Influence of silicon content on ion nitriding of ultrahigh strength steels

The effect of silicon content on the tempering behaviour and ion nitriding of ultrahigh strength AISI 4340 steels has been investigated. Four alloys with varying silicon contents were quenched and tempered for 1, 3, and 5 h, and ion nitrided at different temperature and time conditions. The nitrided layers were characterised by microhardness tests, optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive X-ray spectrometry (EDS), and X-ray diffraction (XRD). An increase in silicon content enhanced tempering resistance of the steels and also significantly improved the hardness of the nitrided layers. The increase in silicon content produced thinner compound layers with better hardness. XRD analysis indicated a mixture of nitrides in these layers, γ-Fe₄N, ϵ-Fe_2-3N, CrN, MoN, and Si₃N₄, with their proportions varying with nitriding conditions.