Study on Forging Process of Large Head for Nuclear Power

Large forgings are an important part of the current large equipment for the country in the economy and national defense, and large high-tech devices have made great contributions; whether or not able to manufacture large forgings to meet the existing demand represents the industrial level of this country. And with the vigorous development of nuclear power energy, new energy is provided for large forgings. The manufacturing level of existing large forgings in China must be higher to manufacture forgings to meet the current extreme working conditions, which is the key factor restricting the existing major equipment in China. In this paper, we focus on the forging and forming process of a large head for nuclear power by analyzing the characteristics of the existing large head forging process to determine the influence of forming temperature, die, and other factors on forging to determine the design scheme to meet the requirements.

0. Introduction

Large forgings are key parts of major equipment and important basic parts for manufacturing special equipment, such as large thick-walled heads, large road rolling machines, large military equipment, etc. Most of the existing large equipment with large forgings are complex in structure, the working conditions to be completed are also very special, and the requirements for their quality are extremely strict, so the level of forging of large forgings has become an important symbol to measure the comprehensive national power of China.
With the improvement of existing plastic forming process simulation technology, the real simulation of actual process conditions through numerical and physical tests provides good technical support for the optimal design of the properties and quality of forging materials. This increases the safety of the actual operation, and its reliability gradually improves after experiments, playing a huge role in the plasticity of metal-forming materials. This paper aims at this condition for the in-depth analysis of existing large forgings forming process and research a new type of head forging forming process for nuclear power that meets the design requirements.

1. Research and technology status of nuclear power large forgings

The forging process of large forgings for nuclear power is extremely complex. It requires high precision, so finding the results to meet the requirements using the traditional classical plastic forming theory isn’t easy. At present, physical and numerical simulation of data is the most commonly used method. Physical simulation methods mainly include plastic clay, sensor, photoelastic, Mishan cloud pattern, etc. This is the simplest way to understand the production process and forging method of nuclear power forgings in depth among existing methods.
However, because the large forgings for nuclear power are too large, it is necessary to scale down the forgings when simulating them, which will produce large errors for the research. The invention of finite cloud simulation technology has solved this problem. This technology has become the most effective tool for studying forging theory. In recent years, the production of large forgings in China has had a good industrial base, its scale is larger, and the type and quality of forgings produced have been improved. So far, the theoretical research and process development of large forgings for nuclear power in China has made great progress. Still, the process flow of forging and forming large heads for nuclear power, including the forging process parameters obtained by numerical simulation, needs to be completed more and needs further study. Given the current research status, this paper will further study the forming process of large-head forging for nuclear power and determine a reasonable and effective research plan.

2. Characteristics of large head forging process

2.1 Characteristics of large head forgings

Large head forgings are key parts of large equipment, and their working and stress conditions are extremely complicated. Their reliability and precision are highly required during the working period. The forgings have complex structures and large individual sizes, and their quality and technical requirements are also very high. The production of its large head forgings has the following characteristics:

• (1) The individual size and weight are relatively large, requiring direct use of large forging equipment;
• (2) Extremely strict quality requirements for forgings because some large forgings are used under special conditions, such as under special stress and temperature, so the requirements for safety and accuracy of forgings are higher;
• (3) The production process is complex and requires a long time. Because of the previously mentioned large size and quality of forgings, the production requires a single small batch forging, and the forging process, raw materials, forging conditions, etc., are different for different requirements, so the production cycle is longer;
• (4) High production cost. Each forging requires a large number of raw materials and a lot of energy and labor, which is under the premise of success; if there are mistakes, it will cost more, and the requirements for production equipment are also higher, and some of them need special tools.

2.2 Features of large head forging and forming process

The large head is an important pressure-bearing element on large boilers and large pressure vessels, and the scope of its application is equipped with high pressure and energy. Currently, the main forming methods of large heads are: stamping forming, spinning forming, welding forming, and explosion forming.

2.2.1 Stamping forming

Stamping forming is a forming method using temperature control, mainly hot stamping, cold stamping, and warm stamping. This forming method can only solve local problems to a certain extent. Still, this method is prone to uneven heating, the big difference between inside and outside, and bad control of its quality.

2.2.2 Spin forming

Spin forming is a way to meet the deformation by changing the shape of the slab, and the thickness of this piece of the slab is unchanged. Generally, hot spinning is used. This method easily leads to an uneven internal structure in the forming process because it adopts point-by-point forming, so the extremely high requirements of this method on performance can hardly meet the technical requirements of large head forgings.

2.2.3 Piece-welding forming

This method only applies to vessels with smaller pressure, and it is difficult to meet the requirements of special large nuclear power equipment because of its uneven internal weld structure, air bubbles, and easy corrosion.

2.2.4 Explosion forming

Explosive forming is a method of metal forming using the explosive blast effect. This method produces a large amount of energy; the advantage is that it does not require much investment, simple operation, and produces good results, but the disadvantage is that it is not easy to control, dangerous, to meet the required energy requirements will require more explosives, which will produce a great danger.
The existing head provided to the nuclear reactor is core equipment with high requirements for the safety and reliability of its components, and it may not be possible to meet the existing technical requirements by using a general forming process. This ensures that each parameter meets the performance requirements in all aspects and realizes the current design. However, China’s nuclear power research started late, and more systematic research on the forging process still needs to be done.

3. Forging and forming process of a large head for nuclear power

3.1 Introduction of a large head for nuclear power

Large head for nuclear power is designed for large pressure equipment using nuclear power energy. Its purpose is to resist internal pressure and ensure the normal operation of internal nuclear reactions. The dimensions of the head forgings are shown in Figure 1.

Figure.1 Dimensional Diagram of Head Forging

3.2 Steps of forging and forming process development for large heads for nuclear power

• Phase I: To use the existing equipment and adopt a suitable forging forming method to produce large head forgings that can meet the requirements of nuclear power, firstly, we should use wax clay plastic and lead as materials in proportion to create the corresponding model to meet the basic test requirements; the stress-strain curve of the experimental material is measured on the simulation experiment machine as the material parameters of finite element, and then the whole production process is simulated by finite element software. The production process will be simulated, and the final results will be recorded.
• Phase 2: Measure the parameters of the model according to the requirements of phase 1, and then use the results of physical and numerical simulations to input the relevant dimensions in the corresponding software to correspond to the model.
• Phase 3: Analysis of the current experimental results and parametric analysis of the test data and materials used.
• Phase 4: Field test for the model according to the production process method and experimental parameters in step 3 to see if the results meet the design requirements.
• Phase 5: If it meets, the production is carried out according to the correct parameters and process in step 3.

3.3 Process of forging and forming large head for nuclear power

• (1) Select suitable head roughing materials;
• (2) Pre-forging of head rough billets;
• (3) First preparatory heat treatment;
• (4) Preparation of billet (round cake) and welding lugs;
• (5) Rotary forging (forming the head with a special tire film);
• (6) Second preparatory heat treatment;
• (7) Roughing, welding lugs, and thermal buffers for quenching and tempering;
• (8) Quenching and tempering for obtaining mechanical properties;
• (9) Removal of the test material and simulated post-weld heat treatment of the test material;
• (10) Material testing;
• (11) Finishing;
• (12) Final parameter inspection.

3.4 Forging parameters of large head forging and forming process for nuclear power

The forging parameters of large head forging and forming process for nuclear power need to consider two parts: forging ratio of ingots and rounding off of forging billets. The influence of the following two factors on the forging and forming process.

3.4.1 Ingot forging ratio

The forging ratio is an important indicator to measure the quality of forgings, it reflects the impact of forging on the internal structure and performance of forgings, so the choice of forging ratio should be reasonable, not too large; too large will affect the internal structure of forgings and destroy the performance of forgings, and not too small, too small to forge internal defects of forgings, not to enhance the mechanical properties of forgings. Therefore, a reasonable choice of forging ratio is essential to enhance the forging level of the entire forgings. The actual production of the minimum forging ratio calculation formula:

k_min = 2.5G_ingot^0.0784

In the formula:

• k_min is the minimum forging ratio;
• G is the ingot mass.

3.4.2 Rounding of forging billets

To facilitate forging, the regular quadrilateral or cylindrical shape is chosen. In this paper, cylindrical forging billets are chosen by the needs of actual large heads. The diameter of the forging billet is 2000mm.

4. Simulation parameters

According to the data requirements of this design, the important parameters of forging are set in the software:

• (1) Determine the principal equation of the material and simulate the high-temperature flow stress-strain curve;
• (2) Setting of the contact conditions between the billet and the convex and concave dies;
• (3) The setting of the operating speed of the hydraulic press convex die, a parameter that takes into account the speed of the operation;
• (4) The initial final forging temperature, the initial and final forging temperature setting will affect the internal grain size and its distribution of the main forgings, thus affecting the performance of the forgings; therefore, when setting the forging material, the temperature should not be too low or too high, too high will be difficult to transform the casting organization, too low will be the forgings cracking phenomenon. The relevant simulation parameters are set in the following table.

5. Analysis of results

From the simulation results, the wall thickness of the head forging increases more and more obvious as it approaches the end, while the bottom of the forging is not in contact with the convex die, and there is no film phenomenon. It can be concluded from the above graph that the trend of thinning at the bottom of the forging increases and then decreases as the die stroke increases, with the maximum at about 1400 mm.
From the above simulation analysis, it can be seen that the main factors affecting the forging results are rounding angle, punching temperature, and die clearance. Among them, the rounding angle affects the equivalent force of the forging, which in turn affects the wall thickness reduction of the forging; the punching temperature affects the forming time of the forging and the effect of slab forming; the die clearance affects the accuracy of the forging because when the clearance is small, the thickening phenomenon of the edge of the head forging is not corrected. The accuracy of the product could be better.
Table. Relevant parameters of finite element simulation

Forging stock material	SA508-3
Initial forging temperature of blank/°C	1200
Final forging temperature of blank/°C	850
Preheating temperature of male and female molds/°C	350
Ambient temperature/°C	25
Coefficient of friction/μ	0.3
Punch pressing speed/(mm.s-1)	50
Single cycle reduction/mm	100
Thermal conductivity coefficient with upper and lower anvils λ  (J/mm.s.°C)	11
Thermal conductivity coefficient with environment (J/mm.s.°C)	0.02

Figure.2 Variation trend of forming force and thinning amount at different stamping stages

6. Conclusion

This design adopts the way of integral forging, uses physical simulation and digital simulation technology to study the whole large head forging and forming process, and designs a research plan that meets the current technical means and the performance of the existing equipment. However, because of time, there are still the following places to be perfected and improved in this design:

• (1) This time only simulates the forging of large forgings, with lets you have been infinitely close to the real situation, but there are still some differences;
• (2) To be universally applied to the actual need for analysis of the results need to be further analysis and demonstration;
• (3) There is still a long way to go for the widespread use of practical applications. This study optimizes large heads’ forging and forming processes for nuclear power generation. The finished heads forged according to the test data meet the process requirements. After testing and evaluating its parameters, the test results of this forging meet the requirements of the forging evaluation outline.

Author: Xuan Yucheng