Search

Abstract: Plasma immersion ion implantation (PIII) has been employed for nitrocarburizing 304 stainless steel. The sample was treated at relatively low gas pressure of nitrogen and acetylene in the range of 10−3 mbar. The microstructure variation, the austenite lattices spacing and the phase transformations were studied in-situ during heating up to 800 °C and after cooling, using synchrotron X-ray diffraction. Glow discharge optical spectroscopy (GDOS), optical microscopy and hardness profile measurements have been used before and after thermal treatment to analyse the nitrocarburized layer. [Copyright &y& Elsevier]

The depth dependence of elemental composition, phase distribution, and cross-sectional morphology of rf plasma nitrocarburized 304 austenitic stainless steel were investigated using glow discharge optical spectroscopy (GDOS), grazing incidence X-ray diffraction (GIXRD), and optical microscopy, respectively. A step-wise mechanical polishing method was used to remove successive sublayers of the treated surface. It was found that the properties of the nitrocarburized layer depend critically on the plasma gas composition, which controls the supersaturation of nitrogen and carbon through the compound layer depth. Iron nitride phases and/or nitrogen-expanded austenite (γN) were detected in the nitrocarburized layer prepared at high plasma nitrogen (N2) content. In the compound layer processed at high plasma carbon (C2H2) content, besides the carbon-expanded austenite phase (γC), carbide phases were found predominantly in the top-layer, in which the carbon concentration has a maximum value of ∼2 wt.%. The lattice expansion of the expanded austenite phases changes with sampling depth, depending on local variations in nitrogen and carbon content. The applied rf plasma processing power influences significantly nitrogen and carbon distribution in the treated sublayers. [Copyright &y& Elsevier]

Abstract: Plasma immersion ion implantation (PIII) has been used to modify the surface properties of 304 austenitic stainless steel (AISI). The influence of working gas pressure, 0.2–1.0 Pa, and substrate temperature, 300–500 °C, on the microstructure, treating rate, nitrogen/carbon concentration depth profile, and surface microhardness was investigated. A gas composition of 25% C2H2, 75% N2, r.f. plasma power input of 350 W, and a negatively biased potential of 30 kV were fixed during the experiment. The experimental results show that the substrate temperature and the diffusion process of nitrogen and carbon depend on the gas pressure inside the plasma chamber. The thickness of the modified layer has been found to be more than 30 μm for samples were treated in the plasma for 60 min. The results show also that the values of diffusion coefficient and surface microhardness of the treated samples are high to be 3.4×10−1 μm2/s and 1880 kg/mm2, respectively. [Copyright &y& Elsevier]

Adding C2H2 gas to nitrogen gas during r.f. plasma processing has significant influence on the properties of 304 austenitic stainless steel. The elemental depth profiles of the compound layer were investigated by glow discharge optical spectroscopy. Different treatment gas compositions of N2/C2H2 mixtures (0–100% in steps of 10 or 5% in some cases) have been investigated. The total gas pressure during the plasma processing was 8.4×10-2 mbar. The sample temperature was approximately 550 °C with an insignificant variation for different gas compositions. A high rate of diffusion of carbonitriding has been obtained without initial sputter removal of the surface oxide layer, which demonstrates that the surface oxide layer does not play a crucial role during r.f. plasma carburizing and carbonitriding of stainless steel. The nitrogen concentration and the compound layer thickness increase, when 10% of C2H2 is added. With further increase in the C2H2 ratio, they decrease again gradually. The carbon depth profiles for the same samples show lower concentration and opposite tendency with respect to the nitrogen content. X-ray diffraction was used to characterize the microstructure of the compound layers. Fe2N, Fe3N, CrN, nitrogen-expanded austenite (γn) and carbon-expanded austenite (γc) phases are detected. The relative proportions of the phases are critically dependent on the N2/C2H2 ratio. Depending on the N2/C2H2 ratio, the microhardness value increases by a factor between 2 and 6. The carbonitrid layer exhibits a corrosion resistance better than pure nitrided and carburized layers. [Copyright &y& Elsevier]

Abstract: Surface of 304 AISI austenitic stainless steel has been modified using duplex treatment technique of nitriding and carbonitriding. A thick modified nitrided layer, of approximately 20 µm, has been achieved when rf inductively coupled plasma was adjusted at 450 W for processing time of only 10 min. After performing the nitrided layer, the nitrided samples were carbonitrided using the same technique at different acetylene partial pressure ratios ranges from 10% to 70%, the balance was pure nitrogen. Different amount of nitrogen and carbon species are diffused underneath the surface through the nitrided layer during carbonitriding process and are found to be gas composition dependent. The treated samples were characterized by glow discharge optical spectroscopy, X-ray diffractometry, scanning electron microscopy and Vickers microhardness tester. The microstructure of the duplex treated layer indicates the formation of γ́-Fe4N, Fe3C, CrN and nitrogen-expanded austenite (γN). The thickness of the duplex treated layer increases with increasing the acetylene partial pressure ratio. The surface microhardness of duplex treated samples has been found to be gas composition dependent and increased by 1.29 fold in comparison to the nitrided sample. [Copyright &y& Elsevier]