Processes Of Carbide Formation Research Articles

ВПЛИВ ВУГЛИЦЮ НА КОРОЗІЙНУ ПОВЕДІНКУ ТЕРМІЧНО ЗМІЦНЕННОГО ПРОКАТУ

In this work, we studied the corrosion behavior of hot-rolled and heat-hardened rolled products made of carbon steel with different carbon content in order to select the optimal conditions for its heat treatment to obtain a satisfactory combination of strength and corrosion resistance during the development of processes of general corrosion with hydrogen depolarization. The studies were carried out on samples of carbon steel (0.14-0.8% C) in hot-rolled and heat-hardened states. It has been established that the corrosion rate (K) in specimens made of steels in the quenched state is lower than in the hot-rolled state; however, even in this state, with an increase in the carbon content within the studied limits, it increases 40 times. An increase in the corrosion rate with an increase in the carbon content in hot-rolled steel is explained by an increase in the amount of pearlite and an increase in the length of the ferite-cementite interphase boundaries. It was determined that in quenched hardened steel in the carbon content range from 0.14 to 0.80 % C, an increase in the corrosion rate is apparently due to an increase in microstresses (accumulation of excess internal energy) associated with an increase in the tetragonality of the martensite lattice and the appearance of concentration inhomogeneity in the solid solution. It was found that during the course of corrosion with hydrogen depolarization, the dependences of the general corrosion rate of thermally hardened structural steels on the tempering temperatures are described by extreme dependences with a maximum corrosivity in the temperature range of 350-500 oC, while, with an increase in the carbon content in steel, rental increases. It was determined that at the tempering temperature corresponding to the maximum K, the corrosion rate is 1,5-3,6 times higher than in the quenched state, and 1,4-2,6 times higher than in hot-rolled, and with an increase in the carbon content this difference is growing. An increase in the rate of corrosion with hydrogen depolarization in the tempering temperature range of 350-450 oC is apparently associated with the process of carbide formation and the appearance of coherent stresses during the release of dispersed particles. An increase in corrosion losses in this interval with an increase in carbon content is due to an increase in the volume fraction of fine particles of the carbide phase and an increase in the level of elastic microdistortions of the crystal lattice (Δa/a).

THERMODYNAMIC MODELING OF THE INTERACTION OF TECHNOGENIC SILICA FUME WITH BROWN-COAL SEMI-COKE

Thermodynamic modeling of high-temperature interactions of microsilica and brown-coal semi-coke was carried out. Calculation of equilibrium structures of Si – O – C and Si – O – C – H systems was defi ned by a “constant” method with the use of the computer modeling program of high-temperature chemical interactions of “PLASMA”. It was established that in both systems the formation process of carbide was dominating. At stoichiometric composition of furnace charge the maximum contents in products of restoration of silicon carbide can be reached at 1700 K, and at 10 % a lack of carbon – 1900 K. The introduction of hydrogen to system doesn’t actually infl uence on the process of carbide formation that is caused by low (less than 0,001 mol) contents in a gas phase at temperatures of carbide formation of hydrocarbons and hydrocarbonic radicals. In Si – O – C system the equilibrium extent of transformation of silicon into carbide doesn’t exceed 0,97, that corresponds to the content of monoxide of silicon in a gas phase of 0,02 mol, owing to what from furnace charge of stoichiometric structure (SiO2 + 3C) it is impossible to receive the single-phase, not containing free carbon, carbide of silicon. It can be avoided using furnace charge with some (~ 10 %) lack of carbon reducer.

Modelling the growth kinetics of carbide phase particles in welded joints in high-alloy steels

The results of modelling the processes of carbide formation in the metal of welded joints in 12Cr18Ni10Ti high-alloy steel, produced by manual arc welding with electrodes with different types of coatings – rutile (OK 61.30) and basic (TsL-11) – are presented. Intercrystalline corrosion can be suppressed by initiating the bonding of carbon in carbides by titanium and niobium transferred into the welded joint from the coating.

SHS - Processes in the Carbonaceous Oxide System at High Nitrogen Pressure Values

<p>This paper contains study results of the nitride containing composites features formed in the compacted Al-TiO<sub>2</sub>-C-Si samples inside a high pressure reactor with various nitrogen pressure values. The nitriding and carbide-formation processes take place simultaneously with aluminothermic reduction of titanium oxide in the self-propagating high-temperature synthesis (SHS) mode. It has been found that the nitrogen pressure effect is manifested as insignificant reduction of the combustion temperature and increased durability of the synthesized composite. The SHS process in the nitric environment leads to practically complete reduction of titanium oxide, the free titanium being absent. The X-ray analysis has revealed that the basic SHS products are refractory compounds such as metal nitrides, carbides, and oxides. Increase in nitrogen pressure results in decrease of the specific electric resistance of the synthesized composites caused by growth of the electroconductive phases i.e. titanium carbides, nitrides, and silicides. Performed electron microscopic study including the energy dispersion analysis of the morphology and structure of the SHS products has revealed formation of the nano-sized titanium silicide crystals distributed between the complex carbonitride particles. The complex carbonitride composites synthesized in the high pressure reactor are of interest as highrefractory and abrasive materials considering their physical and chemical properties.</p>

Degradation of a corrosion and heat resistant steel pipe

Degradation process of a cracking furnace pipe made of the austenitic 25Cr–35Ni alloy was investigated using SEM equipped with energy dispersive probe and full area and selective area X-ray diffraction techniques. The results revealed that the bulk diffusion of C into the tube resulted in a complex process of carbide formation at the grain boundaries, Cr depletion of the matrix and thus the appearance of the ferromagnetic behaviour. The degradation was remarkably enhanced by the abrasive environment and the periodic decoking (elimination of deposed carbon by means of an air–steam mixture) process leading to the formation of spongy Cr-oxide.

Synthesis of niobium carbides in copper melts

The production of cast composite Cu-NbC alloys by synthesizing of niobium carbides in a copper melt is studied. The processes of carbide formation are intensified by the mechanical activation of niobium and graphite powders, which are mixed with a copper melt, in a high-speed ball mill and by the action of low-frequency vibrations on copper melts.

Special Features of Vacuum Carburizing of Heat-Resistant Steel in Acetylene

The mechanism of formation of the diffusion layer due to vacuum carburizing of complexly alloyed steel 16Kh3NVFMB-Sh (VKS5) in a medium of acetylene is considered. It is shown that the process of carburizing occurs in a self-regulation mode without the danger of carbon black due to the high rate of arrival of carbon from the gas medium as a result of catalytic dissociation of acetylene molecules, intense process of carbide formation on the saturated surface, and absence of catalytic activity of carbides. The diffusion layer forms under a thin layer of the carbide phase in a kinetic regime, the intensity of which is determined by the rate of dissolution of carbides.

Interface characterisation and wettability properties of carbon particle reinforced copper alloy

The effect of carbide alloying elements such as chromium and tin upon the bonding state and the interfacial carbon/copper zone in a copper melt have been investigated. The reactivity behaviour, contact angle and the kinetics of wettability have been investigated for each system studied (C/Cu, C/Cu–Sn, C/Cu–Cr and C/Cu–Sn–Cr) using Auger electron spectroscopy and line profile/sessile drop experiments performed under ultra high vacuum conditions. Non-wetting systems such as C/Cu and C/Cu–Sn are characterised by (i) an adhesive solid–solid bond strength and van der Waals interactions and (ii) monotonous behaviour during experiments where the kinetics are controlled by the viscous flow of the liquid drop over the solid substrate. Wetting systems such as C/Cu–Cr and C/Cu–Sn–Cr show a completely different behaviour. For these two systems, chromium tends to segregate at the reinforcement/metal interface, forming a stable carbide compound. Chromium segregation and carbide formation processes lead to different kinetics of wettability which can be summarised in a three step process.

Influence of the carbide phase on fracture of improved steels

We present the results of investigation of the mechanism of fracture of 14KhN3A, 20, 20KhN3A, 40, and 40Kh steels after hardening and high tempering, and after embrittlement. We show that the intergranular fracture of steels is related to inhomogeneity of the processes of carbide formation on tempering and to the primary carbide separation on the boundaries of grains with elevated energy level. Procedures for thermal treatment are proposed, which enable one to reduce the susceptibility of improved steels to intergranular fracture.

Kinetics of carbide reactions in Cr - Mo - V steel

The method of phase analysis is used to investigate the processes of carbide formation in quenched Cr - Mo - V steel in the 550 – 750°C range for aging lasting up to 3000 h. Temperature-time carbide diagrams are constructed. The carbide reactions are analyzed from the standpoint of the formal theory of the kinetics of transformations.

Effect of carbon, chromium, and nickel on carbide transformation in Cr-Ni-Mo-V-steels during tempering

The regression equations suggested make it possible to determine chromium and iron concentration and also their ratio in carbide precipitate in relation to content of carbon, chromium, and nickel in the steel taking account of tempering duration. On the basis of these equations the process of carbide formation has been analyzed in Cr-Ni-Mo-V-steels. Nomograms have been plotted making it possible to determine the amount and type of carbide phase in Cr-Ni-Mo-V-steel structures in relation to carbon and chromium concentration. Sections have been plotted for quaternary diagrams making it possible to determine the change in type of carbide phase in Cr-Ni-Mo-V-steels in relation to content in the steel of carbon, chromium, and nickel, and also tempering duration at 640°C for a time of 1–100 h. Quaternary diagrams make it possible to expand markedly ideas about processes of carbide formation in steels taking account of varying chemical composition and the time factor. This gives the possibility of approaching reality without simplified recommendations with respect to alloying and heat treatment. Currently work is being carried out for calculating regression equations which will make it possible to connect the amount and type of carbide phase, chromium concentration in the matrix with melt content of carbon, chromium, nickel, and vanadium in a steel, and also with the temperature and duration of tempering, i.e. we are talking about obtaining data for plotting sections of six-part diagrams. However, obtaining these equations is connected with considerable difficulties since in practice there are no data in the literature for the chemical composition and type of carbide phases in Cr-Ni-Mo-V-steels.

タングステン-コバルト混合粉における炭化タングステンの生成過程

The carbide formation process in tungsten-cobalt mixed powder was investigated by carburizing of tungsten plates. The carbides are formed uniformly at the whole surface of pure tungsten, where W2C and WC are formed succesively and the rate-determining stage is the diffusion of carbon in WC.While in tungsten covered with cobalt, carbides are formed preferentially at the surface of tungsten being in contact with cobalt. Coexisting cobalt with tungsten M6C(η-W3Co3C etc.) and WC are formed by the reaction of W2C with Co after the formation of W2C, and then WC is formed by the reaction of M6C with C. Cobalt is observed in the M6C layer and at grain boundaries of the WC layer, and it promotes the diffusion of carbon into the inner part of the carbide layer. The rate of carbide forming from tungsten-cobalt mixture deviates from the parabolic law, since the amount of cobalt in the carbide layer increases with the carburizing time and temperature. The rate-determining process of carbide formation in the tungsten-cobalt mixture is the diffusion of carbon in the carbide layer containing cobalt.

Effect of carbide formation processes during tempering on decomposition of martensite and the ductility of steels

1. With increasing tempering temperature the precipitation of carbide from temper martensite is due, according to the variation of the resistivity with tempering temperature, to decomposition of intermediate carbides e(Fe2C) and χ(Fe5C2) and the change in martensite to a condition in metastable equilibrium with the respective carbide phases. 2. It was found that the anomalous change in the mechanical properties is correlated with the decomposition of intermediate carbides and the decomposition of martensite, accompanied by carbide transformations.

Effect of chromium on the rate of carbide formation processes during tempering

1. An increase in the chromium content of steel slows down carbide formation processes. 2. If the steel contains undissolved particles of chromium carbide after quenching then special carbides are precipitated more rapidly with higher chromium concentrations in the steel.

Sintered 13% Cr steel for fast reactor fuel element cans

A study was made of the effects of various metallurgical factors on the structure and strength and creep characteristics of sintered 13% Cr ferritic steel. The beneficial effect of alloying with molybdenum on these properties is noted and discussed, and the influence exerted by various plastic working conditions is examined. Data yielded by determination of various properties of sintered ferritic steels show that the changes observed are attributable to differences in internal oxidation, diffusion, and carbide formation processes, which play an important part in the evolution and variation of properties during the production of the steels and prolonged testing under load. While internal oxidation and diffusion are of importance from the viewpoint of stability of properties in the initial condition, carbide formation is responsible for embrittlement during creep.