Forging is to apply external force to the metal blank (not including plate), make it produce plastic deformation, change the size, shape and improve the performance, for the manufacture of mechanical parts, workpiece, tools or blank forming processing method.
Types and characteristics of forging
When the temperature exceeds 300-400℃ (the blue brittle zone of steel) and reaches 700-800℃, the deformation resistance will be sharply reduced and the deformation energy will be greatly improved. According to forging in different temperature zones, according to the quality of forgings and different forging process requirements, it can be divided into cold forging, warm forging, hot forging three forming temperature zones. The original division of this temperature area does not have strict boundaries, generally speaking, in the temperature area of recrystallization forging is called hot forging, not heated at room temperature forging is called cold forging.
During low temperature forging, the size of the forgings changes very little. When forged below 700℃, the oxidation scale is less formed and the surface has no decarburization. Therefore, as long as the deformation energy is within the range of forming energy, cold forging is easy to obtain good dimensional accuracy and surface finish. As long as the temperature and lubrication cooling are well controlled, good accuracy can be obtained for warm forging under 700℃. During hot forging, large forgings with complex shapes can be forged because the deformation energy and deformation resistance are small. To obtain high dimensional precision forgings, 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. Die life (hot forging 2-5 thousand, warm forging 10-20 thousand, cold forging 20-50 thousand) is shorter than other temperature range forging, but it has greater freedom and lower cost.
The blank will produce deformation and work hardening during cold forging, so that the forging die will bear high load. Therefore, it is necessary to use high strength forging die and adopt hard lubrication film treatment method to prevent abrasion and bonding. In addition, in order to prevent billet cracks, intermediate annealing is carried out when necessary to ensure the required deformation capacity. In order to maintain good lubrication state, the billet can be phosphated. At present, the cross section cannot be lubricated in continuous processing with bars and wire rods, and the possibility of using phosphating lubrication is being studied.
According to the movement mode of billet, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging and closed upsetting. Closed die forging and closed upsetting have high material utilization because of the lack of flicking edges. It is possible to finish complex forgings in one or several processes. Since there is no flash, the forgings are less stressed and the load required is reduced. However, it should be noted that the blank can not be completely restricted, so we should strictly control the volume of the blank, control the relative position of the forging die and measure the forging die, and strive to reduce the wear of the forging die.
According to the movement mode of forging die, forging can be divided into rotary forging, rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling. Rotary forging, rotary forging, and ring forging may also be performed by precision forging. In order to improve the utilization rate of the material, roll forging and cross rolling can be used as the previous process of the elongated material. Like free forging, rotary forging is locally formed and has the advantage that it can be formed with less forging force compared to the size of the forging. In this forging method, including free forging, the material expands from near the die surface to the free surface during processing. Therefore, it is difficult to guarantee the accuracy. Therefore, the movement direction of the forging die and the rotary forging sequence are controlled by the computer, and the products with complex shape and high precision can be obtained with lower forging force. For example, the production of many varieties, large size of turbine blades and other forgings.
The die movement of forging equipment is inconsistent with the degree of freedom. According to the following four deformation limiting characteristics, forging equipment can be divided into the following four forms:
· Form of limiting forging force: hydraulic press that directly drives the slider.
· Quasi-stroke limit mode: hydraulic press with hydraulic drive crank and connecting rod mechanism.
· Stroke limit mode: mechanical press with crank, connecting rod and wedge mechanism driving the slider.
· Energy restriction mode: screw and friction press with screw mechanism.
In order to obtain high precision, attention should be paid to prevent overload at the bottom dead point and control the speed and mold position. Because these will have an impact on the forging tolerance, shape accuracy and die life. In addition, in order to maintain accuracy, attention should also be paid to adjusting slider rail clearance, ensuring stiffness, adjusting the lower dead point and the use of auxiliary transmission device and other measures.
In addition, according to the sliding block movement mode and sliding block vertical and horizontal movement (for the forging of slender parts, lubrication and cooling and high-speed production parts forging), the use of compensation device can increase the movement in other directions. Different methods require different forging forces, processes, material utilization, yield, dimensional tolerances, and lubrication and cooling methods, which are also factors affecting the level of automation.
What are the characteristics of forgings compared with castings
The microstructure and mechanical properties of metal can be improved after forging. Casting organization after forging method of thermal deformation due to metal deformation and recrystallization, make original bulky dendrite and columnar grain to grain is fine and uniform axial recrystallization organization, make the ingot in the original segregation, porosity, porosity, slag compaction and welded, such as its organization become more closely, 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, the metal forging process can guarantee the continuity of fibrous tissue, the forging of fibrous tissue and forging appearance consistent, metal flow is complete, can guarantee the parts with good mechanical properties and long service life with precision die forging, cold extrusion, extrusion temperature and other process of forging, is incomparable by casting.