectic structure layer at the surface of the iron and steel. The eutectic structure layer could be obtained either by coating powdered boron compounds on the surface of the specimens or by producing acicular boride layers with gas boronizing method, prior to the heat treatment at the temperatures above eutectic transformation point. The eutectic structures formed on pure iron consisted of α-Fe and Fe
2B, without coating by tetraboron carbide. Whenever S45C was used for the specimens α-Fe, Fe
2B and Fe
3(B, C) were found in the eutectic structures, without distinction of coating materials and coating methods. The eutectic structure layers showed the microhardness of about 500-800Hv, and about double depth of coated layer or acicular layer.
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