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Surface hardening

Nitriding   4 of 5
 

Nitriding involves the diffusion of nitrogen into the surface layers of a low carbon steel at elevated temperature. The formation of nitrides in the nitrided layer provides the increased hardness. Nitriding is typically carried out in the temperature range of 500 - 575°C, this is in the ferritic state rather than the austenitic used for carburising. This is possible since ferrite has a much higher solubility for nitrogen than it does for carbon.

The advantage of nitriding in the ferritic state is that any previous heat treatment of the steel component is not disrupted and there is little / no distortion of the final component shape. This means that higher carbon steels that have been previously heat treated, such as through hardened steels which have been quenched and tempered, can be surface hardened by nitriding. Nitriding can be carried out using solid, liquid or gaseous media but the most common is gas nitriding using ammonia (NH3) gas as the nitrogen carrying species. To get a hardening effect on nitriding the steel must contain strong nitride forming elements such as Al, Cr and / or V.


 

 
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