Work hardening
Cold forming means the forming of metals at a temperature well below the recrystallization temperature.
Since during plastic deformation, the dislocation density in the metal increases ( up to ), the probability increases that meet dislocations during their movement and thereby interfere with each other. Accordingly, a larger voltage is required for further deformation, which is reflected in an increase of yield strength and hardness. We call this behavior as work hardening.
The increased dislocation density also leads to an increase in the energy stored in the crystal.
Bring to a cold worked metal long enough in a temperature range in which the lattice defects can heal, the strain hardening is built by stress relaxation and recrystallization from again.
See also stress relief, recrystallization annealing.
Compared with hot stamping
Cold forming
- Working temperature below the recrystallization temperature
- Tight tolerances possible
- No descaling the surface
- Increase the strength and reduce the ductility, i.e., elongation at break ( hardening )
- Ductile materials are readily cold formable such as deep drawing, bending or stretching.
Hot forming
- Working temperature above the recrystallization
- Large formability of the materials
- Low deformation forces
- Slight change of strength and elongation at break on the reshaped material
Follow
The by cold forming (cold rolling, deep drawing, bending, peening or hammering and shot peening ) induced dislocations and residual stresses lead only to increase the hardness and yield strength, to different electrical and magnetic properties: the electrical conductivity and the initial decrease, and for steel may result in a permanent magnetization. In this way, overburdened tools (eg twist drills ) can magnetize spontaneously.
By cold forming the structure of the material used is so affected, for example, that the welding processing of this component can not be guaranteed. Annealing can restore by dislocation reduction weldability.
Strain hardening is often desirable and increases eg the life of a scythe by peening. Deliberately introduced superficial compressive stresses lead shot peening to high hardness and improved fatigue resistance, because tensile stresses are transferred to the underlying material and can thus do not form cracks.
The hardening is specially very pronounced in copper. Copper wire and pipes are hard, semi-hard or soft offered. The wire is cold drawn in several stages and is then cold worked ( hard). He is further processed mostly annealed or delivered. The hard tubes allows the installer to soften by heating with a gas flame locally again. When bending, they will solidify again.
Cold extrusion
Cold extrusion ( extrusion ) is a manufacturing process, which is used as an alternative to turning. The components can often be installed without additional postprocessing ( net shape ). Where this is not possible, the production of a blank is at least possible, which is manufactured only with a low to a installation rework finished component ( near net shape ). The starting material is usually a geometrically simple semi-finished products ( wire or rod ), which is formed in a press in several forming to finished parts. The geometry of the cold forged parts is usually rotationally symmetrical, but it is also complex and eccentric components are manufactured. The excellent material utilization, the method especially in middle and high number field is economically attractive.