Ni-Cr/BN在1250℃烧结时的碳化和粉化行为

Ni-Cr/BN在1250℃烧结时发生了碳化和粉化行为，本文分析了这种现象产生的原因和机理，并讨论了碳化动力学.同时，借用Grabke的模型分析了粉化机理，并研究了碳化界面处组织.     

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 对高炉布料槽失效部件挂臂35CrMo的表面形貌、组织、成分、物相和硬度,以及高炉气氛进行了测试分析,结果显示失效部件表面发生严重开裂,表层组织Fe3C质量分数达到87.06%,表层碳质量分数达到6.24%,远远高于正常的35CrMo基体,HV0.2达到600,高炉煤气气氛分析发现布料槽服役环境存在高温和渗碳气氛。综合分析,布料槽挂轴在服役过程中发生了明显的金属碳化现象,主要过程为高炉活性碳气氛和高温造成游离状态的碳原子在表面沉积,并向基体扩散,造成金属高温碳化和表面脆化,诱发表层金属内部产生孔洞和缺陷,高温碳化在裂纹两侧继续进行,造成失效部件产生高温碳化腐蚀沟,导致设备发生失效。     

混凝土假性碳化引起回弹法强度的误判

混凝土的碳化作用能提高其表面硬度，现行无损检测规程把碳化深度作为回弹法测强的一个修正参量来采用。研究发现在某种场合用酚酞试剂测定到的碳化值，不一定是实质意义上氢氧化钙和二氧化碳反应生成的碳酸钙。该疑似混凝土碳化深度值实际是混凝土表层失碱产生的中性化现象，研究揭示了回弹法检测中酚酞试剂指示的假性碳化引起混凝土检测强度评判的误区。     

混凝土的碳化及其防治措施

在现代社会的建筑领域中,混凝土及钢筋混凝土的应用十分广泛,我们对混凝土及钢筋混凝土结构的研究主要包括:混凝土及混凝土结构的强度、刚度及混凝土结构的耐久性等几个方面,而混凝土及其结构的耐久性主要包括混凝土的抗酸碱、抗风化、抗碳化等主要现象。其中,混凝土的碳化,对混凝土及钢筋混凝土耐久性有很大的影响,但至今尚未引起高度重视。本文就混凝土结构及钢筋混凝土结构的基本原理,剖析了混凝土碳化的物理化学机理及混凝土碳化对钢筋混凝土结构的危害性,研究并归纳了防治和减缓混凝土碳化的主要方法和措施等几个方面入手,使人们对混凝土的碳化现象有一些初步了解和认识。     

钨粉碳化防粘涂料的研制

在生产粗颗粒WC粉时，因WC粘舟而引起舟皿非正常破损是比较常见的现象。本文对W粉碳化时WC粘舟原因进行了分析，介绍了一种既具有较好防粘效果，同时也对WC与合金性能无不良影响的W粉碳化防粘涂料。     

26Cr18Mn12Si2N奥氏体耐热不锈钢早期失效分析

就26Cr18Mn12Si2N奥氏体耐热不锈钢制传送带链板使用中出现裂纹,发生早期失效的现象,进行了原因分析.结果表明,同溶处理效果欠佳或锻后冷却不足,将导致同溶碳化物品界大量析出聚集,材料塑韧性降低,高温使用中在不大的荷载下易产生裂纹而失效.     

Mo-La2O3阴极碳化机理

研究了碳化温度、碳化时间、碳化时苯的压强对Mo La2 O3 阴极碳化度大小和碳化层组织的影响。结果表明 :在 172 3K ,苯的压强为 1.5× 10 -2 Pa,碳化 6min后Mo La2 O3 阴极碳化度达到 19.7%,碳化层为疏松多孔的Mo2 C组织 ,有利于阴极热电子发射。应用扫描电镜 (SEM)、X射线衍射仪 (XRD)等手段对Mo La2 O3 阴极碳化层的显微组织和微观结构、物相等进行了观察与分析。并从热力学与动力学两方面对Mo La2 O3 阴极碳化机理进行了探讨。     

砂芯存储输送系统的拥堵分析和对策

针对广泛应用于铸造车间的砂芯存储输送系统,分析了其可能出现的一种拥堵现象产生的过程和条件,提出了避免产生这种现象的对策。     

耐磨涂层用 SiC / PI 复合薄膜的碳化研究

目的拓宽碳化硅增强聚酰亚胺(SiC/PI)复合薄膜在耐磨涂层领域的应用。方法利用流延成膜法制备SiC/PI复合薄膜,在氮气氛围中对复合薄膜进行600~1000℃的碳化处理,并对碳化后的薄膜进行SEM,XRD及FTIR等测试,分析碳化过程中组织结构的变化。结果由于SiC纳米颗粒起到物理交联点的作用,复合薄膜的热稳定性和残碳率得到提高,同时也具有了断裂塑性特征。随着碳化温度升高,复合薄膜六角碳层结构逐步完善。PI在碳化中,芳核自由基聚合成环数更多的分子,且SiC与PI的界面处产生Si—O键。结论碳化过程中,SiC纳米粒子与PI作用形成微弱的化学键合,改善了碳膜的界面结合情况,使得其耐热性得到提高。     

La2O3-Mo阴极表面碳化层作用机制研究

用光电子能谱和Ａｕｇｅｒ电子能谱等分析手段对纯钼丝表面离子注Ｌａ后形成的Ｌａ２Ｏ３－Ｍｏ阴极表面碳化层的作用机制进行了研究。实验结果表明,碳化层主要起到产生活性物质Ｌａ的作用,此外还具有贮存和输运活性物质的作用。改变阴极碳化工艺可使Ｌａ２Ｏ３－Ｍｏ三极管阴极的寿命由初期的１４ｈ提高到满足实际应用水平（１０００ｈ）的１４３６．５ｈ。     

金属材料在高温含碳气氛中的腐蚀

本文综述了材料在高温含碳气氛中的腐蚀,包括金属粉化和碳化,讨论了材料在含碳气氛中的腐蚀机理和腐蚀动力学。     

Metal dusting

Metal dusting is a catastrophic carburization of steels which leads to disintegration of the material to a mixture of powdery carbon and metal particles leaving pits and grooves. The phenomenon was simulated by carburization of low-and high-alloy steels in CO-H₂-H₂O mixtures at carbon activities > 1 in the temperature range 600–700°C. The occurance of an unstable carbide M₃C (M=Fe, Ni), as an intermediate in metal dusting, was proven—the reaction sequence involves over saturation of the metal matrix with carbon, M₃C formation at the surface, subsequent decomposition of the M₃C layer M₃C3 M+ C, leading to carbon with interspersed metal particles which act as catalysts for additional carbon deposition from the gas atmosphere. With increasing Ni content in Fe-Ni alloys, typical metal dusting is suppressed, but another mode of deterioration was observed, involving graphite growth on the grain boundaries. The high-alloy, chromia-forming alloys showed metal dusting only when chromia formation was suppressed by electropolishing the materials.     

Carburization and metal dusting like attack of 9Cr-1Mo steel resulting from sulphide corrosion

The influence of two sulphur-bearing refinery environments: (1) liquid hydrocarbons with steam present, and (2) liquid hydrocarbons with hydrogen present on the corrosion behaviour of commercial 9Cr-1Mo steel has been investigated. It has been stated that the progress of sulphide corrosion is followed by carburization of this steel due to the release of carbon from the sulphur-attacked metal carbides and the diffusion of carbon ahead of the advancing corrosion front. The rapid progress of sulphide corrosion with the presence of hydrogen results in a steep carbon concentration profile and metal dusting of the carburized layer. When metal dusting starts, rapid acceleration of scale growth and localised metal thinning takes place.     

Metal Dusting Protective Coatings. A Literature Review

Metal dusting is a catastrophic form of carburization attack that takes place in carbon-supersaturated gaseous atmospheres, and is most commonly encountered in steam reforming processes such as the production of hydrogen or syngas for ammonia, Fischer?CTropsch and methanol applications. The consequence of metal dusting can be a severe loss of metal from the process units, leading to high-cost maintenance and serious safety issues. The present literature review discusses the latest developments within metal dusting protection of alloys with special emphasis on protective coatings. In the first part of the paper, an overview of the main theories for metal dusting of alloys as well as fundamental studies is provided. In the second part, the paper focuses on the different methods to prevent metal dusting, including surface poisoning, alloying, chemical, mechanical and laser treatments as well as coatings. Particular focus is given to coatings and their composition, and fabrication methods, and a critical analysis of the different materials?? behaviours and the suitability perspectives of deposition techniques are provided.     

Thermodynamics,mechanisms and kinetics of metal dusting

A survey is given on recent research on “metal dusting” i.e. a catastrophic carburization or rather graphitization of metals and alloys occuring in carbonaceous atmospheres at carbon activities a_C>1. The thermodynamics are explained, the mechanisms for iron, low and high alloy steels, nickel and Ni-base alloys are described and the kinetics derived for iron and low alloy steels. Protection against metal dusting is possible by the presence of sulfur in the atmosphere, since adsorbed sulfur retards carbon transfer and hems graphite nucleation. Also dense oxide layers are protective, the preconditions for the formation of Cr-rich protectivee layers on steels and Ni-base alloys are shortly presenteed.     

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 a_C > 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.     

Hochtemperaturschäden bei hitzebeständigem Stahlguß

High temperature failures of heat resistant cast steel Heat resistant cast steel can fail by high temperature, the atmosphere, deposits and mechanical stresses. High temperature corrosion by gas atmospheres can be oxidation, nitriding, carburization, metal dusting, sulphur and chlorine attack. Corrosion by deposits as oil ash or chromates and liquid metals as copper base alloys, lead, tin and zinc leads to a rapid reduction in wall thickness. Besides there are mechanical stresses which lead to failures by stress rupture cracks or thermoshock. The additional effect by material aging should be considered as well.     

Metal dusting of high temperature alloys

Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H₂-H₂O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H₂ at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr₂O₃ scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr₂O₃ scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M₃C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.     

Metal dusting resistance of high chromium alloys

Samples of 5 high Cr‐alloys were discontinuously exposed for 10,000 hours under severe metal dusting conditions, i. e. in flowing 49%CO‐49%H₂‐2%H₂O at 650°C. After each of the 11 exposure periods the mass change was determined and any coke removed and weighed. Metallographic cross sections were prepared after about 4,000 h and 10,000 h. The high Cr‐alloys: 1. PM 2000 (Fe‐19%Cr‐5.5%Al‐0.5%Ti‐0.5%Y₂O₃), 2. Cr‐44%Fe‐5%Al‐0.4%Ti‐0.5%Y₂O₃, 3. Cr‐50%Ni, 4. Cr‐5%Fe‐1%Y₂O₃ and 5. porous chromium showed no or only minute metal dusting attack. Compared to the attack on reference samples of Alloy 601 (Ni‐23%Cr‐14%Fe‐1.4%Al), the metal dusting symptoms were negligible on the 5 high Cr‐alloys, minor coking and pitting and no internal carburization was observed. Because of the high Cr‐content, carbon solution and ingress should be minute, and in addition are inhibited by the formation of a chromia scale, as confirmed for four of the Cr‐rich alloys, and formation of an alumina scale on PM 2000. These alloys could be used for parts exposed to severe metal dusting conditions, and in fact, 50Cr‐50Ni has been applied successfully under such conditions.     

Corrosion of Alloy 600 in a carburization furnace

The failure case of a shielding tube was investigated which had been used in a carburization furnace for protection of an oxygen sensor. The tube made of Alloy 600 showed all kinds of attack by carbonaceous gases: (1) at the hot end within the CO‐H₂‐H₂O atmosphere at a_C ≤ 1, carburization leading to internal carbide formation, followed by internal oxidation of the carbides, (2) graphitisation, i.e. internal graphite formation in a region of lower temperature and a_C > 1 and (3) metal dusting, i.e. disintegration to a dust of graphite and metal particles and pitting. The phenomena are described and discussed. The Alloy 600 is not sufficiently corrosion resistant at high temperatures, due to its low Cr‐content (≤ 16% Cr) a protective scale is not formed for sure. Better corrosion resistance for this alloy may be obtained by surface working or a preoxidation under appropriate conditions.