Characterization of pack-siliconized 31CrMoV9 steel

The present study reports on iron (Fe) silicides produced on 31CrMoV9 steel by using the pack-cementation method. Pack siliconizing was carried out using metallic silicon (Si), ammonium chloride (NH₄Cl) and alumina (Al₂O₃) at 1000–1100–1200°C and with varying deposition times of 3–5–7 h in an open atmospheric furnace. The morphology and structure of silicide layers were analyzed by optical microscopy and scanning electron microscopy–energy-dispersive X-ray spectroscopy. It was observed that there was good bonding between the matrix and silicide layers, which were smooth, homogenous and dense and had no remarkable porosity. Dominant phases of Fe₃Si, as well as some FeSi, were detected by X-ray diffraction analysis. The layer thickness was measured from the surface to the matrix and changed from 40 to 800 μm, which increased with increasing process time and temperature. The observed silicide layer growth obtained at 1200°C was much higher than those at 1000 and 1100°C. The hardness profile showed that there was no diffusion zone. The matrix hardness is 225 HVN, while the hardness of the coating layer rose to 1021 HVN with the process time and temperature. The activation energy for the growth of the siliconizing layer was determined as 292 (kJ/mol)/K according to the kinetic study. It can be claimed that the optimum siliconizing layer was obtained up to 1100°C for 5 h due to the formation of a layer with smoother morphology, less porosities and oxygen impurities and adequate thickness.