A method for molding 9Ni steel LNG tank head flaps

Abstract

A method for compression molding of LNG tank head flaps made of 9Ni steel, the process method includes the design and manufacture of 9Ni steel large-scale LNG tank head flap molding molds and the heating and mechanical property quality control of the 9Ni steel plate during the compression molding process. Based on the design and manufacture of special molds, by controlling the heating temperature of 9Ni steel below the tempering temperature, the heat treatment performance of the steel plate is not damaged, and the mechanical strength of the 9Ni steel plate compression molding can be reduced, making the compression molding process easier, reducing the rebound amount, and improving the molding quality. The amount of rework after one-time compression molding is smaller, and the efficiency is higher. The present invention is a brand-new process route for 9Ni steel molding, which realizes the efficient and high-quality manufacturing of large LNG tank head flaps, saves a lot of heat treatment costs and labor, avoids the damage to the performance of 9Ni steel base material by quenching or normalizing, and improves the compression molding efficiency and molding quality of the head flaps.

Description

Technical Field

[0001] The invention belongs to the field of shipbuilding and design, and specifically relates to a compression molding process method for large butterfly-shaped, elliptical and spherical head flaps suitable for 9Ni steel LNG ship C-type tanks and land LNG pressure container storage tanks. Background Art

[0002] Large LNG fuel carriers or dual-fuel ships use C-type tanks the most. The important components of C-type LNG tanks, including land-based LNG tank pressure vessels, are head seals. The heads at both ends of C-type LNG tanks generally use spherical heads, butterfly heads, and elliptical heads. Since the head specifications of large LNG tanks are too large, they need to be formed in pieces and then welded together. The pressing and forming of the head directly determines the difficulty and quality of head welding. As a pressure-bearing component, the shape tolerance of the head is relatively high. The gap between the 2m sample plates of large LNG head forming pieces is generally controlled to not exceed 3mm. Therefore, the head manufacturing process in the construction of large LNG tanks is the most difficult process with the greatest impact on product quality. The molding process of large LNG tank heads determines the product construction cycle, construction efficiency, and manufacturing cost.

[0003] 9Ni steel material properties Material properties list

[0004]

[0005] At present, the heads of LNG storage tanks are generally spherical and are pressed with small spherical molds. Although the manufacturing cost of the mold is low, it needs to be repeatedly pressed, which is inefficient and has a long cycle. Now, for large LNG elliptical or butterfly heads, especially thicker plate heads, they are basically formed by high-temperature hot pressing with an integral mold and then heat treated to restore the performance (the general 9Ni steel hot pressing process is to heat to above 850℃ for normalizing (the insulation time is 2.4min / mm), then take it out of the furnace for hot pressing, and then perform performance recovery heat treatment, quench at about 850℃ (the insulation time is 2.4min / mm), then quickly water cool, and then temper at about 560℃ (the insulation time is 2.4min / mm), and then air cool). The heat treatment time is long and the cost is high. The deformation is large during the heat treatment process. The high-temperature hot pressing and heat treatment have a great damage to the parent material structure and the mechanical properties of the parent material, especially the low-temperature impact performance of -196℃ will be reduced, which seriously affects the safety of the equipment. This method is too uneconomical. In order to achieve high-quality, high-precision, high-efficiency and low-cost press-forming manufacturing of LNG heads, it is very valuable and necessary to develop an optimal compression molding process method for 9Ni steel large-scale LNG tank head petals. Summary of the invention

[0006] In order to solve the above problems, the present invention provides a method for molding 9Ni steel LNG tank head flaps, and the technical solution adopted is:

[0007] A method for molding 9Ni steel LNG tank head flaps, the specific operation steps are as follows:

[0008] S1: Divide the head into petals, polar zone and temperate zone, perform 3D modeling and lofting according to the curved surface size of the head and the curved surface size of the petals. The head is generally divided into petals according to the size, polar zone and temperate zone, and can be divided into 2 or 3 or more layers according to the size of the head to meet the equipment profiling capacity.

[0009] The mold includes an upper mold and a lower mold, both of which are provided with a base plate, a tire frame and a panel. A frame-type tire frame is seated on the base plate, and a panel is fixed on the tire frame. The top of the tire frame of the upper mold is an arc convex upward, and the top of the tire frame of the lower mold is an arc concave downward. The curvature of the tire frames of the upper mold and the lower mold corresponds to the curved surface size of the petal, and the tire frame is a grid-type frame formed by steel plates staggered horizontally and vertically.

[0010] S2: Use a grid-type frame to perform a press test on the panels of the upper and lower molds. After the panels are completely attached to the tire frame, they are spot welded to the tire frame.

[0011] S3: Measure the rebound amount of the petal, heat to 520°C, keep warm for 1 hour, perform mold repair test, determine the final pressing parameters of the upper and lower molds, and then remove the panels of the upper and lower molds.

[0012] S4: After the upper and lower molds are finalized, cement is filled into the mold frames of the upper and lower molds. The shape of the mold frame after cement filling should be consistent with the shape of the petal. After the cement solidifies, the panel is re-welded to the upper and lower molds to finally form the upper and lower molds. The internal cement filling increases the rigidity. The plate is heated below the medium-temperature tempering temperature before pressing to achieve pressing with a smaller tonnage press. The head petal is pressed and formed in one time, and the performance and quality are qualified.

[0013] S5: Heat and keep the petals warm for warm pressing. The specific process parameters are as follows: the temperature of the pressed sheet entering the furnace is less than 400°C, the heating rate is 150°C / hour, the holding temperature is 520°C±15°C, and the holding time is set according to the thickness. The holding time is 1 hour for a thickness greater than 20mm and 0.5 hour for a thickness less than 20mm. The upper and lower molds are butted up and down, the sheet is taken out of the furnace for pressing, and cooled in the air after pressing.

[0014] S6: The whole piece is pressed and formed at one time, and the formed petals are subjected to ultrasonic thickness measurement inspection. Each petal has no less than 5 points, 4 of which are evenly distributed at a distance of 100 mm from the edge, and 1 point is located in the center of the petal. The measured thickness should not be less than the minimum forming thickness required by the drawing.

[0015] The above-mentioned 9Ni steel LNG tank head petal molding method, further, the head is a spherical, elliptical or butterfly-shaped head of a large pressure-bearing LNG storage tank with a diameter of more than 6m and a thickness of more than 10mm.

[0016] The above-mentioned 9Ni steel LNG tank head flap molding method further, after the flap size and type value are inspected and qualified, flame or plasma cleaning, groove processing, and tensile and impact tests are performed as required.

[0017] The above-mentioned 9Ni steel LNG tank head flap compression molding method, further, in step S3, the temperature pressing mold repair rebound amount in the mold repair test is 10%-15%.

[0018] The above-mentioned method for compression molding the 9Ni steel LNG tank head flap further has the edge of the panel extending beyond the edge of the tire frame.

[0019] The above-mentioned 9Ni steel LNG tank head flap molding method, further, for step S5, requires the corrugated sheet to be preheated when the ambient temperature is lower than 5°C and the humidity is greater than 80%. As shown in Table 1,

[0020]

[0021] Table 1

[0022] The above-mentioned 9Ni steel LNG tank head flap molding method further comprises: in step S5, the plate is taken out of the furnace and pressed for 5 minutes, and then cooled in the air.

[0023] After forming, it is strictly forbidden to use explosives to correct the shape. The relevant requirements are as follows:

[0024] A. After the petals are formed, measurements and border lines should be laid out and marked on the bracket;

[0025] B. The full-size sample should be used for the compression test. The gap between the sample and the petal at any part should be ≤3mm, and the edge area of ​​the petal should be inspected in detail.

[0026] C. The tolerance of the chord length of some petals should be ≤±1.5mm; the tolerance of the circumference should be ≤±1.5mm; the two chords of any petal passing through the axis should be on the same plane, and the vertical distance deviation should not exceed 3mm when measured diagonally with two straight lines;

[0027] D. The diameter tolerance of the straight edge of the bottom petal after pre-assembly is 0~+6mm, there should be sufficient roundness, and the difference between the maximum and minimum inner diameters should not exceed 10mm;

[0028] E. Other tolerance requirements for petal manufacturing must meet the tolerance requirements of ASMEⅧ-1 and GB / T25198.

[0029] The inspection sample box is used to inspect the petal forming condition, and the partial shape is modified until it is qualified. After the petal size and shape value are inspected and qualified, flame or plasma cleaning is carried out as required. The groove size accuracy should meet the precision control process requirements. The groove surface requirements are as follows:

[0030] A. The groove surface must be 100% visually inspected. The surface should be smooth and crack-free after cutting. The surface roughness Ra should be ≤50μm and the flatness should be ≤0.5mm.

[0031] B. The slag and oxide scale should be cleaned up, and there should be no defects such as cracks and delamination on the groove surface;

[0032] C. After cleaning, conduct PT inspection according to ASMEⅧ-1Appendix 8;

[0033] D. Perform ultrasonic thickness measurement on the formed head flaps. Each flap should have no less than 5 points, 4 of which are evenly distributed about 100 mm from the edge, and 1 point is located near the center of the flap. The actual measured thickness should not be less than the minimum forming thickness required by the drawing.

[0034] E. In order to demonstrate the reliability of the above compression molding process, a test plate heat treated in the same furnace was sent to the testing center for tensile test and impact test. The test acceptance standard was the acceptance standard of 9Ni steel plate, and the test was qualified.

[0035] The invention describes a method for integral compression molding of 9Ni steel LNG tank head flaps, which solves the high-cost and low-efficiency molding method of high-temperature hot pressing of 9NI steel heads, and also solves the problems of low molding efficiency and difficult shape control of point-pressed heads. The mold adopts an integral mold for compression molding, and the mold adopts a frame structure that is easy for plate welding. The internal cement filling increases the rigidity. The plate is heated below the medium-temperature tempering temperature before pressing, which can achieve pressing with a smaller tonnage press and ensure the molding quality, greatly reducing the demand for press equipment hardware. One-time molding improves production efficiency, reduces production cycle and production costs, and warm pressing not only ensures compression molding quality, but also reduces the cost of heating the press. The method of the invention has good practicability and a wide range of applications, and can achieve high-quality, low-cost, and high-efficiency integral compression molding of LNG tank head flaps. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a front view of a butterfly head with a diameter of 13m according to an example of the present invention;

[0037] Figure 2 This is a top view of a butterfly head with a diameter of 13m according to an example of the present invention;

[0038] Figure 3 It is a schematic diagram of the structure of the lower tire frame with the head petal piece integrally molded;

[0039] Figure 4 It is a schematic diagram of pouring cement in the lower tire frame structure of the head petal integrally molded in the present invention;

[0040] Figure 5 It is a schematic diagram of a panel of a tire assembly for mounting a tire with a head petal piece integrally molded under the tire of the present invention;

[0041] Figure 6 It is a schematic diagram of the structure of the integrally molded upper tire frame of the head petal of the present invention;

[0042] Figure 7 It is a schematic diagram of pouring cement in the integrally molded upper tire frame structure of the head petal piece of the present invention;

[0043] Figure 8 This is a schematic diagram of the panel of the tire assembly for the head petal integrally molded on the tire of the present invention. DETAILED DESCRIPTION

[0044] like Figure 1 to Figure 2 As shown, the product of this implementation case is a butterfly head with a diameter of 13m. Due to its large size, it needs to be manufactured in three layers. The manufacturing of a large double-curvature head petal with a third layer forming thickness of 19mm is taken as an example for explanation. The specific implementation steps are as follows:

[0045] 1. The mold is laid out according to the curved surface dimensions R8500 and R1550 of the third layer of petals of the head. The inner diameter of the upper opening of the petals is D = 9848mm, the inner diameter of the lower opening is D = 13000mm, divided into 20 pieces, the blanking thickness is 21mm (considering 2mm thinning), and the petal width is 18°;

[0046] 2. Carry out 3D modeling according to the petals, carry out detailed design of the mold according to the curve shape of the head petals (the rebound amount is 10%), the mold size is 150mm larger than the product size, the frame box structure, the material is Q235B, the longitudinal and transverse frame structure size is 200*200mm, the longitudinal bone plate thickness is 30mm, the transverse rib plate thickness is 20mm, the mold panel thickness is 30mm, the mold material is cut and inspected, and the frame mold is manufactured and welded, and the panel is not welded first;

[0047] 3. After the production is completed, use the mold frame to press the upper and lower 30mm panels of the mold. After they are fully fitted, spot weld the upper and lower panels;

[0048] 4. Measure the rebound amount of 9Ni steel petals (heating temperature 520℃, insulation for 1 hour), conduct mold repair test, and determine the final pressing mold parameters;

[0049] 5. Remove the panels of the upper and lower molds;

[0050] 6. After the frame mold is finalized, fill it with cement to increase its strength. After the cement is filled, the shape must be consistent with the shape of the petal pressing mold after inspection. After the cement is solidified, follow-up work can be carried out;

[0051] 7. Reinstall the upper and lower panels of the welding mold;

[0052] Put the petals into the heating furnace for heating, with a set of 21*300*400 test plates accompanying the furnace, the temperature of the profiled plate entering the furnace is less than 400℃, the heating rate is 150℃ / hour, the insulation temperature is 520℃±15℃, the insulation time is 1 hour, and then it is taken out of the furnace for profiled;

[0053] 8. Carry out warm pressing and keep the pressure for 5 minutes. Cool in the air after pressing. It is strictly forbidden to use explosives to correct the shape after forming.

[0054] 9. Check the petal molding of the sample box and make partial modifications until it is qualified;

[0055] 10. After the size and type of the petal are qualified after the sample box inspection, the ultrasonic thickness measurement inspection shall be carried out on the formed head petal. Each petal shall have no less than 5 points, of which 4 points are evenly distributed at a distance of about 100mm from the edge, and 1 point is located near the center of the petal. The measured thickness shall not be less than the minimum forming thickness required by the drawing;

[0056] 11. Carry out flame or plasma cleaning and groove processing as required;

[0057] 12.Perform performance test on the test board. The test results are as follows:

[0058] Tensile test

[0059] Test method: Requirements of various classification societies

[0060]

[0061] Impact test

[0062] Test method: Requirements of various classification societies

[0063]

[0064] After testing, the test board's performance was qualified.

Claims

1. A method for molding 9Ni steel LNG tank head flaps, characterized in that: S1: Divide the head into petals, divide the head into polar zone and temperate zone, perform three-dimensional modeling according to the curved surface size of the head and the curved surface size of the petals, and perform lofting; The mold includes an upper mold and a lower mold. Both the upper mold and the lower mold are provided with a bottom plate, a tire frame and a panel. A frame-type tire frame is seated on the bottom plate, and a panel is fixed on the tire frame. The top of the tire frame of the upper mold is an arc convex upward, and the top of the tire frame of the lower mold is an arc concave downward. The arc of the tire frame of the upper mold and the lower mold corresponds to the curved surface size of the petal. The tire frame is a grid-type frame formed by steel plates interlaced horizontally and vertically. S2: Using a grid frame to perform a press test on the panels of the upper and lower jigs, after the panels are completely attached to the jig frame, spot welding is performed with the jig frame; S3: Measure the rebound amount of the petal, heat to 520°C, keep warm for 1 hour, perform mold repair test, determine the final pressing parameters of the upper and lower molds, and then remove the panels of the upper and lower molds; S4: After the upper and lower jigs are shaped, cement is filled into the jig frames of the upper and lower jigs. The shape of the jig frames after the cement is filled should be consistent with the shape of the petals. After the cement is solidified, the panel is welded to the upper and lower jigs again to finally form the upper and lower jigs. S5: heating and heat preservation of the petals for warm pressing. The specific process parameters are as follows: the temperature of the pressed sheet entering the furnace is less than 400°C, the heating rate is 150°C / hour, the heat preservation temperature is 520°C±15°C, and the heat preservation time is set according to the thickness. The thickness is greater than 20mm for 1 hour, and the thickness is less than 20mm for 0.5 hours. The upper mold and the lower mold are butted up and down, and the sheet is taken out of the furnace for pressing, and cooled in the air after pressing. S6: The whole piece is pressed and formed at one time, and the formed petals are subjected to ultrasonic thickness measurement inspection. Each petal has no less than 5 points, 4 of which are evenly distributed at a distance of 100 mm from the edge, and 1 point is located in the center of the petal. The measured thickness should not be less than the minimum forming thickness required by the drawing.

2. A method for molding 9Ni steel LNG tank head flaps according to claim 1, characterized in that: The head is a spherical, elliptical or butterfly-shaped head for large pressure-bearing LNG storage tanks with a diameter of more than 6m and a thickness of more than 10mm.

3. The method for molding 9Ni steel LNG tank head flaps according to claim 1, characterized in that: After the petal size and type value are inspected and qualified, flame or plasma cleaning, groove processing, tensile and impact tests are carried out as required.

4. The method for molding a 9Ni steel LNG tank head flap according to claim 1, characterized in that: In step S3, the temperature pressing mold repair rebound amount in the mold repair test is 10%-15%.

5. The method for molding 9Ni steel LNG tank head flaps according to claim 1, characterized in that: For step S5, when the ambient temperature is lower than 5°C and the humidity is higher than 80%, the corrugated sheet is required to be preheated.

6. The method for molding 9Ni steel LNG tank head flaps according to claim 1, characterized in that: In step S5, the sheet is taken out of the furnace and pressed for 5 minutes, and then cooled in the air.

7. The method for molding 9Ni steel LNG tank head flaps according to claim 1, characterized in that: The edge of the panel extends beyond the edge of the tire frame.

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