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Optimization of Forming Process for Ball Head

Abstract: Large head forgings are prone to uneven wall thickness, local thinning, and creasing. This paper proposes optimizing the head forging process to overcome the above problems. Through Deform-3D numerical simulation analysis, the forming slab and forming auxiliary tool are optimized to avoid local thinning and wrinkling of the head. The optimized head-forming process meets the requirements.

The head is an integral part of chemical and nuclear power equipment. To ensure the long-term and high-efficiency operation of the equipment under high temperatures and high pressure, the requirements for performance are getting higher and higher.
The integrity of the nuclear power heads will be directly related to the safety and life of the nuclear reactor. The head is subjected to high temperature and pressure in the working process. The particular service conditions also put forward more stringent requirements for the materials used in nuclear reactor pressure vessels:

  • (1) Suitable strength and the high toughness and lowest possible brittle transition temperature at room temperature and working temperature;
  • (2) Good weldability and hot and cold workability;
  • (3) Maximum tissue stability at working temperature;
  • (4) Sufficient hardenability and thick section tissue properties Uniformity.

There are two kinds of joint heads: ellipsoidal and ball heads. This paper introduces the forming of ball head punching. It optimizes the forming auxiliary tool of the head through numerical simulation to ensure the uniform wall thickness of the ball head after punching.

Brief introduction of the forging process of ball end

Ingot cutting ingot bottom and riser press jaw → upsetting and compaction → drawing length down → upsetting and compaction → upsetting out finished product → forging slab post forging heat treatment → slab roughing → slab flaw detection → slab bending and forming.

Deform model establishment

A 3D solid model is established using 3D modeling software UG to establish a 3D solid model of forming the upper die and finishing the ball head, as shown in Figure 1 and Figure 2.
In the process of ball head forming, the forming upper die is connected with the movable cross beam of the hydraulic press, the forming lower die is placed on the four corner pillars, and the punching stroke is H=1300mm. after the punching is completed, the movable cross beam of the hydraulic press is lifted, and the overhead crane lifts the finished head.

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Fig.1 Upper die of ball head forming

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Fig.2 Finishing drawing of the ball head
The material model is SA508-3, which has excellent process stability, weldability, and high strength. There is no data on this material in Deform’s material library, and the natural stress-strain curve of the material was obtained according to the material properties experiment, as shown in Figure 3.

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Figure.3 SA508-3 material stress-strain curve
During the forming process of slab punching, the forming slab is defined as a deformed body, and the forming die is defined as a rigid body. The friction between the formed slab and the die is a very complex physical phenomenon related to various factors on the contact surface, such as the relative hardness between the contact surfaces, surface roughness, temperature, everyday stress, relative sliding speed, etc. The advantage also changes during the deformation process. Deform has two types of shear friction and Coulomb friction; this paper selects the shear friction type. The friction coefficient between the deformer and forming upper die is defined as μ=0.4, and that between the deformer and forming lower die is defined as μ=0.3. The punching temperature is set to 1000.

Numerical simulation analysis of ball head forming

According to the dimensions of the finishing drawing, the forming slab is designed, and the slab and die are optimized through a series of numerical simulations. The slab’s dimensions before forming are verified through the simulation results, and the forming slab is determined, as shown in Figure 4.

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Figure.4 Formed slab dimensions
After punching (stroke 1300mm), the comparison between the ball head forgings and the fine drawing is shown in Fig.5. The head forgings obtained by numerical simulation according to this scheme can meet the finishing process. The allowance at the bottom of the ball end is about 10mm on one side, and the inner and outer circles at the open end of the ball end are about 25mm on one side, and it can be seen from the figure that the ball end forgings meet the finishing dimension requirements.

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Fig.5 Comparison between ball head forging and finishing drawing
The equivalent stress after the completion of forming is shown in Fig.6. The deformation at the open end of the ball end is the largest, which is equivalent to the closing process and causes a concentration of stress of about 40MPa. The deformation at the bottom of the ball end is the smallest, and the forming stress is the smallest. The equivalent effect changes after forming are shown in Fig. 7, and the equivalent effect becomes 0.02~0.2mm/mm. The forming force is shown in Fig. 8, and the maximum forming force is about 2600t.
From the numerical simulation results of ball head punching, it can be seen that the bottom thinning is serious, so the forming angle of the lower die is adjusted, and the bottom arc of the upper die is optimized to ensure the even balance of the head after punching.

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Fig.6 Equivalent force diagram of ball head after punching

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Fig.7 Equivalent force diagram of ball head after punching

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Fig.8 Forming force of ball head after punching

Optimization and numerical simulation of ball head punching die

Firstly, we analyze the influence of forming an angle of the lower die on forming effect of the head slab, adjust the angle of the lower die and compare the size of forming force; there is no significant difference between forging and finishing allowance after forming through simulation and comparison, but considering the small diameter of this pressure regulator head and considerable length of the straight section, the larger the forming angle of the lower die, the smaller the forming force, the better the guiding effect on the slab during forming and the less likely to wrinkle the slab.

Under the premise that the forming temperature is 1000 and the shape of the slab remains unchanged, the forming force comparison is shown in Figure 9, and the larger the die angle is, the smaller the forming force is. The forming force is 2600t for the lower die angle of 16°, 2400t for the lower die angle of 21°, and 2000t for the lower die angle of 35°. The comparison shows that the larger the angle, the smaller the forming force required to form the slab. By comparing the forming force, the forming angle of the lower die for ball head punching is finally determined to be 35°.

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Figure.9 Effect of die angle on forming force
Modify the forming slab, increase the thickness of the slab at the bottom of the ball head, increase the thickness of the slab at the bottom by 25mm to ensure the uniform wall thickness of the ball head slab after punching, increase the allowance at the bottom of the ball head, and the modified forming slab is shown in Fig.10.

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Fig.10 Optimized forming slab.
After optimization of the ball head slab and punching auxiliary tool, the result of ball head punching is shown in Fig.11; the equivalent force at the open end of the ball head is the largest, about 40MPa; after punching, the forgings of the ball head are compared with the finished ones as shown in Fig.12, the balance of each part of ball head forging is even, the contact between the inner wall of forging and convex die is good, the balance is even, the balance at the bottom of the ball head is about 20mm, the balance at both sides of the open end of the ball head is about 20mm. After punching, the forgings of the ball end are shown in Fig. 13.

Conclusion

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Fig.11 Punching shape of ball end

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Fig.12 Comparison of forging and finishing of ball end

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Fig.13 Ball end forging

  • (1) When designing forming slab for the ball end, the wall thickness at the risk point should be increased, and the design angle of the lower die for forming should be between 30° and 40° to avoid local thinning of the end due to the growth of forming force. Through a series of numerical simulations, the slab and forming auxiliary tools are continuously optimized to ensure that the head is formed once and the complete forging flow line is retained to provide a good tissue base for subsequent heat treatment.
  • (2) Optimize the rounding angle of the lower die by adjusting the gap between the punch and the lower die to avoid the bruising of the ball end during punching. Control the press-down speed and punching stroke of the water press. When starting punching, the hydraulic press should use first-class pressure to avoid too fast speed during punching.
  • (3) After the punch touches the head slab, set the forging endpoint when the punching stroke reaches H=600-700mm so that the internal stress of the head slab is fully released in the punching process. If wrinkling and folding are found in the actual punching, it should be stopped in time, and the slab should be returned to the furnace for heating before punching to avoid the thinning of the bottom of the head due to the head slab being stuck on the lower die.

Author: Hu Jie

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