The concept of cold forgingIn the fastener forming process, cold heading (extrusion) technology is a main processing technology. Cold heading (extrusion) belongs to the category of metal pressure processing. In production, at room temperature, external force is applied to the metal to form the metal in a predetermined mold. This method is usually called cold heading.In metallurgy, the forging performed when the material is heated to the recrystallization temperature (about 700°C for steel) is called hot forging.For steel forgings, the forging that is below the recrystallization temperature and above room temperature is called warm forging.
Advantages of cold heading (squeezing)High utilization rate of steel: Cold heading (extrusion) is a less and no cutting processing method, such as processing rod type hexagonal head bolts and cylindrical head hexagon socket screws. The cutting processing method is adopted, and the utilization rate of steel is only 25% to 35%. However, with the cold heading (extrusion) method, its utilization rate can be as high as 85%-95%, which is only the process consumption of the material head, the material tail and the cutting of the hexagonal head edge.High productivity: Compared with general cutting processing, the efficiency of cold heading (extrusion) is dozens of times higher.Good mechanical properties: For parts processed by cold heading (extrusion) method, since the metal fiber is not cut, the strength is much better than that of cutting.Suitable for automated production: Fasteners suitable for cold heading (extruding) methods (including some special-shaped parts) are basically symmetrical parts, suitable for high-speed automatic cold heading machine production, and also the main method of mass production.In short, the cold heading (extrusion) method for processing fasteners and special-shaped parts is a processing method with high comprehensive economic benefits. It is a processing method commonly used in the fastener industry, and it is also a widely used method at home and abroad. Very developed advanced processing methods.
02 Cold forging process
Generally speaking, cold forging is to obtain the final shape of the part through a combination of multiple processes. Figure 2 is an example of cold forging. After the blank is cut, the shaft is positively extruded, the cup tube is reversely extruded, the cup tube is positively extruded, upset, punched, and the tube is extruded.Divided into multi-process processing is to avoid excessive pressure during one-time forming. Since the fewer processes, the lower the cost, reducing the forming pressure and reducing the number of processes is the key to process design.
03 Overview of the main processing methods of cold forging
the free upsetting of the outer surface without being constrained by the mold. The processing pressure increases with friction constraints. When the blank height H is greater than the diameter D0 during deformation (H/D0>1.0), C=1.2, but when the blank becomes thinner, C will rise to about 2.5.When the reduction rate increases, as shown in Figure 4, cracks will occur in the oblique and longitudinal directions of the outer peripheral surface. The occurrence of cracks depends on the ductility of the material. For this reason, it is necessary to use materials specially made for cold forging. When the reduction rate increases, Figure 3 shows the size of the restraint coefficients during various upsetting.
In free upsetting, when the initial height of the blank is more than twice the diameter, as shown in Figure 5, the blank is bent due to material instability, thereby forming a folding defect. In order to prevent material instability, a mold of the shape of Figure 6 is usually used for pre-forming.
Semi-hermetic forging is a method to increase the pressure in the cavity by generating flash and promote material filling. All the big technology guys know about the Chinese screw net screw. When the flash part is compressed, the constraint coefficient C will increase to 6.0 to 9.0, and the thickness of the flash should be controlled above the necessary thickness as much as possible. Figure 8 is an example of cold forging using semi-hermetic forging.
The extrusion of the cup tube is to squeeze the punch into the material to form a cylindrical part with a bottom while the outer diameter of the blank is constrained by the die. This is the most commonly used method in cold forging.Usually the extrusion punch squeezes into the material, and the direction of material flow is opposite to the direction of movement of the punch, so it is called back extrusion, but there is also a forward extrusion method in which the punch does not move through the extruded material to form a cylindrical part.
04 Compound drive cold forging method
In general, cold forging forming requires multiple processes, which is mainly due to the excessive pressure of the mold when forming with one process. High die surface pressure will not only cause strength damage to the die, but also cause elastic deformation of the die, resulting in a decrease in the accuracy of the finished forging.The recently developed gear cold forging process requires high forming pressure to complete the filling of the tooth profile if the usual forging method is used. In order to forge gears with as few steps as possible, it is necessary to use a closed forging or split forging method with a compound motion function.Block forgingFigure 15 shows the principle of closed forging and bevel gear forging. The blank is put into the cavity formed by the upper and lower molds, and the material is compressed and deformed by the upper and lower punches.The contact area between the material and the punch remains almost the same. The material is squeezed to the radial direction. Compared with the compression flash in the semi-hermetic forging, the forming force can be greatly increased.In addition to the movement and clamping force of the upper and lower punches, a specially designed mold base device is required to use this method. Through the closed forging method, we have successfully produced bevel gears and constant velocity joints.
The principle of split forging is to design a space for material flow in both the main direction and the opposite direction of the material flow, thereby reducing the forging pressure.When back-extrusion is shown in Figure 16a, the method of designing an extrusion outlet in the front is called the abandoning shaft method. Figure 16b is to flow the material to the outer toothed part, and at the same time design a cavity inside the material so that the material will generate direction at the same time. The flow on the inside is the so-called hole-making method.
This method is applied to the forging of gears through the use of composite action molds. According to the characteristics of material flow, this method is called the split forging method. In recent years, the precision forging of helical gears has been successfully developed by using this method and using the compound movement of the tooling