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1.
《热加工工艺》2007,36(4):67-67
气体渗碳方法有两道工序,在工序1中.设定温度在δ铁和液相相变为γ铁的包晶点以下、从液相相变为γ铁和渗碳体的共晶点阻上.将钢制的处理对象在含有渗碳气体的渗碳气氛中加热,使其表面碳碳度不超过固溶限值、在工序2中.使渗碳温度从前述设定温度逐渐下降.在不超过固溶限值的范围内使处理对象的表面碳浓度增加的同时,使其渗碳深度增加.  相似文献   

2.
以丁烷加空气为流态化渗碳气源,系统试验研究了充态化渗碳温度气氛碳势、刚玉粒径及表面流态化速度等对碳传递系数的影响。利用单纯形方法回归建立了不同条件下流态化渗碳碳传递系数数学模型和综合影响因素数学模型,试验验证表明,该数学模型具有较高的拟合精度。  相似文献   

3.
在对高浓度深层渗碳的特点进行分析的基础上,考虑到碳化物的析出与溶解、温度及碳浓度对扩散行为的影响,气氛环境对相界面碳传递过程的影响,淬火烈度对渗碳层冷却效果的影响,提出了一种新的模拟高浓度深层渗碳渗层硬度分布的方法。  相似文献   

4.
肖顺枢  钟世雄 《热处理》2009,24(2):4-15
根据当前通用机械零件光亮淬火和渗碳对控制气氛的需要及控制气氛的发展状况,综述了钢铁零件最终热处理用的控制气氛的种类及其基本的控制原理,包括发生炉制备的吸热型气氛、甲醇滴注式气氛、氮-甲醇气氛、直生式气氛和快速渗碳气氛,简要介绍了渗碳气氛的碳势控制原理和方法,紧固件、轴承圈、弹簧等通用件的光亮淬火用气氛及其控制技术。  相似文献   

5.
快速压力渗碳工艺及其机理分析   总被引:1,自引:1,他引:0  
介绍了快速压力渗碳工艺,并对其机理进行了分析。提高渗碳炉内压力,可提高气氛中的碳势,增大工件表面对活性碳原子的物理吸附速度,同时也可提高内扩散速度.从而提高此工艺的渗碳速度,缩短渗碳时间.提高生产效率。  相似文献   

6.
本文研究了煤油加不同配比的空气所得气氛的渗碳特性,包括气氛成分、碳势及可控性、渗碳速度和传质系数,并找到了该渗碳气氛的最佳煤油与空气配比范围.实验结果表明,该气氛具有渗速快、碳黑少、碳势可控和经济实用的特点,值得推广应用.  相似文献   

7.
研究了20Cr Mn Ti钢真空低压渗碳过程中渗碳温度和渗碳时间对渗层深度、渗层硬度分布和表面碳含量的影响,并分析了碳含量对渗层硬度分布的影响规律,比较了真空渗碳和气体渗碳两种渗碳工艺对盲孔结构的渗碳结果和渗层组织。结果表明,随渗碳温度的升高和渗碳时间的延长,渗层深度和表面碳含量增大,但表面硬度下降。碳含量对渗层硬度分布的分析结果表明,碳质量分数为0.78%时,渗碳层具有最高淬火硬度。对于盲孔结构,相较于气体渗碳,真空渗碳能显著减小渗层深度偏差,并改善渗层组织。  相似文献   

8.
韩伯群 《热处理》2009,24(5):71-75
一种可实现气氛控制的真空渗碳或渗氮炉,采用热传导探头和氧探头双探头控制法进行气氛碳势或氮势的闭环控制。当工件形状与装炉量发生变化时,能自动调节原料气的供给量,从而提高了渗碳的均匀性,并有效消除积炭,工艺稳定性和再现性好。这种炉子可广泛用于高温渗碳、低温渗碳、高浓度渗碳、碳氮共渗、氮碳共渗、渗氮等表面改性处理。  相似文献   

9.
流态化渗碳碳传递系数试验研究及数学模型   总被引:2,自引:0,他引:2  
以丁烷加空气为流态化渗碳气源,系统试验研究了流态化渗碳温度、气氛碳势、刚玉粒径及表观流态化速度等对碳传递系数的影响。利用单纯形方法回归建立了不同条件下流态化渗碳碳传递系数数学模型和综合影响因素数学模型,试验验证表明,该数学模型具有较高的拟合精度。  相似文献   

10.
采用称重法和金相法,研究了三种成分不同的Fe3Al合金在不同表面状态下的抗渗碳性,并与1Cr18Ni9Ti不锈钢对比,结果表明,Fe3Al合金具有很强的抗渗碳性.初步探讨了其抗渗碳机理。为Fe3Al合金在渗碳气氛中的应用提供了一定的实验与理论依据。  相似文献   

11.
The pre-oxidation results demonstrate that rare earth elements play an important role on the scale adherence of alloy after the oxidation at elevated temperature. The better scale adherence of the yttrium modified alloy permits a superior cyclic coking and carburizing resistance. This is because the integrity chromia layer, which prevents fast ion diffusion, can retard the outer catalytic coking and the inner carburizing of pre-oxidized alloy. As for the alloy under the cast condition, severe coking and carburization has been detected, which was also observed on the pre-oxidized samples where the scale suffered from severe spalling.  相似文献   

12.
Metal dusting     
This introductory review paper summarizes shortly the research on metal dusting, conducted in the MPI for Iron Research during the last dozen years. Metal dusting is a disintegration of metals and alloys to a dust of graphite and metal particles, occurring in carburizing atmospheres at aC > 1 and caused by the tendency to graphite formation. The cause of destruction is inward growth of graphite planes into the metal phase, or in the case of iron and low alloy steels into cementite formed as an intermediate. The kinetics of metal dusting on iron and steels was elucidated concerning dependencies on time, temperature and partial pressures. High alloy steels and Ni‐base alloys are attacked through defects in the oxide scale which leads to pitting and outgrowth of coke protrusions, after initial internal formation of stable carbides M23C6, M7C3 and MC. A dense oxide layer prevents metal dusting, but formation of a protective Cr‐rich scale must be favored by a fine‐grain microstructure and/or surface deformation, providing fast diffusion paths for Cr. Additional protection is possible by sulfur from the atmosphere, since sulfur adsorbs on metal surfaces and suppresses carburization. Sulfur also interrupts the metal dusting mechanism on iron and steels, causing slow cementite growth. Under conditions where no sulfur addition is possible, the use of high Cr Nickelbase‐alloys is recommended, they are largely protected by an oxide scale and if metal dusting takes place, its rate is much slower than on steels.  相似文献   

13.
When metals or alloys protected by oxide films are exposed to a carburizing atmosphere, carbon may diffuse into the metal substrate because of imperfections in the film. Stresses from the change in carbon concentration in the metal substrate may arise and can result in spallation of the oxide film, crack initiation on the exposed surface and acceleration of metal dusting. In this paper, the stresses at a corner point, where the channel or flaw intersects the interface between the oxide film and metal substrate, are studied by a coupling model between carbon diffusion and elastic deformation. The results show that the stresses of the corner point depend on the oxide film thickness and channel width and are more sensitive to the former. The dependency varies with the time for which the metal substrate is exposed to the carburizing atmosphere. It is expected that the present study may lead to the comprehension of the metal dusting corrosion with regard to the effect of diffusion induced stress.  相似文献   

14.
Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in α‐Fe than in Fe3C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr–Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni–Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni–Sn Fe‐base materials facilitates the formation of a Fe3SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.  相似文献   

15.
块体非晶合金是一类具有高强度、高硬度和大弹性极限的无序金属材料,其优异的力学性能是目前先进金属材料领域研究热点之一,如何提高材料的强度是材料研究领域永恒的主题。系统地总结了已知具有超高强度的一类块体非晶合金材料——钴基块体非晶合金的成分、热学稳定性及力学性能;同时研究了不同非晶合金的断裂强度与其弹性常数、硬度和特征温度的关联。研究结果表明:在非晶合金体系中杨氏模量、维氏硬度、玻璃转变温度与断裂强度之间都存在较好的线性变化关系。基于以上结果,本课题组提出了超高强钴基块体非晶合金的组成设计方法,即选取具有强共价键特性的非金属元素和高模量、高熔点过渡金属元素与钴元素进行组合。  相似文献   

16.
The present study focuses on a new technique for the prevention of metal dusting in carbonaceous gas environments at intermediate temperature. Preliminary laboratory metal dusting test was conducted for transition‐metals and Ni‐x%Cu binary alloys in a simulated 60%CO‐26%H2‐11.5%CO2‐2.5%H2O (in vol.%) gas mixture at 650°C for 100 h. The metal dusting caused no coke deposition on transition‐metals of Cu, Ag, and Pt, while those of Fe, Co, and Ni have a large amount of coke and lost mass. Whether or not coking behavior of Ni‐Cu binary alloys formed any oxide scales in the simulated gas environment depended on the Cu content. Specimens containing low Cu were entirely covered with coke and showed rough metal surfaces due to the degradation of metal. Alloys of 20% and more Cu, on the contrary, had no coke deposition and smooth metal surfaces, suggesting alloys with an adequate Cu do not react with CO in the gas mixture without an oxide scale barrier. Based on these results, we conclude that Cu does not protect by formation of the oxide scale but has a “Surfactant‐Mediated Suppression” against metal dusting. This effect can be explained in terms of atomistic interaction of CO with transition‐metal surfaces by electronic structure analyses. The concept can be also useful for the practical material design of Ni‐Cr base alloy with excellent metal dusting resistance.  相似文献   

17.
焊接残余应力对接头尘化腐蚀影响的有限元模拟   总被引:1,自引:1,他引:0       下载免费PDF全文
金属尘化腐蚀是发生在含碳气氛中的一种高温腐蚀现象,碳在金属基体中的扩散是其过程的第一步.文中开发了碳扩散的耦合计算程序,进行了加热炉炉管焊态和工况温度状态下焊接残余应力对碳扩散影响的数值模拟,并与无应力状态下的碳扩散情况进行了比较.结果表明,在热影响区附近焊接残余应力梯度最大,对碳扩散的影响也最大,并且形成一个碳浓度峰值,在峰值两侧则会形成相应的碳浓度低谷,这是碳向高应力梯度区扩散所致.经过升温至工况温度后,残余应力得到松弛,对碳扩散的影响有所降低,可以延缓炉管的损坏时间.但由于残余应力的影响仍然存在,因此在炉管检修期间要注意焊接接头附近的尘化腐蚀情况.  相似文献   

18.
Investigation was made by SEM examination on metal dusting and coking behaviours of alloy 803 in a flowing gas mixture of H2-CO-H2O. It was found that an oxide scale arisen on the sample surface at the beginning of exposure. Metal dusting started when graphite deposition occurred earlier at the local defects in the oxide scale than the defects were repaired by enough supply of chromium from the interior of alloy matrix. Coke consisted of graphite filaments and metallic particles produced by disintegrating of alloy matrix, and grew up from the defects in the oxide scale with pit left in the sample surface. Increasing chromium content, doping a small amount of silicon and reducing grain size to create fast diffusion paths for chromium and silicon to alloy surface, all promote the formation of a dense oxide scale and favor early self-repairing of the defects in the oxide scale before occurrence of graphite deposition. The resistance of an alloy to metal dusting can be improved generally by means of these methods.  相似文献   

19.
25Cr35Ni耐热合金表面结焦机制   总被引:5,自引:0,他引:5  
分析了25Cr35Ni耐热合金表面的沉积焦炭,探讨了灰的形成机理。结果表明,遄分为三种类型,即丝状焦炭、层片状炭和颗粒状焦崦;局域丝状焦崦的沉积与材料表面化学成分必切相关,其变形成和生长本质上不受材料成分的影响;丝状焦 生长促进材料表面继续结焦和渗碳,而非焦炭的沉积一定程度地抑制催化灰的继续形成和发展,进而减轻材料的碳侵蚀程度。  相似文献   

20.
The HR-120 alloy is a candidate structural material for heat-exchanger reactors of steam methane reforming. Consequently, the behaviour at high-temperature of this alloy in oxidizing/carburizing atmosphere is of considerable interest for industrial applications. In this study, the behaviour of HR-120 alloy was evaluated in Ar, CH4, CO, H2, H2O oxidant/carburizing gas mixtures at 1,223 K, with or without pre-oxidation. In the former case, the as-grown scale layer consisted of inner Cr2O3 layer and an outer MnCr2O4 spinel layer. This scale structure, which is completely transformed into carbide layer in Ar–CH4 atmosphere, exhibits an excellent stability in CH4, CO, H2, H2O gas mixture. However, the oxidation of particles rich in Nb promoted cracking and spalling of protective scale layer, resulting in exposure of substrate metal.  相似文献   

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