Analysis of the free forging process of large long shaft forgings

This paper reveals the characteristics of the free forging process, starting from the improvement of the traditional process for large long shaft forgings to enhancing the free forging process for large long shaft forgings, to reduce production costs, reduce energy consumption, and thus improve the forming quality of large long shaft forgings.

What are large long shaft forgings?

Large long shaft forgings are generally used for transmission shafts, the core and key components of machinery and equipment, and are the basic parts of heavy equipment manufacturing. Hence, its quality requirements are extremely high. The production of large long shaft forgings mainly includes pre-forging heating, forging process, and post-forging heat treatment. The production of large forgings is a multi-departmental collaborative production, so there will be many influencing factors in the production process. Therefore, it is necessary to develop a reasonable free forging process, to ensure the safety of the ingot in the heating process through reasonable heating specifications and to reduce the heating time to achieve the purpose of energy saving and emission reduction and then through the appropriate heat treatment process to obtain parts to meet the requirements, and thus promote the economic development of our country to achieve good and rapid development.

1. The importance of large long shaft forgings

Long shaft forgings are generally installed in the most critical parts of the machine; in the work process, it is heavy and complex, so it does not allow the existence of porosity and need to achieve the required performance. Large forgings are generally made from ingots, which are then forged and heat treated to produce the required parts. The production process of ingots is mainly smelting and ingot injection; due to the solidification characteristics of ingots and metallurgical properties, it is determined that in the ingots, there are inevitable defects such as porosity, segregation, inclusions, etc., and the defects increase with the weight of the ingots. To get the products to meet the requirements, we have to solve three problems, firstly, to eliminate the metallurgical defects of the ingot itself; secondly, to obtain a relatively uniform size of the fine grain organization; and thirdly, to achieve a quantitative distribution of mechanical properties of forgings orientation. The quality of large long shaft forgings is not only the need for industrial development but also the guarantee of the personal safety of operators. The free forging process, with its advantages of simple tools, flexibility, and versatility, can meet the characteristics the traditional forging process does not have and improve the defects in the traditional forging process. By improving the free forging process of large long shaft forgings, we can analyze the large forgings after heat treatment, obtain fine grain organization of uniform size, produce the required part shape, and reduce or even eliminate the internal defects of the ingot itself, thus improving the comprehensive performance of large forgings, meeting the needs of China’s industrial development for large long shaft forgings, and promoting the healthy, benign and efficient operation of China’s industry. 2.

2. Free forging process of large long shaft forgings

The forging process of large long shaft forgings mainly includes pressing, drawing and chamfering, and rounding. The forging process is mainly composed of forging, drawing, and chamfering. The forging process takes 90 degree V-shaped anvil to square and 120 degree V-shaped anvil to chamfer and roll.

2.1 Selection of drawing type

The drawing process has a large proportion in the forging process of large long shaft forgings, and its influence on the quality of the forgings is also relatively large. The forging force required for a flat anvil drawing is 30% smaller than that of a 90 degree V-shaped anvil, making it more convenient to manufacture. Therefore, it is necessary to choose the type of draw length according to the actual equipment used. When using a flat anvil, a rectangular cross section forms a forging a billet with a round cross section. The billet with a rectangular cross section is forged to a certain size, then to an octagonal cross section, and finally rolled to a round shape. In the actual use of equipment, according to the actual use of equipment, the upper anvil should be fixed to the power head, a non-removable flat anvil. In contrast, the lower anvil can be adjusted and replaced according to the production needs, and then choose the appropriate forging process for forming long shaft forgings.

2.2 Define the forging fire of ingots

The appropriate forging temperature range is to ensure that the metal has high plasticity and low deformation resistance within this temperature range and, simultaneously, make the forgings meet the required organization and properties. Under this premise, the forging temperature range should be taken as wide as possible to minimize the number of forging fires, reduce energy consumption, and thus improve production efficiency to facilitate the operation. In the real production process, in order to prevent the phenomenon of cracking caused by large tensile stress due to excessive spreading, usually take the operation method of “two light and one heavy” and avoid repeated hammering in the same place. This not only ensures that the defects inside the material can be effectively eliminated, minimizes the waste of material and work time, but also distributes the deformation amount of each step more reasonably and helps to reduce the forging deformation force. Thus, according to the required shape of the forging part and the distribution of fire times, the required process card for forging the intermediate shaft is drawn up.

2.3 Ensuring the best possible delivery

In order to ensure a smooth forging surface of large forgings, the forging end feed is generally less than 0.75 times the anvil width. In order to ensure production efficiency, the relative feeding amount selected for the first fire is 0.75, and the relative feeding amount selected for the rest of the fire is 0.70. Large forgings are forged together with the cavity, usually choosing the wide anvil forging method. Still, it will need to use a large strike energy equipment due to the extraction of long processing, so the investment in equipment increases. The electro-hydraulic hammer is forged with an anvil that matches the required striking energy of the equipment. At the same time, when the end of the billet is deformed, it is necessary to ensure that there is sufficient compression length and a relatively large compression volume to reduce the deformation of the surface metal due to the small feeding volume of the drawing length, thus ensuring the quality of large forgings, the smooth progress of construction and the personal safety of the operator.

3. Forging Heating Process Specification for Large Long Shaft Forgings

The production process of large shaft forgings includes pre-forging heating, forging, and post-forging heat treatment. Because large forgings are mostly single-piece production, the scrapping of forgings will cause huge economic losses and delay the construction period. Therefore, it is necessary to develop reasonable heating specifications to ensure the safety of steel ingot heating and reduce heating time to achieve the goal of energy conservation.

• (1) When charging at room temperature, the heating of steel ingots can use the maximum heating capacity of the heating furnace. The stress and temperature fields of the steel ingot during heating were obtained through simulation. The stress at the center of the steel ingot is a three-dimensional tensile stress, with the axial stress being the maximum. And there are peak values of axial stress and surface center temperature difference during the low-temperature period and the phase transformation period at the center of the steel ingot. From the perspective of rapid heating, the optimization parameters for heating specifications are: the insulation temperature during the phase change period of the rudder stock steel ingot is 850 ℃, the insulation time during the phase change period is 1 hour, and the forging insulation temperature is 1235 ℃; The insulation temperature during the phase change period of the propeller shaft steel ingot is 850 ℃, the insulation time during the phase change period is 0.8 hours, and the forging insulation temperature is 1220 ℃.
• (2) During high-temperature charging, the heating rate of the steel ingot significantly accelerates during the low-temperature period, but the stress at the core also increases significantly. High-temperature charging can reduce heating time, but the highest charging temperature must be controlled. The insulation time at charging temperature is 2 hours for the rudder stock steel ingot, 1.5 hours for the propeller shaft, and the rest are the same as when charging at low temperature.
• (3) The subsequent heating cycles are different from the previous ones. Due to the high internal temperature of the forging and the low surface temperature, when the furnace temperature reaches the forging insulation temperature, the surface temperature difference inside the forging is insignificant. Therefore, a smaller forging insulation temperature of 1220 ℃ needs to be selected for heating. When the length of the forging is greater than the length of the heating furnace, the forging without furnace installation will significantly impact the temperature of the forging near the furnace door in the furnace. To reduce heating time, it is necessary to increase the furnace installation length.

4. Conclusion

The forging of large forgings with long shafts should be combined with the actual needs of the enterprise and the heating equipment to develop the forging process. To analyze the distribution of cross-sectional stress and surface temperature field of large forgings, as well as forging force and drawing efficiency, the actual situation. The test proves that by selecting reasonable ingots and using a series of processes such as flat anvil pressing, chamfering and rolling, and drawing, large long shaft forgings of large tonnage can be forged with small equipment. The reasonable forging process can eliminate the defects such as internal shrinkage and looseness of the ingots. The forged long shaft forgings can be tempered and heat-treated after forging to obtain highly comprehensive mechanical properties. From the free forging process, the advantages of simple tools, flexibility, and versatility are brought into full play to develop large forgings with long shafts, thus realizing our economy’s sound and rapid development.

Author: Zhang Mingqiao

Source: China Shaft Forgings Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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