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Large punch type free forging rings are widely used in large construction machinery such as shield tunneling machines. These products have the characteristics of large diameter, small inner holes, and thick thickness. Due to the harsh service conditions and complex stress conditions of construction machinery components, users have high requirements for the internal structure, non-destructive testing, and comprehensive mechanical properties of forgings to ensure their service life.

Requirements for large forged rings

Our company has recently undertaken large-scale forging rings made of Q345B, a low alloy high-strength structural steel. The outer diameter of the finished product is about 3.5m, the inner diameter is about 1.3m, and the thickness is 250mm. The shape is shown in Figure 1; The technical specifications require that three cylindrical specimens with a diameter of 300mm be cut from the three small holes shown in the figure, and a group of specimens is taken from the middle of the height direction of the cylinder for testing. Each group of specimens includes one tensile specimen, three impact specimens, and one metallographic specimen. The sampling location is shown in Figure 2.
The test results require that the metallographic structure reach level 1-3, and the average grain size of the metal should be at least level 5. The ultrasonic testing of forgings meets the Class II requirements of GB/T2970, and the mechanical properties meet the requirements of GB/T1591.
The width of the forging plate exceeds 1m, and the thickness is thick. The sampling location is close to the center of the plate and the inner hole. There is a high possibility of steel ingot segregation and shrinkage residue in this area. At the same time, the forging penetration conditions are poor, and the cooling conditions are poor. Therefore, it is easy to have coarse grains and mixed grains. To meet the requirements of product testing and mechanical properties, the difficulty can be imagined.

Our company uses 36tVD steel ingots for forging, and the chemical composition test results are listed in Table 1. Remove the head and tail of the steel ingot, and control the utilization rate within 75%. Cold ingot charging, gas furnace heating, with an average temperature of 850 ℃ for 4 hours, then heating to 1230 ℃ for 8 hours and then forging out of the furnace.

The main deformation process of forging

The forging is completed on a 100MN free forging press, and the main deformation process is as follows:
Steel ingot rounding → upsetting (deformation of about 20%) → steel ingot elongation → upsetting (deformation of about 50%) → re-elongation → upsetting (deformation of about 70%) pressure maintaining → punching → horse bar expanding → shaping → cooling after forging.
During upsetting, as the height of the forging is continuously pressed down, the plane of the forging gradually increases, and the required deformation resistance continues to increase. The entire deformation process requires 5-7 heats, including two heats, to complete the hole expansion of the horse screw.
When the steel ingot is elongated, a wide anvil presses heavily with a large amount of pressure, each time pressing 200-300mm. After pressing down, the upper anvil remains for 15-20s before lifting the anvil. Pay attention to the overlapping amount of the anvil between 100-200mm to ensure a flat connection.

Before punching, the upsetting can be carried out by rotating the local upsetting method. When the upsetting reaches the required height, an additional upsetting plate is added to cover a large area of the blank, and the upper anvil is pressed down. After the pressure reaches equilibrium, the pressure is maintained for 15-20 minutes.

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Figure.1 Large forged ring

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Figure.2 Drawing of large forged ring
Table.1 Chemical Composition (Mass Fraction) of Q345B Forging Steel (%)
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The final heat deformation is greater than 20%, and the final forging temperature is controlled at 750-850 ℃. After forging, it is air-cooled to room temperature.
After cooling, the forging meets the process requirements through dimensional testing and is subjected to furnace heat treatment. The heat treatment process is also a key process to ensure the mechanical properties of the forging. Our company adopts the method of normalizing forced cooling, focusing on controlling the cooling process after the forging is discharged from the furnace, enabling the forging to achieve a cooling speed between quenching and normalizing and improving the cooling conditions at the center of the forging. To prevent the deformation of forgings due to excessive stress and stabilize the structure, we have added a tempering process, which has a good effect on stress relief.
Rough machining after heat treatment, with surface roughness Ra ≤ 6.3 μm. Afterward, ultrasonic testing was conducted using a single crystal straight probe with a diameter of 24mm, a frequency of 2MHz, and initial sensitivity φ 2mm; vertical four-sided inspection was conducted on the upper and lower planes, inner and outer circles of the forging, and no exceeding standard defects were found. The inspection results showed compliance with the Class II requirements of GB/T2970.
After ultrasonic testing, cylindrical specimens are cut using the flame-cutting method and sampled for testing, as shown in Figure 2. The mechanical performance test results are listed in Table 2, and the metallographic test images are shown in Figure 3.
From the values listed in Table 2, it can be seen that the performance of each part of the forging meets the requirements of GB/T1591, and there is no significant difference in the detection results of the three positions; The metallographic examination results show that the metallographic structure of the three test specimens is ferrite + pearlite structure, with fine and uniform grains, without obvious coarse or mixed crystal phenomena, and meets the requirements of GB/T13320 Grade II metallographic structure.
For this type of large punching type free forging ring, we carefully designed the forging process, combined with normalizing controlled cold and heat treatment measures. We ultimately obtained a finished product with a uniform microstructure of fine grains and excellent matching of strength and toughness indicators. The various indicators of the finished product fully meet the customer’s performance requirements. We applied this process to produce three products successfully delivered for use.

The most common quality problem in the production of large punch type free forging rings is uneven microstructure such as coarse grains and mixed grains. Once coarse grains appear, there will be grass-like waves or even no bottom waves during ultrasonic testing, and the toughness index will also decrease significantly. The coarse grain phenomenon can be improved to some extent through repeated heat treatment, but it cannot be completely avoided, and it also causes great energy waste. Therefore, avoiding coarse and mixed grain in large punch type free forging rings is the key and difficult point of quality control. Strengthening the forging process control is the most effective measure, and it also provides a prerequisite guarantee for subsequent heat treatment quality.

Key points to note in the forging process

In the forging process of large punch type free forging rings, the following points should be noted:

  • (1) When stretching, it is necessary to use a wide anvil and strong pressure method to ensure that the loose area at the center of the steel ingot continues to shrink, deform, and be pressed. After each pressing, a slight pause should be made to diffuse further the contact surface of the pressed porous defects and ultimately merge them into one.
  • (2) After upsetting, there should be a 15-20 min full coverage pressure holding process before punching, which can effectively weld defects such as micro cracks and shrinkage holes inside the forging.
  • (3) The final fire should have sufficient deformation, which should be greater than the critical deformation degree, usually taken as 15% -20%.
  • (4) Measures such as reducing the final forging temperature and rapid cooling after forging are also important guarantees for obtaining fine grains in forgings.

Table.2 Mechanical Property Test Results of Large Punched Free Forging Rings

Sample number Sample direction Rm/MPa ReL/MPa A(%) Z (%) AKv (20 °C)/J Grain size Remarks
Acceptance criteria 450-600 ≥265 ≥17 - Average ≥ 27 ≥5 -
Sample 1 Portrait 531 321 33 76 58.742.4 54.5 51.9 8.7 Qualified
Sample 2 Portrait 551 311 30.5 73 65.461.5 50.4 59.1 8.2 Qualified
Sample 3 Portrait 520 335 28 73 94.279.4 87.3 86.9 9 Qualified

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Figure.3 Metallographic test images
Author: Ren Xiufeng

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