case hardened generally refers to a
metal object (usually a
steel) that has been subjected to a process that hardens its outer surface layers. Typically during machining a tool or other piece, it is desirable for it to be soft enough to work into the required shapes, and to cut and mill with a minimum of difficulty. The finished piece however, is generally desired to be hard.
Nitriding is a way to produce this sort of surface hardening. At temperatures between 400°C and 590°C, the piece is placed in a nitrogen atmosphere that will then diffuse into the piece's surface to a certain depth. This depth is controlled by the temperature and the length of time the object is kept there.
The nitrogen combines with nitride forming elements such as Cr, Al, V, Ti, and Mo, forming precipitates that harden the surface region. A compound layer consisting of iron nitride forms at the surface of the component during processing. This layer confers corrosion resistance to the part and improves its adhesive wear resistance.
Automobile crankshafts are often hardened in this manner to make them resistant to wear.