15 / 2025-03-20 12:38:46

Effects of the prior forming process on nitrocarburized low-carbon steels

Nitrocarburizing,Low carbon steel,Oxide layer,Hardness,Compound layer

Nitrocarburizing at 570°C for 2 hours in a controlled atmosphere of ammonia and endothermic gas in the ratio 8:7 was applied to cold-rolled low-carbon steel (SPCC) and hot-rolled low-carbon steel (SPHC) workpieces. The steels have similar compositions but were produced by different forming processes prior to the nitrocarburizing process. The aim was to study the effects of the hot and cold rolled external surfaces and of the presence of iron-oxide surface layers on the nature and thickness of the nitrocarburized compound layer. Vickers microhardness tests at 100 g load were used to measure the surface hardness of all the test samples. Nano-hardness testing at 1 mN load was used to measure the hardness profile Optical microscopy was used to measure the thickness of the compound layers. Scanning electron microscopy (SEM) and wavelength dispersive x-ray spectroscopy (JEOL, JXA-ISP100) were used to evaluate the microstructure and porosity in each layer, as well as the elemental distribution. The results indicate that the surface hardness of treated SPCC is higher than that of SPHC, however the compound layer thickness of SPCC is lower than that of SPHC. The compound layer of SPHC was underneath the prior process-generated oxide layer, which contributed to the lower surface hardness. From the SEM and WDS results, carbon aggregation was found in the prior oxide layer of the SPHC sample. Carbon atoms reduce iron oxide to iron metal, and carbon reacts with oxygen to form carbon monoxide (CO) or carbon dioxide (CO₂). This phenomenon caused high porosity in the iron oxide layer and less carbon diffused into the iron matrix of the SPHC sample. While nitrogen atoms do not react with iron oxide, the diffusion rate of nitrogen atoms through iron oxide is faster than that of carbon, thus allowing nitrogen to penetrate the oxide layer and diffuse into the steel, to form a compound layer underneath the oxide. For the SPCC sample, there was no iron oxide layer that could cause carbon aggregation, so carbon diffused into the substrate, causing higher carbon content in the epsilon phase. From the hardness profile results, the hardness of the epsilon phase containing a higher amount of carbon in SPCC is harder than that of the epsilon phase containing lower carbon in SPHC, the latter having a similar hardness as the gamma prime phase.