Thermochemical Boriding of Fe–5% Cr Alloy

Abstract

Boriding of the Fe–5% Cr alloy in amorphous boron powder with KBF 4 as an activator at 850–950°C 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 Fe 2 B phase. The average chromium content of the FeB layer is 4.2 wt.% (2.7 at.%) and of the Fe 2 B layer is 4.5 wt.% (3.5 at.%). The formation of boride layers is sequential, rather than simultaneous. The Fe 2 B layer forms and grows first. The FeB layer does not show up until Fe 2 B reaches a necessary minimum thickness exceeding 100 μm, for example, at 850°C. The characteristic feature of both layers is a profound texture. Their diffusional growth kinetics is close to parabolic, x 2 = 2k 1 t. The temperature dependence for the growth rate constant of the Fe 2 B layer in the time range 3600–14400 sec, when the FeB layer still has not formed between boron and Fe 2 B, is described by an Arrhenius relation, k 1 = 7.00 ∙ 10 −6 exp(−135.0 kJ × × mol −1 /RT). The microhardness is 15.6 GPa for the FeB layer, 13.0 GPa for the Fe 2 B layer, and 0.87 GPa for the alloy base. The dry abrasive wear resistance of borided Fe–5% Cr alloy samples found from weight loss measurements is more than one 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.