Duplex stainless steel propeller and casting method

Abstract

The present application relates to a kind of duplex stainless steel propeller casting method, the method is using 3D printing sand mould printing propeller sand mould cavity and carries out modeling;SAF2507 duplex stainless steel is melted into metal liquid in medium frequency furnace, pure nitrogen is blown into metal liquid in oxidation period and reduction period in AOD furnace multiple times, and treated metal liquid is obtained;The treated metal liquid is poured in propeller sand mould cavity;After it solidifies and cools, the casting is obtained, and after the casting is annealed and solid solution treated again, duplex stainless steel propeller is obtained;Wherein metal liquid is poured in propeller sand mould cavity and solidified into shape after cooling to 800~1000 ℃, and it needs to be cleaned. The method is applied to the processing and manufacturing of marine propeller using SAF2507 duplex stainless steel, shortens the production cycle, improves the production efficiency, and the casting will not appear the defects such as pore and inclusion, improves the machining precision and performance of propeller casting.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ship propeller processing, and particularly relates to a duplex stainless steel propeller and a casting method. BACKGROUND

[0002] As a key component of the ship propulsion system, the performance of the ship propeller is directly related to the sailing efficiency and safety of the ship. SAF2507 duplex stainless steel has become an ideal material for manufacturing ship propellers due to its excellent mechanical properties and corrosion resistance. However, the metallurgical structure of SAF2507 material is complex, and the flowability is poor. Especially in the casting process of thin-walled parts, it is prone to forming difficulties, which increases the manufacturing difficulty. In addition, the ship propeller usually has a complex geometric shape in design, such as a spiral line shape and the curvature of the blades, which puts higher requirements on the casting process.

[0003] Although the casting process has certain advantages in material forming, for high-performance materials such as SAF2507 duplex stainless steel, the traditional propeller casting process leads to defects such as pores and inclusions in the castings, and has problems such as low machining precision, long manufacturing cycle, and low efficiency, which cannot guarantee the performance and precision of the SAF2507 stainless steel propeller. Further, the traditional casting method cannot meet the high standard requirements for precision and performance.

[0004] In order to overcome the problems in the prior art and improve the manufacturing quality of the ship propeller, the application provides a duplex stainless steel ship propeller casting processing method. SUMMARY

[0005] In view of the deficiencies of the prior art, the purpose of the application is to provide a duplex stainless steel propeller and a casting method, wherein the method can optimize the casting process in the prior art, so that it can be applied to the processing and manufacturing of ship propellers made of SAF2507 duplex stainless steel, shorten the production cycle, improve the production efficiency, and the castings will not have defects such as pores and inclusions, thereby improving the machining precision and performance of the propeller castings.

[0006] To solve the above technical problems, the application adopts the following technical solutions:

[0007] The first implementation scheme of the application provides a duplex stainless steel propeller casting method, which adopts SAF2507 duplex stainless steel to cast the propeller, and includes the following steps:

[0008] The 3D printed sand mold is used to print the propeller sand mold cavity and perform molding; wherein the 3D printed sand mold realizes one-time accurate printing according to the complex geometric shape, reduces waste and unnecessary post-processing time, and saves cost.

[0009] The SAF2507 duplex stainless steel is melted into a molten metal, and pure nitrogen is blown into the molten metal for multiple times during the oxidation and reduction periods of the melting, to obtain a treated molten metal; the purpose of blowing pure nitrogen into the molten metal is to control and optimize the chemical composition of the steel, especially the nitrogen content, which can improve the strength and corrosion resistance of the steel, help the decarburization and refining of the molten steel, remove sulfur and other impurities in the molten steel, reduce the oxidation of the molten steel, protect the alloying elements in the molten steel from excessive oxidation, and thus reduce the burn loss. At the same time, the blowing of nitrogen also helps to remove gas and inclusions in the molten steel, improve the purity of the molten steel, reduce casting defects, and thus improve the smelting efficiency, shorten the smelting time, and reduce the production cost.

[0010] The treated molten metal is poured into the propeller sand mold cavity, and when the molten metal solidifies and forms in the propeller sand mold cavity and cools to 800-1000℃, it needs to be knocked out and cleaned to reduce the temperature difference of each part of the propeller blade, thereby reducing the deformation of the propeller blade and ensuring the stability and performance of the casting. After solidification and cooling, the casting is obtained, and after the casting is taken out of the propeller sand mold cavity, necessary finishing and cleaning processes are carried out to remove surface defects such as gates, risers, burrs, and scales, so that the casting reaches the required appearance quality; then the propeller casting is taken out of the propeller sand mold cavity and subjected to cutting, cleaning, and finishing processes to remove the scales and residual sand and other impurities on the surface of the casting and ensure the geometric dimensions and surface quality requirements. After annealing and solid solution treatment of the casting, a duplex stainless steel propeller is obtained.

[0011] Preferably, the percentage of N can be controlled at 0.14-0.35 by blowing pure nitrogen into the molten metal for multiple times. The nitrogen in the duplex stainless steel can improve the strength and corrosion resistance of the material. The nitrogen element can cooperate with the chromium and molybdenum elements in the steel to enhance the material's resistance to pitting and stress corrosion cracking. The stirring action of nitrogen makes the nitrogen element fully homogenized and alloyed in the molten metal.

[0012] Preferably, the obtained casting must be subjected to ribbing treatment before annealing and solid solution treatment, and the deformation amount of the propeller blade after ribbing treatment is controlled at 2.5-3.5mm, which can effectively release stress without causing insufficient structural strength due to excessive deformation. Ribbing treatment can improve the deformation problem of the propeller blade at high temperature.

[0013] Preferably, before the molten metal is blown with pure nitrogen, silicon iron with a mass percentage of silicon ≤2, manganese iron with a mass percentage of manganese ≤4, chromium iron with a mass percentage of chromium 21-27, nickel plate with a mass percentage of nickel 4-9, and molybdenum iron with a mass percentage of molybdenum 3-5 are sequentially added to the molten metal.

[0014] Silicon iron improves the oxidation resistance and strength of the alloy, manganese iron enhances the strength and hardness of the alloy, chromium iron provides the corrosion resistance of the alloy, nickel plate increases the toughness and corrosion resistance of the alloy, and molybdenum iron enhances the strength and heat resistance of the alloy. It is necessary to strictly control the addition amount of these elements to maintain the balance and stability of the alloy performance.

[0015] Preferably, the molten metal is poured into the propeller sand mold cavity at 1600℃-1620℃, at which temperature the porosity and inclusion defects can be reduced, while ensuring the fluidity of the metal liquid, which is beneficial to obtain high-quality castings. The pouring time is controlled to be 10s-20s to ensure that the metal liquid can flow quickly and uniformly when filling the propeller sand mold cavity, avoiding the metal liquid not being able to completely fill the cavity due to too short pouring time, or the metal liquid temperature dropping too fast due to too long pouring time, resulting in cold shut, porosity and inclusion defects in the cavity. The metal liquid poured into the propeller sand mold cavity needs to be heat treated for 2h-3h to ensure that the temperature gradient is small during the solidification process, reducing the uneven shrinkage of the castings and reducing the risk of deformation and cracking. In addition, heat treatment also helps to reduce the cooling rate of the castings, allowing more time for gases and inclusions in the metal liquid to escape, reducing porosity and inclusion defects in the castings. By controlling the heat treatment time, the microstructure and mechanical properties of the castings can be optimized, improving the overall quality of the castings.

[0016] Preferably, the castings are annealed at 550℃-650℃ to eliminate internal stress, improve the structure, reduce hardness and improve plasticity and toughness, and improve processing performance. Annealing at a temperature range of 550℃-650℃ can effectively eliminate the internal stress of steel castings and welded parts. This temperature range is lower than the recrystallization temperature of steel, and after heat preservation, cooling in air can reduce the brittleness of the material and improve its plasticity and toughness. In addition, this temperature range also helps to reduce the hardness of the material, making it easier to process and form subsequently. After annealing, the castings are subjected to solid solution treatment at 1050℃-1150℃ to eliminate stress caused by cold and hot processing, ensure the full dissolution of the main strengthening phase, and obtain a suitable grain size to ensure the mechanical properties of the alloy. If the annealing or solid solution treatment temperature is too high, the material may overheat and the grain size may be too large, thereby reducing the mechanical properties of the material. If the temperature is too low, the internal stress or strengthening phase may not be fully dissolved, resulting in insufficient improvement of the material's performance. For solid solution treatment, a temperature that is too low may not be able to obtain a uniform solid solution, affecting the subsequent performance of the material.

[0017] Preferably, SAF2507 duplex stainless steel is melted into a metal liquid at 1600℃-1660℃, which can reduce porosity and inclusions in the castings and improve the internal quality of the castings.

[0018] Preferably, before casting using this casting method, a casting shrinkage of 2.2% is selected. During pouring, open pouring and bottom pouring are used, with visible risers for feeding. This operation can reduce porosity and inclusions in the casting and improve the internal quality of the casting.

[0019] Preferably, the sand particles in the 3D printing sand mold are round or elliptical, with a particle size D50 of 170μm and a D90 of 280μm. Appropriate particle size distribution and particle shape can ensure the stability of the sand mold during the printing process, reduce the misalignment phenomenon caused by uneven sand layers, and improve the mechanical properties, printing quality and reliability of the sand mold.

[0020] The second implementation of this invention provides a duplex stainless steel propeller prepared by a casting method.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] The present invention provides a method for casting dual-phase stainless steel propellers. By using 3D printing sand molds to print the propeller sand mold cavity, the production efficiency of the propeller sand mold cavity can be improved, the debugging cost of the propeller sand mold cavity can be reduced, and the casting process design can be made more flexible, thus saving the cost of the propeller sand mold cavity.

[0023] This invention involves repeatedly blowing pure nitrogen gas into molten metal during the oxidation and reduction phases. The stirring action of the pure nitrogen gas ensures that nitrogen is thoroughly homogenized and alloyed within the molten metal. Simultaneously, the repeated blowing of pure nitrogen gas and the use of in-furnace PMI (Process, Measurement, and Control Integration) allow for precise measurement and control of the nitrogen injection amount and pressure, achieving accurate nitrogen content control. This effectively reduces porosity and inclusions in propeller castings, resulting in an ideal phase composition and microstructure, and improving the material's mechanical properties and corrosion resistance. Nitriding treatment, by forming a hard nitride layer on the metal surface, significantly improves the material's hardness, wear resistance, and corrosion resistance. The formation of these nitrides not only increases surface hardness but also generates residual compressive stress on the material surface, contributing to improved fatigue strength and crack resistance. Simultaneously, the nitride layer improves surface roughness, reduces the coefficient of friction, and enhances the material's chemical reactivity, further improving adhesion and corrosion resistance.

[0024] Before annealing and solution treatment, the castings are cooled to 800℃~1000℃ for cleaning and packing. This reduces the temperature difference between different parts of the propeller blades, helping to achieve a uniform temperature distribution and reduce the difference in cooling rates between different parts. This reduces internal stress and propeller blade deformation, further improving its precision and performance. Annealing and solution treatment eliminate internal stress, improve microstructure, enhance corrosion resistance, refine grains, and optimize phase composition, further improving its precision and performance. This is because annealing helps reduce internal stress generated during casting and cooling, increasing dimensional stability; solution treatment, through rapid cooling, allows solute atoms to dissolve in the solvent lattice, improving material homogeneity, reducing grain boundary segregation, and enhancing the material's mechanical properties. These heat treatment processes work together to give the castings superior overall performance in subsequent processing and use.

[0025] 2. In this invention, before heat treatment, necessary tie rods are added to the casting. The tie rods have the same wall thickness as the components, which improves the deformation of the blades at high temperatures. By adding tie rods, the casting can be pre-strengthened and shaped, limiting its free expansion at high temperatures, thereby reducing deformation. The use of tie rods can also help disperse and balance these internal stresses, reducing the risk of cracks in the casting due to stress concentration.

[0026] 3. In this invention, before introducing pure nitrogen gas into the molten metal, ferrosilicon, ferromanganese, ferrochrome, nickel plate, and ferromolybdenum need to be added sequentially. Controlling the mass percentage of each element added can further improve its precision and performance. Ferrosilicon and ferromanganese are commonly used deoxidizers that can remove oxygen from the molten steel, improving its purity. Ferrochrome, nickel plate, and ferromolybdenum, as alloying elements, can adjust the chemical composition of the steel, enabling alloying. These elements can improve the mechanical properties and corrosion resistance of the steel.

[0027] 4. The castings described in this invention require heat preservation treatment for approximately 2-3 hours. The molten metal is poured into the required cavity of the propeller at 1600℃-1620℃, with the pouring time controlled at 10-20 seconds. Maintaining a suitable melting temperature reduces production costs and ensures the quality and fluidity of the molten metal, minimizing the formation of porosity and inclusions. An appropriate pouring temperature ensures good fluidity of the molten metal, reducing cold shuts and incomplete filling defects during the filling process. Higher pouring temperatures also help reduce porosity. Controlling the pouring speed ensures that the molten metal uniformly fills the mold, avoiding turbulence and air or slag retention caused by excessive pouring speed, thus reducing porosity and other casting defects. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the SAF2507 stainless steel propeller casting process of the present invention;

[0029] Figure 2 This is a sample image after mechanical property testing according to the present invention, wherein... Figure 2 a is a spliced ​​image of the sample. Figure 2 b is a sample split diagram.

[0030] Figure 3 This is a metallographic image of the present invention; wherein Figure 3 a is a metallographic image magnified 50 times. Figure 3 b is a metallographic image magnified 100 times. Detailed Implementation

[0031] The specific embodiments of the present invention are described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods described in the embodiments of the present invention are conventional methods.

[0032] The inventors discovered that SAF2507 duplex stainless steel, due to its ultra-low carbon content, favorable phase ratio, and high alloying element content, possesses excellent mechanical properties and resistance to chloride corrosion. However, in traditional casting processes, SAF2507 duplex stainless steel castings are prone to defects such as porosity and inclusions, which can lead to accuracy and performance issues. Porosity is a common defect in castings, reducing the effective load-bearing area and decreasing the strength and toughness of the casting. Porosity can also become a source of stress concentration, increasing the risk of fracture during service. Excessive inclusion content is a major cause of cracking, significantly affecting the elongation, reduction of area, and fatigue strength of SAF2507 stainless steel. Inclusions disrupt the continuity of the matrix, reduce the material's plasticity and toughness, increase crack initiation sites, and thus affect the overall performance and service life of the casting.

[0033] In view of the above problems, the present invention optimizes and improves the casting process, namely by controlling the pouring temperature and cooling rate, adopting a special pouring system design, and improving the molding sand and riser system, which can effectively reduce the generation of porosity and inclusions, thereby improving the precision and performance of SAF2507 duplex stainless steel castings, shortening the manufacturing cycle, and increasing production efficiency.

[0034] Product structure analysis: The SAF2507 super dual-phase stainless steel propeller developed this time has an external dimension of Ø621x239, a propeller shaft diameter of 110mm, a blade hyperbolic wall thickness that gradually changes, with the thinnest part being 1.9mm, and a net weight of 46kg. SAF2507 material has a melting point of 1350℃~1380℃, density of 7.8g / cm³, thermal conductivity of 13.7W / (m·K), and coefficient of linear expansion of 13.7um / m·℃. Its microstructure consists of approximately 45%~55% ferrite and austenite, exhibiting high strength and excellent corrosion resistance. However, its complex metallurgical structure and microstructure can lead to intergranular corrosion and brittle fracture if not properly handled. Most importantly, this material has poor fluidity, making thin-walled casting difficult. Propeller casting requires adjusting the pouring temperature and speed. Higher pouring temperatures reduce the viscosity of the molten metal, decreasing gas entrapment and increasing gas solubility in the molten metal, thus reducing porosity. Appropriate pouring speeds help the molten metal fill the mold cavity smoothly, reducing bubbles and inclusions, thereby improving the density and surface quality of the casting.

[0035] Sand mold printing and molding: 3D printing sand molds are used to print and shape the propeller's sand mold cavity. The advantage of this technology is that it can quickly form the required cavity for the part, allowing for timely verification and adjustment of process parameters, and shortening the debugging cycle. The propeller forming process requires high-precision mold design and manufacturing, and the propeller's geometry is complex and difficult to process. Utilizing 3D printing sand mold casting technology can improve production efficiency, reduce debugging costs, and achieve freedom in casting process design while saving on molds. Adopting 3D printing sand mold technology requires the selection and optimization of sand materials, analyzing the sand's particle size distribution, particle shape, flowability, and other characteristics to improve the printing quality and reliability of the sand mold.

[0036] Casting process design: A casting shrinkage of 2.2% was selected, employing an open gating system and bottom pouring with visible risers for feeding. After parameter determination, casting process simulation analysis was conducted through modeling to determine the optimal casting parameters, including pouring temperature, pouring speed, and cooling rate. Controlling the smelting temperature appropriately can reduce production costs and ensure the quality and fluidity of the molten metal, minimizing the formation of porosity and inclusions. A suitable pouring temperature ensures good fluidity of the molten metal, reducing cold shuts and incomplete filling defects during the filling process. Higher pouring temperatures also help reduce porosity. Controlling the pouring speed ensures that the molten metal fills the mold uniformly, avoiding turbulence and air or slag retention caused by excessive pouring speed, thus reducing porosity and other casting defects.

[0037] Smelting and Casting: A medium-frequency furnace is used for smelting. After the scrap steel is melted, ferrosilicon (silicon ≤2% by mass), ferromanganese (manganese ≤4% by mass), ferrochrome (chromium 21-27% by mass), nickel plates (nickel 4-9% by mass), and ferromolybdenum (molybdenum 3-5% by mass) are added sequentially for initial metal melting. Once the composition is adjusted to the desired level, the metal is transferred to an AOD furnace for refining via an argon-oxygen decarburization furnace. Under standard atmospheric pressure, pure nitrogen gas is repeatedly blown into the molten steel during the oxidation and reduction phases in the AOD furnace. This repeated blowing of pure nitrogen gas allows the nitrogen percentage to be controlled between 0.14% and 0.35%. The gas agitation ensures that the nitrogen element is fully homogenized and alloyed in the molten steel, effectively reducing the carbon content and inhibiting chromium oxidation. With repeated gas blowing and in-furnace PMI (partial nitrogen induction), precise control of the nitrogen content within the alloy is achieved to obtain an ideal phase composition and microstructure, improving the material's mechanical properties and corrosion resistance.

[0038] Cooling and solidification: Due to the shape and material properties of the casting, a temperature gradient is generated inside and outside the blade during the cooling process of the propeller. The temperature changes of different parts are quite different, which will generate thermal stress and cause changes in the shape of the blade. To address this, a hot-release rapid cooling method is adopted to minimize the temperature difference between different parts of the blade, thereby reducing blade deformation.

[0039] Demolding and Cleaning: After the casting temperature is cooled to 800℃~1000℃, it is removed from the mold and cleaned to avoid defects and ensure the stability and performance of the casting. It is then removed from the molding sand and subjected to necessary finishing and cleaning processes. Surface defects such as sprues, risers, burrs, and oxide scale are removed to ensure the casting achieves the required appearance quality.

[0040] Cleaning and finishing: The propeller casting is removed from the mold and subjected to processes such as cutting, cleaning and finishing to remove impurities such as oxide scale and residual sand from the surface of the casting, and to ensure the required geometric dimensions and surface quality.

[0041] Heat treatment: Before heat treatment, necessary tie rods were added. The tie rods had the same wall thickness as the components, which could improve the deformation of the blades at high temperatures. The results showed that the blade deformation was 2.5mm to 3.5mm after the tie rods were flattened. According to the heat treatment process requirements of SAF2507 stainless steel, the propeller casting was annealed and solution treated to improve its microstructure and properties.

[0042] The casting and processing technology of the SAF2507 stainless steel marine propeller disclosed herein will be described in detail below by way of specific embodiments.

[0043] Example 1

[0044] like Figure 1 As shown in the figure, the specific steps of a hot pressing processing method for marine propellers in this embodiment are as follows:

[0045] S1. Use 3D printing sand mold manufacturing technology to manufacture casting molds; the use of 3D printing sand mold process requires the selection and optimization of sand materials, and analysis of sand material particle size distribution, particle shape, flowability and other characteristics, which can improve the printing quality and reliability of sand molds;

[0046] S2. Propellers are manufactured using casting technology;

[0047] S201, the casting shrinkage is selected as 2.2%, an open gating system and bottom pouring are adopted, and the riser is used for feeding. After the parameters are determined, the casting process is simulated and analyzed through modeling to determine the optimal casting parameters. The molten metal is controlled to be tapped at 1600℃ and poured at 1600℃, and the pouring time is controlled to be 10s.

[0048] S202 is smelted using a medium-frequency furnace. After the scrap steel is melted, ferrosilicon, ferromanganese, ferrochrome, nickel plate, and ferromolybdenum are added sequentially. The mass percentage of each element is controlled, with Si≤2, Mn≤4, Cr 21~27, Ni 4~9, and Mo 3~5. After adjusting the composition to the required level, the steel is transferred to an AOD furnace for refining via argon-oxygen decarburization. Pure nitrogen gas is repeatedly blown into the molten steel during the oxidation and reduction phases in the AOD furnace. By repeatedly blowing pure nitrogen into the molten metal, the N percentage can be controlled at 0.14%. The stirring effect of the gas ensures that the nitrogen element is fully homogenized and alloyed in the molten steel. With multiple blowing and PMI in the furnace, the nitrogen content is precisely controlled to obtain the ideal phase composition and microstructure, thereby improving the mechanical properties and corrosion resistance of the material.

[0049] S203. Based on the shape and material properties of the casting, a hot-release and rapid-cooling method is adopted to minimize the temperature difference between different parts of the blade, thereby reducing blade deformation.

[0050] The casting process proposed in this disclosure has the following advantages and characteristics:

[0051] (1) Precise scaling control: By selecting a casting scaling of 2.2%, the dimensional accuracy of the propeller during the manufacturing process can be ensured, which is crucial for ensuring its performance in practical applications.

[0052] (2) Optimize casting quality: The design of open gating system and bottom pouring helps the smooth flow of molten metal, reduces the generation of bubbles and inclusions, thereby improving the internal quality of castings. Using open risers for feeding can effectively compensate for the shrinkage of metal during solidification, reduce shrinkage cavities and porosity defects, and improve the integrity of castings.

[0053] (3) Precise control of composition: The combined use of medium-frequency furnace melting and AOD furnace refining can precisely control the composition of molten steel, especially the nitrogen content, which is crucial for obtaining ideal phase composition and microstructure. Blowing pure nitrogen into the AOD furnace not only helps to homogenize nitrogen, but also promotes the uniform distribution of alloying elements and improves the overall performance of the material.

[0054] (4) Ensure material properties: The hot de-cooling method can effectively control the cooling rate of the casting and reduce the temperature gradient, thereby reducing the risk of blade deformation and cracking.

[0055] Based on the completion of the above casting process, the following steps are performed:

[0056] S3. After casting is completed, the casting is demolded, cleaned and heat-treated.

[0057] S301. Demolding: After the casting temperature is cooled to about 800℃, the casting is removed from the mold and cleaned to avoid defects and ensure the stability and performance of the casting.

[0058] S302. Cleaning: Remove the casting from the molding sand and perform necessary trimming and cleaning procedures. Remove surface defects such as gating, risers, burrs, and oxide scale to ensure the casting meets the required appearance quality.

[0059] S303. Finishing: Remove the propeller casting from the mold and perform processes such as cutting, cleaning and finishing to remove impurities such as oxide scale and residual sand from the surface of the casting, and ensure the requirements of geometric dimensions and surface quality.

[0060] S304. Heat treatment: Add necessary tie rods. After the tie rods are flattened, the blade deformation is about 2.5mm. Anneal and solution treatment are performed on the casting.

[0061] S4. Complete the acceptance process and obtain the finished product; use non-destructive testing, chemical analysis, metallographic microscopy and other methods to conduct strict quality inspection and performance testing on the propeller casting to ensure that it meets the relevant standards and technical requirements.

[0062] Example 2

[0063] like Figure 1 As shown in the figure, the specific steps of a hot pressing processing method for marine propellers in this embodiment are as follows:

[0064] S1. Use 3D printing sand mold manufacturing technology to manufacture casting molds; the use of 3D printing sand mold process requires the selection and optimization of sand materials, and analysis of sand material particle size distribution, particle shape, flowability and other characteristics, which can improve the printing quality and reliability of sand molds;

[0065] S2. Propellers are manufactured using casting technology;

[0066] S201, the casting shrinkage is selected as 2.2%, an open gating system and bottom pouring are adopted, and the riser is used for feeding. After the parameters are determined, the casting process is simulated and analyzed through modeling to determine the optimal casting parameters. The molten metal is controlled to be tapped when it is 1650℃ and poured at 1600℃, and the pouring time is controlled to be 15s.

[0067] S202 is smelted using a medium-frequency furnace. After the scrap steel is melted, ferrosilicon, ferromanganese, ferrochrome, nickel plate, and ferromolybdenum are added sequentially. The mass percentage of each element is controlled, with Si≤2, Mn≤4, Cr 21~27, Ni 4~9, and Mo 3~5. After adjusting the composition to the required level, the steel is transferred to an AOD furnace for refining via argon-oxygen decarburization. Pure nitrogen gas is repeatedly blown into the molten steel during the oxidation and reduction phases in the AOD furnace. By repeatedly blowing pure nitrogen into the molten metal, the N percentage can be controlled at 0.30. The stirring effect of the gas ensures that the nitrogen element is fully homogenized and alloyed in the molten steel. With multiple blowing and PMI in the furnace, the nitrogen content is precisely controlled to obtain the ideal phase composition and microstructure, thereby improving the mechanical properties and corrosion resistance of the material.

[0068] S203. Based on the shape and material properties of the casting, a hot-release and rapid-cooling method is adopted to minimize the temperature difference between different parts of the blade, thereby reducing blade deformation.

[0069] Based on the completion of the above casting process, the following steps are performed:

[0070] S3. After casting is completed, the casting is demolded, cleaned and heat-treated.

[0071] S301. Demolding: After the casting temperature is cooled to about 900℃, the casting is removed from the mold and cleaned to avoid defects and ensure the stability and performance of the casting.

[0072] S302. Cleaning: Remove the casting from the molding sand and perform necessary trimming and cleaning procedures. Remove surface defects such as gating, risers, burrs, and oxide scale to ensure the casting meets the required appearance quality.

[0073] S303. Finishing: Remove the propeller casting from the mold and perform processes such as cutting, cleaning and finishing to remove impurities such as oxide scale and residual sand from the surface of the casting, and ensure the requirements of geometric dimensions and surface quality.

[0074] S304. Heat treatment: Add necessary tie rods. After the tie rods are flattened, the blade deformation is about 3.0mm. Anneal and solution treatment are performed on the casting.

[0075] S4. Complete the acceptance process and obtain the finished product; use non-destructive testing, chemical analysis, metallographic microscopy and other methods to conduct strict quality inspection and performance testing on the propeller casting to ensure that it meets the relevant standards and technical requirements.

[0076] Example 3

[0077] like Figure 1 As shown in the figure, the specific steps of a hot pressing processing method for marine propellers in this embodiment are as follows:

[0078] S1. Use 3D printing sand mold manufacturing technology to manufacture casting molds; the use of 3D printing sand mold process requires the selection and optimization of sand materials, and analysis of sand material particle size distribution, particle shape, flowability and other characteristics, which can improve the printing quality and reliability of sand molds;

[0079] S2. Propellers are manufactured using casting technology;

[0080] S201, the casting shrinkage is selected as 2.2%, an open gating system and bottom pouring are adopted, and the riser is used for feeding. After the parameters are determined, the casting process is simulated and analyzed through modeling to determine the optimal casting parameters. The molten metal is controlled to be tapped when it is 1660℃ and poured at 1620℃, and the pouring time is controlled to be 20s.

[0081] S202 is smelted in a medium-frequency furnace. After the scrap steel is melted, ferrosilicon, ferromanganese, ferrochrome, nickel plate, and ferromolybdenum are added sequentially. The mass percentage of each element is controlled, with Si≤2, Mn≤4, Cr 21~27, Ni 4~9, and Mo 3~5. After the composition is adjusted to the required level, the steel is transferred to an AOD furnace for refining through an argon-oxygen decarburization refining furnace. Pure nitrogen gas is repeatedly blown into the molten steel during the oxidation and reduction phases in the AOD furnace. By repeatedly blowing pure nitrogen gas into the molten metal, the N percentage can be controlled at 0.35%. The stirring effect of the gas ensures that the nitrogen element is fully homogenized and alloyed in the molten steel. With multiple blowing and PMI in the furnace, the nitrogen content is precisely controlled to obtain the ideal phase composition and microstructure, thereby improving the mechanical properties and corrosion resistance of the material.

[0082] S203. Based on the shape and material properties of the casting, a hot-release and rapid-cooling method is adopted to minimize the temperature difference between different parts of the blade, thereby reducing blade deformation.

[0083] Based on the completion of the above casting process, the following steps are performed:

[0084] S3. After casting is completed, the casting is demolded, cleaned and heat-treated.

[0085] S301. Demolding: After the casting temperature is cooled to about 1000℃, the casting is removed from the mold and cleaned to avoid defects and ensure the stability and performance of the casting.

[0086] S302. Cleaning: Remove the casting from the molding sand and perform necessary trimming and cleaning procedures. Remove surface defects such as gating, risers, burrs, and oxide scale to ensure the casting meets the required appearance quality.

[0087] S303. Finishing: Remove the propeller casting from the mold and perform processes such as cutting, cleaning and finishing to remove impurities such as oxide scale and residual sand from the surface of the casting, and ensure the requirements of geometric dimensions and surface quality.

[0088] S304. Heat treatment: Add necessary tie rods. After the tie rods are flattened, the blade deformation is about 3.5mm. Anneal and solution treatment are performed on the casting.

[0089] S4. Complete the acceptance process and obtain the finished product; use non-destructive testing, chemical analysis, metallographic microscopy and other methods to conduct strict quality inspection and performance testing on the propeller casting to ensure that it meets the relevant standards and technical requirements.

[0090] The above-described Examples 1 to 3 all successfully prepared duplex stainless steel propellers. Now, we will select one of the examples, namely Example 1, for testing and inspection.

[0091] Inspection and Testing: SAF2507 stainless steel castings are subjected to rigorous quality inspection and performance testing using methods such as mechanical property testing, chemical analysis, and metallographic microscopy to ensure that they meet relevant standards and technical requirements.

[0092] (0) Mechanical property testing

[0093] The prepared propeller castings, after mechanical property testing, are shown in the sample image below. Figure 2 In Figure 2 a and Figure 2 As shown in b.

[0094] Table 1 shows the mechanical property test results for SAF2507 stainless steel castings.

[0095] Table 1 Mechanical property testing

[0096]

[0097] The test results above show that the SAF2507 stainless steel obtained through this casting process has high tensile strength, meaning that it is less prone to fracture under external forces, i.e., it has higher strength and durability. Its high specified ductile elongation strength means that it can withstand greater stress before permanent deformation begins, i.e., it has a better elastic limit. This demonstrates the high reliability of SAF2507 stainless steel, reducing the risk of fracture or failure, thereby improving the safety and stability of the overall structure.

[0098] (2) Chemical analysis and detection

[0099] Table 2 shows the chemical analysis test results of SAF2507 stainless steel castings. Table 2(a) is the test report of the percentage content of multiple elements; Table 2(b) is the test report of the ppm content of oxygen, nitrogen and hydrogen elements.

[0100] Table 2(a) Multi-element content percentage test report

[0101]

[0102] Table 2(b) Oxygen, Nitrogen, and Hydrogen Element Content (ppm) Test Report

[0103]

[0104] Chemical composition analysis showed that oxygen and nitrogen were present in the SAF2507 stainless steel sample at the ppm level, with a higher nitrogen content, which may enhance hardness but may also increase brittleness. Hydrogen content was extremely low and had little impact on performance. The contents of carbon, silicon, manganese, phosphorus, sulfur, chromium, molybdenum, nickel, copper, vanadium, and nitrogen were expressed as percentages. These elements collectively determine the metal's mechanical properties, corrosion resistance, workability, and heat resistance. High contents of chromium and molybdenum improved corrosion resistance and heat resistance, while manganese and nickel enhanced strength and toughness. Low contents of phosphorus and sulfur may be beneficial, while the contents of carbon and nitrogen had a positive impact on hardness and strength. This indicates that SAF2507 stainless steel possesses good strength and toughness.

[0105] (3) Metallographic testing

[0106] like Figure 3 middle Figure 3 a and Figure 3 Metallographic analysis of sample b shows that the material possesses a dual-phase microstructure of ferrite and austenite. This structure combines the high strength of ferrite with the high toughness of austenite, enabling the material to maintain good strength while also exhibiting excellent plasticity and corrosion resistance. Furthermore, the material's macroscopic grain size is M-8.5, indicating a small grain size and good strength and toughness. Simultaneously, small grains can hinder crack propagation, thereby improving the material's fracture toughness. In addition, small grains can also improve the material's corrosion resistance and wear resistance.

[0107] This method innovatively improves upon traditional casting processes by precisely controlling key parameters such as temperature and speed during casting. It combines 3D printing sand mold technology with advanced melting and refining processes to optimize the propeller casting process. This not only improves product machining accuracy and production efficiency but also ensures the propeller's high-performance requirements in harsh marine environments, meeting the high standards of modern shipbuilding for marine propellers.

[0108] In summary, the SAF2507 stainless steel castings obtained using this casting process not only possess good strength and toughness, but also excellent corrosion resistance and toughness.

[0109] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for casting a duplex stainless steel propeller, wherein the method uses SAF2507 duplex stainless steel to cast the propeller, characterized in that, Includes the following steps: The propeller sand mold cavity was printed and shaped using 3D printing sand mold; SAF2507 duplex stainless steel was melted into a molten metal, and pure nitrogen gas was repeatedly blown into the molten metal during the oxidation and reduction periods of the melting process to obtain the treated molten metal. The above-treated molten metal is poured into the cavity of a propeller sand mold; after solidification and cooling, a casting is obtained, which is then annealed and solution treated to obtain a duplex stainless steel propeller. When the molten metal is poured into the propeller sand mold cavity and solidifies, it needs to be cooled to 800℃~1000℃ and then cleaned by unpacking. The castings obtained must be subjected to a tie-bar treatment before annealing and solution treatment, and the blade deformation of the propeller after tie-bar treatment must be controlled to be 2.5mm to 3.5mm; in addition, the castings are cooled to 800℃ to 1000℃ before annealing and solution treatment and then cleaned. Before pure nitrogen gas is introduced into the molten metal, ferrosilicon with a mass percentage of silicon ≤2, ferromanganese with a mass percentage of manganese ≤4, ferrochrome with a mass percentage of chromium 21~27, nickel plates with a mass percentage of nickel 4~9, and ferromolybdenum with a mass percentage of molybdenum 3~5 need to be added to the molten metal in sequence.

2. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, The percentage of nitrogen can be controlled between 0.14 and 0.35 by repeatedly blowing pure nitrogen into the molten metal.

3. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, The molten metal is poured into the cavity of a propeller sand mold at 1600℃~1620℃, and the pouring time is controlled to be 10s~20s. The molten metal after being poured into the cavity of the propeller sand mold needs to be kept warm for 2h~3h.

4. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, The castings are annealed at 550℃~650℃, and then solution treated at 1050℃~1150℃.

5. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, SAF2507 duplex stainless steel is smelted into liquid metal at 1600℃~1660℃.

6. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, Before casting using this method, a casting shrinkage of 2.2% is selected. Open pouring and bottom pouring are used during casting, with visible risers to compensate for shrinkage.

7. The method for casting a duplex stainless steel propeller as described in claim 1, characterized in that, The sand particles in the 3D printing sand mold are round or elliptical in shape, with a particle size D50 of 170μm and D90 of 280μm.

8. A duplex stainless steel propeller prepared by a casting method for duplex stainless steel propellers as described in any one of claims 1 to 7.

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