On the influence of rare-earth oxide additives on kinetics of borated layer formation and boron diffusion along grain boundaries during steel boriding

Metallographic studies showed that the use of rare-earth oxide (REO) additives during liquid electrolysis-free boriding increases the borated layers depth, with these additives not interacting with the treated product material. Addition of lanthanum and yttrium oxides increases the borated layer depth by 30 – 40 %, while addition of scandium oxide either has no effect or decreases the layer depth. X -ray phase analysis of boriding alloys with REO additives was conducted in this study. It was shown that REO additions to the melt result in formation of low-melting rare-earth borates (LaBO 3 , YBO 3 , ScBO 3 ), which enhance grain boundary diffusion and significantly intensify the boriding process. Estimated values of bulk and grain boundary diffusion coefficients were obtained. The addition of yttrium oxide increased the bulk diffusion coefficient in VKS-5 steel by 280 %. In Kh12MF steel, addition of lanthanum oxide resulted in an 83 % increase in the bulk diffusion coefficient. For 40Kh steel, no increase in the bulk diffusion coefficient was recorded in any of the investigated cases. The grain boundary diffusion coefficient increased in VKS-5 and Kh12MF steels by 1000 % with addition of lanthanum oxide. Addition of yttrium oxide increased the grain boundary diffusion coefficient by 1000 % in VKS-5 steel, by 135 % in Kh12MF steel, and by 87 % in 40Kh steel. Addition of scandium oxide increased the grain boundary diffusion coefficient by 160 % in VKS-5 steel. The diffusion coefficient values at grain boundaries obtained through modeling calculations agree well with the experimental data.