Research on the Whole Forging Forming Process of Large Diameter Disc Ring Forgings

A spinning and expanding hole forming theory was studied, and the production process and technical parameters were determined. It was successfully applied to production, solving the manufacturing problem of large diameter disc shaped ring forgings by whole forging.

With the increasing number of projects in the domestic infrastructure market, such as rail transit, urban underpass tunnels, and comprehensive pipe galleries, shield tunneling machines have been increasingly widely used in industries such as transportation and energy in China. The diverse and large-scale usage demands have also posed new challenges in manufacturing shield machine components in equipment, technology, and other aspects.
The large ring of the shield tunneling machine bears the role of transmitting large torque and thrust under harsh working conditions during the excavation process. The power the driving system provides is transmitted to the panel through the cloud legs welded on the large ring, which drives the cutter head to rotate and feed axially. To achieve better performance, the large ring needs to be processed and manufactured by forging. Its inner and outer diameter dimensions are large in terms of external structure, while its height is very low, making it a typical disc-shaped ring forging. This forging type cannot be finally formed through the conventional method of expanding holes with horse screws like ordinary ring type free forgings. Through process practice, the complete forging of this type of forging was successfully achieved by spinning and expanding the hole.

Technical characteristics

The material of the large ring used in a large shield tunneling machine is Q355, which belongs to low alloy high-strength structural steel and has good comprehensive mechanical properties and welding performance. The forging specification is Ø 7745mm × Ø 4730mm × 320mm, the final forging weight is over 85t, and its shape is shown in Figure 1. The wall thickness ratio to the height of forgings reaches 4.7, which belongs to large-diameter disc ring forgings.
The technical requirements for this forging are as follows:

• (1) The chemical composition shall be executed according to the GB/T1591-2018 standard.
• (2) The mechanical properties should meet the following requirements: R_eH ≥ 275MPa; R_m=470MPa-630MPa; A ≥ 21%; KV₂ (longitudinal) ≥ 34J.
• (3) Ultrasonic testing shall be carried out by JB/T5000.15-2007 standard level II.
• (4) The technical agreement shall conduct hardness, grain size, and inclusion inspection.

Figure.1 Schematic diagram of large ring forgings

Analysis of the whole forging plan

Due to its large wall thickness and low height, it is impossible to expand the hole and form it using a barbed bar like ordinary ring forgings for this type of large-diameter disc ring forging. The user’s design department in the contract allows segmented forming followed by welding for manufacturing. However, this plan faces risks such as increasing the allowance for segmented bending, difficulty bending, and inability to ensure product quality at the weld seam. There are also significant issues in terms of cost and manufacturing difficulty.
Based on the theory of volume invariance and the law of minimum resistance in metal plastic forming, and drawing on the company’s experience in spinning tube sheet forgings, a technical solution is proposed to achieve the complete forging forming of large diameter disc ring forgings through spinning and expanding holes. Starting from the basic law of metal flow, the hollow forging billet is placed on a rotary table, and the method of rotating the rotary table and reciprocating pressing the upper anvil is used to achieve the expansion forming of the forging, as shown in Figure 2. The principle is based on the theory of volume invariance, using the upper anvil and rotary table constraints to reduce the height of the forging, and the restricted metal will flow in the tangential and radial directions. At this point, by controlling process parameters such as the amount of anvil, the blank metal mainly flows in the tangential direction of the hollow forging according to the law of minimum resistance, thereby achieving the goal of synchronously increasing the inner and outer diameters of the forging to the process size.

Figure.2 Schematic diagram of spinning and expanding method forging
In addition, two issues need to be addressed in the implementation of this method:

• (1) Regarding technology, control the relationship between the amount of anvil input and the amount of reduction. To achieve synchronous expansion of the inner and outer diameters of forgings, it is necessary to ensure that the metal billet mainly flows in the tangential direction; that is, the elongation of the metal under the anvil should be much greater than the elongation.
• (2) In terms of equipment, the press equipment’s operating space should meet the spinning requirements of large-diameter hollow billets. Our company’s 185MN hydraulic press is a double column prestressed frame structure that has successfully forged tube plates and ring forgings with a diameter exceeding 8m. Therefore, this aspect meets the requirements.

Production application

Based on the non-destructive testing and performance requirements of forgings, the manufacturing process for large ring forgings is determined as follows: electric furnace initial refining → LF refining → VD vacuum degassing → VC vacuum casting → heating before forging → 185MN hydraulic press forging → post forging heat treatment → rough machining → ultrasonic testing and mechanical property inspection → shipment.

Smelting process

Production practice has shown that in the smelting process of low alloy steel Q355, adopting the smelting process of increasing C and reducing Mn can achieve the goal of reducing alloy consumption while ensuring the chemical composition and mechanical properties of the steel, thereby reducing raw material costs. However, a high content of C element can lead to an increase in the volume fraction of pearlite in the structure, which exhibits brittleness at room temperature, reducing the plasticity of the steel and increasing the tendency for brittle fracture in tensile specimens, ultimately leading to the failure of the forging elongation to meet technical requirements.
This forging is forged using 126t steel ingots. To ensure that the final mechanical properties after normalizing and tempering meet the technical requirements, the following requirements have been made for material composition and smelting process:

• (1) Internal control has been implemented on the chemical composition of some materials, such as controlling the content of C elements between 0.13% and 0.17% and appropriately increasing the element content of V, Nb, etc., which can refine the grains.
• (2) During the smelting and casting processes, dual vacuum treatment is used to improve the purity of molten steel and reduce oxide inclusions and gas content in the ingot. For large steel ingots, improving the purity of molten steel can effectively reduce or eliminate defects such as internal segregation and inclusion aggregation, significantly improving the performance of forged products.

Forging process

For forgings, the forging process first involves compacting the center of the steel ingot, breaking columnar crystals, improving macroscopic segregation, welding internal pores under appropriate temperature and stress conditions, transforming the as-cast structure into a forged structure, and improving the density of the material. In addition to completing the extremely difficult process of forming large rings, the forging process also requires reasonable compaction process parameters to meet non-destructive testing requirements, mechanical properties, and grain size of forgings. Therefore, the forging process of this forging includes the following processes:

• (1) Press the jaw, cut off the ingot tail and excess material from the riser;
• (2) Steel ingot upsetting + WHF elongation and cutting;
• (3) Upsetting and punching of the blank with a diameter of 1300mm;
• (4) Enlarge the hole to the inner hole with a diameter of 2700mm;
• (5) Spin and expand the hole to the process size.

In actual production, as a special process of expanding holes, spinning and expanding holes also have the following operational points:
(1) Selection of anvil width
The selection of the anvil type during the final spinning and expanding process is also very important. For large ring forgings, the flow of the metal billet should mainly be tangential elongation, and the widening in the diameter direction should be minimized as much as possible. Therefore, a narrow flat anvil that matches the height of the billet should be used for spinning (Figure 3).

Figure.3 Spinning and Expanding Holes
(2) Design of pressing parameters
To ensure the regular shape of the forging and avoid surface defects such as cracks and folds at the anvil, the reduction and anvil insertion during the spinning and the expanding process should not be too large, and the rotation angle should be uniform. Based on the production experience of dozens of large ring forgings, the reduction amount per pass can be controlled between 12% -18%. After each reduction, it is advisable to evenly rotate the circular billet by about 15 ° through a rotary table. The specific parameters need to be adjusted according to the anvil width.
(3) Control of fire and final forging temperature
Due to the large diameter size and small height size of large ring forgings, the temperature drop during the forging process is fast. Therefore, spinning and expanding the hole requires multiple heats to be completed. Therefore, it is necessary to plan the forging task for each heat and control the temperature of the forgings when the final product is produced to avoid the problem of the billet exceeding the size of the heating furnace midway or causing coarse grain structure of the forgings.

Heat treatment process

The Q355 series steel can meet the mechanical performance indicators required by the standard by normalizing or quenching and tempering heat treatment after forging. The difference is that quenching and tempering treatment can achieve higher yield strength, and the normalizing strengthening method has a better economy because the obtained ferrite + pearlite structure can provide good microstructure for subsequent welding of large ring forgings.
The heat treatment process of normalizing at 880-910 ℃ and high-temperature tempering at 650 ℃ was used for the large ring forgings made of Q355 material. To better ensure the mechanical properties and grain size of forgings, it is recommended to adopt air blast, spray, and other methods to accelerate cooling after normalizing and air cooling. These methods can all affect the nucleation, grain size, and interlayer spacing of ferrite by increasing the undercooling of austenite.

Processing inspection

After the heat treatment of the large ring forging was removed from the furnace, it passed the ultrasonic testing smoothly and entered the machining process.
The processed large ring parts have undergone final inspection, and their appearance dimensions and performance indicators fully meet the technical requirements. The actual performance inspection data of the sample is shown in Table 1.

Table.1 Sample Performance Inspection Data

Conclusion

This article proposes a new spinning and expanding process for forging large-diameter disc ring forgings and analyzes its metal flow mechanism. After more than ten production practices of large ring forgings in recent years, it has been proven that using this process for the whole forging of large disc ring forgings is practical and feasible. It simplifies the manufacturing process, but more importantly, the whole forging scheme avoids the potential quality hazards that the welding scheme may bring to subsequent product use.
Under the premise of ensuring forming by spinning and expanding, controlling chemical composition, and developing a reasonable post-forging heat treatment plan, the product performance can meet the requirements, omitting the final performance heat treatment, simplifying the process, and reducing costs.
Authors: Zhang Zhiyong, Guo Jian