Characterization and Boron Diffusion Kinetics on the Surface-Hardened Layers of Royalloy Steel

Abstract

The Royalloy steel was boronized at 1173, 1223, 1248, 1273 or 1323 K for 1, 3, 5, 7 or 10 h using a Durborid powder mixture. The boronized samples were analyzed by scanning electron microscopy, X-ray diffraction and Vickers microhardness testing. The kinetic activity of boronized layers growth obeys the parabolic law, and the maximum thickness was 182 ± 10 µm. The thickness of FeB makes up to 40% of the total layer thickness. The obtained layers have two phases, which were composed of FeB and Fe2B phases, except for the sample boronized at 1173 K for 1 h which had an Fe2B layer only. The microhardness of the Fe2B phase had a range of 1370–1703 HV0.1, and that of the FeB phase was within 1727–2231 HV0.1. During the boronizing process, the chromium created extra particles with the highest amount of chromium in the transient region. The highest amount of silicon was observed at the boride layer/substrate interface. The amount of manganese was slightly lower in the boride layers compared to the amount in the substrate. Finally, the integral diffusion model was applied to determine the boron activation energies in the FeB and Fe2B layers, and this was followed by a comparison with the literature data.