Božidar Matijević

Novelty in Diffusion Coating Technology

Diffusion forming of a hard carbide layer has been used for many years, mostly in Japan (Toyota Diffusion Process, TDR) and in the USA (Thermoreactive Deposition, TRD) but not so much in Europe. Carbide layers on constructive parts and tools formed by a diffusion process, especially a vanadium carbide layer, have excellent tribological properties and some technical and economical advantages in comparison with carbide layers formed by chemical vapour deposition (CVD) and physical vapour deposition (PVD) processes. Diffusion formed layers are thicker with very strong bond to steel substrate and are produced using significantly cheaper equipment than for deposition coating. The process has one disadvantage: partial decarburization of the steel surface beneath the carbide layer reducing hardness and loading capacity of the surface. This disadvantage is eliminated by a developed and patented duplex process introducing previous carburizing to prevent decarburization and to form a hardened case under layer beneath the carbide layer. The new developed process should contribute to widely application of diffusion carbide coating for automotive industry. Mathematical model and program for optimization of process is presented as well as some examples of application: extruders for medium with very abrasive particles, aluminium die casting moulds, etc.

Effect of composition on oil quenchant performance

AbstractQuenching is the most common method of hardening metals and involves continuous cooling from a given austenitising temperature to achieve full transformation into a martensitic structure. The cooling rate depends mainly on the thermal characteristics of the metal, the section thickness of the workpieces and the heat removal properties of the quenching medium. Oils, when used as quenching media contain a base oil and different types of additives according to the application requirements. As the base oil, it is possible to use mineral, synthetic and natural oils, either separately or in combination. Mineral base oils are widely used because of their advantages in stability in comparison with natural oils, or lower price in comparison with synthetic oils. There are numerous additives available for use but, besides functional properties, the environmental and safety requirements have to be considered in their selection, e.g. barium additives must be replaced with less harmful compounds. Additionally, ...

Evaluation of Boride Layer Growth on Carbon Steel Surfaces

The kinetics of growth of the boride layer in the boronizing of carbon steels from EKABOR 3 powder is studied. Relations between the thickness of the boride layer and the parameters of the process, i.e., the temperature and the duration of the boronizing operation, and the composition of the saturated steel are derived. The morphology and the types of the borides are determined by the methods of light microscopy and x-ray diffraction. The parameters of the Arrhenius equation for the kinetics of boronizing are computed. The contour diagram obtained for the variation of the thickness of boride layer makes it possible to determine the size of the layer from data on the parameters of the process.

Determination of Steel Carburizing Parameters by Using Neural Network

The article discusses the application of neural networks for calculating the laws of complex processes, with carbon diffusion processes in steel carburizing classified among them. Empirical and mathematical models for the Carbomaag carburizing process, which have been proven in practice, are presented for the determination of technological carburizing parameters resulting in a required carbon concentration profile in the carburized layer. A comparison between the mathematical model (MM) and the neural network model (NNM) and the empirical model < EM) with respect to the time required for shallow and great carburizing depths is given special attention in this article. The results of the empirical carburizing model were used as a neural network training set and compared with the results of computer simulation of the MM. A comparison of results obtained at shallow and great carburizing depths shows that the NNM approximates much better the EM than the MM. It is assumed that the carrying out of a larger number of experiments and the repeated neural network training with new sets of experimental data would result in increasingly better solutions. Thus, the disadvantages of theoretical models, i.e., MMs, would be avoided.

Improvements in thermo reactive deposition of carbide layers

Abstract This paper presents developments contributing to the improvement of the process of producing hard carbide layers, with special attention to the application on tools and dies. The quantitative impact of the chemical composition of the steel and the process temperature on the kinetics of the carbide layer formation has been determined. In conventional processes of diffusion formation of carbide layers the phenomenon of partial decarburisation of steel occurring beneath the carbide layer during the process of its formation has been dealt with by the introduction of controlled precarburisation. Diffusion formation of carbide layers has been improved by the development of duplex processes that enable the formation of a case hardened layer which is harder than the core. Thus, the substrate for the hard and brittle carbide layer is improved. This development should contribute to the improvement of properties of the surface layers produced by the diffusion formation of carbide layers. In this way this process can be applied to the manufacturing of tools and dies.

Influence of Additive Chemistry on the Physical, Chemical, and Cooling Properties of Quenching Oils

Quenching, the most common method of hardening metals, involves continuous cooling from a given austenitization temperature to achieve full transformation into martensite. Thus, optimal mechanical properties are obtained. The quenching media contains a base oil and different types of additives depending on application requirements. The cooling rate mainly depends on the thermal characteristics of metals, section thickness of workpieces, and heat-removal properties of the quenching medium. For the base oil, it is possible to use petroleum oils, synthetic oils, and natural oils separately or in combination. Petroleum base oils are widely used because of their thermal-oxidative stability advantages in comparison to natural oils, or lower prices in comparison to synthetic oils. There are numerous compounds available for potential use based on their functional properties. Environmental and safety requirements also exhibit a strong influence on additive selection. For this reason, the current use ofbarium-containing additives must be replaced with less harmful compounds. Petroleum basestocks should be replaced by renewable basestocks that are biodegradable, as is the current general trend in lubricant development. By changing the composition of quenching oils, base oils, and/or additives, the heat transfer characteristics are also changed. In this study, quenching oils with different types of additives are tested and the results regarding their physical and chemical properties as well as cooling characteristics are presented. Cooling curves of the tested oils have been evaluated according to ISO 9950:1995.

Modification of hot work tool steel surfaces by various thermochemical treatments

The surface of the hot work tool steel X37CrMoV5-1 (W300) was subjected to different thermochemical treatments in order to improve its mechanical properties, corrosion and wear resistance. Nitrocarburising with or without post-oxidation based on the TENIFER procedure and low temperature powder aluminising+nitrocarburising with or without post-oxidation were performed. Nitrocarburised and nitrocarburised+post-oxidised layers were characterised by testing their basic properties (compound layer thickness, nitrocarburising depth and microhardness) as well as their corrosion resistance. First, the microstructures of aluminised+nitrocarburised samples (with or without post-oxidation) were analysed by means of optical microscopy, and then the microhardness distribution across the surface layer. Finally, the corrosion resistance of aluminised+nitrocarburised samples (with or without post-oxidation) were examined. Glow discharge optical spectroscopy was employed to investigate the distribution of elements in the c...

The low temperature aluminising kinetics of hot-work tool steels

Abstract Conventional aluminising procedures are mostly conducted at temperatures well above 900 °C, and can last for hours. When applied to hot-work tool steels, aluminising has to be done at notably lower temperatures in order to prevent grain growth and carbide formation, and to improve creep resistance. The kinetics of aluminium coating formation on hot-work tool steels was studied in the temperature range of 550–610 °C. The pack Al content was varied from 5–15 wt.-% and aluminising time from 1–9 hours. The halide activator AlCl3 was applied. A series of statistically designed experiments were conducted to determine how key process factors influence the aluminide coating formation. A Box-Behnken experimental design was used to evaluate three process factors at three levels. The microstructures of coated samples were analysed by a scanning electron microscope (SEM). Glow Discharge Optical Spectroscopy (GDOS) and energy-dispersive X-ray spectroscopy (EDX) were employed to investigate element distributions in the coating layer