Forging refers to the forming and processing method of applying an external force to metal blanks (excluding plates) to produce plastic deformation, change size, and shape and improve performance, which is used 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 be sharply reduced and the deformation energy will be greatly improved.
The forging in different temperature areas, according to the different forging quality and forging process requirements, can be divided into three forming temperature areas: cold forging, warm forging, and hot forging. 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 without heating at room temperature is called cold forging.
When forging at low temperatures, the size change of forgings is very small. Forging below 700 ℃, the formation of oxide scale is less, and there is no decarburization on the surface. Therefore, as long as the deformation can be 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, warm forging below 700 ℃ can also obtain 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 service life of forging die (hot forging 2000-5000, warm forging 10000-20000, cold forging 20000-50000) is shorter than that of forging in other temperature ranges, but it has large degree of freedom and low cost.
The blank will be deformed and work-hardened during cold forging so that the forging die will bear the high load. Therefore, it is necessary to use a high-strength forging die and adopt the treatment method of hard lubricating film to prevent wear and adhesion. In addition, in order to prevent blank cracks, intermediate annealing is carried out when necessary to ensure the required deformation capacity. In order to maintain good lubrication, the blank can be phosphated. When continuous processing is carried out with bars and wire rods, the section cannot be lubricated at present, and the possibility of using phosphating lubrication method is being studied.
According to the moving mode of the blank, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Because there is no flash in closed die forging and closed upsetting, the utilization rate of materials is high. It is possible to finish machining complex forgings with one or several processes. Because there is no flash, the stress area of forgings is reduced, and the load required is also reduced. However, it should be noted that the blank should not be completely limited. Therefore, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, and the forgings should be measured to try to reduce the wear of the forging die.
