Work hardening

The work hardening of a Métal corresponds to the modifications which it undergoes when the constraints which are applied to him are sufficiently strong to cause plastic deformations, permanent. These modifications are of a metallurgical nature (modification of the internal structure of metal) and generally have an influence on its mechanical properties.

Work hardening occurs only on the ductile materials and in the plastic field.

Work hardening is characterized by an increase in the Limit elastic and hardness, which becomes also more fragile. According to metals considered the mechanical properties can evolve to an increase in resistance (case of the allied Acier S) up to a certain point (threshold of rupture) or contrary worms its reduction (case of not very allied steels).

Stress concentration and local work hardening

When the part comprises a variation of section or a defect - cavity, inclusion (Précipité) harder or less hard than the remainder of material - it can locally occur a Concentration stresses. Whereas one thinks of being in the elastic field, one enters the plastic range locally.

It can thus occur a local work hardening. This phenomenon is one of principal the causes of the birth of cracks in the phenomena of tiredness.

Multiplication of dislocations

The plastic deformation of a metal part is done by the movement of the Dislocation S.

These dislocations multiply according to the mechanism of Franck and Reed:

1. They are pinned (by specific defects, precipitates or other normal dislocations in the plan of movement);
2. the pinned parts remain fixed, the moving parts extend around the fastening;
3. When the moving parts meet, that forms a loop of dislocation which moves (extends) freely while the pinned part starts again the cycle.