Development of Super Large Ring Forgings

Large ring forgings have a wide range of applications, and their production capacity and technical level are one of the main indicators to measure a country’s level of heavy industry development and self-sufficiency of major key technical equipment. China attaches great importance to the development of the manufacturing industry of large ring forgings. Although a large amount of funds and manpower have been invested over the past 40 years, the industry has undergone significant changes from small to large and from low to high. However, so far, there is still a significant gap in the manufacturing capacity and technical level of domestic large ring forgings compared to foreign countries.
Our company has developed 9m super large complex ring forgings to improve the manufacturing strength of the company and enhance the market adaptability. From the perspective of size, it is the first attempt in China and the trial production has met the technical requirements. The production process for 9m ultra large complex ring forgings is: raw material procurement → raw material inspection → cutting → heating → billet making → ring rolling → heat treatment → inspection → machining. Figure 1 shows the actual ring component.

Figure.1 Physical object of ring components

Development process

Raw materials

The raw material is 42CrMoA steel ingot, and the smelting method is electric furnace + external refining + vacuum degassing. The chemical composition, surface quality, and mechanical properties of raw materials shall meet certain requirements and shall be inspected to prevent the quality problems of raw materials from affecting the subsequent Hot-working. The chemical composition of the raw materials is shown in Table 1.
Table.1 Chemical Composition (Mass Fraction) (%)

Hot working

(1) G42180 automatic saw machine is used for forging to cut and cut materials: diameter 800mm, length 1965mm continuous casting billet, weight 8000kg. The TRQ3000/3000/1500 natural gas furnace is used to heat the steel ingot, and the heating curve is shown in Figure 2. The heating process is a very important step, which can improve the plasticity of the billet, reduce deformation resistance, make the billet easy to deform and flow, and enable smooth plastic deformation. Steel should be properly insulated at a certain temperature range during heating. Due to the large size of the billet, there is a significant temperature difference between the surface and the center during the heating process. If not insulated, it will generate significant internal stress inside the material, resulting in cracks during forging or subsequent heat treatment and machining processes, leading to scrapping.
From the figure, it can be seen that the heating temperature of the material is 1250 ℃ and the insulation time is 8 hours. The initial forging temperature of this material is 1150 ℃, and the final forging temperature is not less than 850 ℃.

Figure.2 Heating Curve of Steel
Use a 60000 kN YTD96-1600 hydraulic press to forge the blank. The forging process is completed in two separate heats. The first heat includes one upsetting of the billet to 1000mm and one stretching of the billet to 1300mm. The second heat includes secondary upsetting of the billet to 850mm and punching with a 400mm punch, with an eccentricity of ≤ 20mm. The dimensions of the forging blank are shown in Figure 3. Due to the susceptibility of forgings to coarsening, it is necessary to increase the forging ratio for each upsetting and drawing to ensure thorough forging of the internal structure. However, during the upsetting and drawing process, the height to diameter ratio of the billet must be controlled between 1.5 and 2.5, so the upsetting and drawing forging ratio must be controlled to be ≥ 3.5.

Figure.3 Dimensions of Forging Blank
(2) The rolling of the ring is carried out on the RAM9000 radial and axial ring rolling machine, with the initial rolling temperature ≤ 1150 ℃ and the final rolling temperature ≥ 850 ℃. Due to the large size of the ring, to ensure sufficient plasticity of the forging billet during the rolling process, the initial rolling was carried out to 2400mm × 1600mm × 600mm, it needs to be reheated. Then the second heat is rolled into 9020mm × 8690mm × 230mm. During the rolling process, pay close attention to the deformation of the workpiece. If defects such as dents, peeling, and burning are found on the surface, the rolling should be stopped immediately, and the workpiece should be cooled and repaired before reheating and rolling.
During rolling, the initial rolling temperature deviates from the lower limit of the initial forging temperature. In the early stage of rolling, to eliminate the eccentric wall thickness difference, the radial feed speed should be small, and the axial feed may not be necessary (because at this time, the ring temperature is high, the grain size is large, and plastic deformation has a small impact on the grain size). When the temperature drops to above the final forging temperature (900-950 ℃), the feed speed is rapidly increased, and both radial and axial rolling are carried out simultaneously (the large plastic deformation generated at this temperature causes the grains to refine under continuous local shear force). When the size approaches the size of the forging, the feed rate is reduced, and only radial rounding is performed. Figure 4 shows the schematic diagram of the ring components.

Figure.4 Schematic diagram of ring components
Due to the susceptibility of large ring forgings to coarsening, grain refinement can be achieved through appropriate methods. Under the condition of constant rolling force, increase the rotational speed of the main roller to increase the strain rate of the ring; Increase the feed force of rolling to increase the feed per revolution to increase the strain; When the temperature of the ring is close to the final forging temperature, the main rolling is carried out to cause more strain to occur in the ring at this temperature.
(3) Heat treatment was carried out in a 9m×9m×3m RTD-5000 natural gas heat treatment furnace to heat the ring parts, and the ring parts were tempered according to the heat treatment and tempering curve (see Figure 5). During the heat treatment process, the quenching and heating speed of large forgings should be controlled to avoid excessive temperature difference, which further expands the original internal defects (such as micro cracks, inclusions, and porosity). During the heating process, there will be two peaks in temperature difference between the surface and the center. The first peak of temperature difference occurs at a surface temperature of 600-700 ℃. At this time, the surface of the forging has entered a plastic state, while the core temperature is only 350-500 ℃, still in an elastic state. At this time, the thermal stress is high, and the workpiece is prone to forming cracks in the core or expanding the original microcracks. Therefore, step heating is adopted, and the heating speed should be controlled below 600 ℃ to reduce the temperature difference between the surface and the center, thereby reducing thermal stress. Insulate around 650 ℃ to ensure that the surface and core temperatures are similar. The second peak of temperature difference occurs near the surface temperature of 800 ℃. At this time, the core temperature is 600 ℃ or slightly higher, and the material has entered a plastic state. The temperature difference in thermal stress can be significantly relaxed, and the tendency of the workpiece to crack and deform is very small. After quenching, large forgings should be immediately tempered, and the intermediate stay should generally not exceed 2-3h. On the one hand, it is due to the high internal stress of large forgings after quenching; On the other hand, there is still undercooled austenite in the core that has not fully transformed after quenching. Quickly transferring the workpiece to a tempering furnace for maintenance at low temperatures can continue to cool the core.

Figure.5 Heat Treatment Tempering Curve

Results and Discussion

Mechanical properties of ring components

Three specimens were taken from the surface and center of the ring for mechanical performance testing, and the mechanical performance values of the ring in different directions were obtained, as shown in Table 2.

Table.2 Mechanical Properties of Forgings

From the data in the table, it can be seen that the mechanical properties of the surface of the ring exceed the development indicators, while the mechanical properties of the core are lower than the development indicators, indicating a decreasing trend in the mechanical properties of the material from the surface to the inside, with the attenuation of yield strength being particularly significant.

Metallographic structure

Take metallographic samples from the surface and center of the ring forging for metallographic inspection. Firstly, grind and polish the sample, and observe its internal structure after being corroded by corrosive solution; Then put the sample into the saturated aqueous solution of Picric acid and detergent, and after constant temperature corrosion at 60 ℃ for a long enough time, observe its grain size (1000 ×). It can be seen that the structure of the surface hardened layer of 42CrMoA steel after quenching and tempering maintains tempered sorbate in the martensite direction, as shown in Figure 6a; The core is mainly composed of coarse sorbate and block ferrite, as shown in Figure 6b. This explains why the yield strength equivalent of the surface is higher than that of the core.

Figure.6 Metallographic Structure of Ring after Quenching and Tempering

Ultrasonic testing

The macroscopic inspection found no cracks or visible defects on the surface of the ring forging. Through ultrasonic testing on its surface, it was found that the ultrasonic wave was strongly attenuated and the clutter scattering was strong, making the waveform image appear Exponential type, further indicating that the internal structure of the ring was uneven and the grain was relatively coarse.

Conclusion

The development results of super large ring components indicate that although ultrasonic testing has found uneven internal structure and relatively coarse grains, the mechanical properties and metallographic structure of the surface of the ring components have reached the level required by the development technology. This indicates that the selected materials’ chemical composition and processing technology for development are feasible.

• (1) Ultra-large rings should be upset and drawn multiple times during billet making to ensure that the internal structure is forged as thoroughly as possible. Rolling the ring requires two fires to complete, with small initial deformation and large main deformation, thereby refining the grain size.
• (2) After quenching and tempering treatment, the mechanical properties from the surface to the center of the ring forging show a decreasing trend. The microstructure of forgings is unevenly distributed. The surface is fine tempered sorbate with martensite orientation, and the core is coarse sorbate and massive ferrite.

Authors: Dai Yutong, Chen Hong, Zhu Qianhao, Fan Yu