Method of forming a two-phase stainless steel propeller

Abstract

The application discloses a forming method of a duplex stainless steel propeller, and is specifically implemented according to the following steps: step 1, a casting process of the duplex stainless steel propeller is designed, and a sand mold used in the casting process is designed by using three-dimensional modeling software; step 2, a designed sand mold is formed by using a 3D printing technology, then the printed sand mold is detected and trimmed, then the sand mold is assembled into a sand box and is filled with sand around the sand mold, and thus a mold cavity required for forming the propeller can be obtained; step 3, duplex stainless steel liquid metal is poured into the mold cavity, and a complete propeller casting is obtained; and step 4, the propeller casting obtained in step 3 is subjected to heat treatment, and a final propeller casting is obtained. The method of pouring metal into a mold cavity formed by 3D printing a sand mold solves the problems of sand dropping, forming difficulty and low strength in the existing 3D printing resin model re-sand casting or 3D direct printing metal propeller technology.

Description

Technical Field

[0001] This invention belongs to the technical field of marine parts manufacturing methods, and relates to a method for forming duplex stainless steel propellers. Background Technology

[0002] The propeller is a core component of a ship's propulsion system, and its strength and corrosion resistance directly affect the ship's propulsion efficiency and navigation performance. With the rapid development of marine engineering and the shipbuilding industry, higher demands are being placed on the material properties and manufacturing processes of propellers; especially for high-performance ships, such as high-speed yachts and naval vessels, the requirements for propeller corrosion resistance, strength, and weight are even more stringent. Traditional propeller materials such as manganese bronze and nickel-aluminum bronze, while possessing certain strength and corrosion resistance, are no longer sufficient to meet the needs of modern large and special-purpose ships in harsh marine environments such as high salinity and high current velocities. Duplex stainless steel has a much higher strength than traditional manganese bronze and nickel-aluminum bronze, and its good toughness and corrosion resistance make it extremely suitable for manufacturing high-performance propellers.

[0003] Currently, duplex stainless steel propellers are mainly manufactured using two methods: separate forging followed by welding and integral casting. The separate forging and welding method involves forging duplex stainless steel material separately into the approximate shapes of the hub and blades, assembling and welding them, and then machining them into the desired shape. This method suffers from drawbacks such as low material utilization, poor isotropy, long manufacturing cycle, and high cost. In particular, intergranular corrosion at the weld seams poses a significant safety hazard to propeller operation. The integral casting method, using casting, additive manufacturing, or other methods, directly forms the blades and hub together. This effectively avoids the problems of low material utilization, poor isotropy, long manufacturing cycle, high cost, and intergranular corrosion at the weld seams associated with the separate forging method. Chinese patent "A Method for Manufacturing Propellers by 3D Printing" (Application Date: July 20, 2016; Application No.: 201610579419.6; Publication No.: CN106166599A; Publication Date: November 30, 2016) discloses a method for manufacturing marine propellers by 3D printing. The method involves creating a model of photosensitive resin that perfectly matches the shape of a propeller using 3D printing. The resin model is then placed in a sand mold, filled with sand, and cured. After curing, the resin model is removed, creating the cavity required for casting the propeller. Solid metal is then melted and poured into the cavity to obtain the propeller casting. While this method can quickly print propeller models, the curved shape of the propeller blades makes it prone to sand loss due to insufficient sand filling, resulting in sand inclusion defects and product scrap. Furthermore, removing the resin model of the curved blades from the cured sand mold without damaging the mold is quite challenging and requires a high level of operator skill. This patent has not yet been published. Chinese patent “A 3D additive manufacturing method for casting marine propellers” (application date: December 30, 2017, application number: 201711493691.3, publication number: CN107999770A, publication date: May 8, 2018) discloses a 3D additive manufacturing method for marine propellers. This method involves mixing titanium alloy, cobalt-chromium alloy, stainless steel, aluminum, ceramic powder, and copper alloy in a certain proportion, then layering the powders, and bonding the layers with adhesive to form the desired propeller. While this method has high material utilization, it fails to achieve metallurgical bonding between the printed layers. Furthermore, the adhesive evaporates after post-processing, creating numerous pore-like defects inside, resulting in low overall propeller strength and failing to meet the performance requirements of high-speed or large ships. Summary of the Invention

[0004] The purpose of this invention is to provide a method for integral molding of a dual-phase stainless steel propeller. This method uses 3D printing of sand molds to form a cavity and then pouring metal to form an integral mold, which solves the problems of sand shedding, molding difficulties, and low strength in existing technologies such as 3D printing of resin models followed by sand casting or 3D direct printing of metal propellers.

[0005] The technical solution adopted in this invention is a method for forming a duplex stainless steel propeller, which is implemented according to the following steps:

[0006] Step 1: Design the casting process for the duplex stainless steel propeller and use 3D modeling software to design the sand mold used in the casting process.

[0007] Step 2: Use 3D printing technology to form the designed sand mold, then inspect and trim the printed sand mold, then assemble it into a sand box and fill it with molding sand to obtain the cavity required to form the propeller.

[0008] Step 3: Inject molten duplex stainless steel into the mold cavity to obtain a complete propeller casting;

[0009] Step 4: Heat treat the propeller casting obtained in Step 3 to obtain the final propeller casting.

[0010] The invention is further characterized in that:

[0011] The casting process in step 1 is as follows:

[0012] Austenitic and ferritic duplex stainless steel is selected, and the scaling factor is chosen based on the structural characteristics of the propeller casting, ranging from 1.0% to 2.5%. A parting surface is provided, from which the blades separate from the curved surface. A semi-closed gating system is adopted. 直 :ΣF 横 :ΣF 内 =(1.0-1.2):(0.8-0.9):1.0, where ΣF 直 , ΣF 横 , ΣF 内 These refer to the cumulative cross-sectional areas of the sprue, runner, and ingate in the gating system. Bottom pouring is used, with molten metal injected from the bottom of the propeller shaft through a circular cross-section ceramic gating pipe. The diameter of the ingate is 1 / 2 to 1 / 3 of the propeller shaft diameter. The riser diameter is selected as 1.2 to 1.6 times the diameter of the hot spot circle of the propeller shaft, and the riser height is 1.1 to 1.7 times the riser diameter. The riser is placed on the top surface of the propeller shaft. The machining allowance of the propeller is determined according to the dimensions of the propeller, and the machining allowance value should meet the machining requirements.

[0013] In step 1, 3D modeling software is used to design the sand mold. Specifically, the propeller is divided into upper and lower molds along the split plane of the blade, and a radial core head is added. The core head size is 100-300mm larger than the propeller outline size. A positioning groove is set in the center of the bottom surface of the core head. The positioning groove adopts a frustum-shaped male and female pin hole fit. The vertical projection area of ​​the positioning groove is 1 / 8-1 / 6 of the bottom surface area of ​​the sand mold. The upper sand cover and lower sand cover of the sand mold are designed. The edges of the upper sand cover and lower sand cover are flush with the outer edge of the blade sand mold.

[0014] Step 2, which uses 3D printing technology to form the designed sand mold, involves: processing the three-dimensional model data of the sand mold, upper sand cover, and lower sand cover obtained from the mold splitting; importing the processed data into the sand mold printer control system; feeding the molding sand, which is uniformly mixed with resin binder and curing agent, into the array nozzle device; under the control of the computer, the nozzle device densely accumulates molding sand particles within the sand mold layer cross-sectional contour range to form the sand mold structure; and loosely accumulates molding sand particles outside the sand mold layer cross-sectional contour range to form the support structure; and printing the designed sand mold layer by layer.

[0015] Step 2 involves inspecting and trimming the printed sand mold as follows: a scanner is used to collect data points on the surface of the printed sand mold, and Geomagic Qualify software is used to fit and compare the scanned data with the 3D model used for printing. Areas where the sand mold dimensions are out of tolerance are ground down, and the sand mold dimension deviation is controlled within "nominal size ±0.3mm".

[0016] Step 2, specifically the filling of molding sand, involves: spraying alcohol-based zircon powder coating onto the surface of the sand mold, drying the coating, adding a sand box with a sand intake of 100-200mm, placing the assembled sand mold and gating system inside the sand box, and filling the surrounding area with back sand; after the lower sand mold has been made and cured, the mold is turned over, and risers, sprues, and pouring cups are placed, and back sand is filled in. After the back sand has hardened, the cavity is baked with a hot air blower.

[0017] When printing the sand mold in step 2, the weight ratio of molding sand, resin binder and curing agent is 100:(1.5-3):(0.5-1);

[0018] In step 2, the hot air blower is used to bake the cavity. The hot air blower is heated to 150-200℃ and hot air is sent into the cavity through the pipe until the surface of the cavity is dry.

[0019] Step 3 specifically involves:

[0020] Austenitic and ferritic duplex stainless steel is smelted in an intermediate frequency furnace to obtain duplex stainless steel molten metal. After adjusting the alloying elements to the required composition range, the molten duplex stainless steel is transferred from the intermediate frequency furnace to an AOD furnace for argon-oxygen decarburization refining. High-purity nitrogen gas is then repeatedly blown into the molten metal in the AOD furnace. The nitrogen element is fully homogenized and alloyed in the molten metal through gas stirring. After the nitrogen content meets the requirements, the casting is poured out of the furnace. After the propeller casting cools and solidifies, it is cleaned to obtain a complete propeller casting.

[0021] In step 3, the purity of high-purity nitrogen gas is 99.99-99.9999%. High-purity nitrogen gas is introduced into the duplex stainless steel molten metal through a heat-resistant pipe. After the nitrogen content meets the requirements, the duplex stainless steel molten metal is taken out of the furnace at 1640-1660℃ and poured at 1600-1620℃.

[0022] Step 4 is as follows:

[0023] Weld anti-deformation ribs onto the blades of the propeller casting obtained in step 3. Then heat the propeller casting in a box-type resistance furnace to 1145-1155℃ and hold it for 2 hours (the wall thickness of the casting is δ(mm) / 20(mm) + 2 hours) to allow the alloying elements to fully dissolve in the austenite and ferrite. After holding, furnace cool to 1065-1075℃ and remove from the furnace. Place in water at 35-45℃ for rapid cooling. After cooling, remove the anti-deformation ribs.

[0024] In step 4, the anti-deformation bracing is installed in both the circumferential and axial directions. The circumferential bracing uses a globe-like parallel bracing to horizontally encircle the propeller blade tip and weld it to the edge of the blade tip. One bracing is installed every 150-200mm in height. One end of the axial bracing is welded to the side of the propeller shaft, and the other end is welded to the central area of ​​the top surface of the blade. This prevents the blade from softening and bending downwards due to heat. Each blade needs to be welded with an axial bracing.

[0025] The beneficial effects of this invention are:

[0026] (1) The present invention directly forms the required propeller cavity by printing sand molds, which can effectively avoid the problems of loose filling and difficulty in removing the resin model caused by printing the resin propeller model and then filling the sand mold. At the same time, with the help of laser scanning detection, it can ensure that the dimensional accuracy of the sand mold is highly consistent with the required propeller model, and the product consistency and accuracy are significantly improved.

[0027] (2) The propeller forming method of the present invention can directly cast the propeller in one piece, and add certain heat treatment and anti-deformation measures. It can effectively avoid the problems of low overall strength of the propeller caused by non-metallic bonding between layers and non-dense internal structure caused by direct printing of alloy powder. The overall structure is dense and the strength is high.

[0028] (3) This invention can significantly improve the production efficiency of propellers. Using 3D printed sand molds can reduce molding time and increase the yield. Compared with traditional casting, 3D printed sand molds have high compactness and good consistency, shorten the manufacturing cycle by 20-30%, and increase the yield by 10-20%. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the casting process structure of the duplex stainless steel propeller of the present invention.

[0030] Figure 2 This is a microstructure image of the duplex stainless steel propeller prepared in Example 2.

[0031] In the diagram: 1. Pour cup, 2. Riser sleeve, 3. Sand mold upper sand cover, 4. Sand mold, 5. Sand mold lower sand cover, 6. Ceramic pouring pipe, 7. Back sand. Detailed Implementation

[0032] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0033] Example 1

[0034] The method for forming a duplex stainless steel propeller according to the present invention is implemented according to the following steps:

[0035] Step 1: Design the casting process for the duplex stainless steel propeller and use 3D modeling software to design the sand mold used in the casting process.

[0036] Step 2: Use 3D printing technology to form the designed sand mold, then inspect and trim the printed sand mold, then assemble it into a sand box and fill it with molding sand to obtain the cavity required to form the propeller.

[0037] Step 3: Inject molten duplex stainless steel into the mold cavity to obtain a complete propeller casting;

[0038] Step 4: Heat treat the propeller casting obtained in Step 3 to obtain the final propeller casting.

[0039] The casting process in step 1 is as follows:

[0040] Austenitic and ferritic duplex stainless steel is selected, and the scaling factor is chosen based on the structural characteristics of the propeller casting, ranging from 1.0% to 2.5%. A parting surface is provided, from which the blades separate from the curved surface. A semi-closed gating system is adopted. 直 :ΣF 横 :ΣF 内 =(1.0-1.2):(0.8-0.9):1.0, where ΣF 直 , ΣF 横 , ΣF 内 These refer to the cumulative cross-sectional areas of the sprue, runner, and endogate in the gating system, respectively, with bottom pouring selected as the pouring method. Figure 1 As shown, a circular cross-section ceramic gating pipe is used to inject molten metal from the bottom of the propeller shaft. The diameter of the ingate is 1 / 2 to 1 / 3 of the diameter of the propeller shaft. The riser diameter is selected as 1.2 to 1.6 times the diameter of the hot spot circle of the propeller shaft, and the riser height is 1.1 to 1.7 times the riser diameter. The riser is placed on the top surface of the propeller shaft. The machining allowance of the propeller is determined according to the size of the propeller, and the machining allowance value is sufficient to meet the machining requirements.

[0041] In step 1, 3D modeling software (such as UG, CATIA, SolidWorks, etc.) is used to design the sand mold. Specifically, the propeller is divided into upper and lower molds along the split surface of the blade, and a radial core head is added. The core head size is 100-300mm larger than the propeller outline size. A positioning groove is set in the center of the bottom surface of the core head. The positioning groove adopts a frustum-shaped male and female pin hole fit. The vertical projection area of ​​the positioning groove is 1 / 8-1 / 6 of the bottom surface area of ​​the sand mold. The upper sand cover 3 and the lower sand cover 5 of the sand mold are designed. The edges of the upper sand cover 3 and the lower sand cover 5 are flush with the outer edge of the blade sand mold.

[0042] In step 2, the sand mold designed using 3D printing technology is formed as follows: the three-dimensional model data of sand mold 4, sand mold upper cover 3, and sand mold lower cover 5 obtained from the mold splitting are sliced ​​and processed. The processed data is imported into the sand mold printer control system. The molding sand uniformly mixed with resin binder and curing agent is fed into the array nozzle device. Under the control of the computer, the nozzle device and the nozzle extrusion device densely accumulate molding sand particles within the sand mold layer cross-sectional contour range to form a sand mold structure. Outside the sand mold layer cross-sectional contour range, the molding sand particles are loosely accumulated to form a support structure. The designed sand mold is printed layer by layer.

[0043] Step 2 involves inspecting and trimming the printed sand mold as follows: a scanner is used to collect data points on the surface of the printed sand mold, and Geomagic Qualify software is used to fit and compare the scanned data with the 3D model used for printing. Areas where the sand mold dimensions are out of tolerance are ground down, and the sand mold dimension deviation is controlled within "nominal size ±0.3mm".

[0044] In step 2, the filling of molding sand is as follows: the surface of the sand mold is sprayed with alcohol-based zircon powder coating, the coating is dried, a sand box is added with a sand intake of 100-200mm, the assembled sand mold 4 and sprue 6 are placed in the sand box, and the back sand 7 is filled around the sides; after the sand mold in the lower box is completed and cured, the box is turned over, the riser 2, sprue 6 and pouring cup 1 are placed, and the back sand 7 is filled. After the back sand 7 hardens, the cavity is baked with a hot air blower.

[0045] When printing the sand mold in step 2, the weight ratio of molding sand, resin binder and curing agent is 100:(1.5-3):(0.5-1);

[0046] In step 2, the hot air blower is used to bake the cavity. The hot air blower is heated to 150-200℃ and hot air is sent into the cavity through the pipe until the surface of the cavity is dry.

[0047] Step 3 specifically involves:

[0048] Austenitic and ferritic duplex stainless steel is smelted in an intermediate frequency furnace to obtain duplex stainless steel molten metal. After adjusting the alloying elements to the required composition range, the molten duplex stainless steel is transferred from the intermediate frequency furnace to an AOD furnace for argon-oxygen decarburization refining. High-purity nitrogen gas is then repeatedly blown into the molten metal in the AOD furnace. The nitrogen element is fully homogenized and alloyed in the molten metal through gas stirring. After the nitrogen content meets the requirements, the casting is poured out of the furnace. After the propeller casting cools and solidifies, it is cleaned to obtain a complete propeller casting.

[0049] In step 3, the purity of high-purity nitrogen gas is 99.99-99.9999%. High-purity nitrogen gas is introduced into the duplex stainless steel molten metal through a heat-resistant pipe. After the nitrogen content meets the requirements, the duplex stainless steel molten metal is taken out of the furnace at 1640-1660℃ and poured at 1600-1620℃.

[0050] Step 4 is as follows:

[0051] Weld anti-deformation ribs onto the blades of the propeller casting obtained in step 3. Then heat the propeller casting in a box-type resistance furnace to 1145-1155℃ and hold it for 2 hours (the wall thickness of the casting is δ(mm) / 20(mm) + 2 hours) to allow the alloying elements to fully dissolve in the austenite and ferrite. After holding, furnace cool to 1065-1075℃ and remove from the furnace. Place in water at 35-45℃ for rapid cooling. After cooling, remove the anti-deformation ribs.

[0052] In step 4, the anti-deformation bracing is installed in both the circumferential and axial directions. The circumferential bracing uses a globe-like parallel bracing to horizontally encircle the propeller blade tip and weld it to the edge of the blade tip. One bracing is installed every 150-200mm in height. One end of the axial bracing is welded to the side of the propeller shaft, and the other end is welded to the central area of ​​the top surface of the blade. This prevents the blade from softening and bending downwards due to heat. Each blade needs to be welded with an axial bracing.

[0053] Example 2

[0054] Based on Example 1, this example uses a certain type of 5-bladed propeller as a reference example. The external dimensions of this propeller are as follows: Net weight 46kg, main wall thickness 20mm, maximum wall thickness 120mm, minimum wall thickness 2mm, composition (percentage content) control range: C≤0.04, Si≤2, Mn≤4, Cr 21-27, Ni 4-9, S≤0.02, P≤0.035, Mo 3-5, N 0.14-0.35.

[0055] Step 1, Casting Process Design: The propeller is selected with a reduction scale of 1.8%; the propeller is placed vertically, and the parting surface is set on the top surface of the propeller shaft; a bottom-pouring gating system with a filter screen is adopted, and the gating system is composed of 6 ceramic gating pipes. The cross-sectional ratio of the gating system is sprue: runner: ingate = 1.2:0.9:1; one visible riser is set on the top surface of the propeller shaft, the riser adopts 2 heat-insulating riser sleeves, the riser diameter is 155mm, and the riser height is 1.3 times the riser diameter; the machining allowance is 8mm on each side.

[0056] Step 2, Sand mold design: The propeller is parted along the mid-section of the blades, and the parting surface is used as the outer surface of the sand mold. The 5-bladed propeller produces 5 sand molds 4. A 150mm long core head is added radially, and a frustum-shaped positioning groove is set on the bottom surface of the core head. Sand mold upper cover 3 and sand mold lower cover 5 are designed on the upper and lower surfaces of the sand mold. The outer contours of sand mold upper cover 3 and sand mold lower cover 5 are flush with the radial side of the sand mold.

[0057] Step 3, Sand Mold Printing: The 3D model data of the sand mold and sand cover obtained from the mold division is sliced ​​and processed. The slice layer height is set to 0.4mm, and the printing layer height is also set to 0.4mm. The processed data is then imported into the sand mold printer control system. 0.1mm sand particles, uniformly mixed with curing agent and phenolic resin binder, are fed into an array nozzle device at a weight ratio of sand:binder:curing agent = 100:2.1:0.7. Under computer control, the nozzle device densely packs sand particles within the sand mold layer cross-sectional contour area to form a sand mold / core structure, while loosely packing sand particles outside the sand mold layer cross-sectional contour area to form a support structure. The designed sand mold and upper and lower sand cover plates are printed layer by layer until the sand mold printing is completed.

[0058] Step 4, Sand mold inspection: Use a portable laser scanner to collect data points on the surface of the sand mold, and use GeomagicQualify software to fit and compare the scanned data with the 3D model to be printed. Grind the deformed parts of the sand mold and control the size deviation of the sand mold within "±0.3mm".

[0059] Step 5, Sand Filling and Shaping: Spray paint onto the printed sand mold surface, dry the paint, select a sand box with a sand capacity of 180mm, place the assembled sand mold and sand cover in the sand box, install the gating system, and fill the surrounding area with back sand 7; after the lower sand mold is completed and cured, turn the box over, place the riser sleeve 2, ceramic gating system 6 and pouring cup 1, fill with back sand 7, and after the back sand 7 hardens, heat it to 180℃ with a hot air blower to bake the cavity for 3 hours.

[0060] Step 6, Melting and Casting: Melting is carried out in an induction furnace. After the molten metal is liquefied and adjusted to the required composition, it is transferred to an AOD furnace for argon-oxygen decarburization refining. Then, 99.999% high-purity nitrogen gas is repeatedly blown into the molten metal in the AOD furnace for 15 minutes at a flow rate of 30 L / min. The stirring effect of the gas ensures that the nitrogen element is fully homogenized and alloyed in the molten metal. After the nitrogen content meets the requirements, the metal is removed from the furnace and poured into the mold. The furnace temperature is controlled at 1660℃, and the casting temperature is 1620℃. After the propeller casting cools and solidifies, it is cleaned.

[0061] Step 7, Heat treatment: Weld a ring of tie rods around the propeller circumferentially, and weld anti-deformation ribs to the blades and shaft in the axial direction; Heat the propeller casting to 1155℃ and hold for 3 hours to fully dissolve the alloying elements in the austenite; cool it in the furnace to 1070℃ and remove it from the furnace quickly; then cool it in 40℃ water to inhibit the precipitation of harmful phases.

[0062] Step 8, Performance Testing and Verification: Randomly select one sample for performance testing and analysis; wire-cut tensile and metallographic specimens, and grind the metallographic and tensile specimens; test the tensile strength using a universal tensile testing machine, and observe the austenite and ferrite phase ratio using a metallographic microscope, such as... Figure 2 The image shown is a microstructure of the duplex stainless steel propeller prepared in this embodiment. The microstructure of the propeller casting is austenite + ferrite, with austenite accounting for 45%. The tensile strength Rm is 752 MPa, which meets the requirements for metallographic and mechanical properties.

[0063] Example 3

[0064] Based on Example 1, this example uses a certain type of 3-bladed propeller as a reference example. The external dimensions of this propeller are as follows: Net weight 38kg, main wall thickness 25mm, maximum wall thickness 80mm, minimum wall thickness 3mm, composition (percentage content) control range: C≤0.04, Si≤1.8, Mn≤3, Cr 21-25, Ni 4-7, S≤0.02, P≤0.02, Mo 3-4, N 0.15-0.35.

[0065] Step 1, Casting Process Design: The propeller is selected with a 2% reduction in size; the propeller is placed vertically, and the parting line is set on the top surface of the propeller shaft; a bottom-pouring gating system with a filter screen is adopted, and the gating system is composed of 6 ceramic gating pipes. The cross-sectional ratio of the gating system is sprue: runner: ingate = 1.1:0.9:1; one visible riser is set on the top surface of the propeller shaft, the riser adopts an insulated riser sleeve 2, the riser diameter is 150mm, and the riser height is 1.3 times the riser diameter; the machining allowance is 10mm on each side.

[0066] Step 2, Sand mold design: The propeller is parted along the mid-section of the blades, and the parting surface is used as the outer surface of the sand mold. The 3-bladed propeller produces 3 sand molds 4. A 150mm long core head is added radially, and a frustum-shaped positioning groove is set on the bottom surface of the core head. Sand mold upper cover 3 and sand mold lower cover 5 are designed on the upper and lower surfaces of the sand mold. The outer contours of sand mold upper cover 3 and sand mold lower cover 5 are flush with the radial side of the sand mold.

[0067] Step 3, Sand Mold Printing: The 3D model data of the sand mold and sand cover obtained from the mold division is sliced ​​and processed. The slice layer height is set to 0.4mm, and the printing layer height is also set to 0.4mm. The processed data is then imported into the sand mold printer control system. 0.1mm sand particles, uniformly mixed with curing agent and phenolic resin binder, are fed into an array nozzle device at a weight ratio of sand:binder:curing agent = 100:2.1:0.7. Under computer control, the nozzle device densely packs sand particles within the sand mold layer cross-sectional contour area to form a sand mold / core structure, while loosely packing sand particles outside the sand mold layer cross-sectional contour area to form a support structure. The designed sand mold and upper and lower sand cover plates are printed layer by layer until the sand mold printing is completed.

[0068] Step 4, Sand mold inspection: Use a portable laser scanner to collect data points on the surface of the sand mold, and use GeomagicQualify software to fit and compare the scanned data with the 3D model to be printed. Grind the deformed parts of the sand mold and control the size deviation of the sand mold within "±0.3mm".

[0069] Step 5, Sand Filling and Shaping: Spray paint onto the printed sand mold surface, dry the paint, select a sand box with a sand capacity of 180mm, place the assembled sand mold and sand cover in the sand box, install the gating system, and fill the surrounding area with back sand 7; after the lower sand mold is completed and cured, turn the box over, place the riser sleeve 2, ceramic gating system 6 and pouring cup 1, fill with back sand 7, and after the back sand 7 hardens, heat it to 180℃ with a hot air blower to bake the cavity for 3 hours.

[0070] Step 6, Melting and Casting: Melting is carried out in an induction furnace. After the molten metal is liquefied and adjusted to the required composition, it is transferred to an AOD furnace for argon-oxygen decarburization refining. Then, 99.999% high-purity nitrogen gas is repeatedly blown into the molten metal in the AOD furnace for 15 minutes at a flow rate of 30 L / min. The stirring effect of the gas ensures that the nitrogen element is fully homogenized and alloyed in the molten metal. After the nitrogen content meets the requirements, the metal is removed from the furnace and poured into the mold. The furnace temperature is controlled at 1660℃, and the casting temperature is 1620℃. After the propeller casting cools and solidifies, it is cleaned.

[0071] Step 7, Heat treatment: Weld a ring of tie rods around the propeller circumferentially, and weld anti-deformation ribs to the blades and shaft in the axial direction; heat the propeller casting to 1155℃ and hold for 3.25 hours to fully dissolve the alloying elements in the austenite, furnace cool to 1070℃ and quickly remove from the furnace, then place it in 40℃ water for cooling to inhibit the precipitation of harmful phases.

[0072] Step 8, Performance Testing and Verification: Randomly select one piece for performance testing and analysis; wire cut tensile and metallographic specimens, and grind the metallographic and tensile specimens; test the tensile strength using a universal tensile testing machine, and observe the austenite and ferrite phase ratio using a metallographic microscope; the microstructure of the propeller casting is austenite + ferrite, with austenite accounting for 46%; the tensile strength Rm is 755MPa, which meets the metallographic and mechanical property requirements.

Claims

1. A method for forming a duplex stainless steel propeller, characterized in that, The specific steps are as follows: Step 1: Design the casting process for the duplex stainless steel propeller and use 3D modeling software to design the sand mold used in the casting process. The casting process specifically includes: Austenitic and ferritic duplex stainless steel is selected, and the reduction scale is chosen based on the structural characteristics of the propeller casting, ranging from 1.0% to 2.5%. A parting surface is set, from which the blades separate from the curved parting surface. A semi-closed gating system is adopted, ΣF 直 :ΣF 横 :ΣF 内 =(1.0-1.2):(0.8-0.9):1.0, where, ΣF 直 , ΣF 横 , ΣF 内 These refer to the cumulative cross-sectional areas of the sprue, gating system, and ingate, respectively. Bottom pouring is selected as the pouring method. A circular cross-section ceramic gating pipe (6) is used to inject molten metal from the bottom of the propeller shaft. The diameter of the ingate is 1 / 2 to 1 / 3 of the diameter of the propeller shaft. The diameter of the riser (2) is selected to be 1.2 to 1.6 times the diameter of the hot spot circle of the propeller shaft. The height of the riser (2) is 1.1 to 1.7 times the diameter of the riser. The riser (2) is placed on the top surface of the propeller shaft. The machining allowance of the propeller is determined according to the size of the propeller. The machining allowance value should meet the machining requirements. Among them, 3D modeling software is used to design the sand mold. Specifically, the propeller is divided into upper and lower molds along the split surface of the blade, and a radial core head is added. The core head size is 100-300mm larger than the propeller outline size. A positioning groove is set in the center of the bottom surface of the core head. The positioning groove adopts the form of a frustum-shaped male and female pin hole. The vertical projection area of ​​the positioning groove is 1 / 8-1 / 6 of the bottom surface area of ​​the sand mold. Design the upper sand cover (3) and the lower sand cover (5) of the sand mold. The edges of the upper sand cover (3) and the lower sand cover (5) of the sand mold are flush with the outer edge of the blade sand mold. Step 2: Use 3D printing technology to form the designed sand mold, then inspect and trim the printed sand mold, then assemble the trimmed sand mold and fill it with molding sand to obtain the cavity required to form the propeller. Among them, the sand mold designed by 3D printing technology is specifically as follows: the three-dimensional model data of the sand mold, sand mold upper cover (3), and sand mold lower cover (5) obtained by mold splitting are sliced ​​and processed. The processed data is imported into the sand mold printer control system. The molding sand uniformly mixed with resin binder and curing agent is fed into the array nozzle device. Under the control of the computer, the nozzle device densely piles up molding sand particles within the sand mold layer section contour range to form a sand mold structure. Outside the sand mold layer section contour range, the molding sand particles are loosely piled up to form a support structure. The designed sand mold is printed layer by layer. Specifically, the process of inspecting and adjusting the printed sand mold involves: using a scanner to collect data points on the surface of the printed sand mold; using Geomagic Qualify software to fit and compare the scanned data with the 3D model used for printing; grinding the areas where the sand mold dimensions are out of tolerance; and controlling the sand mold dimension deviation within "nominal size ±0.3mm". Specifically, the filling of the molding sand involves: spraying alcohol-based zircon powder coating onto the surface of the sand mold, drying the coating, adding a sand box with a sand intake of 100-200mm, placing the assembled sand mold and gating system inside the sand box, and filling the surrounding area with back sand; after the lower sand mold is completed and cured, the box is turned over, and risers, sprues and pouring cups are placed, and back sand is filled in. After the back sand hardens, the cavity is baked with a hot air blower. Step 3: Inject molten duplex stainless steel into the mold cavity to obtain a complete propeller casting; Step 4: Perform heat treatment on the propeller casting obtained in Step 3 to obtain the final propeller casting, specifically as follows: Weld anti-deformation ribs onto the blades of the propeller casting obtained in step 3. Then heat the propeller casting in a box-type resistance furnace to 1145-1155℃ and hold it for 2 hours for a time equal to the casting wall thickness δ (mm) / 20 (mm) + 2 hours. This allows the alloying elements to fully dissolve in the austenite and ferrite. After holding, furnace cool the casting to 1065-1075℃ and remove it from the furnace. Place it in water at 35-45℃ for rapid cooling. After cooling, remove the anti-deformation ribs. The anti-deformation bracing is installed in both circumferential and axial directions. The circumferential bracing uses a globe-like parallel bracing to horizontally encircle the tip of the propeller blade and is welded to the edge of the tip. A bracing is installed every 150-200mm in height. One end of the axial bracing is welded to the side of the propeller shaft, and the other end is welded to the central area of ​​the top surface of the blade. This prevents the blade from softening and bending downwards due to heat. Each blade needs to be welded with an axial bracing.

2. The forming method of the duplex stainless steel propeller according to claim 1, characterized in that, In step 2, when sand molding is performed, the weight ratio of molding sand, resin binder, and curing agent is 100:(1.5-3):(0.5-1). In step 2, the hot air machine baking cavity involves heating the hot air machine to 150-200℃ and sending hot air into the cavity through a pipe until the cavity surface is dry.

3. The forming method of the duplex stainless steel propeller according to claim 1, characterized in that, Step 3 specifically involves: Austenitic and ferritic duplex stainless steel is smelted in an intermediate frequency furnace to obtain duplex stainless steel molten metal. After adjusting the alloying elements to the required composition range, the molten duplex stainless steel is transferred from the intermediate frequency furnace to an AOD furnace for argon-oxygen decarburization refining. High-purity nitrogen gas is then repeatedly blown into the molten metal in the AOD furnace. The nitrogen element is fully homogenized and alloyed in the molten metal through gas stirring. After the nitrogen content meets the requirements, the casting is poured out of the furnace. After the propeller casting cools and solidifies, it is cleaned to obtain a complete propeller casting.

4. The forming method of the duplex stainless steel propeller according to claim 3, characterized in that, In step 3, the purity of the high-purity nitrogen gas is 99.99-99.9999%. The high-purity nitrogen gas is introduced into the duplex stainless steel molten metal through a heat-resistant pipe. After the nitrogen content meets the requirements, the duplex stainless steel molten metal is taken out of the furnace at 1640-1660℃ and poured at 1600-1620℃.

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