5.13.6 Concept of Hardening
Some words that are universally understood are often misused. Hardening is one of these words. We
frequently use the word “hard” to describe something that is firm or solid. In people’s mind a hard
substance will not wear easily. This is only partially true and to understand what hardness really
means, we will look at how hardness is achieved.
The hardening process is often associated with heat treatments like quenching or aging. The process
is described as the increasing of hardness by suitable treatments, usually involving heating and cooling
or cold working.
Hardness, in fact, is a measure of the resistance of a material to plastic deformation usually by
indentation. Plastic deformation is a change in shape (no matter how small), which will remain
permanent after removal of the force which caused it. The term may also refer to stiffness or temper,
or to resistance to scratching, abrasion or cutting. Indentation hardness may be measured by various
tests such as Brinell (B), Rockwell (HR) and Vickers (HV).
Hardness testing methods measure the size of an indentation made in the surface of a material. The
indentation size made with the same load and indentor is compared (soft material has a large indent,
hard material a small indent).
Hardness is achieved by a hardening process, and the effects of this treatment will depend on the
grade of steel being treated. The response of a given steel to a hardening treatment is called
hardenability.