In modern manufacturing, the diversity and efficiency of metalworking technologies play a crucial role in determining the quality of products and the efficiency of production. Radial forging, as an advanced metalworking process, has gained widespread application in various fields due to its unique advantages. This article will provide a detailed introduction to the principles, advantages, application scope, and different types of radial forging processes, helping readers gain a comprehensive understanding of this efficient and precise machining technology.
Radial forging is a forming method that applies multiple forging forces evenly distributed in the circumferential direction to shaft parts simultaneously in the same plane. Specifically, multiple hammers move in a radial direction under the drive of the driving mechanism, while the workpiece is fed axially under the drive of the holding mechanism. During the feeding process, the rotation speed of the workpiece is controlled according to the process requirements. This process subjects the workpiece to uniformly distributed high-speed, high-frequency, and short-stroke forging forces in the circumferential direction, causing the material to flow in the axial and radial directions under a state of high hydrostatic pressure.
Radial forging, as an advanced metalworking process, stands out among various metalworking technologies due to its unique processing principles and a series of significant advantages. These advantages make radial forging an indispensable part of modern manufacturing.
The workability of materials is influenced by various factors, including intrinsic factors such as the material's composition and microstructural state, as well as extrinsic factors such as stress state, temperature, and deformation rate during processing. Radial forging applies pressure in the circumferential direction during processing, generating high hydrostatic pressure within the material, thereby increasing its ductility. This makes brittle materials, which are prone to cracking, less likely to develop cracks during deformation, and even existing cracks can be pressed together. Moreover, due to the deformation heat generated by high-frequency forging, the temperature drop during the forging process is minimal, making it easier to achieve isothermal forging, which is particularly suitable for high-alloy steels that are prone to segregation at high temperatures. Isothermal forging can better control grain size, enabling fine-grained forging across the entire section, thereby enhancing the overall performance of the part.
Parts processed by radial forging are characterized by high precision and low surface roughness. The dimensional tolerance of the inner diameter of hot-forged pipe fittings can reach ±0.1 mm, while the dimensional tolerance of the outer diameter of cold-forged workpieces can reach ±0.1 mm, with an inner diameter tolerance of ±0.025 mm, and a surface roughness of 3.2–0.4 μm. Since the temperature drop during radial forging is small and the cooling effect of the mold is weak, no deformation dead zones are formed on the workpiece surface. On the contrary, surface deformation is greater than core deformation. This ensures that most of the material in the workpiece is in the same deformation state, thereby guaranteeing the uniformity and consistency of the part.
Radial forging machines have a high degree of automation, high production efficiency, and low material consumption. Currently, numerical control technology has been widely adopted in radial forging machines, with high forging frequency and no interruptions between steps, significantly improving production efficiency. Due to the high precision of parts processed by radial forging, the machining allowance is small, which can significantly save raw materials. In particular, for hollow stepped shafts, radial forging can save 30%–50% of material compared to machining.
Despite its many advantages, radial forging also has some inherent disadvantages and limitations. First, the structure of radial forging equipment is complex, and the reciprocating motion stroke of the hammer is limited to a specific range. Therefore, without changing the hammer, the deformation space of the workpiece is relatively small. Second, compared to a free-forging hydraulic press, the maximum forging force of the hammer of a radial forging machine of the same tonnage is relatively smaller. For high-strength materials, high-frequency forging and the maximum forging force of the equipment limit the amount of deformation per forging, and the deformation area is easily confined to the region near the surface of the part. In addition, radial forging machines are complex in structure, expensive to build, have high initial investment and maintenance costs, and are limited in the types of parts that can be forged, mainly shaft parts, with specific limitations on the optimal forging diameter and size of the blank. Although radial forging has high production efficiency, it is only suitable for mass production and is not economically viable for single-piece or small-batch production. If the metallurgical quality of the blank is poor, the ability of radial forging to forge and close core defects in the blank is inferior to that of a forging hammer.
Radial forging processes are diverse and can meet the needs of different industrial scenarios and material processing requirements.
Depending on the temperature of the workpiece during forging, radial forging can be divided into hot forging and cold forging processes. Currently, most radial forging workpieces use the hot forging process, with the pre-forging blank being either a casting ingot or a preformed shaft blank. The cold forging process is mainly used for workpieces that are close to the final size and require high dimensional accuracy, and it is often used in automated production lines.
Depending on the feeding method of the workpiece during forging, radial forging can be divided into axial feeding, die feeding, and local upsetting.
Axial Feeding The workpiece enters from one side of the forging machine, and the chuck controls the passage of the workpiece through the rotation and feeding speed of the hammer on one or both sides of the forging machine. This method is the most commonly used for processing round bars and round blanks.
Die Feeding The workpiece is held at one end, and a specific area of the die forms the desired shape. This method is often used in conjunction with a core die to forge hollow parts and for closed die forging.
Local Upsetting An axial force is applied to a locally heated rod or tube, and the heated area is locally upset and then radially forged into shape. During the upsetting process, a core die may be used to control the shape and size of the inner hole.
Depending on the equipment configuration, radial forging can be divided into three major categories: mechanically driven, hydraulically driven, and hybrid mechanical-hydraulic driven.
An eccentric shaft installed on the periphery drives a double-slider mechanism connected to it to convert rotation into linear motion. The motion is transmitted to the hammer through a lead screw driven by a worm gear. The synchronization of the hammer is ensured by a gear system that drives four eccentric shafts. The stroke position of the forging hammer can be synchronously changed simultaneously or in pairs by four sets of worm gears, allowing the processing of workpieces with circular, square, or rectangular cross-sections. The stiffness of a fully mechanically driven radial forging machine is relatively high, resulting in higher dimensional accuracy of the workpiece.
The hammer and die are directly connected to the hydraulic-driven actuator. The deformation amount and deformation rate can be well controlled. Unlike mechanical drive, the mold movement speed varies depending on the size, shape, and material of the workpiece. For high-strength materials requiring large deformation forging, the forging machine needs a greater forging force. Compared to mechanically driven radial forging machines, the forging frequency of hydraulically driven radial forging machines is often lower, mainly due to the characteristics of the hydraulic system. Also, due to the compressibility of hydraulic oil, the final accuracy of the workpiece processed by a hydraulically driven radial forging machine is relatively lower.
In this equipment, four eccentric shafts are arranged on an octagonal frame and driven by a synchronous gear system integrated in the forging box. The main driving force for the hammer is still provided by the eccentric shaft, and a hydraulic cushion is connected between the hammer and the eccentric shaft to adjust the mold stroke position. The hydraulic cushion also serves the functions of overload protection and real-time measurement of forging force. The hydraulic cushion requires very little installation space, making the design of this type of forging machine quite compact. The forging frequency of this type of equipment is determined by the drive system of the eccentric shaft. Compared to purely hydraulically driven forging machines, it can achieve a higher forging frequency, thereby obtaining higher dimensional accuracy, surface quality, and production efficiency.
The application scope of radial forging is very broad. Different processes can be used to process shaft and tube parts of various shapes. These parts include stepped shafts and conical shafts of large-diameter long rotary bodies; necking and reducing the diameter of thin-walled tubular parts; internal holes with specific shapes, such as internal splines and rifled gun barrels; and profiles with shapes like rectangular, hexagonal, octagonal, and other polygonal bars. In addition to ordinary carbon steel, radial forging can also be applied to alloy steel, tool steel, copper alloy, aluminum alloy, and magnesium alloy. It is particularly suitable for low-plasticity, high-strength refractory metals such as tungsten, molybdenum, zirconium, niobium, and their alloys for billet making and forging. It can also forge materials with poor forgeability, such as powder metallurgy ingots and white cast iron.
Radial forging, as an efficient and precise metalworking technology, holds an important position in modern manufacturing. Through its unique processing principles, it enhances material workability, achieves high precision and excellent surface quality, and improves production efficiency while reducing material consumption. Despite some limitations, radial forging can meet a variety of industrial needs through different processes and equipment configurations. With the continuous progress of technology, radial forging will play a greater role in more fields and make significant contributions to the development of manufacturing.
