Papers by Keyword: Carbonitrides

Abstract: Niobium is added to carbon steels in small amounts (< 0.1weight %), thus being called a microalloying element, to increase mechanical strength and toughness. When added to steel, niobium is partly soluble in the matrix and another part combines with carbon and nitrogen forming a family of Nb_xC_yN_z precipitates (niobium carbides, nitrides or carbonitrides), where the values ​​of x, y, z depend on the temperature and the chemical composition of the steel. The solubility equations for niobium in austenite available in the literature only provide the niobium content that could be solubilized at a given temperature. But when niobium is added above the solubility limit, the excess niobium will not only form the Nb_xC_yN_z family of precipitates. This is what the proposed model calculates. The proposed model is easy to apply and provided results are very close to those determined experimentally by different researchers, who used different techniques such as atom probe, or matrix dissolution with precipitate filtration, for example. Although the proposed model has been used to calculate niobium in solution in austenite, the same can be applied to any other precipitate, such as carbides, nitrides or carbonitrides of vanadium and titanium, for example.

Abstract: The technology of restoring worn-out engine parts using modern techniques for applying wear-resistant diamond-like (DLC) coatings has been analyzed. To simplify the coating technology and reduce the wear of the highly adhesive deposited layer, applying metal-containing adhesive undercoats has been proposed. The camshaft surface wear types have been studied.The dependence of the change in the oil film thickness in the camshaft -follower contact zone on the cam rotation angle at a rotation frequency of 1,500 rpm is given. The contact causes an unsteady friction surface lubrication mode and intensive wear of the material in the contact zone.To apply a wear-resistant coating, ion etching has been proposed, followed by spattering a tungsten layer with a thickness of up to 10 μm on the product surface with subsequent 20 μm thick tungsten carbide-based coating using microwave plasma. Such a gradient structure over the cross-section of the wear-resistant coating allows achieving the best product performance.The values ​​for the change in surface temperature in the cam-follower friction pair contact zone are given. Nitride, carbonitride, and other types of wear-resistant coatings, their recommended thicknesses, and maximum allowable operating temperatures are given. The hardness of various coatings and possible types of their destruction are shown.The reasons for adhesive and cohesive delamination of wear-resistant coatings are shown, and ways to eliminate this phenomenon are proposed.

Abstract: Nanostructured smart coatings (NSC) based on the TiAlSi-CN composite structure elements were deposited using reactive high-power physical vapor deposition (PVD) technique. The advanced modular deposition system included up to 8 high-power magnetron sputtering devices (MSD) allowing operate them simultaneously and exceed power density of 120 W/cm² within each device erosion zone. The novel designed NSC on bearing steel 100Cr6 substrates demonstrated enhanced mechanical and tribological properties comparably with bearing steel ones required for multifunctional high-tech applications. The deposited NSC containing TiAlSi-CN nanoparticles strengthened by elemental additives Cr and Nb exhibited microhardness as high as 2500 HV values in comparison with 750 HV of 100Cr6 steel substrates. Load-displacement curves obey Meyer’s power-law surprisingly well because power-trendline fitted ones by R-squared value of 0.9999 for all the film-samples. Tribological properties were measured under dry friction conditions between the bearing steel ball of Ø 6 mm and the film-samples’ flat surface. Coefficient of friction (CoF) ranges between 0.22-0.56 depending on a sample and load. Tribotracks worn under the friction indenter were too shallow to evaluate them by Mitutoyo profilometer SJ-500. Therefore, the wear rate was estimated as ball wear of the friction indenter.

Abstract: Nanostructured smart coatings (NSC) based on the TiAlSi-CN composite structure elements were deposited using advanced reactive physical vapor deposition (PVD) method. The novel NSC on steel substrates demonstrated enhanced wear and corrosion resistance required for multifunctional high-tech applications. The deposited NSC containing TiAlSi-CN nanoparticles strengthened by Cr, Nb and Hf additives exhibited a coefficient of friction (CoF) less than 0.2 and wear rate as low as 10E^-8 mm³/Nm. In addition, some self-healing properties were observed preventing denitrification of the core carbon-nitride (CN) layer due to specific tribochemical reactions of Al and Si constituents. Thus, the chemical composition and tentative nano-structure of the NSC can be described by a stoichiometric formula TiAlSi-CN:Me/a-C_xN_y:Si₃N₄, where Me=Cr, Nb or Hf.

Abstract: Based on the mean field approximation, a model has been worked out for the description of evolution of carbonitride precipitate ensemble with various composition in steels at the stages of their growth, dissolution and coarsening. Based on the numerical realization of this model, the calculations of growth and dissolution kinetics of carbonitrides in a Fe-Nb-V-C-N system have been carried out.

Abstract: The secondary phases such as carbonitrides in the microalloyed steel play very important roles in retarding the austenitic grains growth during reheating. The dissolving process of carbonitrides containing Nb, Ti, Mo will affect the austenitic grain sizes directly. In the present work, the dissolving behaviors of secondary phases in low carbon microalloyed steel during isothermal holding at different temperatures were investigated by electrolytic experiment, carbon extraction replicas, TEM and EDX analysis. Meanwhile, the austenitic grain sizes were measured corresponding to the temperatures. The experimental results indicate that there are two types of carbonitrides in as-forged steel. One is the coarsened Ti-rich precipitates originated from solidification, and the other is the finer Nb-rich particles attributed to strain-induced process. The strain-induced precipitates disappear after being held for 1 h at 1000°C. At 1000~1220 °C, the austenitic grains grow obviously due to rapid dissolving of the carbonitrides containing Nb and Mo. However, some undissolved Nb, Ti carbidenitrides still hinder the grain boundary migration. When the reheating temperature rises to 1270°C, the grain size grows abnormally after being held for 2h. At the holding temperature, few Nb-bearing TiN precipitates can be stable while the pinning effect weakens markedly.

Abstract: The creep behavior of a TiCxN1-x-Co-Mo2C cermet has been investigated at temperatures between 1100-1200°C in an inert atmosphere to assess the one step mechanically induced self-sustaining reaction synthesis and pressureless sintering process, and the influence of the Mo2C additive in the high temperature mechanical properties of this cermet. The samples deform plastically at the chosen temperatures, and values of the stress exponent (n=1.70.6) and activation energy (Q= 4.30.5 eV) have been estimated from uniaxial compression tests. No significant grain growth has been detected after deformation. The reproducibility of the creep tests compared to other compositions indicates that the Mo2C addition contributes to increase notably the resistance to high temperature oxidation of the samples, so that the plastic behavior is not affected by oxidation when deformation experiments are performed in an inert atmosphere.

Abstract: Approaches used for simulation of precipitates evolution in multicomponent systems are analyzed in brief. Simulation of precipitates growth and dissolution in such systems, simulation of a polydisperse particle ensemble evolution at different stages and possibilities of taking into account nucleation during precipitates growth from a supersaturated solid solution are considered by an example of iron-carbon alloys. The problems to be solved are formulated.

Abstract: The model describing the behavior of precipitates of variable composition at heat treatment is suggested. The specific feature of this model is that it enables to take into account the effect of diffusion inside the particles on the kinetics of the process. The application of this model is demonstrated by the example of vanadium carbonitrides V(C,N) evolution in austenite at heat treatment of vanadium-containing steels. The necessity to take into account the diffusion modification of carbonitride composition is considered. Examples of particles composition evolution at heat treatment are demonstrated.

Abstract: High Resolution Transmission Electron Microscope and Electron Energy Loss Spectroscopy and have been used to characterize the structure and chemical composition of niobium carbonitrides in the ferrite of a Fe-Nb-C-N model alloy at different precipitation stages. Experiments seem to indicate the coexistence of two types of precipitates: pure niobium nitrides and mixed sub-stoichiometric niobium carbonitrides. In order to predict the chemical composition of these precipitates, a thermodynamical formalism has been developed to evaluate (i) the nucleation and growth rates (classical nucleation theory) and (ii) the chemical composition of nuclei and existing precipitates. A model based on the numerical resolution of former equations, is used to compute precipitates size distribution evolution at a given temperature. The predicted compositions are in very good agreement with experimental results.