INTERACTION OF 25% CHROMIUM STEEL WITH BORON POWDER

Abstract

Thermochemical treatment of industrial chromium steel ((15Kh25T–25% Cr)) in amorphous boron powder in the temperature range 850–950 °C and reaction times up to 43200 sec (12 h) results in the formation of two boride layers at the steel–boron interface. The microstructure of the outer layer bordering the boron phase consists of elongated crystals of the (Fe,Cr)B and (Cr,Fe)B compounds, while that of the inner layer adjacent to the steel base consists of elongated crystals of the (Fe,Cr)₂B and (Cr,Fe)₂B compounds. Both layers reveal a pronounced texture. Their diffusional growth kinetics is close to parabolic x² = 2k₁t, where x is the total thickness of both layers (m), k1 is the layer growth-rate constant (м² · с^-1), and t is time (sec). The temperature dependence of the growth-rate constant of boride layers on the steel surface is described by a relation of the Arrhenius type k₁ = 1,72 · 10^-8 exp (–137,1 kJ x mole^-1/RT). Microhardness values are 17.8 GPa for the outer layer, 15.9 GPa for the inner layer, and 1.70 GPa for the steel base. The dry abrasive wear resistance of borided steel samples is more than 250 times greater than that of non-borided ones.