Method for hot-upsetting and cold-bending of reactor primary coolant variable diameter austenitic stainless steel main pipe

Abstract

The application discloses a method for forming a reactor primary coolant variable-diameter austenitic stainless steel main pipe by hot breakdown and cold bending integral forging, which comprises the following steps: smelting a steel ingot by adopting an electric furnace smelting+AOD+electroslag remelting, then sequentially performing hot breakdown and straight pipe forging including the first heating time, the second and third heating times, the fourth to sixth heating times and the seventh to tenth heating times, performing cold bending forming under room temperature conditions on a greater than or equal to 5000-ton press, and performing solid solution heat treatment on the pipe under the conditions that the entering-furnace temperature is less than or equal to 600 DEG C, the heating rate is less than or equal to 10 DEG C / min, the temperature is kept at 1050-1070 DEG C for 3 hours and then water cooling. The grain size of the reactor primary coolant variable-diameter austenitic stainless steel main pipe forged by the integral forging forming method is improved to more than 5, the impact toughness is greater than or equal to 290 J, the production cycle is shortened by more than 20%, the energy consumption is saved by more than 50%, and the safety and reliability of the nuclear power equipment in service are ensured.

Description

Technical Field

[0001] The invention belongs to the field of material processing, and in particular relates to a hot blanking-cold bending integral forging method for a reactor primary loop coolant variable diameter austenitic stainless steel main pipeline. Background Art

[0002] The main pipeline of the reactor's primary circuit is a first-level nuclear safety component. It is large in size and operates under harsh operating conditions (about 300°C, 16MPa, high-temperature and high-pressure water containing phosphoric acid and boric acid). It has extremely high requirements for material performance. In addition to having good comprehensive mechanical properties (sufficient strength, high plasticity and toughness), it is also required to be resistant to corrosion in high-temperature and high-pressure water, have good fatigue resistance, easy processing and welding performance.

[0003] Austenitic stainless steel with a duplex structure can better meet the above performance requirements and is widely used in the primary circuit main pipelines of nuclear power plants. Most of the primary circuit pipelines of early nuclear power plants abroad used ASME304 austenitic stainless steel. Later, in order to improve the high temperature and high pressure intercrystalline corrosion resistance of 304 stainless steel, ASME316 stainless steel containing 2%-3% Mo was gradually used. In order to further extend the operating life of nuclear power plants, the international community has conducted further in-depth research on primary circuit pipeline materials in recent decades. The United States and Japan have developed nuclear-grade nitrogen-controlled 316L stainless steel. France has invested heavily in the research of a series of austenitic stainless steel materials and developed Z2CND18.12 and Z3CN20.09M main pipeline materials.

[0004] The primary circuit main piping of a pressurized water reactor (PWR) can be forged or cast. Forged austenitic stainless steel provides uniform microstructure and excellent mechanical properties. However, due to the limitations of traditional split-piece forging processes, the length of straight pipe sections is limited, increasing the number of welds and the welding workload. Furthermore, due to the inherent characteristics of the material, welding defects are prone to occur during welding. Therefore, developing a process for integrally forging the curved sections of the primary circuit main piping to reduce the number of welds while maintaining the full streamlined shape of the forged fiber is a goal of nuclear-grade safety components.

[0005] In the existing technology, Zhang Lingfang et al. of Shanghai Heavy Machinery Plant and Zhang Yongsheng et al. of Jilin Haoyu Petrochemical Power Equipment Manufacturing Co., Ltd. proposed a forging method for AP1000 straight tube with branch nozzle (invention patent number: ZL200910057934.8, ZL201010259575.7); Park Zhensheng et al. of Anshan Iron and Steel Heavy Machinery Co., Ltd. proposed a straight forging forming method for 316LN nuclear power main pipeline (invention patent number: ZL201110158358.3); Tian Caihong et al. of Shanghai Heavy Machinery Plant proposed a hot section bend method for AP1000 nuclear power main pipeline. A mechanical subtractive machining method (invention patent number: ZL201010524766.1) was proposed by Lai Changde et al. of Jiangyin Nangong Forging Co., Ltd. and Si Xingkui et al. of Tongyu Heavy Industry Co., Ltd. for the integral hollow forging process of straight sections of nuclear power main pipelines (invention patent numbers: ZL201210078226.4 and ZL201310091963.2). Men Zhengxing et al. of Erzhong Group (Deyang) Heavy Equipment Co., Ltd. proposed a short-process manufacturing method for straight sections of nuclear power main pipelines (invention patent number: ZL201410356220.8). In summary, there are currently very few reports on the integral forging of curved sections of austenitic stainless steel variable-diameter main pipelines for reactor primary circuits. The integral forging process parameters available for reference at home and abroad are very limited. There are no reports on the integral cold bending technology of austenitic stainless steel nuclear power main pipelines with a diameter of ≥350 mm. Summary of the Invention

[0006] The purpose of the present invention is to propose a method for hot blanking and cold bending integral forging of a variable diameter austenitic stainless steel main pipe for the primary coolant circuit of a reactor, which mainly solves the following two key technical problems: (1) Since austenitic stainless steel does not undergo phase transformation during the heat treatment process, that is, the grain structure after forging directly determines the grain structure of the service component, large austenitic stainless steel forgings undergo a complex thermal deformation history. How to control the evolution of grains during deformation and meet the requirements of grain size under service conditions is one of the key technical problems faced; (2) The primary coolant circuit main pipe has a large curvature bend structure. How to meet the geometric accuracy of the integral forging of the main pipe bend section also poses a huge challenge to the rational formulation of the forging process. This is the second key technical problem to be solved. Based on this, the present invention addresses the above two major technical problems faced by the integral forging of variable diameter austenitic stainless steel main pipe for the primary coolant circuit of a nuclear reactor, and proposes a "hot blanking + cold bending integral forging method for large austenitic stainless steel forgings".

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for integrally forging a reactor primary circuit coolant variable diameter austenitic stainless steel main pipe by hot blanking and cold bending, which comprises the following steps in sequence:

[0009] (1) Steel ingot smelting: adopt electric furnace smelting + AOD + electroslag remelting combined process;

[0010] (2) Hot open mill and straight pipe forging, technical route includes: ① 1st heating → ② 2nd-3rd heating → ③ 4th-6th heating → ④ 7th-10th heating; wherein:

[0011] The process parameters of the 1st heating include: ingot head cutting ≥10%, tail cutting ≥10%, the forging piece is heated to 1150-1180℃ at a heating speed of 56-62℃ / h, and the temperature is kept for 4-5h; in the open mill process, the initial forging temperature of the forging piece is ≥1150℃, the final forging temperature is ≥800℃, and the first heating elongation ratio is ≥1.2;

[0012] The process parameters of the 2nd-3rd heating include: the return to the furnace is kept at 1150-1180℃ for ≥0.5h, the temperature during the whole deformation process is ≥800℃, 2 times of upsetting and 2 times of elongation are experienced, the upsetting ratio is 1.4, the first elongation ratio is ≥2.0, and the second elongation ratio is ≥1.5;

[0013] The process parameters of the 4th-6th heating include: the elongated blank is numbered, and the local part is upset and compacted with a special die, two variable diameter connectors are extruded, the upsetting ratio is ≥1.2, the initial forging temperature is ≥1150℃, and the return to the furnace is kept for 0.5-1h when the temperature decreases to ≤800℃;

[0014] The process parameters of the 7th-10th heating include: the pre-prepared blank with connector is elongated to the target size, and when the surface temperature of the forging piece is <800℃, it is returned to the furnace for keeping temperature in time, the keeping temperature is 1100-1180℃, and the keeping time is 0.5-1h each time;

[0015] (3) Cold bending pipe: cold bending forming is carried out on a ≥5000 ton press under room temperature conditions, the bending pipe axis is in the same horizontal plane, the whole does not twist, and the axis deviation is <2mm;

[0016] (4) Solid solution heat treatment: the pipe piece after bending forming is kept at 1050-1070℃ for 3h under the conditions of ≤600℃ of furnace temperature and ≤10℃ / min of heating rate, and then water cooled, the cooling water temperature is ≤35℃, and the whole process is controlled within ≤70 seconds; at the same time, forced cooling is combined to ensure that the actual temperature increase value of the water temperature during the whole quenching process is ≤3℃.

[0017] As preferred, it further includes the rough machining before heat treatment and the semi-finishing and finishing processes after the solid solution heat treatment.

[0018] As preferred, it further includes the size detection and UT self-check processes before and after steps (2)-(4).

[0019] As preferred, the technical route of the steel ingot smelting is: ① electrode blank preparation and appearance inspection → ② electrode welding → ③ pre-production preparation → ④ arc striking slagging → ⑤ metal electrode hanging and preheating → ⑥ normal remelting and feeding of electrode blank → ⑦ demolding → ⑧ cover cooling → ⑨ inspection; wherein:

[0020] The arc striking slagging adopts the same steel type metal electrode arc striking as the austenitic stainless steel main pipeline, the arc striking agent is steel chips, the voltage after the formation of the molten pool is ≥60V, and the slag adding time is ≥30min.

[0021] When the metal electrode is hung, the electrode blank is first adjusted to be centered, and the distance between the periphery and the crystallizer is ≥50mm.

[0022] As preferred, the deformation amount of the straight pipe forge piece in the 7th-10th fire is greater than the recrystallization critical deformation degree, and the deformation amount is ≥13%.

[0023] As preferred, the straight pipe forge piece of the hot open-butt and straight pipe forging is trimmed at above 800℃, wherein the cold forge piece is preheated to 900-950℃ before trimming.

[0024] The application designs a hot open-butt-cold bending integral forging forming method of a reactor primary coolant variable-diameter austenitic stainless steel main pipeline, compared with the current casting or split forging and re-welding method, has the following beneficial effects:

[0025] (1) The grain size of the forge piece is improved from less than 2 grade of the casting grain size to above 5 grade, the integral forging forming avoids the weld, and reduces the in-service detection frequency of the nuclear grade forge piece.

[0026] (2) The integral forging forming ensures the continuity of the full-fiber streamline of the forging, greatly improves the mechanical property surplus of the main pipeline, and the impact toughness is ≥290J, thereby ensuring the safety and reliability of the nuclear power equipment in service.

[0027] (3) After the cold bending forming, the inner wall of the main pipeline does not need additional mechanical processing, and the production cycle is shortened by more than 20%.

[0028] (4) The main pipeline bending section does not need multiple fire heating during the cold forming stage, and the energy saving is ≥50%. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a flowchart of the hot open-butt-cold bending integral forging forming method of the reactor primary coolant variable-diameter austenitic stainless steel main pipeline in the embodiment.

[0030] Figure 2 It is a structural schematic diagram of the cold bending forming die of the main pipeline bending section with split mandrels and split ship models in the embodiment.

[0031] Figure 33 is a schematic structural diagram of the position distribution of the tube blank and the die after the cold bending deformation is completed in the embodiment. DETAILED DESCRIPTION

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the described embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0033] Example 1

[0034] like Figure 1 As shown, a method for hot blanking and cold bending integral forging of a reactor primary circuit coolant variable diameter austenitic stainless steel main pipe is exemplarily described. The technical route is as follows: electric furnace smelting → refining outside the furnace → electroslag remelting → raw material delivery → material composition re-inspection → hot blanking → straight pipe forging → processing before bending → dimension inspection → UT self-inspection → cold bending → dimension inspection → UT self-inspection → rough processing before heat treatment → dimension inspection → UT self-inspection → solution heat treatment → sample cutting → sample processing → physical and chemical test → semi-finishing → dimension inspection (small nozzle) → UT test → finishing → dimension inspection → VT test → PT test → UT test → hydrostatic test → removal of hydrostatic allowance → dimension inspection → cleaning → sample summary → sample dissection → dissection physical and chemical test → trial product identification. The specific steps are as follows:

[0035] (1) Hot blanking process

[0036] (1a) Ingot smelting:

[0037] To ensure the strength and toughness requirements of the material, target values ​​for chemical composition control are proposed. On the basis of meeting the technical conditions for development, the main elements are controlled to improve the hardenability and strength of the material and reduce the content of elements such as P and S. The "electric furnace smelting + AOD + electroslag remelting" process is used to reduce the contact between molten steel and the atmosphere during smelting and pouring, thereby achieving the purpose of reducing the segregation of raw material components. The specific process control requirements for ingot smelting are as follows:

[0038] ① Electrode blank preparation and appearance inspection: There are no obvious cracks on the surface of the blank, and the composition of the electrode blank meets the requirements of electroslag ingot smelting.

[0039] ②Electrode welding: Weld the riser of the electrode blank. Use a grinder to polish the surface of the electrode blank before welding. The welding should be firm. After welding, the weld slag should be cleaned.

[0040] ③Pre-production preparation: Heat the slag before use, with a temperature ≥ 400°C and a heating time ≥ 3 hours. Use it immediately after heating. At the same time, inspect the equipment and auxiliary equipment such as water pipes and gas pipes, as well as the high-voltage and low-voltage power supply systems, hydraulic systems, mechanical systems, and water supply systems.

[0041] ④ Arc striking and slag forming: Use the metal electrode of this steel grade to strike the arc, the arc starting agent is steel chips, the voltage is ≥60V after the molten pool is formed, and the slag adding time is ≥30min.

[0042] ⑤ Hanging and preheating of metal electrodes: When hanging the electrode, adjust the center of the electrode blank and the distance between the periphery and the crystallizer should be ≥50mm.

[0043] ⑥ Normal remelting and feeding of electrode blanks: Computer automatic control is used in the normal remelting stage. When the electrode blanks are melted to the riser line, the electrodes are exchanged. The electrodes must be exchanged quickly and accurately. After the exchange, the electrodes must be centered and the exchange time must be controlled to no more than 3 minutes. In the final stage, the current and voltage are gradually reduced to achieve the electroslag ingot feeding.

[0044] ⑦ Demolding: After the electroslag ingot is smelted, the steel ingot is completely solidified in the water-cooled crystallizer and then demolded.

[0045] ⑧Hood cooling: After the electroslag ingot is demoulded, it is put into the hood for cooling. The hood cooling time is 24 hours.

[0046] ⑨ Inspection: After the electroslag ingot is cooled, clean the surface and take samples from both ends of the head and bottom for inspection. The composition must meet the requirements of smelting analysis, and the appearance must meet the surface quality requirements of electroslag remelting steel ingots.

[0047] (1b) Hot blanking integral forging:

[0048] ① First heat: Ingot head trimming ≥ 10%, tail trimming ≥ 10%. Heat the forging to 1150–1180°C at a rate of 56–62°C / h and hold for 4–5 hours. During the billeting process, the initial forging temperature must be ≥ 1150°C and the final forging temperature must be ≥ 800°C. The first heat drawing ratio must be ≥ 1.2.

[0049] ② Second-third heats: Due to the surface temperature drop after the first drawing heat, the forging needs to be returned to the furnace and held at 1150-1180°C for ≥0.5h. The temperature during the entire deformation process should be ≥800°C. This heat undergoes two upsetting and two drawing heats, with an upsetting ratio of 1.4. The first drawing ratio is ≥2.0, and the second drawing ratio is ≥1.5.

[0050] After the above three rounds of large deformation and blanking, the problems of looseness, segregation and shrinkage holes that may exist in the steel ingot are completely eliminated.

[0051] ③ 4th–6th Fires: Mark the drawn billet and further upset and compact a portion using a dedicated die to extrude two reducers. Upset ratio ≥ 1.2. Initial forging temperature ≥ 1150°C. When the temperature drops to ≤ 800°C, return to the furnace and hold for 0.5–1 hour.

[0052] ④ 7th-10th heats: Lengthen the prefabricated blank with nozzle to the target size. When the surface temperature of the forging is less than 800°C, return it to the furnace and keep it warm at 1100-1180°C for 0.5-1 hour each time.

[0053] ⑤ In order to obtain fine grains and fully weld the micro cracks and pores in the central area, the last fire should have a sufficiently large forging ratio, the deformation should be greater than the critical deformation degree of recrystallization, and the deformation requirement should be ≥13%.

[0054] ⑥ Austenitic stainless steel has a large cold shrinkage rate. When the forging is finally formed, a larger shrinkage rate (1.5%-1.7%) should be considered to avoid scrap due to insufficient size after cooling.

[0055] ⑦ The forging temperature should be above 800℃ before trimming (cold forgings should be preheated to 900-950℃ before trimming).

[0056] ⑧ In order to avoid the precipitation of carbides along the grain boundaries of austenitic stainless steel and increase the tendency of intergranular corrosion, it is required that the steel should not stay in the austenite sensitization temperature range (485–810℃) after forging, and water quenching should be used for rapid cooling.

[0057] ⑨ Cutting is used to remove defects such as fine cracks and wrinkles on the surface of forgings and the inner wall of straight pipes due to forging, so as to avoid macro cracks induced by the cold bending process.

[0058] (2) Cold bending process

[0059] Before cold bending, straight pipe forgings are first subjected to UT testing and then cold bent on a 5000 ton press at room temperature. There should be no wrinkles on the inside of the bent pipe, the axis of the bent pipe should be in the same horizontal plane, the entire pipe should not be twisted, and the axis deviation should be less than 2mm. The specific steps for cold bending at room temperature are as follows:

[0060] ① Place the pipe fitting (3, D1 ≥ 350 mm) on the lower die (2), with the long pipe nozzle facing vertically downward (D3 ≥ 350 mm) and the short pipe nozzle (D2 ≥ 240 mm) placed horizontally.

[0061] ② The center line of the upper mold (1) coincides with the center line of the inner ship mold (4), and the inner ship mold (4) is placed inside the sample. The distance between the center of the inner ship mold (4) and the center of the long connecting pipe is 840mm±2mm. Core rods (5) are placed at both ends to ensure the roundness requirements of the sample. The function of the inner ship mold is to ensure the roundness of the tube during the cold bending deformation process, and it can be split and easily taken out.

[0062] ③ The press applies pressure through the upper die, and the press speed is ≤10mm / min. After the bending is completed, the forging is subjected to dimensional inspection and UT inspection. In addition to meeting the requirements of dimensional inspection, the wall thickness of the bending part shall be ≥28mm, and the ovality of the bending part shall be ≤5% of Φ325mm. Figure 2 As shown, the core rods at both ends and the split ship model are taken out.

[0063] (3) Performance heat treatment process

[0064] ① After bending, the temperature of the pipe entering the furnace is ≤600℃, the heating rate is ≤10℃ / min, and it is kept at 1060℃±10℃ for 3h and then water-cooled. The cooling water temperature is ≤35℃.

[0065] ② In order to ensure rapid cooling of forgings, the time from opening the heat treatment furnace door to immersing the sample in the water tank is shortened as much as possible, and the entire process is controlled within 70 seconds or less. At the same time, in order to enhance the cooling speed of the sample, two quenching water tanks with a flow rate of 200m 3 / h external circulation pump, and 10 internal circulation pumps are equipped on the four sides of the water tank to directly force cooling of the workpiece. The actual temperature rise of the water temperature during the entire quenching process is ≤3℃.

[0066] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for forming a reactor primary circuit coolant variable diameter austenitic stainless steel main pipe by hot blanking and cold bending, characterized in that: The following steps are involved: (1) Ingot smelting: adopting electric furnace smelting + AOD + electroslag remelting combined process; (2) Hot blanking and straight tube forging, the technical route includes: 1st fire → 2nd-3rd fire → 4th-6th fire → 7th-10th fire; among which: The process parameters of the first fire include: ingot head trimming ≥10%, tail trimming ≥10%, heating the forging to 1150-1180°C at a heating rate of 56-62°C / h and holding for 4-5 hours; during the cogging process, the initial forging temperature of the forging is ≥1150°C, the final forging temperature is ≥800°C, and the first fire drawing ratio is ≥1.2; The process parameters of the second and third firings include: reheating at 1150-1180°C for ≥0.5h, the temperature of the entire deformation process is ≥800°C, undergoing two upsetting and two drawing, the upsetting ratio of the two times is 1.4, the first drawing ratio is ≥2.0, and the second drawing ratio is ≥1.5; The process parameters of the 4th to 6th fires include: marking the drawn blank, upsetting and compacting a portion of the blank with a special die, extruding two reducers, an upsetting ratio ≥ 1.2, an initial forging temperature ≥ 1150°C, and returning to the furnace for 0.5-1h when the temperature drops ≤ 800°C; The process parameters of the 7th to 10th fires include: drawing the prefabricated pipe blank to the target size, returning to the furnace for insulation when the surface temperature of the forging is less than 800°C, the insulation temperature is 1100-1180°C, and the insulation time is 0.5-1h each time; (3) Cold bending: cold bending is performed on a ≥5000 ton press at room temperature. The axis of the bent pipe is in the same horizontal plane, the whole pipe is not twisted, and the axis deviation is <2mm; (4) Solution heat treatment: The bent pipes are kept at 1050-1070℃ for 3h and then water-cooled under the conditions of a furnace temperature of ≤600℃ and a heating rate of ≤10℃ / min. The cooling water temperature is ≤35℃, and the entire process is controlled within ≤70 seconds. At the same time, combined with forced cooling, the actual temperature rise of the water temperature during the entire quenching process is ensured to be ≤3℃.

2. The reactor primary coolant variable diameter austenitic stainless steel main pipe hot blanking-cold bending integral forging method according to claim 1 is characterized in that: It also includes rough machining before heat treatment before the solid solution heat treatment and semi-finishing and finishing processes after the solid solution heat treatment.

3. The reactor primary coolant reduced diameter austenitic stainless steel main pipe hot cogging-cold bending integral forging method according to claim 1, characterized in that: It also includes setting up size detection and UT self-inspection processes before and after each step (3) and (4).

4. The method for integrally forging austenitic stainless steel main pipes for a reactor primary coolant circuit with variable diameter by hot blanking and cold bending according to claim 1, characterized in that: The technical route for smelting the steel ingot is: electrode blank preparation and appearance inspection → electrode welding → pre-production preparation → arc ignition and slag formation → metal electrode hanging and preheating → normal remelting and shrinkage feeding of the electrode blank → demoulding → hood cooling → inspection, wherein: The arc ignition and slag forming method uses the same steel grade metal electrode as the austenitic stainless steel main pipe to ignite the arc, the arc starting agent is steel chips, the voltage is ≥60V after the molten pool is formed, and the slag adding time is ≥30min; When hanging the metal electrode, first adjust the center of the electrode blank so that the distance between the periphery and the crystallizer is ≥50mm.

5. The method for integrally forging austenitic stainless steel main pipes for a reactor primary coolant circuit with variable diameter by hot blanking and cold bending according to claim 1, characterized in that: The deformation of the straight tube forgings in the 7th to 10th firings is greater than the critical deformation degree of recrystallization, and the deformation is ≥13%.

6. The reactor primary coolant reduced-diameter austenitic stainless steel main pipe hot blanking-cold bending integral forging method according to claim 1, characterized in that: The straight tube forgings produced by hot blanking and straight tube forging are trimmed at a temperature above 800° C., and the cold forgings are trimmed after being preheated to 900-950° C.

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