Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part III. Control of furnace atmosphere composition with a zirconia oxygen sensor

Gas carburizing experiments were conducted in a batch-type sealed-quench furnace using furnace atmospheres produced by the reaction of propane and air within the furnace. The air-propane ratio of the inlet gases was automatically controlled to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. The results of carburizing trials at 843 and 927 °C are described. The effect of inlet gas flow rate on furnace atmosphere composition and the amount of carburizing is illustrated.     

Automatic control of the carbon potential of furnace atmospheres without adding enriched gas

An approach has been proposed in this article for controlling the carbon potential of furnace atmosphere without the need to add enriched gas for the sake of simplifying the process of carbon potential control and consequently lowering the cost of atmosphere. The atmosphere was generated by reacting an N₂ + H₂O mixture with hot charcoal. An oxygen sensor and a computer were used for controlling the carbon potential of the atmosphere through controlling the temperature of charcoal instead of adding an enriched gas to the atmosphere. The control result was evaluated from (a) the stability of oxygen partial pressure of the furnace atmosphere and (b) the equilibrium carbon content of the steel heated in the furnace atmosphere. The result of controlling carbon potential with this method was indicated by observed experiments to be satisfactory;i.e., the deviation of carbon potential was below 0.06 pct. There was especially no need to add enriched gases to the atmosphere.     

Analytical models for the gas carburizing process

Mathematical models for carburizing in batch and continuous furnaces are described. Both the steady-state model for continuous furnaces and the time-dependent model for batch furnaces are based on material balances, with thermodynamic equilibrium of the constituents of the furnace atmosphere assumed. The instantaneous rate of carburizing is taken to be proportional to the difference between the carbon potential of the furnace atmosphere and the surface carbon content of the work load. Computer programs incorporating these models were written which predict furnace operating characteristics for any assumed process. The continuous furnace model predicts the pattern of internal gas flow within the furnace and computes the natural gas (or air) additions to each zone needed to achieve the desired carbon potentials and satisfy the carbon demand. The batch furnace model describes how the furnace atmosphere changes in composition during carburizing as a result of the interaction of the instantaneous carbon demand and the rate of supply of carburizing gases to the furnace. Examples of the use of these programs are given, and the limitations of the predictions are discussed.     

Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part II. Analysis of the characteristics of gas flow in a batch-type sealed quench furnace

Gas flow dynamics in a batch-type sealed quench carburizing furnace were studied for operations utilizing low inlet gas flow rates. By analyzing the rate of change of furnace atmosphere composition when a sudden change is made in the inlet gas composition, it is shown that a significant amount of gas circulation occurs between the hot furnace chamber and the unheated vestibule. This circulation has the effect of increasing the mean residence time of gases within the furnace. A long mean residence time is advantageous for carburizing when the inlet gases consist of an airJhydrocarbon blend rather than prereacted endothermic gas.     

高温渗碳齿轮钢的研究进展

常用齿轮钢渗碳温度为930℃,提高渗碳温度至1000～1050℃能显著缩短渗碳时间,但易引起晶粒长人,因此发展了通过Nb、Ti、B微合金化,细化钢原奥氏体晶粒的高温渗碳齿轮钢。文中介绍了国内外高温渗碳齿轮钢的钢种成分、工艺特点、高温渗碳层组织控制和钢的疲劳性能的研究进展。     

bdGas carburizing of steel with furnace atmospheres formedin situ from methane and air and from butane and air

Carburizing experiments were conducted at 927°C (1700°F) and 843°C (1550°F) using furnace atmospheres formed from methane and air and from butane and air introduced directly into the carburizing furnace. Gas flow rates were low to promote equilibration of the reaction products within the furnace. The air flow rate was held constant while the methane or butane flow was automatically regulated to maintain a constant oxygen potential, as measured by a zirconia oxygen sensor, within the furnace. In comparing the results of these experiments with earlier results obtained using propane and air, several differences were noted: (a) The methane content of the furnace atmosphere, measured by infrared analysis, was about twice as great when methane was the feed gas rather than propane or butane. This was true despite the fact that the mean residence time of the gas within the furnace was greater in the methane experiments. Methane appears to be less effective than propane or butane in reducing the CO₂ and H₂O contents to the levels required for carburizing. (b) There was a greater tendency for the CO content of the furnace atmosphere to decrease at high carbon potentials when methane is used instead of propane or butane. The decrease in CO content is due to hydrogen dilution caused by sooting in the furnace vestibule. These differences in behavior make propane or butane better suited than methane forin situ generation of carburizing atmospheres. However, there is no difference in the amount of carburizing occurring at a specified carbon potential when methane, propane, or butane are used as the feed gas in this process. J.A.Pieprzak, formerly a member of the Engineering and Research Staff     

Microprocesses of metal dusting on iron

In strongly carburizing atmospheres iron and steels are subject to ‘metal dusting’, a catastrophic carburization, leading to disintegration into a dust of carbon and metal particles. TEM investigations of iron foils in different states of this corrosion process have clearly shown now that unstable cementite is an intermediate in the reaction. The carbide is formed at the iron surface after supersaturation (a_C > 1), its disintegration is started by graphite deposition on its surface (a_C = 1). The carbon atoms from Fe₃C disintegration are attached to the graphite planes which are vertically oriented to the cementite surface, the iron atoms diffuse through the graphite and agglomerate to small particles (～ 20 nm) which act as catalysts for further carbon deposition from the atmosphere, till they are densely covered with graphite. The TEM investigations are in good agreement with preceding studies concerning kinetics and mechanisms of metal dusting.     

Modeling Growth and Dissolution Kinetics of Grain-Boundary Cementite in Cyclic Carburizing

In vacuum carburizing of steels, short-time carburizing is usually followed by a diffusion period to eliminate the filmlike cementite (θ _GB ) grown on the austenite (γ) grain boundary surface. In order to obtain the θ _GB amount during the process, the conventional model estimates the amount of cementite (θ) with the equilibrium fractions for local C contents within a framework of the finite difference method (FDM), which overestimates the amount of θ _GB observed after several minutes of carburizing. In our newly developed model, a parabolic law is assumed for the growth of θ _GB and the rate controlling process is considered to be Si diffusion rejected from θ under the isoactivity condition. In contrast, the rate constant for the dissolution of θ _GB is considered to be controlled by Cr diffusion of θ. Both rate coefficients (α) were validated using multicomponent diffusion simulation for the moving velocity of the γ/θ interface. A one-dimensional (1-D) FDM program calculates an increment of θ _GB for all grid points by the updated diffusivities and local equilibrium using coupled CALPHAD software. Predictions of the carbon (C) profile and volume fraction of cementite represent the experimental analysis much better than the existing models, especially for both short-time carburization and the cyclic procedure of carburization and diffusion processes.     

Mathematical analysis of reactions in metal oxide/carbon mixtures

A mathematical study has been made of the rates of reactions in metal oxide/carbon mixtures. The rate equations derived take into account the rate control by carbon-CO₂ oxidation and by diffusive and viscous flow of the reaction products, CO₂ and CO, through the packed bed where a pressure gradient exists. As the thickness of the powder mixture increases, the pressure buildup during reduction increases. If the furnace atmosphere is a neutral gas, the rate of reduction of metal oxides by carbon decreases, because of the dilution of CO₂ in the interparticle pores of the bed by back diffusion of the furnace atmosphere.     

焦炭床内铁水渗碳行为

摘要：高炉冶炼过程中铁水最终渗碳量接近饱和,相比而言,闪速炉中没有固体炉料的压迫作用,无法发生像高炉炉缸内死料柱根部与铁水之间的渗碳反应,最终渗碳量难以预料。为此,以电解铁和化学纯石墨为原料,利用管式电阻炉升温到1855K熔化铁块,以高纯Ar作为保护气体研究焦炭床内铁水渗碳行为,并对铁水渗碳行为进行数值模拟,为闪速炼铁工业化打下基础。实验结果表明,终铁C含量随焦炭粒度增大和渗碳床高度降低而减小,且铁水初始C含量对终铁C含量具有较大影响。对于不同条件下铁水渗碳反应中C元素的迁移规律,基于VB编程技术进行了数值模拟,模拟结果与高温实验结果较吻合,相对误差在3%以内。渗流速度控制因子始终控制在05左右,即铁水在焦炭床中的平均速度为初始滴落速度的一半左右。     

The Gas Carburization of Linear Cellular Alloys as a Novel Alloy Development Tool

Investigations of the production of thin-walled steel alloys through the gas carburization of structures made from reduced and sintered metal oxide powders were performed. Extrusions with low-alloy steel composition were produced successfully without the occurrence of metal dusting, yielding a novel technique for the production of thin-walled steel structures. Thin strip geometries (~200 to 300 μm final thickness) of samples with the composition of 4140 steel, without carbon, were produced through the extrusion of a paste of metal-oxide powders. Full reduction and sintering in a 10 pct H₂/90 pct Ar atmosphere yielded a metal part containing all necessary alloying elements except carbon. Gas carburization in a controlled CO/CO₂ atmosphere was then used to introduce carbon through the thickness of the structure while carburization parameters were controlled such that metal dusting was not observed. It has been shown in this study, through heat treatment and microstructural investigations, that structures with 4140 composition displaying microstructures and mechanical properties comparable with conventionally made steels can be reached in approximately 30 minutes during gas carburization. The research shows that carbon contents above the eutectoid composition can be reached in less than 30 minutes. As a result, a novel alloy development tool has been introduced.     

Effects of rapid heating on solutionizing characteristics of Al-Si-Mg alloys using a fluidized bed

Effects of rapid heat transfer using a fluidized bed on the heat-treating response of Al-Si-Mg alloys (both unmodified and Sr modified) were investigated. The heating rate in the fluidized bed is greater than in conventional air convective furnaces. Particle size analyses of eutectic Si showed that the high heating rate during fluidized bed solution heat treatment causes faster fragmentation and spherodization of Si particles compared to conventional air convective furnaces. The mechanism of Si fragmentation through fluidized bed processing is through both brittle fracture and neck formation and its propagation. In contrast to this, the mechanism of Si fragmentation using a conventional air convective furnace is through neck formation and propagation. The Sr-modified D357 alloy showed a faster spherodizing rate than the unmodified alloy. Thermal analyses showed an exothermic reaction during solution heat treatment using a fluidized bed due to recrystallization, and coarsening of eutectic Al grains. Whereas the alloy solutionized using a conventional air convective furnace showed two exothermic reactions, one due to annihilation of point defects and the other due to recrystallization, and coarsening of the eutectic grains in the aluminum matrix. The recrystallization temperature of the alloy solutionized in the fluidized bed is lower than those in the conventional air convective furnace. Both tensile strength and elongation of fluidized bed solutionized alloys are greater than those solutionized using the air convective furnace. The optimum heat-treatment time for T4 temper using a fluidized bed for unmodified and Sr-modified alloy was reduced to 60 and 30 minutes, respectively.     

Carburizing performance and kinetics of carbon sources in blast furnace hearth

It is of great significance to improve the carbon saturation of molten iron which weakens the erosion of sidewall carbon bricks, so as to realize the hearth longevity. Carburizing properties of various carbon sources in molten iron by static method was systematically investigated. The apparent reaction rate constant K of samples was calculated. The results show that the carburizing properties of different carbon sources from strong to weak are NMA carbon brick > carbon rod > pulverized coal > coke > 9RDN carbon brick. The carburizing performance of carbon sources decreases with the increase in its graphitization degree. Ash has a negative effect on the carburizing performance of carbon sources, but ash in the form of agglomerated large particles cannot weaken the carburizing performance of carbon sources. The Al2O3 phase can obviously degrade the carburizing performance of carbon sources. The 9RDN carbon bricks with low carburizing properties can be adapted to more complex furnace conditions. The carburizing properties of pulverized coal and coke are relatively weak. When the pulverized coal and coke powder formed in the tuyere raceway enter the hearth, the porosity of the deadman will be reduced, resulting in hearth inactivity and frequent fluctuation of furnace condition, and it is not conducive to the carburization of the deadman. Therefore, it is necessary to properly improve the coke particle size and increase the air volume for weakening the negative effects of unburned coal powder and coke powder on the longevity of the hearth sidewall carbon bricks.     

Carburization of iron by CO-based mixtures in nitrogen at 925 °C

The rates of carburization by mixtures of CO, CO₂, H₂ and H₂O in N₂ have been measured gravimetrically at 925 °. The rate equation which best describes the experimental data is based on a mechanism which involves a rapid surface dissociation of CO into carbon and oxygen atoms, and a subsequent rate determining step between this atomic oxygen and either CO or H₂. The CO-H₂ system carburizes much faster than CO alone, because H₂ combines faster with atomic oxygen than does CO. The carburizing rate constant for CO-H₂ is 44 times that for CO alone. The mechanism is confirmed by the additivity of the separate rates for the CO-H₂ mixtures and for CO alone.     

Effects of Magnetic Field Annealing on Carburizing in Pure Iron

Effects of magnetic field annealing on carburizing behavior in pure iron in the γ‐Fe temperature region were investigated. Specimens were subjected to isothermal annealing at 930 °C for 25 min with a heating rate of 5 °C /min, and then cooled in the furnace. A magnetic field with different intensity was applied during the whole heating, isothermal holding and cooling processes. The results showed that the magnetic field annealing obviously hindered the carburization in γ‐Fe in the direction parallel and anti‐parallel to the magnetic field direction and this effect increased with the enhancement of the magnetic field intensity.     

Studies on oxychlorination of MoS2 in a fluid bed reactor

Studies on the chlorination kinetics of molybdenum sulfide in the presence of a mixture of oxygen, chlorine, and nitrogen gases have been carried out in a fluidized bed reactor at temperatures of 250 to 350 ° and particle size range of 75 to 200 μm. The reaction rate with respect to surface area of MoS₂ particles as well as the composition of reactant gases has been determined and the specific rate constants evaluated. The oxychlorination of MoS₂ has been determined to be of first order with respect to surface area of particles and the overall reaction is found to be controlled by chemical reaction.     

Studies on oxychlorination of MoS2 in a fluid bed reactor

Studies on the chlorination kinetics of molybdenum sulfide in the presence of a mixture of oxygen, chlorine, and nitrogen gases have been carried out in a fluidized bed reactor at temperatures of 250 to 350 ° and particle size range of 75 to 200 μm. The reaction rate with respect to surface area of MoS₂ particles as well as the composition of reactant gases has been determined and the specific rate constants evaluated. The oxychlorination of MoS₂ has been determined to be of first order with respect to surface area of particles and the overall reaction is found to be controlled by chemical reaction.     

高炉炉缸碳源渗碳性能及动力学

摘要：提高铁水碳饱和度,减弱侧壁炭砖侵蚀,实现炉缸长寿对高炉炼铁具有重要意义。利用静态法系统地研究了高炉系统中进入炉缸铁水的碳源的渗碳性能,并计算了表观反应速率常数K。研究结果表明,不同碳源的渗碳性能由强到弱为：NMA炭砖>碳棒>煤粉>焦炭>9RDN炭砖。碳源的渗碳性能随其石墨化程度的升高而降低。碳源中的灰分会极大影响其渗碳性能,但以团聚大颗粒形式存在的灰分并不能减弱其渗碳能力,同时Al2O3可明显降低碳源的渗碳速率。9RDN炭砖的渗碳性能低,预示着其可适应更加复杂的炉况条件。煤粉和焦炭渗碳性能偏差,煤粉以及炉料下行过程形成的焦粉进入炉缸会降低死料柱空隙度,造成炉缸不活跃,使得炉况波动频繁,并不利于死料柱渗碳和侧壁保护层的稳定。因此,要适当提升入炉焦炭粒度,增大风量,减弱未燃煤粉及焦粉对炉缸侧壁炭砖长寿的负面影响。     

Corrosion behaviors of inconel 617 in hydrogen base gas mixture

The corrosion behavior of Inconel 617, a candidate for the structural material of heat exchanger in the high temperature gas-cooled reactor (HTGR), has been investigated at elevated temperatures in the hydrogen base gas mixture (80 pct H₂ + 15 pct CO + 5 pct CO₂). This gas mixture simulates the reducing gas in the direct steel making system that uses heat from HTGR in Japan. This gas has relatively high oxidizing and carburizing potentials. In the temperature range of 650 to 1000 °C Inconel 617 oxidized to form a Cr₂O₃ scale containing titanium oxide. The activation energy for this process is estimated to be 50 to 60 kcal/mol. The time dependence of the growth of the surface oxide scale was parabolic. The aluminum in Inconel 617 was internally oxidized. The time dependence of the internal oxidation was noticed to obey a 0.4 power rate law. Carburization was noticed at 650 and 900 °C. At 900 °C, carbides containing Si, Ti, and Mo precipitated beneath the oxide scale for gas exposure times up to 200 h. After 200 h, the formation and growth of the surface scale suppresses carburization. The thermodynamic analysis of gas atmosphere proposed by Gurry could be applied successfully to the experimental results. Some inconsistency existed mainly because of the scale formation and direct gas-metal interactions.     

Gas carburizing of steel with furnace atmospheres formedIn Situ from propane and air: Part I. The effect of air-propane ratio on furnace atmosphere composition and the amount of carburizing

Gas carburizing experiments were conducted in a batch-type sealed quenched furnace at 843 and 927 °C using furnace atmospheres produced by reacting propane and air within the furnace chamber. With low, constant gas flow rates it is shown that the amount of carburizing varies regularly with air-propane ratio. Furnace atmosphere composition was monitored as a function of temperature and air-propane ratio and compared with the composition expected at thermodynamic equilibrium.