Hardening and softening

Surface hardening involves selective hardening of just the surface of a part or changing the composition of the surface by diffusing a small atom into the surface (usually C or N), followed by heat treatment of the surface. An example is case hardening of steel by increasing the carbon content of the surface by heating the part for about one hour in contact with some source of carbon (such as methane gas) followed by heat treatment. 

The advantage of surface hardening over through-hardening is that the bulk of the part may remain relatively soft for shock resistance while the surface is hardened for wear resistance. 

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In the high temperature regime for covalent crystals where the hardness drops rapidly, its values are affected by impurities (dopants). Both hardening and softening occur depending on whether the dopant is is a donor, or an accepter... 

This is similar to the variation in the skin hardening and softening as discussed in Sect. 24.3. For instance, the Y value of ZnO nanobelts of 50-140 nm thick and... 

Stress-strain relationships for the steel material including strain hardening and softening... 

Finally, the Cogswell [5] and Binding method [6] were used for determination of the extensional viscosity in the strain hardening and softening parts, respectively. Of course, effective entry length correction [8] has been applied to deal with all extensional viscosity data. 

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