Forging technology and types

A forming method for manufacturing mechanical parts, workpieces, tools, or blanks by applying external forces to metal billets (excluding sheets) to cause plastic deformation, change size, shape, and improve performance.

Types and characteristics of forging

When the temperature exceeds 300 ℃ -400 ℃ (in the blue brittle zone of steel) and reaches 700 ℃ -800 ℃, the deformation resistance will sharply decrease, and the deformation energy will also be greatly improved. According to the forging carried out in different temperature regions, it can be divided into three forming temperature regions: cold forging, warm forging, and hot forging, depending on the quality of the forging and the requirements of the forging process. Originally, there was no strict boundary for the division of this temperature region. Generally speaking, forging in the temperature region with recrystallization is called hot forging, and forging at room temperature without heating is called cold forging.

During low-temperature forging, the size variation of the forging is minimal. Forging below 700 ℃ results in less formation of oxide scale and no decarburization on the surface. Therefore, as long as the deformation energy is within the range of forming energy, cold forging can easily achieve good dimensional accuracy and surface smoothness. As long as the temperature and lubrication cooling are well controlled, forging below 700 ℃ can also achieve good accuracy. During hot forging, large forgings with complex shapes can be forged due to their low deformation energy and resistance. To obtain high-precision forgings, hot forging can be used in the temperature range of 900 ℃ -1000 ℃. Additionally, attention should be paid to improving the working environment for hot forging. The lifespan of the forging die (hot forging 2-5000, warm forging 10-20000, cold forging 2-50000) is shorter compared to forging in other temperature ranges, but it has greater freedom and lower cost.

The billet undergoes deformation and work hardening during cold forging, causing the forging die to bear high loads. Therefore, it is necessary to use high-strength forging dies and use hard lubricating film treatment methods to prevent wear and adhesion. In addition, to prevent cracks in the billet, intermediate annealing is necessary to ensure the required deformation capacity. To maintain good lubrication, the billet can be phosphatized. At present, lubrication treatment cannot be applied to the cross-section when using bar and wire rod for continuous processing, and the possibility of using phosphating lubrication method is being studied.

According to the movement of the billet, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Closed die forging and closed upset forging have high material utilization due to the absence of burrs. It is possible to complete the precision machining of complex forgings with one or several processes. Due to the absence of burrs, the stress area of the forging is reduced, and the required load is also reduced. However, it should be noted that the billet cannot be completely restricted. Therefore, it is necessary to strictly control the volume of the billet, control the relative position of the forging die, and measure the forgings, striving to reduce the wear of the forging die.

According to the movement mode of the forging die, forging can be divided into swing forging, swing rotary forging, roll forging, cross wedge rolling, ring rolling, and oblique rolling. Rotary forging, rotary forging, and ring forging can also be processed by precision forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the preceding processes for processing slender materials. Rotary forging, like free forging, is also locally formed, and its advantage is that it can be formed even under smaller forging forces compared to the size of the forging. This forging method, including free forging, involves the expansion of materials from the vicinity of the mold surface to the free surface during processing, making it difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the rotary forging process with a computer, complex and high-precision products can be obtained with lower forging force. For example, producing forgings such as steam turbine blades with multiple varieties and large sizes.

The mold movement and degrees of freedom of forging equipment are inconsistent. According to the characteristics of deformation limitations at the bottom dead center, forging equipment can be divided into the following four forms:

Limited forging force form: an hydraulic press that directly drives the slider with oil pressure.

Quasi stroke limit method: an hydraulic press that drives the crank connecting rod mechanism by hydraulic pressure.

Stroke limitation method: a mechanical press with a crank, connecting rod, and wedge mechanism driving the slider.

Energy limitation method: Utilize the spiral mechanism of the screw and friction press.

In order to achieve high accuracy, attention should be paid to preventing overload at the bottom dead center, controlling the speed and mold position. Because these will have an impact on the tolerance, shape accuracy, and die life of forgings. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance between the slider guide rails, ensuring stiffness, adjusting the lower dead center, and utilizing auxiliary transmission devices.

In addition, according to the movement mode of the slider, there are also vertical and horizontal movements of the slider (used for forging, lubrication and cooling of slender parts, and forging of high-speed production parts), and the use of compensation devices can increase movement in other directions. The above methods differ in terms of required forging force, process, material utilization, output, dimensional tolerance, and lubrication and cooling methods, which are also factors that affect the level of automation.