4. Ageing tests

Intrinsic aging cannot be measured directly, as it is not a property in itself, but the evolution of properties.

Measurement will therefore depend on the property being measured, even if we can hope that variations over time, i.e. kinetics, are common to all properties.

There are two families of properties:

• mechanical properties, which are macroscopic and correspond to steels' properties of use: hardness, yield strength, ductility, toughness, etc.

• physical properties, which correspond to changes in microstructure measurable by physical quantities such as resistivity, internal friction, etc.

In fact, this is almost always an indirect measure: the precipitation of carbides or nitrides will result in a depletion of the matrix...