Carburizing and Nitriding

Steel Ultimate strength T (MPa) a/Ou ratio Bending Traction Torsion 

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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. 

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. 

Thermodynamic considerations are useful for determining conditions for carburization and nitridation. For example, in the nitridation of niobium the particular phase of the product (e.g. NbN, / -Nb2N, or y-Nb4N3) will depend on temperature and pressure. This holds true also for carbides. However, for carbides, competing reactions such as... 

Modern techniques are currently available using carburizing and nitriding systems under vacuum. In these processes of vacuum carburizing and plasma carburizing, the components are heated under vacuum to around 950 °C. Methane is leaked into the chamber to a pressure of between 3 and 30 mbar to add carbon to the system. In the absence of a plasma, the methane will only decompose to the extent of about 3%, probably on the surface of the components according to a sequence such as that shown in Equation (11.19) ... 

Since residual stresses counteract the external loading to the material, they can increase resistance to destruction perceptibly. In this sense, strain hardening has a similar effect to carburization and nitriding. 

To reduce further the potential of distortion and to avoid structural modifications of the substrate, and without repeating the quench and temp>ering treatments, these carburizing and nitriding treatments have evolved, in the past few years, towards lower temperature processes (350-450°C for austenitic stainless steels). This reduction in the treatment temperatures had to include specific treatments for removing oxide layers, which act as a barrier to the diffusion of nitrogen and carbon. 

Furthermore, the nitrided-only 8N specimens have deeper layers than combined processed specimens. The depth of the simultaneously carburized and nitrided specimen, 8(C+N), reaches only 50% that of the nitrided specimen, and the thickness of 4C-4JS1 specimen had only about 45% compared to the nitrided-only 8N sp>ecimen after being processed for the same duration of 8 h due to the half nitriding duration. 

Fig. 8 shows the hardness depth profiles of the treated specimens. The carburized 8C specimen developed a maximum hardness of about 500 Hv, which is much lower than the hardnesses of 1230 to 1588 Hv for other three nitrided and nitrocarburised specimens. The nitrided layer of the 8N specimen produced a hard layer of 1588 Hv with an abrupt layer-core interface, while the 8C carburizing produced a gradually decreased hardness profile. Two combined carburized and nitrided specimens, 8(C + N) and 4C-4N developed a similar tendency to bulge in hardness profiles at inner carburized layer as shown in Fig. 7. The most gradual decrease in hardness from 1230 Hv level to substrate hardness was displayed by the 4C-4N specimen.

Corrosion tests using the electrochemical technique demonstrated that the precipitation free carburized and nitrided layers have very good corrosion resistance in the corrosive environments. 

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. 

Both carburizing and nitriding are also oxidizing processes, which are important in industrial applications for enhancing the hardness and strength of steels. Therefore, these methods are out of the scope of this chapter. 

Although all high-temperature corrosion is considered oxidation, there are other terms that are also encountered, such as oxidation-reduction, sulfidation, fuel ash corrosion, carburization, and nitridation, to name a few. 

Processes that induce compressive stresses into the surface of the metal such as shot-peening, carburizing, and nitriding are frequently beneficial in preventing corrosion fatigue and fretting corrosion. 

It is exceptionally resistant to SCC in petrochemical applications and to carburization and nitriding. All of the product forms are excellent in terms of forming and welding. Although this alloy is primarily noted for heat and oxidation resistance, it also has good resistance to chloride stress corrosion cracking. 

Chun C M, Ramanarayanan T A, The metal dusting corrosion of steels with varying concentrations of chromium , in Grabke H J and Schnltze M, Corrosion by Carbon and Nitrogen Metal Dusting, Carburization and Nitridation, Cambridge, Woodhead, 2007... 

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