External force is applied to metal blanks (excluding plates) to cause plastic deformation, change size, shape and improve performance, so as to manufacture mechanical parts, workpieces, tools or blanks.
When the temperature exceeds 300-400℃ (blue brittle zone of steel) and reaches 700-800℃, the deformation resistance will decrease sharply and the deformation capacity will be greatly improved. According to the forging in different temperature zones, according to the different requirements of forging quality and forging process, it can be divided into three forming temperature zones: cold forging, warm forging and hot forging. Originally, there was no strict boundary for the division of such temperature zones. Generally speaking, forging in temperature zone with recrystallization is called hot forging, and forging at room temperature without heating is called cold forging.
When forging at low temperature, the size of the forging changes very little. When forging below 700℃, less scale is formed and there is no decarburization on the surface. Therefore, as long as the deformation energy is within the forming energy range, cold forging can easily obtain good dimensional accuracy and surface finish. As long as the temperature and lubrication cooling are well controlled, warm forging below 700℃ can also achieve good accuracy. During hot forging, large forgings with complex shapes can be forged because the deformation energy and deformation resistance are very small. To obtain forgings with high dimensional accuracy, hot forging can be used in the temperature range of 900-1000℃. In addition, attention should be paid to improving the working environment of hot forging. The life of the forging die (2,000-5,000 pieces for hot forging, 10,000-20,000 pieces for warm forging, and 20,000-50,000 pieces for cold forging) is shorter than that of forging in other temperature ranges, but it has a large degree of freedom and low cost.
The billet will deform and work harden during cold forging, so that the forging die will bear high loads. Therefore, it is necessary to use a high-strength forging die and adopt a hard lubricating film treatment method to prevent wear and adhesion. In addition, to prevent the billet from cracking, intermediate annealing is required to ensure the required deformation capacity. In order to maintain a good lubrication state, the billet can be phosphated. When using bars and wire rods for continuous processing, the cross section cannot be lubricated at present, and the possibility of using phosphating lubrication method is being studied.
According to the movement mode of the billet, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Since closed die forging and closed upsetting have no flash, the utilization rate of materials is high. The finishing of complex forgings can be completed with one or several processes. Since there is no flash, the force-bearing 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. For this reason, the volume of the billet, the relative position of the forging die, and the forging should be strictly controlled, and efforts should be made to reduce the wear of the forging die.
According to the movement mode of the forging die, forging can be divided into swing rolling, swing rotary forging, roll forging, wedge cross rolling, ring rolling, and oblique rolling. Swing rolling, swing rotary forging, and ring rolling can also be processed by fine forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the front process of slender materials. Like free forging, rotary forging is also partially formed. Its advantage is that it can be formed under a smaller forging force than the size of the forging. In this forging method, including free forging, the material expands from the vicinity of the die surface to the free surface during processing. Therefore, it is difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the rotary forging process with a computer, products with complex shapes and high precision can be obtained with lower forging force. For example, forgings such as turbine blades with many varieties and large sizes are produced.
The die movement and degree of freedom of the forging equipment are inconsistent. According to the characteristics of the bottom dead point deformation limitation, the forging equipment can be divided into the following four forms:
• Limiting forging force form: hydraulic press with hydraulic direct drive of the slider.
• Quasi-stroke limitation method: hydraulic press with hydraulic drive of crank-connecting rod mechanism.
• Stroke limitation method: mechanical press with crank, connecting rod and wedge mechanism driving the slider.
• Energy limitation method: spiral and friction press using spiral mechanism.
In order to obtain high precision, attention should be paid to preventing overload at the bottom dead point and controlling the speed and die position. Because these will affect the tolerance, shape accuracy and life of the forging die. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance of the slider rails, ensuring rigidity, adjusting the bottom dead point and using auxiliary transmission devices.
In addition, according to the movement mode of the slider, there are also vertical and horizontal movement modes of the slider (used for forging of slender parts, lubrication and cooling, and high-speed production of parts forging), and the use of compensation devices can increase movement in other directions. The above methods are different, and the required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different. These factors are also factors that affect the level of automation.
Metals can improve their organizational structure and mechanical properties after forging. After the casting organization is deformed by hot working by forging, due to the deformation and recrystallization of the metal, the original coarse dendrites and columnar grains are transformed into equiaxed recrystallized structures with finer grains and uniform size, so that the original segregation, looseness, pores, slag inclusions, etc. in the ingot are compacted and welded, and its organization becomes tighter, which improves the plasticity and mechanical properties of the metal.
Generally speaking, the mechanical properties of castings are lower than those of forgings of the same material. In addition, forging processing can ensure the continuity of metal fiber structure, keep the fiber structure of forgings consistent with the shape of forgings, and ensure that the metal streamlines are complete, which can ensure that the parts have good mechanical properties and long service life. Forgings produced by precision die forging, cold extrusion, warm extrusion and other processes are incomparable to castings.
