Thermochemical Boriding of Fe–5% Cr Alloy

Abstract

Boriding of 5% Cr alloy in amorphous boron powder with a KBF₄ activator at 850–950 °С and reaction times of 3600–28800 sec results in the formation of two boride layers at the interface between the reagents. The outer layer bordering boron consists of the FeB phase, while the inner one adjacent to the alloy base consists of the Fe₂B phase. The average content of chromium is 4.2 wt.% (2.7 аt.%) in the FeB layer and 4.5 wt.% (3.5 аt.%) in the Fe₂B layer. The formation of boride layers is sequential, rather than simultaneous. The Fe₂B layer forms and grows first. The FeB layer does not show up until the Fe₂B layer reaches a necessary minimal thickness exceeding 100 µm, for example, at 850 °С. The characteristic feature of both layers is a profound texture. Their diffusional growth kinetics is close to parabolic, x² = 2k₁t. The temperature dependence of the growth-rate constant of the Fe₂B layer in the time range 3600–14400 sec when the FeB layer still has not formed between boron and the Fe₂B layer is described by an Arrhenius relation, k₁ = 7,00 • 10^–6exp(–135,0 kJ • mol^–1/RT). The microhardness is 15.6 GPa for the FeB layer, 13.0 GPa for the Fe₂B layer, and 0.87 GPa for the alloy base. The dry abrasive wear resistance of borided Fe–5% Cr alloy samples found from mass loss measurements is more than an order of magnitude greater than that of their base. Surface boride coatings may be employed in manufacturing products, parts, and materials for functional applications to enhance their service characteristics.