What is Forging?

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Forging: It refers to a forming process which is used for manufacturing metal parts, where an external force is applied to a metal blank (excluding a sheet) and make its plasticity deformed, size changed, form shaped as well as performance improved.

About Forging and Its Types

What is Forging

The deformation resistance will be significantly reduced and the deformation degree is also greatly improved when the temperature exceeds 300-400 ° C (blue brittleness area of steel), especially reaches 700-800 ° C. It can be divided into three types : cold forging, warm forging and hot forging according to the forging in different temperature ranges, which have different forging quality and forging process requirements. Originally, there is no strict limit to the division of such temperature ranges. Generally speaking, forging in a temperature range where recrystallization is performed is called hot forging, and forging at room temperature is called cold forging.

The dimensional change of the forging is small when low temperature forging is performed. When forging below 700℃,the mill scale is less and there is no decarburization on the surface . Therefore, excellent dimensional accuracy and surface finish are easy to obtained in cold forging as long as the deformation can be in the forming range. Perfect accuracy under the warm forging below 700 °C can also be obtained if the temperature and lubrication cooling are well controlled. In hot forging, large forgings with complicated shapes can be made since the deformation and the deformation resistance are small. For high-precision forgings, hot forging can be performed in the temperature range of 900-1000 °C. Furthermore, attention should be paid to improve the working environment of hot forging. Forging die life (hot forging 2-5 thousands, warm forging 10-20 thousands, cold forging 20-50 thousands) is shorter than other temperature range forging, but it has a large degree of freedom and low cost .

In order that the blank is deformed and hardened during cold forging, and the forging die is subjected to high loads. Therefore, it is necessary to use a high-strength forging die and a hard lubricating film treatment method for preventing wear and adhesion. Moreover, to prevent blank cracking, intermediate annealing is performed as needed to ensure the required deformability. The blank can be phosphatized so as to maintain a good lubrication state.

The forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging and closed upsetting according to the movement mode of the blank. The material utilization of closed die forging and closed upset forging is high due to the absence of flash. Finishing of complex forgings is possible with one or several processes. The area of force applied to the forging is reduced and the required load is also reduced since there is no flash. However, the blank should not be completely limited. To this end, the volume of the blank is strictly controlled, the relative position of the forging die is controlled, and the forging is measured to reduce the wear of the forging die.

Forging can be divided into pendulum rolling, pendulum forging, roll forging, transverse rolling, ring rolling and skew rolling according to the movement mode of the forging die. Pendulum rolling, pendulum forging and ring rolling can also be used for precision forging processing. In order to improve the utilization of materials, roll forging and cross rolling can be used as a front-end process for slender materials. Just like free forging, the rotary forging is also partially formed, which has the advantage that it can be formed in the case of a smaller forging force compared with the forging size. This type of forging, including free forging, expands from the vicinity of the die surface to the free surface during processing. Therefore, it is difficult to ensure accuracy and the products with complex shapes and high precision can be obtained with a low forging force when the movement direction of the forging die and the rotary forging process are controlled by computer. For example, steam turbine blade forgings with a large variety and large size are produced.

The die motion and degree of freedom of the forging equipment are inconsistent. According to the characteristics of the bottom dead center deformation, the forging equipment can be divided into the following four modes:

  • Forging force limited mode: the oil hydraulic press that directly drives the slider.
  • Quasi-stroke limiting mode: the oil hydraulic press that drives the crank-link mechanism.
  • Stroke limiting mode: the mechanical presses that drive the slider with cranks, connecting rods and wedge mechanisms.
  • Energy limiting mode: the screw and friction press by using the screw mechanism.

In order to achieve high accuracy, attention should be paid to prevent overload at the bottom dead center,and its speed and die position should be controlled, because these factors will have an impact on forging tolerances, shape accuracy and forging die life. Additionally, in order to maintain accuracy, it is necessary to pay attention to adjust the slider guide clearance and the stiffness should be guaranteed, and measures, such as adjusting the bottom dead center and using the auxiliary transmission should be taken.

What is more, the compensation devices can be used to improve the movement in other directions according to the way the slider moves and the vertical and horizontal movement of the slider (for the forging of elongated part, lubrication cooling as well as part forging of high-speed production). The required forging force, process, material utilization, production, dimensional tolerance and lubrication cooling method are different owing to their different methods. These factors are also affecting their automation level.

How Forging Compared to Casting?

Metals’ structural and mechanical properties are improved when they are forged. After the hot-working deformation by the forging method, the cast structure makes the original coarse dendrites and columnar grains become the equiaxed recrystallized structure with fine grains and uniform size due to the deformation and recrystallization of the metal and makes the original segregation, looseness, porosity, slag inclusion in the steel ingot are compacted and welded, and the structure becomes more compact, which improves the plasticity and mechanical properties of the metal.

Generally speaking, as for the the same material, the mechanical properties of castings are lower than those of forgings. Besides, the forging process can ensure the continuity of the metal fiber structure, which makes the fiber structure of the forging piece is consistent with the shape of the forging piece. Its metal streamline is complete, which can ensure the good mechanical properties and long service life. No matter hot forging, warm forging or cold forging, casting can always not compare with forging.

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