Hot forging refers to all those forming steps which take place above the recrystallization temperature of a metal. The hardening that takes place during forming is cancelled out again by timed relaxation phases. This reduces the dislocation density in the crystal lattice and the material remains formable. As a result, very high degrees of deformation can be applied.
- Working temperature above the recrystallization temperature
- High formability of the materials
- Low forming forces
- Little change in strength and elongation at break in the formed material
- Dimensional tolerances and surface finish not as good as with cold forming
- Can produce anisotropy
- Does not cause hardening
The hot forging process forging on forging presses for small to medium quantities and diameters up to M 200. The round material inserted into a bar mold must first be cut to length in a preparatory process. The sections are then heated in whole or in part (in gas, oil or induction furnaces) to forging temperature (depending on the material; up to 1250 °C) and partially formed in presses. To finish such bolts, machining processes (overturning, thread cutting) are then used, with the threads nowadays being produced mostly non-cutting on thread rolling machines.
Cold forging is the targeted forming of metals at a temperature significantly below the respective recrystallization temperature. Cold forging is the term used for uncontrolled plastic deformation (e.g. car accident). Plastic deformation increases the dislocation density in the crystal lattice. As a result, the dislocations in the crystal lattice meet more frequently during their movement and are thus mutually hindered. Consequently, greater mechanical work is required for further deformation.
Cold forging is therefore also noticeable in the mechanical properties of the finished screw or nut. The increase in dislocation density in the crystal lattice increases the tensile strength or yield strength and hardness of the part. This process is commonly known as strain hardening. The process is used, for example, in the stainless steel sector to produce screws of strength class A2-70 or A4-70 or A2-80 or A4-80, which cannot be increased in their strength class by heat treatment due to the material properties.
The increased dislocation density leads to stresses in the crystal lattice. Among other things, this also means that weldability of the bolt or nut cannot be guaranteed. If the bolt or nut is brought to the material-specific recrystallization temperature for a sufficiently long time after strain hardening, the microstructure relaxes again. However, the recrystallization then also significantly reduces the mechanical properties again. With this process, relatively narrow dimensional tolerances are possible and, in contrast to hot forging, the surface remains without a scale layer.
Cold forging is usually carried out on multi-stage presses and is suitable for large quantities and diameters up to M 30. The starting material is delivered coiled and uncoiled and straightened by an upstream straightening machine. Modern cold extrusion presses operate in such a way that several operations are carried out in succession on the machine and the screw leaves the line completely finished. In this process, several operations are carried out in succession during each stroke. (e.g. hexagon head preforming, upsetting, deburring and reducing threaded part).
In the subsequent process, the threads are rolled onto the defined threaded parts by thread rolling machines with flat dies or roller and segment tools without cutting. In many cases, presses with integrated threading are used. Depending on the diameter and length of the screws, such machines achieve production rates of more than 300 pieces per minute.
Compared with the forming processes, the machining of screws and nuts with lathes and milling machines is only a marginal area in the production of fasteners. Nevertheless, this process has its right to exist.
This means that small quantities in particular can be produced without the need for expensive forming tools. This advantage is particularly significant when the fasteners have to be made of special materials designed for the application. Furthermore, the process is useful when special dimensions or tolerances (deviating from standards) have to be observed.
A combination of machining and forming processes is often used, especially in the production of screws or bolts. For example, the screw blanks (without thread) are produced by machining and then the thread is inserted in a cold forming process.
One way of producing threads is thread cutting (also called thread chasing).
In this process, the thread is introduced by chip removal.
Thread rolling is the non-cutting production of threads by cold forming, in which the profile is rolled into the surface of the corresponding blank. According to DIN 8580, thread rolling belongs to forming, more precisely pressure forming, and there to rolling.
The forming process is based on the generation of compressive stresses by one or more tools imaged on the workpiece. This process is much faster and, for large quantities, less expensive than other methods of producing threads, namely thread cutting. Knurling and serrations are also produced using this process.
All materials with a minimum elongation of 6% and a tensile strength not exceeding 1400 N/mm² can be processed with this method. This also includes high-alloy steels, brass and special aluminum alloys. Particularly brittle (e.g. gray cast iron) and extremely soft materials (e.g. lead) as well as plastics cannot be rolled.
Other advantages of thread rolling:
- The grain boundaries are not interrupted. (=> unbroken material fiber).
- Surface hardening is achieved by cold forming. (=> higher strength)
- Press-polished thread flanks (=> smooth surface)
- Higher wear resistance
- Reduced notch sensitivity
- No chips, thus lower material requirement.
Damage to the thread can occur both during production (e.g. small overrolling or profile deviations, tempering, coating in the barrel) and in the further course (unpacking, storage, transport).
Minor damage such as nicks, knocks or dents which make it difficult to move in thread gauges or in the mating thread are technically unavoidable and do not constitute a material defect.
These production-related surface defects and damages are permissible up to certain limits and are described in the corresponding standards. If particularly smooth-running threads are required for certain applications, either larger tolerances or subsequent "smoothing" with thread protection should be considered.
DIN EN 26157-1 - Fasteners; surface defects; bolts for general requirements
DIN EN ISO 6157-2 - Fasteners - Surface defects - Part 2: Nuts
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