A diffusion model for tungsten powder carburization

A mathematical model describes the carburization kinetics of tungsten powders mixed with carbon and heated in hydrogen. It is based on diffusion of carbon through a shell of WC growing into particles which are modeled as spheres. The activation energy is 58 kcal/mole in the temperature range 1056 to 1833 °C. Hydrogen gas is important to transport carbon as methane or acetylene, but increased hydrogen pressure increases the rate of carburization so little that an adsorbed species such as CH probably controls the carbon concentration at the particle surface.     

含钒铁水增碳与增碳剂的选择

通过分析含钒铁水增碳的热力学,证明了含钒铁水增碳的可行性。然后在1 500℃的增碳温度下,将5种增碳剂加入到含钒铁液,进行增碳试验。由试验结果可知,5种增碳剂的增碳效果优先顺序为:低氮增碳剂、石墨、无烟煤、碳粒、焦炭,增碳反应的平衡常数K的数量级为10~(-4)s~(-1)。分析认为,增碳效果的不同主要与增碳剂中硫含量和灰分含量的差别有关。综合分析表明无烟煤是含钒铁水增碳的最佳增碳剂,在实际生产中,吨铁加入13.2 kg无烟煤时,增碳量为0.5%。     

A diffusion model for tungsten powder carburization

Metallurgical and Materials Transactions A - A mathematical model describes the carburization kinetics of tungsten powders mixed with carbon and heated in hydrogen. It is based on diffusion of...     

Diffusion modeling of the carburization process

Mathematical models have been developed for simulating the carburization process. One model simulates carburization in low alloy steels where temperature, time, surface carbon content, and diffusion coefficient vary during the process. Two step and vacuum carburization are among the treatments considered. The other model simulates the ef-fect of major ternary alloying additions such as Mn, Cr, Ni and Si during carburization. The importance of the off diagonal or cross diffusion coefficientD ₁₂ ³ on carbon diffusion is calculated. The Crank-Nicolson finite difference equations are used to provide numeri-cal stability and flexability. Calculated carbon profiles for low alloy steels were com-pared with experimental data available in the literature. Agreement between calculated and measured data was very good. Chromium and silicon have large cross coefficient ef-fects and it is predicted that they have a large influence on the amount of carburization which will occur. Experimental data for carburization treatments of Fe-C-Cr alloys are in excellent agreement with model predictions of major increases in effective surface carbon content and the formation of carbides in austenite at the carburization tempera-ture. These computer models are relatively easy to apply and can be used to design car-burization treatments for specific alloy steels. Formerly Graduate Assistant, Metallurgy and Materials Engineering Department, Lehigh University. This paper is based on a presentation made at a symposium on “Carburizing and Nitriding: Fundamentals, Processes and Properties” held at the Cincinnati Meeting of the Metallurgical Society of AIME, November 11 and 12, 1975 under the sponsorship of the Heat Treatment Committee.     

Numerical Simulations of Carbon and Nitrogen Composition-Depth Profiles in Nitrocarburized Austenitic Stainless Steels

Unusual composition-depth profiles have been observed after low-temperature nitrocarburization of austenitic stainless steels. When nitridation is performed after carburization, the carbon concentration in the nitrogen diffusion zone is reduced from ≈10 to ≈2 at. pct. Conversely, the carbon concentration in advance of the nitrogen diffusion zone is as high as 10 at. pct. This has been called a “push” effect of nitrogen on carbon, but this concept is non-physical. The profiles can be better understood from conventional thermodynamic principles, recognizing that (1) diffusion always occurs in response to gradients in chemical potentials and (2) the diffusivity of interstitial solutes in austenite is strongly concentration dependent, increasing dramatically with higher solute concentrations. Parameters from the CALPHAD literature quantitatively indicate that interstitial nitrogen and carbon in austenitic stainless steel mutually increase their chemical potentials. Based on these data, we have conducted numerical simulations of composition-depth profiles that correctly account for the chemical potential gradients and the concentration dependence of the diffusion coefficients for nitrogen and carbon. The simulations predict the “push” effect observed on nitridation after carburization, as well as the corresponding composition-depth profiles for other scenarios, e.g., carburization followed by nitridation or simultaneous nitridation and carburization (nitrocarburization).     

不同增碳剂对含钒铁水增碳的理论分析和试验

 用热力学方法计算并分析了铁水增碳过程中碳溶解反应自由能变化以及碳氧化反应自由能变化的关系,认为碳的氧化是阻碍铁液增碳的不利因素,增碳反应必须在氧气较少的气氛中进行。在此基础之上,进行增碳试验。首先利用过量石墨增碳,分别得到含钒铁水在1 450、1 500、1 550 ℃时的碳饱和溶解度,并拟合得到碳溶解度与温度的关系式,即[w(Cs)＝2.7×10－3T＋0.38。]然后将不同的增碳剂加入到由铁屑熔化而得到的含钒铁液中,并保温一定时间进行增碳试验。由试验结果可知,5种增碳剂的增碳效果优先顺序为：低氮增碳剂、石墨、无烟煤、碳粒、焦炭。由动力学计算可知,在本次试验条件下增碳反应速率常数的数量级为10－4 s－1。通过分析认为,提高温度可以促进增碳反应的进行,而增碳剂中的硫和灰分抑制增碳反应的进行。     

Carburization of crude steel from a superpowerful arc furnace

Optimal carburization principles are formulated for the production of pipe steel when using a superpowerful arc furnace. Laboratory and industrial research confirms the significance of combining reduction and carburization, the quality of the material used for carburization, and the time at which the carbon content is corrected. The use of silicon carbide for combined carburization and reduction of the crude steel proves effective.     

抗氧化添加剂对预氧丝原位碳化及强度的影响

研究了不同抗氧化添加剂Ｂ４Ｃ粉、ＳｉＣ粉和铝粉对预氧丝在氧化铝／碳复合材料中原位碳化的影响及其碳化后纤维对材料的强化效果。结果表明,在氧化铝／碳复合材料中添加Ｂ４Ｃ后,由于高温下在材料表面形成了一薄层致密的玻璃相物质,抗氧化效果明显,预氧丝原位碳化后达到同类碳纤维的强度,复合材料强度也大幅度提高。其次,添加少量铝粉也能起到好较好的保护作用。     

直接还原碳化法制备超细碳化钨粉的工艺研究

采用三氧化钨直接还原碳化法制备超细碳化钨粉,研究了碳化温度,碳化时间及配碳量等工艺参数对合成产物的物相和颗粒形貌的影响,并对反应机理进行了探讨。结果表明,在氮气气氛下,采用直接还原碳化法制备超细碳化钨粉的最佳工艺参数为:碳化温度1200℃,碳化时间3h,配碳量0.21。     

胶状碳包覆前驱体工艺制备超细、纳米碳化钨粉的研究

介绍了一种以高团聚态纳米W为原料,采用胶状碳包覆前驱体工艺,低成本、高效率制备高稳定性、低氧、均质、粒状超细和纳米WC粉末的新方法,并对碳化工艺条件进行了探讨。结果表明,氢气氛可以加速C-W反应过程;WC粉对原料W粉的粒度分布特征具有继承性;采用比表面积平均粒度为46.1nm的W粉为原料,当碳化温度由1 000°C增加至1 300°C时,WC的BET比表面积平均粒度由68.6 nm增加至339.4nm,氧含量由0.44%降低至0.10%。     

Reduction-Carburization of NiO-WO<Subscript>3</Subscript> Under Isothermal Conditions Using H<Subscript>2</Subscript>-CH<Subscript>4</Subscript> Gas Mixture

Ni-W-C ternary carbides were synthesized by simultaneous reduction–carburization of NiO-WO₃ oxide precursors using H₂-CH₄ gas mixtures in the temperature range of 973 to 1273 K. The kinetics of the gas–solid reaction were followed closely by monitoring the mass changes using the thermogravimetric method (TGA). As a thin bed of the precursors were used, each particle was in direct contact with the gas mixture. The results showed that the hydrogen reduction of the oxide mixture was complete before the carburization took place. The nascent particles of the metals formed by reduction could react with the gas mixture with well-defined carbon potential to form a uniform product of Ni-W-C. Consequently, the reaction rate could be conceived as being controlled by the chemical reaction. From the reaction rate, Arrhenius activation energies for reduction and carburization were evaluated. Characterization of the carbides produced was carried out using X-ray diffraction and a scanning electron microscope (SEM) combined with electron dispersion spectroscopy (SEM-EDS) analyses. The grain sizes also were determined. The process parameters, such as the temperature of the reduction–carburization reaction and the composition of the gas mixture, had a strong impact on the carbide composition as well as on the grain size. The results are discussed in light of the reduction kinetics of the oxides and the thermodynamic constraints.     

LZ50车轴钢边部增碳低倍缺陷的检验和分析

通过对LZ50车轴钢边部增碳缺陷进行金相检验和扫描电镜检验,发现增碳处组织异常,全部为珠光体,伴随大量MnS和硅酸盐夹杂物。现场排查发现在保温帽和铸模之间有较大的缝隙,缝隙中残留有大量粉渣,而且使用的保护渣为高碳型保护渣。由此判断,浇铸过程中钢水与缝隙中的粉渣接触造成钢锭帽口线增碳,而且钢水中未被保护渣完全吸收的夹杂物也被捕捉残留在钢坯表面。     

The kinetics of carburization of γ-iron in CO-He and CO-H2 atmospheres at 920°C

Investigation of the carburization of thin iron foils in CO-He or CO-H₂ gas mixtures at 920°C in a flow apparatus by a resistance method. Discussion of the kinetics, mechanism of the chemical surface reaction steps in the carburization. Influence of the carbon diffusion in the solid iron on the kinetics of the carburization of thick samples.     

Reduction and Carburization of Iron Oxide by Carbonaceous Materials

The iron nugget making process, where mixtures of iron ore powder and pulverized coal are made to heat rapidly, is regarded as a new ironmaking process. Since the iron carburization reaction is an especially important step from the viewpoint of energy saving in this process, the purpose of this study is to reveal the carburization mechanism of reduced iron formed during the heating process of the mixtures. Fe₂O₃ and H₂‐CO‐CO₂ gas mixture were made react with each other to simulate the formation of iron ore and the generation of coal gas from coal during the heating process. An electric resistance furnace was used to heat the samples. A cut surface of the quenched iron product was observed by microscopy. Carbon content in the iron was analyzed after reduction and carburization of Fe₂O₃. It is considered that there are many possibilities in the real process to carburize the reduced iron during the heating process, such as carburization with CO formed by coal combustion, solution loss reaction, and direct carburization with coal. It was observed in the present work that the carbon content in the reduced iron became the highest as Fe₃C was formed during the heating process. Therefore, it became clear that the key technology for production of pig iron containing high carbon was to control the formation of Fe₃C in the heating period of the mixture. The carburization mechanism was discussed in this study based on these experimental results.     

Activity of carbon in nickel-rich Ni-Mo and Ni-W alloys

Effects of molybdenum and tungsten on carbon activity in nickel have been experimentally determined at 1000, 1100, and 1200 °C. Seventeen nickel-molybdenum and thirteen nickel-tungsten binary alloys were carburized in a flow of purified methane and hydrogen mixed gas. A sealed capsule technique was also employed for carburization of a few series of nickel-molybdenum alloys. The carbon concentration was determined either by hot extraction techniques (LECO and Coulomatic) or by weight gains of these specimens. The carbon concentration at a constant carbon activity decreases with increasing either molybdenum or tungsten concentration in nickel. The effect of tungsten on the carbon solubility in nickel is slightly larger than that of molybdenum. The experimental data were analyzed in terms of the regular solution model with two sublattices due to Hillert and Staffansson. Temperature dependence of the interaction coefficients between carbon and molybdenum or tungsten was expressed as DG_Mo/RT = −4.45 + 11650/T andDG _W /R = 1.21 + 9010/. Formerly Graduate Student, Department of Metallurgical Engineering, Tokyo Institute of Technology Formerly Professor, Department of Metallurgical Engineering, Tokyo Institute of Technology     

超低碳钢连铸过程中增碳机理的探究

应用质量和动量传输理论,对连铸结晶器内含碳熔渣所引起的超低碳钢钢水增碳的机理进行了研究。研究发现,尽管用于超低碳钢的结晶器保护渣的原始碳含量已经很低,而且碳在熔渣中的溶解度也不高,但熔渣中这些碳对钢水的增碳却是明显的。     

Effect of sulfur compounds on the decomposition of carbon monoxide on red mud

The utilizing of red mud via the carburization of its ferrous components by CO-containing gases of ferroalloy furnaces is investigated. As a result of our experiments, it is established that the red mud of alumina production and exhaust gases of ferroalloy furnaces are quite suitable for performing the decomposition reaction of carbon monoxide after the required preparation. It is found that the presence of hydrogen sulfide has a toxic effect on the catalytic abilities of iron oxides as catalysts of the thermal catalytic decomposition of CO. The mechanism behind the effect of sodium aluminum silicate hydrates entering the red mud on the degree to which sulfur compounds are removed from the ferroalloy gas is revealed. It is established that purification of the ferroalloy gas allows us to increase the carbon content in the final product by a factor of 5?C10.     

Control of surface carburization and improvement of dynamic fracture behavior in tungsten heavy alloys

A carburization technique using a Cr powder layer has been developed to control the diffusion depth of carbon in W-Ni-Fe heavy alloys. The aged heavy alloy samples were covered with a Cr powder layer of about 1-mm thickness and then packed with carbon black powder. The packed samples were heat-treated at 1150 °C for 10 minutes in H₂ and then for 50 minutes in N₂. The carburization treatment resulted in the formation of Cr₇C₃ and Fe₃W₃C around the tungsten grains from the sample surface with a thickness of 40 to 50 μm. This carburized layer was much thinner than that formed without a Cr powder layer under the same experimental conditions. With the surface carburization, the surface hardness increased by ∼75 pct, from 508 to 888 VHN, and the impact energy decreased by ∼31 pct, from 123 to 85 J. After the carburization treatment, the main fracture behavior in a dynamic torsional test changed from smearing of the matrix to cleavage of the tungsten grains. A high-speed impact test showed that the surface carburization of penetrators induced the formation of many cracks around the penetrator surface, enhanced the self-sharpening, and improved the penetration performance. It appears that the developed technique provides an easy method of carburization without serious deterioration of the toughness of the material.     

The kinetics of gas-liquid metal reactions involving levitated drops. Carburization and decarburization of molten iron in Co-Co2 gas mixtures at high pressures

The kinetics of decarburization and carburization of levitated molten iron-carbon alloy drops at 1650° in CO-CO2 gas mixtures were studied at a pressure of 40 atm. The results showed that, under the experimental conditions used, the rates were controlled by transport in the gas phase (decarburization) or by mixed transport control (carburization). The levitated drops behaved as static spherical particles as far as transport in the liquid phase was concerned since the effective diffusivity of carbon in the liquid was close to the atomic diffusivity. The transport model developed in the paper was applied to explain the data from some earlier investigations.     

Surface Hardening of Powder Injection Molded 316L Stainless Steels Through Low-Temperature Carburization

The surface hardness of powder injection molded (PIM) 316L is generally low due to the inherited austenitic structure and large grains induced during high-temperature sintering. To increase the surface hardness and the wear resistance, low-temperature carburization (LTC) was applied to PIM 316L. With carburization at 773 K (500 °C) for 24 hours, the resulting hardness at the surface increases from 160 to 810 HV due to the “colossal” supersaturation of interstitial carbon and M₅C₂ carbide, and the corrosion resistance is not deteriorated. The carburized depth is about 40 μm, and the carbon content in this layer is about 3.25 wt pct or 13.5 at. pct, which causes lattice expansion of the austenite. When the carburization temperature increases to 823 K (550 °C), or the carburization time increases to 72 hours, chromium carbides are observed and the corrosion resistance is impaired. One distinct advantage of applying LTC to PIM 316L is that no acid cleaning process is required, unlike for wrought counterparts, because of the clean surface of the sintered materials.