Compression molding method of thick-wall super duplex stainless steel seal head

Abstract

The invention discloses a compression molding method of a thick-wall super duplex stainless steel end socket, which comprises the following process steps: designing and determining the size, shape and material thickness of a container, marking, blanking, carrying out punch molding on a material sheet by using a punching grinding tool, carrying out solid solution, and carrying out compression molding on the material sheet to obtain the thick-wall super duplex stainless steel end socket. Wherein the punch forming process and the solid solution process are specially applicable to the process for manufacturing the end socket by using the thick-wall super duplex stainless steel, the forming quality can be improved, and the quality and the performance of the end socket are ensured by strictly detecting and controlling the process flow; the problems that the end socket cracks and is scrapped due to a traditional forming mode and ferrite and hardness cannot meet the requirements are solved, the manufacturing period can be shortened, and material loss and labor cost can be reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of head forming processes, and particularly relates to a pressing forming method for a thick-walled super duplex stainless steel head. Background Art

[0002] With the development of industries such as chemistry, petrochemical, paper-making, and petroleum, the demand for corrosion-resistant and high-strength materials is increasing day by day. As a high-performance super duplex stainless steel, S25073 is widely used in the manufacture of pressure vessels due to its excellent corrosion resistance and mechanical properties. However, traditional head forming methods have problems such as low material utilization rate, long manufacturing cycle, and high cost, which limit their application in the industry. Moreover, when the super duplex stainless steel head is subjected to hot pressing at 1050°C + solution treatment at 1050°C using traditional processes, the head cracks throughout the circle immediately after hot pressing or during solution treatment. When using traditional warm forming and hot forming processes, different degrees of cracking occur after the head is formed. The reason is that when traditional processes are used to process ordinary stainless steel, the requirements for the properties of the stainless steel itself, such as hardness, ferrite, and whether there are defects, are not high. In addition, the process and process parameter control during stamping cannot be applied to the forming process of stainless steel heads made of S25073 material. Summary of the Invention

[0003] The purpose of the present invention is to provide a pressing forming method for a head that can adapt to super duplex stainless steel (material S25073), which can avoid problems such as head cracking and deformation.

[0004] The technical solution adopted by the present invention is as follows:

[0005] A pressing forming method for a thick-walled super duplex stainless steel head, comprising the following steps:

[0006] The design determines the size, shape, and material thickness of the container. The thickness is greater than 20 mm. A steel plate of S25073 material is selected. The steel plate blanks are subjected to 100% PT inspection and are qualified at Grade I. It is required that the ferrite content is 45 - 55% and the hardness is ≤310 HBW. A thickness greater than 20 mm is considered the rear wall. The reason for using the steel plate with the above requirements is that the strength and phase balance of the original steel plate blank are uneven, which will cause local stress concentration and excessive strength during the initial pressing, resulting in crack defects in stress concentration areas such as the R part. Mainly to improve mechanical properties, super duplex stainless steel consists of austenite and ferrite phases. At phase balance, the austenite phase provides high strength and hardness, while the ferrite phase endows good plasticity and toughness. Super duplex stainless steel can achieve a good balance between strength and toughness, has high yield strength, tensile strength, and impact strength, while maintaining good ductility and fatigue resistance, and can withstand heavy loads and impacts. To improve corrosion resistance, phase balance helps enhance the corrosion resistance of super duplex stainless steel. The reasonable distribution of alloying elements such as chromium, molybdenum, and nitrogen in the two phases is crucial for corrosion resistance. Chromium increases the pitting potential and reduces the corrosion rate; molybdenum inhibits anodic dissolution and improves the stability of the passive film; nitrogen combines with hydrogen to form ammonium ions, promoting the repassivation of small holes. At phase balance, these elements act synergistically in the austenite and ferrite phases, enabling the steel to have excellent resistance to pitting corrosion, crevice corrosion, intergranular corrosion, and stress corrosion cracking, and can resist the corrosion of various inorganic acids, organic acids, and alkalis. To improve machining performance, phase balance also has an important impact on the machining performance of super duplex stainless steel. The balanced duplex structure can reduce the work hardening rate of the material, making it easier to deform during cold forming, hot working, welding, etc., reducing the generation of cracks and defects, and improving machining efficiency and product quality.

[0007] (2) Marking: Mark the cutting size line, processing line, and inspection line of the top elliptical head on the steel plate blank. According to the corresponding blank lofting size, it can ensure the size of the head during and after the stamping process, and for the micro-defects at the edge, they can be directly removed by secondary marking and cutting according to the surplus material, ensuring the size and quality of the head.

[0008] (3) Cutting: Cut the large piece of material into the required size and shape, ensuring the final overall size of the head and the rapid removal of micro-defects.

[0009] (4) When using a stamping die to stamp and form the sheet metal, it needs to be gradually formed through multiple stampings, gradually adjusting the curvature and diameter of the sheet metal. The number of stampings ≤ 4 times. Before the first stamping and forming, the sheet metal is heated in the furnace. After the sheet metal enters the furnace, the heating rate: rapidly heat up with the furnace, the heating rate is 140 °C / h, the holding temperature is 270 - 295 °C, and after holding for 20 - 30 min, it is taken out of the furnace and sent to the stamping die for pressing. The height of the bulge of the sheet metal in the first pressing is 1 / 3 of the overall height of the head. When heating in the furnace before the 2nd - 4th stampings, the sheet metal enters the furnace while it is hot and rapidly heats up, the heating rate is 140 °C / h, the holding temperature is 270 - 295 °C, and the holding time is 20 - 30 min. After each heating, it is taken out of the furnace for pressing. The height of the bulge in the 2nd and 3rd pressings is both at a depth of 1 / 3 of the overall height of the head. The fourth pressing is for trimming the opening. The hardness increases before and after each stamping, and the ferrite decreases before and after each stamping. Because of the super duplex stainless steel material, the austenite phase provides high strength and hardness, and the ferrite phase endows good plasticity and toughness. It achieves a good balance between strength and toughness, has high yield strength, tensile strength and impact strength, while maintaining good ductility and fatigue resistance, and can withstand heavy loads and impacts. If the temperature and number do not meet the standards, it will affect the strength and toughness of the head quality.

[0010] (5) Solution treatment: Weld support rings on the stamped and formed head, use E2594 electrode for ring welding to prevent the size of the head port from changing during solution treatment, eliminate the stress of the head, and detect the residual stress level after elimination. As long as the residual stress distribution is uniform, place the head on the solution treatment fixture, and place a thermocouple on the head. Based on the actual temperature measured by the thermocouple, heat it with a natural gas furnace and perform heat treatment according to the solution treatment process. When performing solution treatment, the furnace inlet temperature > 850 °C, rapidly heat up with the furnace, the heating rate is 200 °C / h, the holding temperature: 1150 - 1180 °C, the holding time: 25 - 30 min, and water-cool after taking out of the furnace, ensuring that the water entry time is less than 1 min. After solution treatment, detect ferrite and hardness. After solution treatment, the required hardness ≤ 330 HBW, and ferrite ≥ 40%. Because the head deforms after cold forming, it will generate restraint force and cause large stress concentration. It is necessary to release the stress generated by the head forming through heat treatment for restoring properties (solution treatment) to ensure the quality of the head and the service performance of the product in the process equipment.

[0011] (6) Conduct ferrite detection on the head. After forming, conduct ferrite detection on the head. The ferrite content in the metallographic structure being 45% - 55% is qualified.

[0012] A further technical solution is that if the size of the head after stamping and forming in step 4 does not meet the design requirements, it can only be trimmed by die pressing. When trimming by die pressing, the furnace inlet temperature > 850 °C, heat up with the furnace, the holding temperature is 1150 - 1180 °C, the holding time is 30 - 40 min, take out of the furnace and water-cool, and the water entry time ≤ 1 min.

[0013] A further technical solution lies in that, in step (5), the opening at the exact center of the bottom of the head is ≤ φ150 mm, which serves as an exhaust port during solution heat treatment.

[0014] A further technical solution lies in that, in step (1), it is ensured that the surface of the steel sheet blank has no oil stains, no impurities, and no damage.

[0015] A further technical solution lies in that, in step (3), after blanking, the burrs and hard objects on the edge of the blank are cleaned, small notches must be polished smoothly, the hardened layer of 3 mm at the cut is removed by grinding, after the grinding and cleaning are completed, the end groove is subjected to PT inspection to confirm no surface defects, and the deformation generated during the cutting process is corrected to ensure the flatness and straightness of the material.

[0016] A further technical solution lies in that, in step (5), a vibration aging device or a thermal aging device is used for stress relief.

[0017] In summary, due to the adoption of the above technical solutions, the beneficial effects of the present invention are as follows:

[0018] 1. Improve the forming quality: Through the process steps in the present invention: designing and determining the size, shape, and material thickness of the container, marking, blanking, stamping the blank with a stamping die, and solution treatment. Among them, the stamping forming and solution treatment processes are specifically suitable for manufacturing heads with thick-walled super duplex stainless steel, which can improve the forming quality. By strictly detecting and controlling the process flow, the quality and performance of the head are ensured, avoiding the situation where the head cracks and is scrapped caused by traditional forming methods and the problems that the ferrite and hardness cannot meet the requirements.

[0019] 2. Shorten the manufacturing cycle: Simplify the head forming process and reduce the manufacturing time.

[0020] 2. Reduce costs: Ensure the forming quality of the head, which can reduce material losses and labor costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a physical picture of the formed thick-walled super duplex stainless steel head manufactured by the method of the present invention;

[0022] Figure 2 is a physical picture after the first stamping;

[0023] Figure 3 is a physical picture after the second stamping;

[0024] Figure 4 is a physical picture after the third stamping;

[0025] Figure 5 is a physical picture after the fourth stamping;

[0026] Figure 6 It is the metallographic structure diagram of the formed head.

[0027] Figure 7 It is the diagram of the change of ferrite and hardness during the four-time stamping process.

[0028] Figure 8 It is the picture of the crack after forming the thick-walled super duplex stainless steel head using the existing stamping process.

[0029] Figure 9 It is also the picture of the crack after forming the thick-walled super duplex stainless steel head using the existing stamping process.

[0030] Figure 10 It is the picture of the crack after forming the thick-walled super duplex stainless steel head using the existing stamping process.

[0031] Figure 11 It is the picture of welding and installing the support ring on the formed head.

[0032] Figure 12 It is the picture of buckling the head on the solution treatment fixture. Specific implementation mode

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0034] As Figures 1 - 12 shown.

[0035] A method for pressing and forming a thick-walled super duplex stainless steel head includes the following steps:

[0036] (1) Design and determine the size, shape and material thickness of the container. The thickness is greater than 20 mm. Select a steel plate of S25073 material. The steel plate sheet is subjected to 100% PT inspection and is qualified at level I. It is required that the ferrite is 45 - 55% and the hardness is ≤ 310 HBW.

[0037] (2) Mark the material. Mark the cutting size line, processing line and inspection line of the top elliptical head on the steel plate sheet.

[0038] (3) Cut the material. Cut the large piece of material into the required size and shape.

[0039] (4) When using a stamping die to stamp and form the sheet material, it needs to be gradually formed through multiple stampings, gradually adjusting the curvature and diameter of the sheet material. The number of stampings ≤ 4 times. Before the first stamping and forming, the sheet material is heated in the furnace. After the sheet material enters the furnace, the heating rate: rapidly heat up with the furnace, the heating rate is 140 °C / h, the holding temperature is 270 - 295 °C, and after holding for 20 - 30 min, it is taken out of the furnace and sent to the stamping die for pressing. The bulging height of the sheet material in the first pressing is 1 / 3 of the overall height of the head. When heating in the furnace before the 2nd - 4th stampings, the sheet material enters the furnace while it is hot and rapidly heats up, the heating rate is 140 °C / h, the holding temperature is 270 - 295 °C, and the holding time is 20 - 30 min. After each heating, it is taken out of the furnace for pressing. The bulging heights of the 2nd and 3rd pressings are both at a depth of 1 / 3 of the overall height of the head. The fourth pressing is for trimming the opening. The hardness increases before and after each stamping, and the ferrite decreases before and after each stamping, as Figure 7 shown. The heating uses a natural gas furnace.

[0040] (5) Solution treatment: Weld a support ring on the stamped and formed head, use E2594 electrode for the welding of the fixed ring to prevent the size of the head port from changing during solution treatment, eliminate the stress of the head, and detect the residual stress level after elimination. As long as the residual stress distribution is uniform, place the head on the solution treatment fixture, place a thermocouple on the head, and take the actual measured temperature of the thermocouple as the standard. Use a natural gas furnace for heating, and perform heat treatment according to the solution treatment process. When entering the furnace during solution treatment, the temperature > 850 °C, rapidly heat up with the furnace, the heating rate is 200 °C / h, the holding temperature: 1150 - 1180 °C, the holding time: 25 - 30 min, and water-cool after taking out of the furnace, ensure that the water entry time is less than 1 min. After solution treatment, detect ferrite and hardness. After solution treatment, the required hardness ≤ 330 HBW, and ferrite ≥ 40%.

[0041] (6) After forming, the ferrite of the head is detected. The ferrite content in the metallographic structure being 45% - 55% is qualified.

[0042] If the size of the head after stamping and forming in step 4 does not meet the design requirements, it can only be trimmed by die pressing. When trimming by die pressing, the furnace entry temperature > 850 °C, heat up with the furnace, the holding temperature is 1150 - 1180 °C, the holding time is 30 - 40 min, take out of the furnace and water-cool, and the water entry time ≤ 1 min.

[0043] Die pressing trimming is to avoid the generation of local residual stress concentration due to secondary spinning. The die pressing trimming process is as follows:

[0044] Check the die: Ensure that the cavity, core and other components of the die are not damaged or deformed, the surface finish meets the requirements, and the connections of all components are firm.

[0045] Prepare materials: According to the material and requirements of the part to be repaired, prepare appropriate blanks and check whether their dimensions, surface quality, etc. meet the starting processing conditions. At the same time, prepare auxiliary materials such as lubricants to reduce friction during the molding process.

[0046] Debug the equipment: Debug the molding equipment, such as hydraulic presses, etc., to ensure that parameters such as pressure and stroke can be accurately controlled and meet the process requirements:

[0047] Clean the blank: Clean the oil, dust and other impurities on the surface of the blank to prevent them from entering the mold and affecting the repair quality.

[0048] Position the blank: Place the blank accurately in the cavity of the mold, and ensure that the blank does not displace during the molding process through devices such as positioning pins and positioning blocks to guarantee the accuracy of the repair position.

[0049] Close the mold: Start the molding equipment to slowly lower the upper mold to close with the lower mold and apply pressure to the blank. During the mold closing process, control the mold closing speed to avoid impacting the blank and the mold due to too fast speed.

[0050] Apply pressure: According to the material, thickness of the blank and the requirements of the repair, gradually increase the molding pressure to the set value and maintain it for a period of time, so that the blank undergoes plastic deformation under the action of the mold to achieve the purpose of repair. During the pressure application process, closely monitor the change of pressure to ensure the pressure is stable.

[0051] Hold pressure and cool: After reaching the set pressure, hold the pressure for a period of time to allow the blank to fully form in the mold. Then carry out cooling, which can be done by natural cooling or forced cooling (such as passing through cooling water) to make the part cool and solidify in the mold, improving the dimensional accuracy and surface quality.

[0052] Demold: After cooling is completed, start the demolding device of the molding equipment to raise the upper mold and take out the formed part from the mold. The demolding process should be carefully operated to avoid damaging the repaired part of the part and the mold.

[0053] Trim the edge: Use trimming tools, such as scissors, punching presses, grinding wheels, etc., to trim the flash, burrs, etc. on the edge of the part to make the edge of the repaired part smoother and neater, meeting the design requirements.

[0054] In step (5), a hole with a diameter ≤ φ150mm is opened at the exact center of the bottom of the head, which serves as a vent during solution heat treatment.

[0055] In step (1), ensure that the surface of the steel sheet blank is free of oil, impurities and damage.

[0056] After blanking in step (3), clean the burrs and hard objects on the edge of the sheet. The small notch must be polished smoothly. Grind and remove the 3-mm hardened layer of the cut. After the grinding and cleaning are completed, perform PT inspection on the end groove to confirm no surface defects, and correct the deformation generated during the cutting process to ensure the flatness and straightness of the material.

[0057] In step (5), use a vibration aging device or a thermal aging device to eliminate stress.

[0058]

[0059]

[0060] The above are only the preferred embodiments of the present invention.

Claims

1. A method for pressing and forming a thick-walled super duplex stainless steel head, characterized in that: The following steps are involved: (1) Design and determine the size, shape and material thickness of the container. The thickness should be greater than 20 mm. Select S25073 steel plate. The steel plate material should be 100% PT tested and qualified at level I. The ferrite content should be 45-55% and the hardness should be ≤310 HBW. (2) Marking: Mark the cutting size line, processing line and inspection line of the top elliptical head on the steel plate. (3) Cutting: cutting large pieces of material into the required size and shape, (4) Use stamping molds to stamp the blanks. It needs to be stamped and formed gradually through multiple stampings. The curvature and diameter of the blanks are gradually adjusted. The number of stampings is ≤ 4 times. Before the first stamping, the blanks are heated in the furnace. The heating rate after the blanks enter the furnace: quickly heat up with the furnace, the heating rate is 140℃ / h, the insulation temperature is 270~295℃, and the insulation time is 20-30min. After that, the blanks are taken out of the furnace and sent to the stamping mold for pressing. The bulge height of the blanks for the first pressing is 1 / 3 of the overall height of the head. When the blanks are heated in the furnace before the second to fourth stamping, the blanks are put into the furnace while hot, and the heating rate is 140℃ / h. The insulation temperature is 270~295℃, and the insulation time is 20-30min. After each heating, the blanks are taken out of the furnace for pressing. The bulge height of the second and third pressings is 1 / 3 of the overall height of the head. The fourth pressing is for trimming. The hardness increases before and after each stamping, and the ferrite decreases before and after each stamping. (5) Solution treatment: install and weld a support ring on the stamped head, use E2594 welding rod for solution treatment to prevent the size of the head port from changing during solution treatment, perform stress relief on the head, and detect the residual stress level after relief to achieve uniform residual stress distribution. Buckle the head on the solution treatment tooling, place a thermocouple on the head, and use the actual temperature measured by the thermocouple as the standard. Use a natural gas furnace for heating and perform heat treatment according to the solution treatment process. During solution treatment, the furnace temperature is >850℃, and the temperature rises rapidly with the furnace at a rate of 200℃ / h. The insulation temperature is 1150-1180℃ and the insulation time is 25-30min. Cool with water after leaving the furnace to ensure that the water entry time is less than 1min. After solution treatment, test the ferrite and hardness. After solution treatment, the hardness is required to be ≤330HBW and the ferrite is ≥40%. (6) After forming, the head is tested for ferrite, and the ferrite content in the metallographic structure is qualified at 45% to 55%.

2. The method for pressing and forming a thick-walled super duplex stainless steel head according to claim 1, characterized in that: If the size of the head after stamping in step 4 does not meet the design requirements, it can only be trimmed by molding. During molding and trimming, the furnace temperature is greater than 850°C, and the temperature rises with the furnace. The insulation temperature is 1150-1180°C, the insulation time is 30-40 minutes, and the head is water-cooled after being taken out of the furnace. The water entry time is ≤1 minute.

3. The method for pressing and forming a thick-walled super duplex stainless steel head according to claim 1, characterized in that: In step (5), a hole ≤φ150mm is opened in the center of the bottom of the head to serve as exhaust during solution heat treatment.

4. The method for pressing and forming a thick-walled super duplex stainless steel head according to claim 1, characterized in that: In step (1), ensure that the surface of the steel sheet is free of oil, impurities and damage.

5. The method for pressing and forming a thick-walled super duplex stainless steel head according to claim 1, characterized in that: After cutting in step (3), the burrs and hard objects on the edge of the sheet are cleaned, small notches are polished to be smooth, and the 3 mm hardened layer of the cut is removed by grinding. After grinding and cleaning, the end groove is subjected to PT inspection to confirm that there are no surface defects and to correct the deformation caused by the cutting process to ensure the flatness and straightness of the material.

6. The method for pressing and forming a thick-walled super duplex stainless steel head according to claim 1, characterized in that: In step (5), vibration aging equipment or thermal aging equipment is used to eliminate stress.

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