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Study on Forging Forming of Incoloy 825 Nickel Base Alloy Flange

Incoloy 825 is a nickel-iron-chromium solid solution reinforced alloy with molybdenum, copper and titanium invented by American Special Metals. The alloy has the ability to resist the stress corrosion of chloride ions in acidic media and also has good corrosion resistance to sulfides, thus it is widely used in oil and gas development and other fields. At present, there is not much research on its forging and forming in China. This paper explores the forging and forming of flanges using Incoloy 825 alloy using the plastic forming finite element software Deform, analyzes the reasons for failure and gives suggestions for modification to achieve satisfactory results in actual production.

The main problems of 825 alloy material forming

The deformation resistance of incoloy 825 alloy at high temperature is high, and even at the starting forging temperature of 980°C, the flow stress is above 240 MPa. This makes forming more difficult. In order to prevent grain boundary corrosion and improve the resistance to hydrogen sulfide stress corrosion, the smaller the grain size, the better. The 825 alloy does not undergo phase change during heating and cooling, so the grain cannot be refined by heat treatment, but only by plastic deformation and recrystallization. When plastic deformation is carried out, special attention should be paid to the deformation amount in each place to ensure that it is greater than the critical deformation degree of 25% to prevent the generation of coarse grains.

Simulation and experiment of incoloy 825 flange forging scheme

Because of the high price of nickel alloy, in order to save material and considering the large deformation resistance of incoloy 825 alloy in forging, the initial process of pre-forging plus final forging is adopted. The initial billet is a cylindrical bar that has undergone billet opening, as shown in Figure 1(a). During the pre-forging process, the lower end of the billet is subjected to the action of the pre-forging die with a narrowing opening, which continuously produces extrusion deformation. At this time, the relative motion of the billet and the inner wall of the die cavity generates friction, but the downward force on the billet in the vertical direction is greater than the frictional force, so the billet initially moves mainly in the axial direction, filling the majority of the cavity, as shown in Figure 1(b). However, as the die opening narrows and the area of contact between the billet and the cavity wall becomes larger, it becomes more and more difficult for the billet to move in the axial direction. With the axial motion inhibited, the upper part of the billet starts to undergo upsetting deformation in the radial direction, as shown in Fig. 1(c). The actual shape after the end of pre-forging is shown in Fig. 1 (d).

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Figure.1 Pre-forging simulation process mesh diagram

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Fig.2 Bad material at the end of pre-forging

After the pre-forging, the resulting billet is loaded into the final forging die, as shown in Figure 4(a). From Fig. 3, it can be seen that the load variation of the final forging process can be divided into three stages: OA, AB and BC. When the lower end of the billet is in contact with the cavity of the die, the billet moves against the inner wall of the cavity and generates friction, but the vertical force on the billet is greater than the frictional force, so the billet first moves in the vertical direction and fills the cavity of the lower end of the die continuously. As the contact area between the billet and the cavity at the lower end of the mold increases, the resistance to downward movement becomes greater and the movement of the billet in the vertical direction is gradually suppressed, and the contact between the billet and the step surface of the mold cavity begins to occur, as shown in Figure 4(b). When the billet fills most of the cavity and starts to contact with the upper end of the mold cavity, as shown in Fig. 4(c), if the billet continues to deform, the force required at this time rises abruptly, which corresponds to Fig. 3BC. The force applied at this stage mainly deforms the upper part of the billet to fill the upper end of the mold cavity, while the lower part of the billet deforms very little in the vertical direction, resulting in the lower part of the billet not filling the mold cavity completely, as shown in Fig. 4(d). As shown in Figure 4(d). In actual production, due to the limited force exerted by the equipment, the incomplete forming of the lower end of the blank is more obvious, as shown in Fig. 5.

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Fig.3 Final forging load-step curve of the original scheme

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Figure.4 Mesh diagram of the final forging simulation process

Simulation and experiment of the improved forging solution

Through the above simulations and experiments, it was found that the main problem of the original solution was the difficulty of forming the lowermost part of the billet. Since the lowermost part of the billet is formed at the last stage of deformation, when most of the billet is already formed and the contact area with the inner wall of the die cavity is already very large, it becomes very difficult to deform the lowermost part of the billet and the force required increases sharply. In view of the disadvantages of the lowermost part of the billet being in the later stages of deformation, the idea of forming the lowermost part of the billet first was adopted. For this purpose, the billet is pre-forged and the shape of the billet after pre-forging is shown in Figure 7(a). The pre-forging is done without a die and is a partial elongation of the cylindrical billet.
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Figure.5 Final forgings obtained from the original solution

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Figure.6 825 weld neck flange forgings obtained from the improved solution

In the modified final forging process, the deformation of the billet can be divided into three stages. First, the lower end of the billet undergoes upsetting forming and the neck of the billet, including the lowermost end of the billet, is fully formed, a process that corresponds to the OA section of Figure 8 on the load-step curve. After the billet and the step surface of the mold cavity are in contact, there is a large increase in the applied load, from point A to point B in Figure 8, because of the large contact area at the beginning, when the upper part of the billet undergoes upsetting deformation and flows in the radial direction on the step surface of the mold cavity, corresponding to segment BC in Figure 8. When the radial flow of the blank reaches the edge of the upper end of the mold cavity, the radial flow is then constrained by the inner wall of the mold cavity, and the load applied to fill the zone III of Fig. 7(c) also rises abruptly, which corresponds to the section CD of Fig. 8. When the lower surface of the upper die and the upper surface of the upper die are in contact, the shape ends and the blank is completely filled with the cavity. At the same time, the equivalent deformation of the final forging is observed, and the deformation of all parts is more than 25%, which meets the requirements. At the same time, the maximum forming load was reduced to 86% of the original solution. And in the next experiments, the forgings that meet the requirements were also obtained, as shown in Figure 6.

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Figure.7 Equivalent variation diagram of the improved forging simulation process

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Figure.8 Load step curve of improved forging scheme

Conclusion

  • 1) In the original solution, the main reason why the billet could not be fully filled was that the lowermost deformation of the billet occurred in the last stage, which was extrusion forming, so it became very difficult to fill. In the improved pre-forged billet, the lowermost part of the billet is formed by the photo during the forging process, so it becomes easy to be filled.
  • (2) The deformation of the forged part obtained from the improved pre-forged billet is greater than the critical deformation at all places, which meets the requirements.
  • (3) The maximum load required in the improved forging is also smaller than the initial solution, which reduces the requirement for equipment.
  • 4) Forgings meeting the requirements were obtained according to the process proposed in this paper.

Source: China Alloy Flange Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

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