With the increase of power generation power and the continuous improvement of safety factor of nuclear power plants (such as AP1000, EPR, etc.), the requirements for their basic parts (nuclear power forgings) are becoming higher and higher, and the manufacturing is more and more difficult. The manufacturing technology and production capacity of super-large forgings have become a bottleneck restricting the rapid development of global nuclear power. Key nuclear power large forgings are:
Integrated top cover and container flange takeover section in the pressure vessel;
Pipe plate, water chamber head, conical cylinder in the steam generator;
Integrated forged stainless steel main pipeline (with pipe nozzle) in the main pipeline;
Whole forging turbine low pressure rotor and generator rotor in conventional island.
Since the vast majority of nuclear power plants in operation and under construction in China are PWR nuclear power plants, it mainly introduces the nuclear island and large forgings of PWR nuclear power plants.
The main system of nuclear island is composed of six categories: reactor coolant system, special safety facilities, nuclear auxiliary system, three waste treatment system, nuclear island ventilation and air conditioning system and nuclear fuel loading, unloading, storage and process transportation system. Among them, large nuclear power forgings are mainly concentrated in the reactor coolant systems (commonly known as the nuclear island main equipment). The reactor coolant system consists of three loops and its nuclear island main equipment pressure vessel, steam generator, voltage regulator, main pipeline and main pump.
Ⅰ. pressure vessel
1. Structure of the pressure vessel
Its main function is to fix and contain the core and inner components, so that the fission reaction of nuclear fuel is limited to a sealed container. It and the primary pipeline form the pressure boundary of high pressure coolant, and is one of the second barriers to prevent radioactive substances from escaping. The reactor pressure vessel and its internal components are assembled from large forgings. Large forgings of pressure vessel are mainly composed of header, cylinder, flange and receiver forgings. The forgings in the core area (barrel body, transition section, takeover section) will be bombarded by neutrons from the core and cause irradiation embrittlement during the service period.2. Manufacturing of large forgings for pressure vessels
(1) Materials
The materials used for large forgings of pressure vessels are low alloy steel (corresponding to ASME standard SA508 Gr. 3 Cl. 1, RCC-M standard 16 MND 5). Because the pressure vessel plays an extremely important role in the operation of the whole nuclear power plant, the steel used for pressure vessels is matched
Learning composition has strict requirements.
The influence and requirements of the main elements are as follows:
C: Main elements to ensure that the strength meets the requirements of the specification. Low C content, the strength is difficult to meet the requirements, and high C content will reduce the weldability of steel, and improve the embrittleness of irradiation. Therefore, the C content should be best controlled at about 0.18%.
Mn: in addition to the role of reinforcement matrix, it can also effectively improve the quenability of steel, and Mn should be controlled in the range of 1.40%.
Ni: obviously increases the low temperature toughness of steel, which is a necessary element to ensure the quenability of thick section steel, but the test proves that high Ni improves the embrittleness of irradiation. Therefore, the Ni content should be controlled at the lower limit of the specification under the condition that the low temperature toughness meets the requirements.
Mo: It can improve the heat resistance and reduce the tempering fragility. In the actual production, the Mo is generally controlled at about 0.50%.
Si: For reinforcing elements, but higher Si increases irradiation brittleness. Therefore, the Si content is better controlled at the lower limit of the specification.
V: Pressure vessel steel requirements is essential fine grain steel, plus a small amount of V has the effect of refining grain, can improve the strength. But V is easy to cause the welding heat to affect the embrittlement and increase the sensitivity of the "reheat crack" of steel. The V content is generally controlled below 0.05%.
Cu, S, P: belong to harmful elements, the lower the content, the better.
As, Sn, Sb: belong to the residual elements, which will increase the tempering brittleness and irradiation brittleness of steel, so the less the content, the better.
(2) Smelting
As the size and weight of the forgings increase, the weight of the ingot required for the forgings also increases (for example, the integrated top cover of AP1000 and the cylinder forging in the core area need 400t grade ingot). With the increase of ingot weight, high requirements are put forward for multi-package pouring technology, antisegregation compensation technology, inclusions, gas elements and harmful elements control technology, which increases the difficulty of smelting.
(3) Forge
With the increase of forging size, weight and strengthen the degree of integration (such as the integration of cover actually from the top head and cover flange two forgings, has now changed to overall forging molding), the auxiliary of forging molding control, tooling design and manufacturing, hardware facilities (press, machine, machine, driving, etc.) limit ability are put forward new challenges, forging difficulty increased significantly.
(4), heat treatment
With the improvement of the safety level and operation life of nuclear power plant, higher requirements are put forward for the performance assessment indicators of forgings (for example, the non-plastic transition temperature NDTT of the cylinder in AP1000 core area has been increased to-23.3℃ non-fracture). However, the increase of the size of the forging, especially the wall thickness, aggravates the heat treatment conditions and limits the improvement of the forging performance, which puts forward more stringent requirements for the heat treatment technology. For example, the uniformity of the temperature field and the control of temperature control accuracy, the circulation conditions of quenching tank and water temperature control measures, which will have a significant impact on the final performance of forgings. To sum up, smelting, forging and heat treatment are the three key links for the successful manufacturing of large forgings of pressure vessels. Each link affects and restricts each other. Problems in any link may cause the final scrap of the forgings. In order to effectively improve the qualified rate of forgings, we need to strictly control each process.
3. Large forgings of pile internal components
Large forgings of pile components are mainly composed of plate parts and ring parts. Its functions are mainly support and fixed core components; the drive line is medium, the control rod movement guide; the coolant channel and flow distribution; provide the pressure vessel and reduce the neutron γ -ray irradiation; provide the installation and fixing conditions for the in-reactor measurement; and provide the test place for the irradiation supervision test of the pressure vessel.
The materials used for large forgings are austenitic and martenitic stainless steel (SA-336 F304, Z2CN19-10.NS and Z3CN18-10.NS for RCC-M; SA-182 F6a and Z12CN13 for RCC-M).
Compared with the large forgings of the pressure vessel, the forgings of the reactor inner components are relatively small (the weight of the ingot is not more than 100t), but due to the characteristics of stainless steel itself, its manufacturing also has unique features.
(1) For austenitic stainless steel, due to the low C content, to ensure the tensile strength, other strengthening elements (Mn, Ni, Cr, Si, etc.) need to be strictly controlled, due to the mixing and forging cracks in the manufacturing process
Accurately grasp the time of cleaning the crack, and strictly control the deformation amount and deformation uniformity of the last two fires.
(2) for martensite stainless steel, due to the tensile strength and impact toughness are higher, for both performance, on the one hand, to optimize the ratio of chemical composition (such as C, Mn, Ni, etc.), on the other hand, according to the chemical elements (C, Mn, Mo, Si, etc.) the content of the tempering temperature and tempering heat preservation time.
Pile component large forgings is only in Shanghai heavy machinery factory co., LTD. Realize the localization, heavy is unique large electric slag remelting smelting technology, relative to the double vacuum ingot, heavy independent development of electric slag remelting patent technology in alloy composition control has unique advantages, can obtain the double vacuum ingot tissue more compact, more uniform high quality steel ingot, subsequent manufacture of stainless steel forgings provides excellent foundation.
Ⅱ, steam can
The main function of the steam generator is to transfer the heat from the primary coolant to the second circuit to feed water as a heat exchange equipment, so that it produces saturated steam to supply the second circuit power plant. Its function is to form a second protective barrier to prevent radioactive leakage between the first and second circuits.
The material used for steam generator large forging is low alloy steel (corresponding to ASME SA508 Gr. 3 Cl. 2, RCC-M 18 MND 5). Compared with the materials used in large forgings of pressure vessels, the composition requirements are basically the same, but there are differences in the performance assessment (for example, the strength assessment index of pressure vessel is 550~725MPa, and the strength assessment index of steam generator is 620~795MPa), which requires the design of steam generator chemistry
When controlling internal components, appropriately increase the content of reinforcing elements (mainly C and Mn elements).
Large forgings of steam generator are mainly composed of sealing head type, cylinder body type and tube plate. Compared with large forgings of pressure vessels (except pipe plate forgings), its wall thickness is thinner, with good quenability, and it is easy to obtain better low temperature impact toughness and strength. The difficulty of forgings is mainly concentrated on conical cylinder, tube plate and water chamber head.
1. Pipe board
Because the pipe plate forgings are particularly thick (such as the maximum heat treatment wall thickness of AP1000 tube plate exceeds 1000mm), the quenability is very poor, the water cooling strength of heat treatment should be improved as far as possible. At the same time, because it is solid parts, in order to ensure that there are no internal excessive defects, it needs to be strictly controlled in smelting
Make inclusions, and adopt multiple processes to ensure central compaction during forging.
2. Conical cylinder
Because the conical cylinder forgings are special-shaped parts (both ends are straight sections and the middle is inclined sections), it is very difficult to forge, and two sets of complex forging tooling need to be designed for manufacture. At the same time, how to realize the fine control forging process is also an important factor to ensure the successful formation of conical cone forging.
3. Water chamber seal head
Because the sealing head of the water chamber is a forging with branch pipe (the sealing head of AP1000 project), the shape is very complex. At present, the manufacturing scheme of the forging at home and abroad is formed as a whole, which causes the very low utilization rate of the steel ingot (the weight of the steel ingot is near
400t, delivery weight of 38t, utilization is less than 10%). The early manufacturing process is similar to the tube plate, and the machining after forging is another major manufacturing difficulty. Due to the complex shape, it takes at least about 8 months to process the water chamber header, resulting in a very long delivery period of the forgings.
Ⅲ, manostat
The main function of the regulator is to connect the bottom of the regulator (liquid area) with a loop heat pipe section of the RCP through the wave tube, and achieve a new balance through the change of the balance state and balance of the vapor phase and liquid phase. And through the difference of water, steam density, volume space, volume change, resulting in pressure change.
The materials and assessment indexes used in the large forging of the voltage regulator are consistent with those of the steam generator. The external size and structure are simpler than those of the steam generator, and its manufacturing difficulty is also simple.
Ⅳ, trunk line
The main pipeline is one of the ten main equipment of nuclear power plant and is the main component of the primary reactor of nuclear power plant. It connects the nuclear reactor pressure vessel and the main evaporator pump to form a ring loop. The heat generated by the nuclear reactor pressure vessel is transmitted to the evaporator through the main channel heat section. After cooling, the medium is pumped back to the nuclear reactor pressure vessel through the main channel cold section to form a ring-circuit heat exchange system. The main pipeline is composed of three parts: cold section, hot section and excessive section, with straight pipe and curved pipe.
As the pressure boundary of the primary circuit, it works under the conditions of high temperature, high pressure, high flow rate and strong radioactive medium, under transient working conditions and variable load superposition conditions in accident working conditions. Once the pipeline leaks or destroys the accident, the harm caused will be unimaginable. Therefore, ensuring its safety must be paid special attention to in the design, manufacturing and operation of nuclear power.
Therefore, it requires good mechanical properties, strong corrosion resistance, good process properties, good plasticity and fracture toughness.
In the past, the main pipeline of the PWR nuclear power plant was cast, and the service life of the third-generation AP1000 nuclear power equipment was 60 years. The design idea is to ensure safety, prevent the aging of materials, requiring nuclear power equipment to minimize the number of welds, and the safety level is the highest level. Hence the Lord
The pipe has been changed from the past casting molding to the whole forging molding.
The material used in the main pipeline is austenitic stainless steel (SA-376 TP316LN corresponding to ASME). The hot section with an integrated nozzle is the most difficult forging to manufacture.
(1) Forging: for the main pipeline steel is austenitic stainless steel, which is easy to produce forging cracks in the forging process. The timeliness and method of crack cleaning is the problem that needs special attention for the main pipeline in the forging process. In addition, for the hot section with an integrated nozzle, due to the particularity of its shape (the two pipes are not on an axis), it needs to be eccentric forging, which has more stringent requirements for the precise control and operation of forging. At the same time, due to the grain size requirement of the main pipe straight section and the control area.
(2) Bending: due to the size accuracy of the main pipeline and the high requirements of ellipticity at the bend, the bending type needs to design a special mold, supplemented by a special bending process.
V, main pump
The main pump (see Figure 9) is used to drive the coolant circulating through the RCP system to continuously transfer the heat generated in the core to the secondary side of the evaporator. The main pump ensures proper flow through the core and cothe core.
At present, the main pump is mainly made of stainless steel castings. As with the main pipeline, to further improve the safety and service life of the nuclear power plant, the designers have begun to consider using forgings instead of castings.
, Steam turbine, generator rotor
The steam turbine generator unit is the main equipment of the second loop system.
It consists of a saturated steam turbine, generator, condenser and an intermediate soda separation heater. Steam turbine is a single shaft, four cylinder six exhaust steam, condensing saturated steam turbine. Between the turbine high pressure cylinder and the low pressure cylinder, there are two steam separation reheaters to dehumidification and heat the steam. The generator is a hydrogen, water-cooled, brushless excited three-phase AC generator. The unit rotation speed is 1,500 r/min. The circulating cooling water of the condenser uses seawater, so the heat transfer material of the tube and tube plate of the condenser are made of titanium alloy.
Large forgings of steam turbine and generator rotor are higher than those of nuclear island main equipment because of their internal quality requirements (flaw detection requirements).
(1) Smelting
Large nuclear power rotor needs double vacuum high alloy steel ingot of 300t class and above with extremely high purity requirements, among which the double vacuum steel ingot of large low pressure rotor of 1000MW nuclear power class weighs 600t. The smelting and pouring technology of super large steel ingot has put forward extremely high requirements for the equipment limit capacity and technical level of the forging manufacturing enterprises. At the same time, in order to meet the uniformity requirements of performance, the component segregation needs to be strictly controlled to ensure the uniform distribution of its components.
(2) Forge
There are inevitably serious loose, air hole and other casting defects. For the cast tissue of the broken ingot, the loose pores and other defects inside the welded ingot, to obtain uniform and dense metal tissue to meet the strict internal quality requirements of the rotor, in forging
A series of complex forging process methods such as multiple pier thick, WHF and JTS are often adopted.