Martensitic surface layer

Other diffusion coatings include manganising to produce austenitic or martensitic surface layers on steel. Mixed Mn/Cr diffusion coatings are readily produced by pack techniques. Carbide coatings based on Ti, V and Cr, singly or in combination, are mainly produced for their wear-resisting properties There are now several commercial boronising processes Con-... 

Specimen preparation involves wet grinding to remove the surface layer, which may be decarburized or otherwise nonrepresentative of the bulk of the specimen, followed by standard metallographic polishing and etching. This procedure ensures a flat, reproducible surface for the x-ray examination, and allows a preliminary examination of the specimen to be made with the microscope. In grinding and polishing, care should be taken not to produce excessive heat or plastic deformation which would cause partial decomposition of both the martensite and austenite. 

Figure 11.18 a) Transformation zone ahead and around crack tip. (6) Surface grinding induces the martensitic transformation, which in turn creates compressive surface layers and a concomitant increase in strength. 

In his early classical work Welsh [10] reported the friction coefficient and wear rate variations for the mild-severe-mild wear transition as a function of the load growth (Fig. 4.1). The rise in local temperatures on loading P2 leads to transformation of the surface layer material into a martensite that forms an oxide layer. This reduces the friction coefficient and abruptly returns the tribosystem into the mild wear mode. 

Fig. 16. SEM images of different microstructures of the modified layers on HS 6-5-2 steel surface a - dendritic microstmcture in the phase transformations zone, b - martensitic microstructure in the phase transformations zone.

For passivation treatments other than for scale removal, less aggressive acid solutions are used. The purpose of these treatments is to remove any contaminants that may be on the component s surface that could prevent the formation of the oxide layer locally. The most common contaminant is embedded or free iron particles from forming or machining tools. A10% nitric add solution is effective in removing free iron. For martensitic, ferretic, and predpitation-hardening grades, a nitric acid solution inhibited with sodium dichromate is used so as not to attack the stainless steel too aggressively. 

Fast reactor core structures are exposed to various environments during their in-reactor life liquid metal for the exterior surfaces and MOX (U, Pu)02 fuel for the internal layer of the cladding. Moreover, it is also necessary to check the impacts of these materials on the cleaning of fuel assemblies and on fuel reprocessing, especially during fuel dissolution in nitric acid. Only a few results are available on the behavior of ODS alloys in these environments, they can be complemented by results available on ferritic-martensitic steels. The influence of the fine dispersion of nanoparticles on the behavior of ferritic-mattensitic steels in the environment has to be considered. 

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