Thermodynamics of inclusion formation in Fe-Cr-Ti-N alloys

The thermodynamics of titanium in Fe-Cr alloys and of inclusion formation in Fe-Cr-N-Ti alloys was investigated. A metal-nitride-gas equilibration technique was used to measure the activity of titanium. The equilibrium titanium content of the metal that is in equilibrium with pure solid titanium nitride and nitrogen gas at 1 atm was determined. The activity coefficients of titanium it(f_Ti) relative to 1 wt pct standard state in Fe were calculated for Fe-Cr alloys from the experimental results. The first-order interaction coefficient between titanium and chromium, e _Ti ^Cr , was determined to be 0.024 at 1873 K. The solubility of nitrogen in Fe-Cr alloys was measured and was found to increase with chromium content, which is in agreement with previous work. Thermodynamic calculations were made in order to predict under what conditions titanium nitride will form in 409 stainless steel and was compared with inclusions found in plant samples. The inclusion stability diagrams for 304 stainless steel and Fe-18 pct Cr and Fe-9 pct Cr alloys were computed.     

Strength-toughness relations for precipitation-strengthened low-alloy steels containing columbium

An investigation conducted to determine the value of the precipitation-hardening vectors (Δ transition temperature/Δ yield strength) for columbium (niobium) carbide and for columbium nitride in low-alloy steel showed that the values of these vectors were not significantly different. Precipitation hardening by columbium carbide and by columbium nitride both resulted in an average increase of 6.8 ± 1.01* in transition temperature for each ksi increase in yield strength. Columbium carbide and nitride are completely isomorphous in commercial steels and any columbium present usually exists as a carbonitride. It has been shown that the change in matrix nitrogen accompanying formation of the carbonitride results in a decrease in the apparent vector for precipitation hardening. Knowledge of the values of the precipitation-hardening vectors for columbium carbide, nitride, and carbonitride in low-alloy steels will facilitate future interpretation of mechanical-property data by the vector-sum method. J. M. GRAY, formerly with U. S. Steel Research, Monroeville, Pa.     

Thermodynamics of TiO x in blast furnace-type slags

Equilibrium studies between CaO-SiO₂-10 pct MgO-Al₂O₃-TiO_1.5-TiO₂ slags, carbon-saturated iron, and a carbon monoxide atmosphere were performed at 1773 K to determine the activities of TiO_1.5 and TiO₂ in the slag. These thermodynamic parameters are required to predict the formation of titanium carbonitride in the blast furnace. In order to calculate the activity of titanium oxide, the activity coefficient of titanium in carbon-saturated iron-carbon-titanium alloys was determined by measuring the solubility of titanium in carbon-saturated iron in equilibrium with titanium carbide. The solubility and the activity coefficient of titanium obtained were 1.3 pct and 0.023 relative to 1 wt pct titanium in liquid iron or 0.0013 relative to pure solid titanium at 1773 K, respectively. Over the concentration range studied, the effect of the TiO _x content on its activity coefficient is small. In the slag system studied containing 35 to 50 pct CaO, 25 to 45 pct SiO₂, 7 to 22 pct Al₂O₃, and 10 pct MgO, the activity coefficients of TiO_1.5 and TiO₂ relative to pure solid standard states range from 2.3 to 8.8 and from 0.1 to 0.3, respectively. Using thermodynamic data obtained, the prediction of the formation of titanium carbonitride was made. Assuming hypothetical ‘TiO₂,’ i.e., total titanium in the slag expressed as TiO₂, and using the values of the activity coefficients of TiO_1.5 and TiO₂ determined, the equilibrium distribution of titanium between blast furnace-type slags and carbon-saturated iron was computed. The value of [pct Ti]/(pct ‘TiO₂’) ranges from 0.1 to 0.2.     

Synthesis of complex carbonitrides Ti1−x WxC1−y Ny

We have studied the effect of process factors (temperature, nitrogen pressure, composition) on synthesis of carbonitrides Ti_1?xW_xC_1?yN_y from the starting components (WC, TiC, TiN) with particle size 10–12 μm. Under realistic conditions, we have determined the range of existence for homogeneous cubic carbonitrides. As the tungsten carbide content increases, the temperature needed for synthesis of homogeneous carbonitrides increases. Synthesis of complex carbonitrides occurs in two steps: via formation of titanium carbonitride followed by dissolution of WC in the latter, and by formation of the complex titanium-tungsten carbonitride. We have established that the major reason for the appearance of the W₂C phase is thermal dissociation of titanium nitride.     

A Model to Simulate Titanium Behavior in the Iron Blast Furnace Hearth

The erosion of hearth refractory is a major limitation to the campaign life of a blast furnace. Titanium from titania addition in the burden or tuyere injection can react with carbon and nitrogen in molten pig iron to form titanium carbonitride, giving the so-called titanium-rich scaffold or buildup on the hearth surface, to protect the hearth from subsequent erosion. In the current article, a mathematical model based on computational fluid dynamics is proposed to simulate the behavior of solid particles in the liquid iron. The model considers the fluid/solid particle flow through a packed bed, conjugated heat transfer, species transport, and thermodynamic of key chemical reactions. A region of high solid concentration is predicted at the hearth bottom surface. Regions of solid formation and dissolution can be identified, which depend on the local temperature and chemical equilibrium. The sensitivity to the key model parameters for the solid phase is analyzed. The model provides an insight into the fundamental mechanism of solid particle formation, and it may form a basic model for subsequent development to study the formation of titanium scaffold in the blast furnace hearth.     

Formation of TiN/TiC-Fe composites from ilmenite (FeTiO3) concentrate

The production of a ceramic hard material-metal composite directly from a mineral concentrate has great potential application. An homogenizing pretreatment of a mixture of ilmenite (FeTiO₃) and graphite, followed by annealing under an argon ambient, showed the formation of titanium carbide and elemental iron. Annealing of the same powder in nitrogen resulted in the formation of a composite of elemental iron and titanium nitride. The nitride was formed at a lower temperature than the carbide with almost complete conversion after 1 hour at 1000 °C. The rate of carbide formation was controlled by carbon diffusion, whereas the nitridation reaction was controlled by either oxygen or nitrogen diffusion. The TiC was found to form via TiC_0.5, which slowly increased its carbon content until near stoichiometric TiC was formed; stoichiometric TiN formed directly with no intermediate phases. Titanium carbide showed the presence of a second phase with a slightly smaller unit cell size; this was due to interdiffusion between the iron and TiC. The titanium carbide composite was found to be composed of 3 to 4 μm anhedral iron grains dispersed in the titanium-rich matrix. There was no segregation in the iron/titanium nitride composite with apparently submicron distribution.     

控轧控冷工艺条件下Nb—V钢碳氮化物的析出行为

本文通过热模拟实验和电子显微技术等方法,系统地研究了控轧控冷对铌钒钛复合微合金化低碳热轧钢板的铌、钒、钛碳氧化物的析出行为的影响,研究结果对开发高强度船体用钢板具有参考价值。通过研究表明,在奥氏体区和铁素体区都用Ni(C,N）析出,对Nb、V的析出起了诱导作用,并与Nb、V形成复杂的碳氮化物。在铁素体中主要以基体均匀沉淀析出和位错沉淀析出。     

Application of thermodynamic computations to the solution behaviour of niobium and vanadium carbonitrides

On the basis of well-known thermodynamic equations, a model is proposed which allows the computation of both the solution behaviour and the composition of the carbonitride precipitates of the microalloying elements in steel. Apart from the concentration of the microalloying elements and the carbon and nitrogen content, the following aspects were taken into account: the non-stoichiometric composition of carbonitrides, a regular solution behaviour of the mixture of carbide with nitride, the effect of third elements on the activity of carbon and nitrogen, the possible primary binding of nitrogen by aluminium. The calculation is based on the knowledge of the solubility products of pure carbides and nitrides. Comparison with experimental results of carbonitride precipitates in microalloyed niobium and vanadium steels support the model-based predictions concerning the temperature-dependent composition and solubility of carbonitrides under equilibrium conditions.     

The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron

A systematic study of carbide and nitride additions on the heterogeneous nucleation behavior of supercooled liquid iron was undertaken. It was found that titanium nitride and titanium carbide were very effective in promoting heterogeneous nucleation. These compounds were followed by silicon carbide, zirconium nitride, zirconium carbide, and tungsten carbide in decreasing order of effectiveness. The degree of potency of the nucleation catalysts is explained on the basis of the disregistry between the lattice parameters of the substrate and the nucleating phase. Through the inclusion of planar terms the Turnbull-Vonnegut “linear” disregistry equation was modified to more accurately describe the crystallographic relationship at the interface during heterogeneous nucleation.     

Composite Ceramic Materials and Coatings for Tribological Use

The tribological properties over a broad range of sliding velocities and loads under dry friction in air of promising composite materials and coatings based on titanium carbide and carbonitride were studied. The dominating effect of secondary structures arising in the process of tribooxidation was investigated. Isomorphous oxide phases which form solid solutions and are strongly adherent to the surface of the material or coating promote the development of a continuous and dense screening film.     

微合金钢中钛析出相的氯化钾-柠檬酸体系提取及各相的分离测定

对含钛微合金钢的钛相进行分离和测定,建立了一种快速分离微合金钢中钛析出相的定量方法。通过优化试验选择75g/L氯化钾溶液-5g/L柠檬酸-去离子水体系代替传统的甲醇电解液提取微合金钢中钛析出相,于0℃电解可以克服低温电解速度慢的问题。用2.4mol/L HCl分解细粒碳化钛,1mol/L HNO₃分解粗粒碳化钛,硝酸和过氧化氢分解氮化钛,采用电感耦合等离子体原子发射光谱法(ICP-AES)测定各相中钛的含量。差减法得到固溶钛以及用碳硫分析仪测定残渣中硫含量换算得到硫化钛含量。细粒碳化钛占钛相的34.27%,粗粒碳化钛占钛相的22.38%,氮化钛占钛相的14.68%,硫化钛占钛相的6.29%,固溶钛占钛相的16.78%。方法的钛析出相回收率为94.41%。     

Ti在铁素体中的平衡固溶度

基于Thermo-Calc和相关文献测试数据所推导出的Fe2Ti在铁素体中的平衡固溶度公式,并结合Fe2Ti的形成自由能,推导出了钛在铁素体中的平衡固溶度公式。这对于钛的碳化物、氮化物及碳氮化物等第二相颗粒在铁基体中的固溶度积求算以及相关的固溶析出行为等方面的研究具有重要的作用。     

On the kinetics of carbide precipitation during reaction between graphite and Fe-Ti liquids under microgravity

Experiments on the reaction between graphite and liquid Fe-Ti alloys were performed with a mirror furnace on board an airplane during parabolic flights. Small Fe-Ti alloy samples were melted in contact with graphite and held for some seconds at a temperature of 1550 °C. The samples were melted and solidified during a microgravity period. Carbon and titanium atoms reacted in the melt and titanium carbides were formed. In the experiments, a precipitation zone with faceted titanium carbide crystals dispersed in high carbon Fe-C-Ti alloy matrix was obtained near the graphite/alloy interface. The thicknesses of the carbide precipitation zones were measured and effects of alloy composition on the growth rates of the carbide zones were revealed by experiments and calculations. It was shown that the process was controlled by the diffusion of titanium in the liquid at low titanium concentrations and by diffusion of carbon through the precipitation layer at high titanium concentrations in the melt. Supersaturation of the carbide in front of the reaction interface was predicted from the calculations. The analysis showed that homogeneous nucleation of titanium carbide can readily occur in the alloys. Carbide morphologies were analyzed, and the mechanisms which lead to their formation are discussed.     

On the precipitation of TiC in liquid iron by reactions between different phases

In order to study the in situ formation of carbide-reinforced metal matrix composites and the inoculation reactions of some low alloy steels, titanium carbide precipitation in liquid iron was investigated. TiC particles were formed directly in liquid iron solution through reactions between different phases. The high-temperature reactions studied in the present work are reactions between (a) solid graphite and liquid Fe-Ti alloys, (b) Fe-Ti and Fe-C liquid alloys, and (c) pure solid titanium and liquid Fe-C alloys. Small samples for these reactions have been processed using a mirror furnace facility in a precisely controlled manner. Samples of Fe-Ti alloys reacted with graphite were also processed by a high frequency induction furnace and other facilities. The precipitation behavior of titanium carbide in various reactions and the distribution of carbide particles in iron melts were investigated. Different growth morphologies of the carbide were observed under various conditions. The effects of convection on the reactions were observed and discussed, and by processing the samples with various orientations relative to gravity, various patterns of carbide distribution were obtained.     

碱度对中钛型高炉渣矿相结构的影响

采用偏光显微镜对不同碱度条件下中钛型高炉渣的矿相结构进行了研究。结果表明,炉渣显微结构为斑状、似斑状结构;碱度为0.92时,矿物组成主要为玻璃质、黄长石、安诺石,少量的碳化钛、氮化钛及其固溶体;碱度为1.02,1.12,1.32时,出现了钛辉石、巴依石及钙钛矿,未见安诺石。随碱度的升高,碳化钛、氮化钛及其固溶体先降低后升高,钙钛矿、含钛辉石(巴依石及钛辉石)含量升高;黏度先降低后升高,承钢高炉渣碱度易控制在1.02左右。     

Structure Formation in Alloys of the Cr3C2–TiN System and the Properties of Materials Based on Them

The addition of 10-20% TiN to chromium carbide made it possible to obtain almost pore-free Cr₃C₂–TiN alloy specimens by hot pressing in the range from 1500 to 1850°C at a pressure of 35.7 MPa for 25 min. The 90% Cr₃C₂–10% TiN alloy had the maximum hardness (89 HRA) while the alloy containing 15% TiN had the maximum bending strength (480 MPa). Activation of solid-phase sintering is shown to cause a Cr₃C₂ solid solution to form in the titanium nitride. In alloys containing 50% or more TiN sintering is activated by the formation of a liquid phase based on the Ti–Cr–C–N eutectic.     

Thermodynamic calculation and experimental verification of the carbonitride-austenite equilibrium in Ti-Nb microalloyed steels

The sublattice-regular solution model has been adapted to describe the thermodynamics of complex carbonitrides. This model has been applied to titanium- and niobium-bearing microalloyed steels for calculation of the mole fraction and composition of the carbonitride precipitates and the residual solute levels in the austenite. Both experimental results and calculations show that titanium nitride predominantly forms at very high temperatures and titanium-niobium carbides go to completion at low temperatures. Quantitative agreement between the experimental measurements and the predictions for carbonitride compositions as a function of temperature is demonstrated. Formerly Research Associate, with the Institute for Materials Research, McMaster University Research Scientist     

TiO_2对高炉渣矿相结构及冶金性能的影响

采用偏光显微镜对不同TiO_2含量(7%~16%)的高炉渣矿相结构进行系统研究。结果表明,炉渣显微结构为斑状结构、似斑状结构;斑晶矿物主要为巴依石和黄长石,基质为玻璃质、细小钛辉石、钙钛矿以及少量的金属Fe、TiC、TiN及其固溶体;随着TiO_2配加量的增大,炉渣中巴依石和钛辉石含量先降低后升高,而黄长石含量先升高后降低;当TiO_2含量超过12%以后,炉渣基质中TiC、TiN及其固溶体的含量有所增加,这些高熔点化合物会使炉渣的黏度和熔化性温度升高,导致高炉渣的流动性变差。该研究成果可为改善含钛高炉渣的流动性能提供重要的理论依据。     

Formation of the phase composition of tungstenless hard alloy powder with mechanochemical synthesis

Carbon and nickel are studied for their effect on the formation of phase composition of tungsten-free hard alloy powder prepared by mechanochemical synthesis. It is established that the phase formation of this powder is governed by the initial titanium carbide stoichiometry. If titanium carbide has a nonstoichiometric composition, the alloy structure corresponds to the three-phase equilibrium TiC_1–x-Ni₃Ti-Ni, and otherwise it corresponds to the equilibrium TiC-Ni.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 21–24, November, 1992.