Chromium carbide abrasive wear

Thermal spray processes can be used to give coatings of chromium carbide or nickel chromium for erosion resistance, copper nickel indium for fretting resistance, tungsten carbide cobalt for wear and abrasion resistance, and even aluminum siHcon polyester mixtures for abradabiHty. 

These steels resist oxidation scaling up to 825°C but are difficult to weld and, thus, are used mainly for items that do not involve welded joints [17]. They are thermally hardened and useful for items that require cutting edges and abrasion resistance in mildly corrosive situations. However, they should not be tempered in the temperature range of 450 to 650°C. This reduces the hardness and wear resistance and also lowers the corrosion resistance because of the depletion of chromium in solution through the formation of chromium carbides. 

Carbides (wolfram carbide [WC], cobalt-chromium carbide [Co-Cr3C2], and nickel chromium-titanium carbide-niobium carbide [Ni-Cr-TiC-NbC]) for protection against abrasive wear and erosion. 

But wear resistance and corrosion resistance are fundamentally contrary material properties, since in the case of corrosion-resistant steels, adequate corrosion protection is to be anticipated only if the chromium content in the matrix is above 12% by mass. Carbon has a high affinity to chromium and removes it from the matrix to form carbide, so an increase in wear resistance is to be anticipated with increased carbon content, but combined with a reduction in corrosion resistance. This situation is exacerbated by the fact that wear processes and corrosion processes normally occur together. Under certain circumstances there may therefore be corrosive attack, since abrasive wear removes passive layers, which cannot be regenerated quickly enough. Conversely, corrosion can also create wear particles. 

Prevention of abrasive wear is possible through proper material selection and the use of surface engineering treatments. A number of material families have demonstrated good resistance to abrasive wear. They are typically hard materials that resist scratching, and include ceramics, carbide materials, alloyed white cast irons containing hard chromium carbides (see Fig. 8), and hardened alloy steels. 

Fig. 12.1.8. Cyclone wear plate consisting of a mild steel plate with an abrasion resistant metal overlay such as chromium carbide. Such a plate is typical of that which may be installed in the cyclone s inlet target zone (just downstream, and in the line of sight, of the inlet duct)...

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