Boriding of Chromium Steels

Abstract

Two boride layers, having different kinds of microstructure, are formed on the surface of industrial chromium steel (13 and 25% Cr) samples during their boriding at 850–950ºC and reaction times 3600–43200 sec (1–12 h). In the case of 13% Cr steel, the outer layer bordering boron consists of the FeB phase, whereas the inner adjacent to the solid substrate consists of the Fe₂B phase. Each layer is a homogeneous phase. It is a microstructure of the first kind. With 25% Cr steel, each of the boride layers is two-phase. The outer layer comprises the FeB and CrB phases, while the inner the Fe₂B and Cr₂B phases. It is a microstructure of the second kind. Both boride layers on both steels are characterized by a profound texture. The strongest reflections are {002} and {020} for the FeB phase and {002} for the Fe₂B phase. The growth rate of boride layers obeys a parabolic relation. Boride coatings with the microstructure of the second kind exhibit a much higher dry abrasive wear resistance than those with the microstructure of the first kind. Theratioof wear resistance of the outer boride layer on 13% Cr steel to that of the steel base is around 10. For 25% Cr steel,this ratio exceeds 250.