Depth of Hardening

the degree to which steel acquires a martensitic structure in a layer of some thickness upon hardening. It is conditioned to some extent by the composition of the steel and the conditions of heating and cooling, but it is primarily determined by the kinetics of the transformations of austenite; thus, it increases with the increasing stability of austenite at the temperatures of the pearlite and bainite transformations.

The depth of hardening can be determined experimentally, for example, by the end-quench test, as well as by calculation on the basis of diagrams of austenite transition. In most cases, hardening must extend to the object’s center in order to obtain uniform mechanical properties over the entire cross section because the presence of nonmartensite products of austenite transformations (ferrite, pearlite, and bainite) in the structure leads to a reduction in plasticity and impact toughness after hardening and tempering.