Duplex surface treatment of sintered iron by plasma nitriding and plasma carburizing at low temperature

The aim of this study is to investigate the combination of plasma nitriding and plasma carburizing at low temperature. Samples of sintered pure iron were nitrided at 500 degrees C for 3 h under three different atmospheres: (1% N-2 + 99% H-2), (10% N-2 + 90% H-2), and (90% N-2 + 9% H-2 + 1% CH4), all followed by plasma carburizing at 500 degrees C for 3 h under a mixed atmosphere of 2% CH4 + 98% H-2. Microstructural characterization and chemical analysis of the layers were performed by scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. Phase identification was carried out by grazing incidence X-ray diffraction. The layer hardness was verified by nanohardness and the case depth by Vickers microhardness. The results show that the post-carburizing treatment promotes the formation of a thin outermost cementite layer over the nitride layers, leading to a nitrided-carburized layer on the surface. The morphology and phase composition of the nitride layer influence the structural and mechanical properties of the nitrided-carburized layers. The best surface hardness improvement, approximately 72%, was achieved by the duplex treatment of nitriding with 1% N-2 content followed by a plasma carburizing at low temperature. Moreover, duplex nitriding-carburizing leads to deeper hardening depths when compared to simply nitrided samples.