A diffusion model was suggested to analyse the growth kinetics of Fe 2 B layers formed on AISI D2 steel by the pack-boriding process. It was used to estimate the boron diffusion coefficients of Fe 2 B in the temperature range of 1123-1273 K by applying the mass balance equation at the (Fe 2 B/ substrate) interface. The proposed model was validated experimentally at 1253 K for a treatment time of 5 h by comparing the experimental Fe 2 B layer thickness with the predicted value. Furthermore, the pack-borided AISI D2 steel was characterised by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis. A contour diagram describing the evolution of Fe 2 B layer thickness as a function of the process variables was also proposed. In addition, the boron activation energy for AISI D2 steel was found to be equal to 201?50 kJ mol 21 , on the basis of our experimental results.
This work focused on the determination of boron diffusion coefficient through the Fe
The nitriding of iron and steel is of considerable technological importance, because it can make a pronounced improvement in the fatigue, the wear, and the corrosion resistance of these materials. Nitriding on the surface of ferrous alloys results in the formation of a compound layer of γ´-Fe4N1-x and ɛ-Fe3N nitrides or a mixture of γ´ and ɛ with a nitrogen diffusion zone beneath the nitride layer. The broad range of nitride layer properties needed for different applications requires good control of nitriding process [1-3]. In the powder-pack nitriding process, which is similar to the powder-pack carburizing process, samples are placed in an annealing box with a powder mixture that consists of nitrogen-rich material and an activator. The nitriding temperatures are between 723 K and 823 K, a range in which the nitriding potential is a function of the mount of activator used in the powder mixture. The powder-pack method is a low-cost process particularly suited for the formation of uniform nitride layers on structural alloy components with complex shapes and of various sizes [4-5]. In this study, the microstructure of the γ´-Fe4N1-x and ɛ-Fe3N layers formed on an ARMCO pure iron surface have been investigated at different temperatures by the powder-pack process.
Surface hardening, a process that includes a wide variety of techniques (Carburizing, Nitriding, Nitrocarburizing, Boriding, and Thermal diffusion process), is used to improve the wear resistance of parts without affecting the more soft, tough interior of the part [1][2][3]. In particular, resistant layers of borides are produced in ferrous and no-ferrous materials though the well-developed process of boriding. In ferrous materials, this thermochemical diffusion treatment generally possesses superior hardening features than those found in conventional processes like carburizing, nitriding or chromising, due to the formation of single Fe2B or a double (FeB + Fe2B) hard phases. On the other hand, the thermochemical process of nitriding is also used to improve the wear and corrosion resistance of engineering components, producing a hard case and a soft and tough core. Nevertheless, significant variation has been identified in the hardness gradient resulting in ablated tribological performances. To mitigate the brittleness and those variations in microhardness, two multicomponent surface treatments such as boronitriding are being investigated. Nonetheless, very little work has been devoted to assess both the microstructural and mechanical characteristics of boron nitride coatings on ferrous materials [4][5]. In this study, the microstructure of the ɛ -Fe3N and Fe2B layers formed on an ARMCO pure iron surface have been investigated at different temperatures by the powder-pack process.Cubic commercial samples were cut from an ARMCO iron bar with composition: Mn, 800 ppm; C and P, 200 ppm; and S, 150 ppm. The substrate pure iron used in this work was selected to curb the effect of alloying elements in order to solely analyse the characteristic boride and nitride layers and some of their mechanical effects. The boro-nitriding treatment was carried out in two stages: boriding and then nitriding. Powder-pack boriding and powder-pack nitriding procedures were preferred in this study for its cost-effectiveness, and simplicity of the required equipment. The samples were embedded in a closed in a closed cylindrical case (AISI 304L stainless steel) having a boron powder mixture inside with an average particle size of 30 µm. The boriding agent contained an active source of boron (B4C), an inert filler (SiC), and an activator (KBF4). The powder-pack boriding process was carried out in a conventional furnace under a pure argon atmosphere at 1223 and 1273 K for 8 h of exposure for each temperature. Once the boriding treatment was finished the container was removed from the furnace and slowly cooled to room temperature. In the second step, the pre-boriding iron samples were nitrided by the pack method in the powder mixture consisting of calcium cyanamide (CaCN2, ~24% of N) and calcium silicate (CaSi, ~35 wt.% of the mixture) as an activator. The samples were directly immersed in the powder mixture in another stainless steel cylindrical case. The nitriding temperatures were 773 K and 823 for 8 h using the same furnace and conditions...
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