3. Aging mechanisms

Qualitatively, the aging phenomenon can be interpreted simply by the diffusion of interstitials: nitrogen or carbon.

To establish quantitative laws, we need to combine several approaches:

• the thermodynamic approach: solubility and diffusion laws; these laws determine the quantity of active interstitials, the aging rate and the time-temperature equivalence;

• the mechanical approach involving the blocking or braking of dislocations by interstitials; this approach should make it possible to introduce the rate of deformation and explain the intensity of the ageing measured;

• the phenomenological approach to interpreting indicators which are the consequences of ageing; this is the case in particular for the return of the bearing after skin pass or fragilization....