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.     

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.     

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.     

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     

渗碳炉喷嘴流场的数值模拟

利用有限体积法对控制方程进行离散,并采用非结构化网格划分流动区域,对某大型渗碳炉喷嘴内及其周围区域流场进行了数值模拟研究.分别比较分析了两种不同结构喷嘴在渗碳炉内所形成的流场和温度场分布,探讨了不同流量工况对喷嘴周围流场和温度场的影响.计算结果表明:多孔喷嘴比简单的单孔喷嘴能更好地组织起渗碳炉内的流场和温度场,不同的流...     

含碳球团熔融渗碳机理

使用CaO-SiO_2-Al_2O_3合成渣与磁铁矿、石墨粉制备含碳球团,考察了球团中渣相成分、渣量以及碳含量对铁水渗碳量的影响,并探讨了含碳球团内铁的熔融渗碳行为和机理。结果表明,熔融渗碳分为熔融还原渗碳以及铁熔体聚合过程渗碳2个阶段,前一阶段熔渣中Fe2+含量与铁的渗碳量处于动态平衡过程,主要受熔渣化学成分的影响;后一阶段的渗碳过程决定铁的物理渗碳极限,主要与铁、碳颗粒的接触条件有关,且熔渣中Fe2+的还原极限与渣相碱度和SiO_2绝对含量有关,铁的熔融渗碳量(质量分数)可以达到4%左右,满足铁锍分离要求。     

High-rate carburizing in a glow- discharge methane plasma

Carburizing at high temperature (1040°C) in a glow-discharge methane plasma signifi-cantly reduces carburizing time. Sufficient carbon to produce a 1.0 mm case on conven-tional carburizing steels can be introduced in 10 min at methane pressures in the range of 1.3 to 2.7 kPa (10 to 20 Torr). To reduce the carbon concentration at the surface to an acceptable level (007E1.0 wt pct) the plasma carburizing stage is followed by a short (007E30 min) diffusion step. To aid in optimizing the division of time between the carburizing and diffusion stages, and in separating the effects of plasma carburizing and high-temperature diffusion, a computer program was written to model the diffusion equation for appropriate initial and boundary conditions. Both model and experimental results show that a diffusion time/carburizing time ratio of at least 3: 1 is required. The diffusion model indicates that the exceptionally high carburizing rates observed arise from the rapid infusion of carbon into the surface from the plasma, during the carburizing stage and not from anomalously high diffusion rates. Exceptionally uniform cases on surfaces of irregular geometry are achieved, and significant amounts of natural gas are saved owing to both the reduced car-burizing time and the low pressures employed.     

Decreased gas consumption of a fluidized bed furnace

The feasibility of utilizing a closed circulatory system to generate gases for a fluidized bed furnace was investigated with the primary concentrations of both economizing on the raw materials used for producing furnace atmospheres and decreasing the air pollution caused by exhaust gases. Air humidified with water vapor was first introduced into a charcoal furnace for causing a reaction with hot charcoal to form a carburizing atmosphere. This atmosphere was then introduced into a fluidized bed furnace to carburize steels. The exhaust gases from the fluidized bed furnace were recycled by repassing them through the hot charcoal layer in the charcoal furnace with a gas pump. The charcoal furnace and the fluidized bed furnace formed a closed circulatory system during the carburization of steels. Experiments were performed with various parameters of this system, including content of water vapor in the humid air, temperature of the charcoal, rate of recirculation of the atmosphere,etc. The effect of each parameter on the carburizing behavior in the fluidized bed furnace was investigated on the basis of the rate of carburization and the carbon potential of the atmosphere. The feasibility of applying this system to a fluidized bed furnace was assessed from the aspects of the fluidization of A1₂O₃ powder, the result of carburizing steel, and the rate of consumption of charcoal. The closed system employed in generating atmosphere was demonstrated by the experimental results to have enabled the fluidized bed furnace to operate normally and to have significantly decreased both the consumption rate of charcoal and the environmental pollution.     

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.     

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。     

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.     

Internal oxidation during carburizing and heat treating

The internal oxidation of the surface layers of ferrous engineering components during carburizing was known for many years, but post heat treating machining and grinding operations have ensured that this oxidation phenomenon has had no deleterious effects on their properties. Recently, however, economical considerations have demanded that where possible such finishing operations should be eliminated and hence there is now considerable interest in avoiding internal oxidation during carburizing—or heat treating steel in general. It is the purpose of the present paper to discuss the factors which in-fluence the depth and configuration of internal oxidation, in particular the effects of steel composition and the thermochemical processing conditions. Internal oxidation and the associated metallurgical difficulties cannot be sufficiently avoided during industrial car-burizing, and hence the factors necessary to minimize internal oxidation are discussed, including the use of oxygen free carburizing media and silicon reduced steels. 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.     

Cr35Ni45钢高温长期服役过程的氧化与渗碳机理

采用扫描电镜、电子探针和X射线衍射等手段对不同服役时间（原始态、1.5a和6a）Cr35Ni45乙烯裂解炉管内壁的氧化与渗碳机理进行了系统分析.结果表明:高温长时服役后炉管内壁出现了氧化层、碳化物贫化区和碳化物富集区三个区域,其氧化行为包括Cr₂O₃外氧化和SiO₂内氧化,且服役过程中外氧化膜发生反复破坏和重建;炉管服役过程的渗碳行为主要由内表面结焦引起,外氧化膜的反复破坏可以加重渗碳,但外氧化膜在破坏后能自动修复,所以服役态两个炉管的渗碳程度较轻;外氧化膜的反复破坏和重建使亚表层贫铬,导致形成碳化物的临界碳浓度增加,在内壁亚表层形成贫碳化物区,多余的碳原子在其内侧析出,形成碳化物富集区.      

高炉内铁?焦界面的渗碳润湿行为研究

高炉内铁水渗碳过程是影响冶炼效率及未饱和铁水对炉缸炉衬侵蚀的重要因素。本文通过高温真空润湿性测试装置模拟了高炉炉缸区的铁水渗碳反应,分析了不同碳质量分数（3.8%、4.3%、4.8%）的Fe?C熔体与质量分数为99.9%的石墨基体在高温下界面间的润湿反应,同时利用扫描电镜（SEM）和能谱仪（EDS）研究了渗碳界面的微观形貌及渗碳距离。结果表明:界面接触角随着Fe?C熔体中碳含量的增加而变大;熔化过程中,接触角随着反应时间延长而减小,并最终趋于稳定;4.8%碳质量分数的Fe?C熔体中由于含碳量已至饱和,处于不润湿状态。扫描电镜分析显示,Fe?C熔体与石墨基体的接触界面形成了“球帽状”凹陷,凹陷半径与体积随碳含量的增加而减小。能谱线扫描分析显示,随着Fe?C熔体中初始碳含量的增加,石墨基体中的碳素溶解量减少,渗碳效果变差,良好的润湿性有利于碳的传质。通过计算表面能发现,石墨基体中碳素溶解进入Fe?C熔体后,有效减小了两者之间的表面能,使得表面张力减小,接触角在熔化期间递减。      

高温渗碳齿轮钢的晶粒粗化行为

  为了开发适合980 ℃高温渗碳的齿轮钢，利用伪渗碳方法，研究了铌质量分数为0、0.036%、0.060%和0.100%的18Cr2Ni2Mo渗碳齿轮钢在930和980 ℃的晶粒粗化行为。结果表明，由于析出NbC钉扎晶界，铌微合金化可以显著细化试验钢在930和980 ℃奥氏体化后的晶粒尺寸，且随着铌质量分数增加，铌微合金化明显抑制试验钢在980 ℃长时间奥氏体化晶粒粗化倾向。添加0.100%Nb的18Cr2Ni2Mo齿轮钢在980 ℃奥氏体化20 h后，平均晶粒尺寸仍然在26 μm左右，适合于980 ℃高温长时间渗碳。     

武钢高炉炉尘中碳来源的变化特点

利用岩相显微分析、化学分析及拉曼分析技术,对武钢高炉炉尘的粒度组成、化学成分变化及碳的来源进行了研究。研究结果表明,武钢高炉炉尘的粒度组成主要以70~900μm为主,比例占80%左右;岩相显微分析表明,高炉炉尘中未消耗的煤粉和焦炭存在不同的结构形态,主要包含粗粒镶嵌、细粒镶嵌、纤维加片状,并含有矿物质、惰性物、煤焦等物质。利用图像分析和拉曼光谱技术共同对高炉炉尘中碳来源进行测定,2种方法所测结果接近,未燃煤粉占高炉炉尘总碳量的10%左右。     

Simultaneous plasma treatment for carburizing and carbonitriding using hollow cathode discharge

Ion carburizing and nitriding are effective processes for saving energy and providing polutionless surface treatment but have the disadvantage of using much electric energy. A cylindric subsidiary cathode was set up around a rod-shaped workpiece with a gap, and hollow cathode discharge for ion carburizing was studied. Thus, simultaneous plasma treatments for ion carburizing and ion car-bonitriding in one workpiece were researched using Cr-Mo steel to save electric treatment power. First, the effects of the gap between the test piece and subsidiary cathode and the pressure of electric discharge gas, including methane gas, on fundamental plasma treatment conditions were experimen-tally researched. It was found that the temperature for ion carburizing in a H₂-N₂-Ar-CH₄ gas mixture was 1123 to 1193 K with a gap of 3 to 5 mm under a gas pressure of 133 to 532 Pa. Next, the test piece was ion carburized with hollow cathode discharge and carbonitrided with normal glow dis-charge simultaneously. The ion-carburized layer was formed in the area covered by the subsidiary cathode. The surface hardness was 800 Hv, the effective case depth was 0.6 mm, and the surface carbon content was 0.75 wt pct. An ion carbonitriding layer was formed in the area without the subsidiary cathode. The surface hardness was 700 Hv and the case depth was 0.1 mm. It is useful to form the different layers of ion carburizing and ion carbonitriding in one treatment process and to give different mechanical and tribological properties on one workpiece simultaneously.     

高炉炉缸含钛保护层物相及TiC0.3N0.7形成机理

基于高炉破损调查取样分析, 借助X射线荧光分析、X射线衍射分析、电子探针分析、扫描电子显微镜结合能谱分析等手段分析了高炉炉缸、炉底不同部位形成的含钛保护层化学成分、物相组成和微观形貌, 并建立正规溶液热力学模型对Ti (C, N)形成的热力学条件进行分析, 然后针对高炉的实际工况, 明晰高炉炉缸TiC_0.3N_0.7形成的条件.结果表明, 高炉炉缸侧壁最薄处炭砖残余厚度仅为200 mm; 炉缸炉底炭砖表面普遍存在含钛保护层, 保护层平均厚度在300~600 mm左右, 高炉炉缸不同部位形成的保护层中Ti(C, N)主要以TiC_0.3N_0.7形式存在, 并与Fe相聚集在一起.Ti (C, N)固溶体实际混合摩尔生成吉布斯自由能显著低于标准混合摩尔生成吉布斯自由能和理想混合摩尔生成吉布斯自由能.在不同温度条件下, TiC和TiN在固溶体中存在的比例不同, 高温时以析出TiC为主, 低温时以析出TiN为主.Ti (C, N)固溶体的形成与高炉热力学状态条件直接相关, TiC_0.3N_0.7在该高炉炉缸中的形成温度为1423℃.      

高强度汽车渗碳齿轮钢的发展及应用

渗碳齿轮钢是汽车用主要结构钢之一,国内主要汽车生产厂家的高强度齿轮钢材料一直使用不同国家多个牌号的渗碳钢。随着技术和应用的发展有必要开发新品种的高强度齿轮钢。分析了高强度汽车渗碳齿轮钢在国内外的发展及应用现状,并根据我国合金材料的特点展望了其发展趋势。     

COREX竖炉炉料黏结的机理研究和影响因素分析

竖炉内炉料黏结是COREX炼铁工艺中的一个重大难题,国内外COREX生产操作都表明预还原竖炉内部的炉料容易黏结成块或黏结在竖炉的炉墙上,累积到一定程度后,会引起竖炉悬料,影响COREX还原煤气的利用和预还原竖炉的顺行,严重时甚至堵塞竖炉,使还原煤气难以接受或无法排料而被迫清空竖炉。在竖炉清空期间,对竖炉内黏结物进行了取样和分布情况调查,借助于化学成分检测、SEM和XRD等手段,分析了竖炉内黏结物形成的机理。在上述分析的基础上,通过实验室的试验研究考察了竖炉内炉料黏结的影响因素,结果表明,温度、金属化率、挤压时间、煤气成分等是黏结的主要影响因素。