Effect of Silicon and Chromium Additions on Surface Oxide/Nitride Layers in a Fe-0.4 wt.% C Alloy by Novel Salt-Bath Nitrocarburizing Treatment Free

A novel salt-bath nitrocarburizing process recently developed forms a lithium-iron compound-oxide layer on the surface of steel in concurrence with a nitride layer by adding lithium ions to the molten salt. This process has already been successfully applied to mass-produced products. However, the microstructure and its formation process of the surface layer in alloyed steels during the nitrocarburizing process have not yet been fully understood. In this study, we focus on the effect of Si and Cr, which are included in a common die steel, on the microstructure of an oxide layer of a nitrocarburized alloy. The alloys used in this study are Fe-0.4wt%C, Fe-0.4wt%C-2.0wt%Si, and Fe-0.4wt%C-2.0wt%Cr. These alloys were arc melted into button ingots under an Ar atmosphere. The ingots were annealed at 1123 K for 1.0 h, followed by air cooling and double tempering at 873 K, similar to the heat treatments employed to hot-die steels. Salt-bath nitrocarburizing was carried out at 823 K for 0.1-10 h. The microstructures of the cross-sectional surface layers of the samples were examined using an optical microscope and FE-SEM. Elemental mapping as well as phase identification of the surface layers were done by EDS, XRD, and GD-OES. In the Fe-C binary alloy, a thin continuous oxide layer of α-LiFeθ₂ formed first on the outermost surface, and a thick iron nitride layer developed underneath the oxide layer, with aligned oxide particles along the grain boundaries of the nitrogen compound layer. In the case of Si addition, the outermost oxide layer became thinner and an additional oxide layer consisting of α-LiFeθ₂ and (Li,Fe)₃Siθ₄ formed between the outer oxide layer and nitrogen compound layer, and the formation of the oxide particles in the nitrogen compound layer was fully suppressed. In the case of Cr addition, internal oxide particles formed in the nitrogen compound layer, similar to those in the binary steel, although an continuous oxide layer of CrfN,O) formed in between those layers. On the basis of these results, the inner oxide layer formed with Si addition contributes to improving the frictional wear characteristics in die steels.