Cold forming

Cold forming is the plastic forming of metals below the recrystallization temperature . Due to the strain hardening that occurs , the material strength increases continuously. If the increase in strength is undesirable, it must be reduced again by a subsequent heat treatment. Cold forming is mainly used when tight dimensional tolerances and good surface properties are required, or to specifically increase the strength of the materials.

In contrast to cold massive forming , cold forming is not only carried out at room temperature. Conversely, not all forming processes that take place at room temperature are included. It is important whether the recrystallization temperature is exceeded, which is 40% for pure metal and 60% for alloys of the absolute melting temperature. Forming steel at 200 ° C is still considered cold forming, as this temperature is below the recrystallization temperature, while forging lead at room temperature is already hot forming , since the recrystallization temperature for lead is only 3 ° C lower .

Comparison with hot forming

Cold forming

Working temperature below the recrystallization temperature
Tight dimensional tolerances are possible
No scaling of the surface
Increase in strength and decrease in ductility , d. H. Elongation at break (work hardening)
Ductile fabrics are easy to cold form, such as B. by deep drawing , bending or stretching .

Hot forming

Working temperature above the recrystallization temperature
Great formability of the materials
Low forming forces
Little change in strength and elongation at break on the formed material

Work hardening

Since the dislocation density increases (up to ) as a result of the plastic deformation , the probability increases that dislocations interfere with one another in their movement. Correspondingly, greater stress is necessary for further deformation, which is noticeable in an increase in the yield strength and strength . One calls this behavior as cold solidification . ${\ displaystyle 10 ^ {12} \, \ mathrm {m} ^ {- 2}}$

This effect can be used to increase the strength of a material by low pre-deformation z. B. to increase by rolling or pulling .

Tensile test

The work hardening ensures that the flow curve of a metal increases in the plastic range. If the material is relieved after plastic deformation, the stress-strain curve follows a line parallel to the elastic straight line. With renewed loading, the yield point is increased by: ${\ displaystyle \ sigma _ {v}}$

{\ displaystyle \ Delta \ sigma _ {v} = k_ {v} \ cdot M \ cdot G \ cdot b {\ sqrt {\ rho}}}

Here is

${\ displaystyle k_ {v}}$ the pre-factor for strain hardening (usually ) ${\ displaystyle k_ {v} \ approx 0 {,} 1 \ dots 0 {,} 2}$
${\ displaystyle M}$ the Taylor factor
${\ displaystyle G}$ the shear modulus
${\ displaystyle \ rho}$ the dislocation density.

At the above When the load is renewed, the stress-strain curve ideally runs on the same straight line as the previous release. The elongation up to the constriction or up to the break is correspondingly reduced, i. that is, the material has clearly lost its ductility . This is why work hardening is only suitable for materials that already have a high initial ductility.

consequences

By cold forming ( cold rolling , deep drawing , bending , peening or hammering and shot peening ) caused dislocations and internal stresses lead in addition to increasing the hardness and yield strength also changed electrical and magnetic properties: the electrical conductivity and the initial permeability decrease, and Steel can become permanently magnetized. In this way, overloaded tools (e.g. twist drills ) can magnetize themselves spontaneously .

Work hardening is often desirable and increases e.g. B. the service life of a scythe by pounding. Targeted superficial compressive stresses lead to high hardness and improved fatigue strength during shot peening , as tensile stresses are shifted to the underlying material and so no cracks can form.

The work hardening is particularly pronounced with copper . Copper wire and tubing are offered hard , semi-hard or soft . The wire is cold drawn in several stages and is then cold worked (hard). It is mostly processed further annealed or delivered. The fitter can locally soften the hard pipes again by heating them with the gas flame. When they are bent, they solidify again.

literature

Industrieverband Massivumformung eV (Hrsg.): Cold massive forming: precision in series. Revised edition. Information point for the massive forming industry association, Hagen 2012, ISBN 978-3-928726-29-0 .