2.1.4 Determination of forging times
In cold forging, products usually need to go through two or more forging processes before forming. Reasonable determination of forging times will fully utilize the allowable deformation degree of the metal, improve the service life of the mold, and ensure the quality of the product. Determine the number of forging cycles, taking into account the following factors:
a. Upsetting forging ratio
The ratio of the length to diameter of the deformed part of the billet is too large, and longitudinal bending will occur in one forging. After flattening, interlayers will appear, as shown in Figure 36-9. To avoid these defects during forging, it is necessary to increase the number of forging cycles. Firstly, the billet is pre upset into a cone shape, and then precision upset is carried out until the desired shape is achieved.
Figure 36-9 Schematic diagram of interlayer generated due to longitudinal bending
The forging frequency is generally determined based on the following data:
, it can be forged in one go;
, forge twice;
, forge three times.
b. Consider the ratio of the diameter D of the workpiece head to the height H.
As shown in Figure 36-10, it is a large-diameter thin flat head thin rod part with a larger head diameter and a smaller height. The required billet h0/d0 is above 2 for the large-diameter thin rod part. If it is formed by one-time forging, cracks will occur at the edge of the head. Similar workpieces can only be formed gradually by increasing the number of upsetting times.
c. Consider the surface roughness requirements of the workpiece and the complexity of the external geometric shape
For machine screws with shapes such as semi-circular heads and cylindrical heads, although the ho/do value of the required blank for the head is generally less than 2.5, in order to fully fill the head during deformation and meet the standard requirements, two upsets are generally used. Pre heading conical head creates good metal flow conditions for precision heading head forming. For example, when using wire with large diameter and small deformation to upset nuts, a wire diameter of 0.9s (s is the size of the opposite side of the hexagonal nut) is used. Generally, the degree of deformation of the product is about 25%. However, due to the complex shape of the hexagonal nut, there are many deformation methods in the upset process. It has both cold heading and composite extrusion and punching. In order to facilitate the flow of metal during deformation, 3-4 times of upset forming is selected.
Figure 36-10
It is worth emphasizing that not all products with complex shapes can be solved by increasing the number of forging cycles. Often, some products have an increased number of upsets, making them easy to form in the first and second upsets. However, due to cold work hardening, it is difficult for the products to undergo subsequent upsets. Manifested as cracking or damage to the mold during forging of the workpiece. The key to solving such problems lies in reducing deformation, increasing the plasticity of steel, and adopting more effective lubrication. Large diameter wire and small deformation process are selected for bolts and screws in cold forging process. Generally, the diameter of the wire is close to the diameter D of the screw thread, and one or two rod reductions are used to achieve the size of the screw blank. For medium carbon steel and alloy steel, spheroidizing annealing is used in material modification to improve the cold heading plasticity of the steel, and phosphating and saponification treatments are used to ensure surface lubrication of the steel and minimize friction during deformation. In addition, adding strength and toughness to the mold allows it to withstand complex deformations with rigidity, as well as sufficient toughness and wear resistance.
2.1.5 Calculation method of force in cold heading process
2.1.5.1 Cold heading force
Cold heading force is the main basis for determining process parameters, designing molds, designing cold heading machines, and selecting specialized equipment.
There are many factors that determine the size of cold heading force, mainly in the following aspects:
a. Mechanical properties of metals
The cold heading force increases with the increase of material strength and hardness.
b. Workpiece shape and degree of deformation
The cold heading force increases with the increase of workpiece deformation.
c. Friction
Due to the frictional force on the contact surface between the mold and the workpiece, the direction and magnitude of the force are changed to varying degrees, resulting in an impact on the cold heading force.
d. Mold shape
The different shapes of molds result in differences in the flow resistance of metals in various directions, which in turn affects the cold heading force.
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