High temperature carburization

In a vacuum, uncoated molybdenum metal has an unlimited life at high temperatures. This is also tme under the vacuum-like conditions of outer space. Pure hydrogen, argon, and hehum atmospheres are completely inert to molybdenum at all temperatures, whereas water vapor, sulfur dioxide, and nitrous and nitric oxides have an oxidizing action at elevated temperatures. Molybdenum is relatively inert to carbon dioxide, ammonia, and nitrogen atmospheres up to about 1100°C a superficial nitride film may be formed at higher temperatures in the latter two gases. Hydrocarbons and carbon monoxide may carburize molybdenum at temperatures above 1100°C. 

In ancient India, a steel called wootz was made by placing very pure kon ore and wood or other carbonaceous material in a tightly sealed pot or cmcible heated to high temperature for a considerable time. Some of the carbon in the cmcible reduced the kon ore to metallic kon, which absorbed any excess carbon. The resulting kon—carbon alloy was an excellent grade of steel. In a similar way, pieces of low carbon wrought kon were placed in a pot along with a form of carbon and melted to make a fine steel. A variation of this method, in which bars that had been carburized by the cementation process were melted in a sealed pot to make steel of the best quaUty, became known as the cmcible process. 

The equation for the rate of oxidation of the transition metals at high temperatures, which form a solid solution of oxygen before the oxide appears at the surface has die same form as that derived for die carburizing of die metal, and... 

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Figure 6 illustrates a more complicated situation. The sample was a plain iron-carbon steel—an iron foil carburized to about 5 atomic % carbon and then quenched. One sees a rather complex pattern. There is a large central peak from some untransformed high temperature face-centered phase of iron containing carbon in solid solution, retained austenite. There is a strong six-line pattern coming from martensite, a distorted body-centered solid solution of carbon in iron. We also see a... 

Thoriated tungsten, whose surface has been carburized at high temperatures, have lower work functions than pure tungsten and emit equivalent electron beam currents at lower temperatures thereby extending the life and stability when compared to regular tungsten sources. These sources require a stable, high vacuum (10 to 10 torr) and are more difficult to fabricate. 

The first syntheses of transition metal nitrides and carbides were derived from metallurgical processes,1,3 and consisted in the nitridation or carburization of the metal or of the oxide at severe conditions, in particular at high temperatures (>1500 K). Consequently, the resulting powders generally had low specific surface areas (Sg). 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

Si Improves oxidation, nitriding, sulfidation and carburizing resistance. Detrimental to nonoxidizing chlorination resistance Synergistically acts with chromium to improve high-temperature degradation... 

High-temperature corrosion frequently shows general corrosion, as in oxidation, sulfidation, carburization, hot corrosion and hydrogen effects, etc. It should be noted that subsurface corrosion or internal corrosion at high-temperature corrosion is a highly localized corrosion phenomenon. 

The most common failures are associated with oxidation, carburization and metal dusting, sulfidation, chlorination, and nitridation. The most common high-temperature degradation mode is oxidation, and the protection against oxidation, in general, is given by the formation of a chromium oxide scale. The presence of a small amount of aluminum or silicon in the alloy may improve the resistance against oxidation of a... 

HP microalloys were developed during the 1990s. The microalloys enhanced carburization resistance and improved high-temperature creep-rupture resistance.88... 

The above changes in the microstructure upon annealing do not only apply to the bulk material, but also for the surface. If an iron material is placed at high temperature in the presence of carbon vapor, a procedure known as carburization occurs. 

For carbonaceous gases such as CO and CH4 at relatively high temperatures (ca. > 800°C), carburization of steel surfaces takes place in the form of brittle interstitial carbides that may cause surface cracking. Cementite may also form on the surface of steel since its melting point is lower than the underlying metal, it may cause melting of the steel surface that is subsequently eroded by the gas stream. 

The details of the preparation of the iron oxide catalyst precursors are described elsewhere [15-17], For those carbides made by exsitu carburization, the oxide was loaded into a 1 diameter quartz tube and heated in a 1/1 H2/CO mixture at a space velocity of 10,000 v/v/hr at 350°C for 24 hours. Iron carbide catalysts were also prepared by laser pyrolysis of iron carbonyl and ethylene using a 150 watt continuous wave CO2 laser to provide both a rapid high temperature reaction (, 1 sec with T 1000°C) and quench [18],... 

Carbides are produced by reacting elemental carbon, or gases which yield carbon, with metals or metal compounds at sufficiently high temperatures (1200 to 2300°C). This carburization is preferably carried out in a protective gas atmosphere or in a vacuum. The commonly used processes are ... 

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