Carburization mechanism

Ramanarayanan T A, Srolovitz H J, Carburization mechanisms of high chromium alloys , J. Eiectrochem. Soc., 1985 132 2268-2274... 

Environments. Among the environmental factors that can shorten life under thermal fatigue conditions are surface decarburization, oxidation, and carburization. The last can be detrimental because it is likely to reduce both hot strength and ductility at the same time. The usual failure mechanism of heat-resistant alloy fixtures in carburizing furnaces is by thermal fatigue damage, evidenced by a prominent network of deep cracks. 

Fusidic acid, bacterial resistance mechanisms, 3 32t Fusinite, 6 707t, 719, 828 Fusion, PVC, 25 663-664. See also Cold fusion Deuterium fusion Fusion-bonded-epoxies (FBE), 10 440 Fusion carburization, 4 674-675 Fusion-cast refractories, 21 504 shapes of, 21 481-482 Fusion method, for tin content assays, 24 791, 792... 

Figure 9. Schematic diagram showing the proposed nucleation mechanism diamond nuclei form on a carbide interlayer on a carbide-forming refractory metal substrateJ Initially, carburization consumes all available C to form a carbide surface layer. A minimum C surface concentration required for diamond nucleation cannot be reached on the substrate surface. With increasing carbide layer thickness, the C transport rate stows and the C surface concentration increases. When the C surface concentration reaches a critical level for diamond nucleation, or a surface C cluster attains a critical size, a diamond nucleus forms. (Reproduced with permission.)...

To account for the carburization of iron we propose the following four-step mechanism ... 

Carbon control. Carbon needs to be controlled for evaluating carburization of structural materials because of possible impact on their mechanical properties. Diffusion and electro-chemical cells similar to those mentioned above are used for carbon control, with the salt mixtures (Na2C03-Li2C03 and CaC2-LiCl) used as electrolytes. 

C O were observed with no peaks of C O2, COO and C O2 with respect to the formation of the C-labelled reaction products. They proposed that the WGSR on the carburized 4.8 and 8.5 wt% M0/AI2O3 catalysts preceded the redox route based on the dissociation of CO and H2O as well as the dissociation-association mechanism. 

Figure 9.7 shows the hardness and modulus distributions of the MoDTC/ZDDP and ZDDP tribofilms, carburized steel disc, and fused silica as a function of the contact depth [1]. Since the fused silica as the standard sample for tip-shape calibration retained constant hardness and modulus values in a contact depth range from 5 nm, the hardness and modulus of the tribofilms were examined at contact depths greater than 5 nm. It was found that both kinds of tribofilms had the same hardness and modulus distributions relative to the contact depth. When the contact depth was greater than 30-40 nm, the mechanical properties were constant, with hardness staying at the fused silica level of 10 GPa and the modulus at the carburized disc level of 215 GPa. However, when the contact depth was reduced from 30 nm to 5 nm near the film surface, film hardness decreased from the fused silica level of 10 GPa to 6 GPa, and the modulus decreased from the carburized steel level of 215 GPa to 150 GPa. 

The mechanisms of nitriding and carburizing involve the transfer of the diffusing species to the surface, the establishment of a diffusing species activity gradient which drives the diffusion process, and the diffusion for itself, may be accompanied by the formation of nitrides or carbides (on the surface or in the core). The diffusion of interstitial species into a metal can only proceed if it exists a chemical potential (or activity) gradient of those species between the surface and the core of the material. 

The friction coefficient is a mechanical parameter, which depends on the surface material composition and the nature of the surface itself. Fig. 4 presents the relative friction coefficient for the samples treated at different gas composition. It relates the friction coefficient of the treated sample to the value of the untreated stainless steel (0.78). The measurement of the friction coefficient has been done for different number of tracks. For pure nitriding, after the first 2000 tracks, at which the wear depth is lower than 0.6 pm, in all examined treated samples, the friction coefficient is reduced to 59 %. While the C2H2/N2 gas ratio increases, the values of the friction coefficient decrease significantly and reaching approximately 14 % for pure carburizing. As a function of gas composition, the friction... 

If no protective oxide scale can be formed on the surface, or if the scale shows cracks or other types of channels for rapid gaseous transport through the scale, internal corrosion or oxidation can become possible. Besides oxidation, it is mostly internal sulfidation, nitridation, and carburization that are observed under technical conditions. Naturally, internal corrosion is not desired as it changes the optimized mechanical properties of a material. Furthermore, it may lead to grain-boundary weakening, which in time may result in the initiation of surface cracks in the case of stresses in a component. This is usually equal to the end of life of a component. Internal corrosion depends on several prerequisites (Rapp, 1965), as follows ... 

High-temperature corrosion is considered to be electrochemical in nature, with the high-temperature scale formed acting as an electrolyte. Corrosion is usually uniform in nature. The predominant effects are oxidation and carburization/decarburization. Changes in mechanical properties, specifically a loss of ductility due to phase changes, also take place. 

In a similar time frame, another superaustenitic alloy was introduced based on the wrought version of the heat-resistant cast alloy, HT. This alloy, identified as RA330 stainless, contains about 35% nickel and 20% chromium with an addition of silicon. This superaustenitic stainless also was assigned a nickel-based UNS number (N08330). N08330 offers excellent oxidation and carburization resistance in combination with good elevated temperature mechanical properties. 

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