Chromium diffusion in Fe–Cr–C–N system under complex thermochemical treatment: activation energy and effective diffusion coefficient

The formation of functional gradient diffusion layers on structural steels by chemical heat treatment methods makes it possible to increase the surface hardness and wear resistance of critical parts. However, during diffusion alloying with chromium from liquid metal media solutions, a decarburized zone appears under the carbide layer, reducing contact strength. Combined chemical heat treatment (precarburizing or nitriding + diffusion alloying) eliminates this drawback, but the diffusion parameters of the Fe–Cr–C–N system under conditions of competing diffusion of carbon and nitrogen toward adsorbed chromium have remained unexplored. Three processing variants were studied on samples of AISI 1015, AISI 420 and AISI 321steels: diffusion alloying without pretreatment, and after precarburizing and after prenitriding. Diffusion alloying was carried out in a Pb–Li melt with 5 % Cr at 1000 –1070°C for 1– 8 h. The effective diffusion coefficients of chromium and activation energies (Ea) were calculated from the thickness of the formed layers and the Arrhenius equation. It was found that precarburizing reduces Ea for all steels. For AISI 420 steel, Ea decreases from 237 to 153 kJ / mol, for AISI 321 — from 223 to 158 kJ / mol, for AISI 1015 — from 147 to 138 kJ / mol. Prenitriding gives intermediate values: for AISI 420 — 194 kJ / mol, for AISI 321 — 204 kJ / mol, for AISI 1015 — 143 kJ / mol. For AISI 321steel without pretreatment, no carbide layer is formed, Cr diffusion occurs in austenite with Ea = 223 kJ / mol. The obtained Arrhenius equations make it possible to predict the thickness of the diffusion layer under combined chemical heat treatment in the temperature range 1000 –1070°C. The results can be used to optimize the hardening treatment conditions for machine parts operating under high contact loads.

1. Russian Science Foundation - 26-29-20112, https://rscf.ru/project/26-29-20112/
2. Kuban Science Foundation - 26-29-20112