Cold forming Basic principles If the billets used in the forming process are not pre-heated and the forming takes place at room temperature, the process is called cold forming. As no shrinkage or high-temperature oxidation occurs with this process, cold-formed parts demonstrate greater form and dimensional stability than comparable hot forged parts. However, with cold-forming there is less design freedom compared with the hot-forming process

due to the lower formability of the "cold" material and the bigger contact stresses between workpiece and tooling. The most important cold-forming process is extrusion. With this process, the material is pressed through a tool nozzle. The parts produced are generally axisymmetric. It is less common for parts with secondary elements to be produced by cold forming. As the final geometry cannot be achieved via a single forming process, cold massive forming processes are generally used in combination, namely in sequential forming stages. The animation shows the production sequence for a cold-extruded transmission shaft which is produced by forward direct extrusion, free reduction and upsetting. Therefore, a suitable choice of material (steel) is essential to cold-forming, as is the special pre-treatment of the workpiece in order to optimize microstructure and surface for the subsequent forming process, as well as a special tool technology. In addition to various non-ferrous metals, unalloyed and low alloyed steels are the preferred materials for cold forming. These materials should exhibit a carbon content of less than 0.5% and the content of the other alloying components should be less than 5%. Other material elements such as sulfur or phosphorous should only be present in minimal amounts (max. 0.035%). The formability of the microstructure is thus optimized prior to forming by annealing on spheroidal cementite (so-called spherodize annealing).

Sequence of forming stages in the production of a transmission shaft

Parts produced by cold-forming offer great dimensional and geometric accuracy. Consequently, the machining allowances can be kept to a minimum. In special cases it is possible to produce complex functional areas such as splines or ball tracks in constant velocity joints, which are ready to install. As with forged (hot and warm) workpieces the fiber orientation is more favorable in cold formed parts than in machined parts. With optimized grain flow and utilization of the work hardening, the fatigue strength can be increased by as much as 30% when using the same material.

Three-stage forming tool for producing transmission shafts Stage process of a cold-extruded transmission shaft