Low-temperature carburization

The low-temperature carburization LTA process developed by VTA meets the worldwide need for a process which economically reduces petrochemical and hydrocarbon residues into recyclable products, feedstocks or clean fuels. The indirectly fired rotary kiln system can be operated up to 850°C. The throughput varies in between 800 and 2000 kg/h. 

Further evidence which indicates that bulk cobalt carbide is not of importance in the synthesis is furnished by x-ray analysis of catalysts which have been used in the sjmthesis. It has been showm (91) that on low-temperature carburization with carbon monoxide, reduced cobalt... 

A surface-treatment process developed by Lee and co-workers [35, 36] showed the beneficial effect of treated 316L in simulated PEMFC environments and low ICR. Stack tests of 300h showed no power degradation with the treated 316L SS bipolar plates [35, 36]. A low-temperature carburization-treated 316 SS gave much better corrosion resistance in simulated PEMFC environments [103]. 

Hertz, et al., (2008) Technologies for low temperature carburizing and nitriding of austenitic stainless steel. International Heat Treatment and. Surface Engineerng, vol. 2, No. 1. Hurricks, P. L. Some aspects of the metallurgy and wear resistance of surface coatings, Wear (1972)... 

M. Tsujikawa, D. Yoshida, N. Yamauchi, N. Ueda, T. Sone, S. Tanaka, Surface material design of 316 stainless steel by combination of low temperature carburizing and nitriding. Surf. Coat. Tech. 200 (2005) 507-511. 

Nikam et al. (2008) proposed a low-temperature carburization (Cao, 2003) for the improvement of corrosion resistance and electric properties of 316L specimens. In potentiostatic tests, corrosion current densities were obtained to be 4 pA cm in anodic and 1.5 pA cm" in cathodic PEM fuel cell environment. Interfacial contact resistance of LTC 316L was lowered by approximately 24% compared to untreated material. 

Nikam, V. V., Reddy, R. G., Collins, S. R. et al. 2008. Corrosion resistant low temperature carburized SS 316 as bipolar plate material for PEMFC application. Electrochimica Acta 53(6) 2743-2750. 

The iron carbide process is a low temperature, gas-based, fluidized-bed process. Sized iron oxide fines (0.1—1.0 mm) are preheated in cyclones or a rotary kiln to 500°C and reduced to iron carbide in a single-stage, fluidized-bed reactor system at about 590°C in a process gas consisting primarily of methane, hydrogen, and some carbon monoxide. Reduction time is up to 18 hours owing to the low reduction temperature and slow rate of carburization. The product has the consistency of sand, is very britde, and contains approximately 6% carbon, mostly in the form of Fe3C. 

The chemical reactions used in CVD are pyrolysis, hydrolysis, disproportionation, reduction, oxidation, carburization, and nitridization [16]. The selection of the precursors is regulated by general features that can be summarized as follows [17] stability at room temperature, enough volatility at low temperature, high purity, ability to react plainly on or with the support, and ability to react without the production of side or parasitic reactions. 

Podgurski, Kummer, DeWitt, and Emmett (14) carburized reduced fused catalysts with propane, butane, and pentane at 325°C. Although x-ray patterns of Hagg carbide were found, the carbon content of these preparations approached 7.5 weight-% as a limit rather than 9.1% which was obtained upon carburization with carbon monoxide. Hall (15) found that during carburization of a reduced fused catalyst (Bureau of Mines number D3001) in n-butane at 300°C., C increased only to 0.22, and the carbide phase was cementite rather than Hagg carbide. Methane is too stable thermodynamically to carburize iron rapidly at low temperatures however, at 500°C. relatively pure cementite may be prepared with methane (15). 

Decarburization occurs in steels and cast irons in hydrogen gas by the reaction of H with C in the steel. The decarburization rate is primarily dependent on the diffusion rate of C in the steel, but is also affected by the carbon content of the steel, alloying elements in the steel, such as chromium, impurities in the hydrogen, and of course time and temperature. Carburization of steels, the reverse of decarburization, is usually conducted at temperatures of about 900°C, but decarburization can occur at temperatures as low as 800°C. " ... 

The percentage of high-temperature carburized WC is low. Main applications are in road planning tools and soft rock drilling tools. Part of the coarse crystallized WC for these applications is also supplied by Menstruum WC. 

Halle and Herbst obtained a hexagonal carbide by carburization of iron-copper catalysts, and later also by carburization of copper-free catalysts (reduction and carburization at low temperatures). The x-ray pattern is not identical to that described by Hagg. On the basis of their x-ray investigations Hofer, Cohn, and Peebles believe that the carbide of Halle and Herbst is identical to the Fe2C carbide with a Curie point at 380°C. of Pichler and Merkel (see Sec. III.4.d). 

Chromium carbide is among the compounds detected as precipitating the low temperature regions of liquid metal circuits, and the system Na—Cr—C is one of the most intensively studied systems There is some evidence that the most stable chromium carbide CrjsCg is formed at temperatures between 550 and 700 °C even in stainless steels, where the chemical activity of chromium is well below unity. This reaction is the chemical process causing the carburization of austenitic CrNi steels. CrjjCs precipitates in the surface zones of the material. 

Then they proposed Ni-Mo carbide catalyst [35] for the low temperature WGS reaction. Among the various catalysts Nio.25Moo.75 catalyst carburized at 873 K was more active than the other catalysts. Usually, Mo carbide catalysts deactivate with time-on-stream. However, Nio.25Moo.75 catalyst carburized at 923 K exhibits stable activity for 300 min of time-on-stream. The Ni contents of 15% and 25% increased the catalytic activity but further Ni content led to a drop in activity. They proposed that the promotion of the WGS reaction activity was due to the formation of Ni-Mo oxycarbide. 

It is nowadays widely accepted that hard, wear and corrosion resistant surface layers can be produced on Austenitic stainless steel by means low temperature nitriding and/or carburizing in a number of different media (salt bath, gas or plasma), each medium having its own strengths and weaknesses (Bell, 2002). In order to retain the corrosion resistance of austenitic stainless steel, these processes are typacally conducted at temperatures below 450 °C and 500 °C, for nitriding and carburizing respectively. The result is a layer of precipitation free austenite, supersaturated with nitrogen and/or carbon, which is usually referred to as S-phase or expanded austenite (Sun et al, 1999 Li, 2001 Li, et al., 2002 Christiansen, 2006). 

F. Ernst, Y. Cao, G.M. Michal, AH. Heuer, Carbide precipitation in austenitic stainless steel carburized at low temperature, Acta Mater. 55 (2007) 1895-1906. 

T. Bell and Y. Sun, Low temperature plasma nitriding and carburizing of austenitic stainless steels. Heat Treatment of Metals 29 (3) (2002) 57-64 T. Bell, Bodycote-AGA Seminar, Lidingo, 2005. 

Y. Sun and T. Bell Effect of layer thickness on the rolling-sliding wear behavior of law- temperature plasma-carburized austenitic stainless steel. Tribology Letters (2002) 13,1, 29-34 Y. Sxm, Kinetics of low temperature plasma carburizing of austenitic stainless steels, J. Mater. Proc. Tech. 168 (2005) 189-194. 

Y. Sun, X.Y. Li and T. Bell, Low temperature plasma carburizing of austenitic stainless steels for improved wear and corrosion resistance. Surf. Eng. 15 (1999) 49-54. 

Cao Y, Ernst, E, and Michal, G. M. 2003. Colossal carbon supersaturation in austenitic stainless steels carburized at low temperature. Acta Materialia 51 4171-4181. 

An insidious aspect of carburization is its nonuniform nature. Just as for other forms of localized corrosion, it is extremely difficult to predict and model localized carburization damage. As a rule of thumb, carburization problems only occur at temperatures above 815°C, because of unfavorable kinetics at lower temperatures. Carburization is therefore not a common occurrence in most refining operations because of the relatively low tube temperatures of most refinery-fired heaters. 

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