Thin-wall shell, preparation process thereof and casting process of small ductile iron casting

Abstract

The invention discloses a thin-wall shell, a preparation process thereof and a casting process of a small ductile iron casting, and relates to the field of precision casting. The preparation process of the thin-wall shell comprises the following steps: preparing surface layer slurry, transition layer slurry, reinforcing layer slurry and sealing slurry; a wax mold set is subjected to dip-coating of the surface layer slurry to manufacture a surface layer, dip-coating of the transition layer slurry and the reinforcing layer slurry to manufacture a back layer, coating of sealing slurry is conducted, and a shell blank is obtained after drying; and after dewaxing of the shell blank, high-temperature roasting is conducted, and the thin-wall shell is obtained. The casting technology comprises the following steps that smelted base iron is heated to 1630-1660 DEG C and then transferred into a preheated casting ladle, and spheroidizing inoculation is carried out; after slag removal, molten iron with the temperature reduced to 1400-1500 DEG C is poured into the thin-wall shell; and after cooling, the thin-wall shell is crushed, a casting head is removed, and a small ductile iron casting is obtained. The chemical pollution of a workplace is eliminated; the problem of local shrinkage porosity of the casting is solved; the surface quality of castings is improved; and the total consumption of shell materials is also reduced.

Description

Technical Field

[0001] This invention relates to the field of precision casting technology, specifically to a thin-walled shell and its preparation process, and a casting process for small ductile iron castings. Background Technology

[0002] Currently, the mainstream production process for small ductile iron castings mostly adopts the water glass investment casting process. This process uses water glass as a binder and quartz sand and powder as refractory materials, and a high-temperature resistant shell is made through a chemical hardening process. This process has the advantages of low raw material cost, short production cycle, and stable process, and is very suitable for the mass production of high-precision small castings such as ductile iron with relatively low casting temperatures.

[0003] However, the water glass investment casting process has the following significant drawbacks. First, the water glass mold shell must be chemically hardened. During the hardening process, hardeners such as ammonium chloride and magnesium chloride easily decompose, producing volatile pollutants such as ammonia, resulting in serious chemical pollution in the production environment and endangering workers' health. Second, the high-temperature strength of the water glass mold shell is relatively low, necessitating the use of a thicker shell wall (usually 10-12 mm). Consequently, heat dissipation during the casting cooling process is poor, easily leading to defects such as shrinkage porosity in complex castings where heat dissipation is difficult. Third, the hardening process of the water glass mold shell easily produces sodium bicarbonate salt bloom, resulting in generally poor surface quality of castings produced by the water glass process. Summary of the Invention

[0004] The purpose of this invention is to provide a thin-walled shell and its preparation process, as well as a casting process for small ductile iron castings, to improve the problem of local shrinkage porosity in castings caused by poor heat dissipation in existing shells.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A process for preparing a thin-walled shell includes the following steps:

[0007] S100, prepare surface layer slurry, transition layer slurry, reinforcement layer slurry and sealing slurry respectively;

[0008] S200. The wax model tree is dipped in the surface layer slurry to make the surface layer, then the transition layer slurry and the reinforcing layer slurry are dipped in the surface layer to make the back layer, and finally the sealing slurry is applied and dried to obtain the shell blank.

[0009] S300. After dewaxing the above-mentioned shell blank, it is fired at high temperature to obtain a thin-walled shell.

[0010] In S100, the surface layer slurry uses fused silica powder and zircon powder as the main raw materials and silica sol as the binder; the transition layer slurry and the reinforcing layer slurry both use mullite powder as the main raw material; in S200, the surface layer sand is made of quartz sand with a particle size of 80~120 mesh, the transition layer sand is made of mullite sand with a particle size of 60~80 mesh, and the reinforcing layer sand is made of mullite sand with a particle size of 16~30 mesh.

[0011] This invention utilizes a high-strength, thin-walled composite shell structure, leveraging its excellent thermal conductivity and overall rigidity to actively intervene in and optimize the solidification process of the casting, thereby significantly improving casting quality. Structurally, the surface layer, transition layer, and reinforcing layer together form a functional gradient system, allowing the shell to gradually transition from a surface layer with good surface quality to a strong and tough back layer providing overall support. This thin-walled structure, controlled at 4–6 mm, effectively increases the heat dissipation efficiency per unit area and significantly accelerates heat transfer.

[0012] Using silica sol as a binder fundamentally avoids the environmental pollution problems caused by chemical hardening in traditional water glass processes. The introduction of zircon powder into the surface layer further ensures high surface smoothness and dimensional accuracy of the casting; while the mullite material system used in the back layer guarantees excellent high-temperature stability and thermal strength of the shell, ensuring that it does not deform or crack during casting.

[0013] When molten metal is poured into the mold shell, the thin-walled composite structure rapidly conducts heat dissipation, achieving rapid cooling of the casting. Rapid cooling is particularly important for small ductile iron castings. On the one hand, a higher cooling rate helps refine the matrix grains, thereby improving the material's mechanical properties; on the other hand, it effectively suppresses shrinkage defects caused by insufficient feeding, enhancing the density of the casting. Furthermore, by controlling the solidification process, the as-cast microstructure directly meets performance requirements, eliminating the need for subsequent energy-intensive heat treatment processes.

[0014] The process system of this invention integrates structural design, material selection and metallurgical control to form a new precision casting technology route characterized by thin-walled rapid cooling, which ensures the quality of castings while also being environmentally friendly and economical.

[0015] Unlike existing technologies that are usually limited to cost control in a single stage (such as surface slurry) through material replacement or powder gradation, this invention is dedicated to solving a more systematic and complex engineering problem: how to simultaneously achieve multiple goals in the precision casting process, including environmentally friendly production, internal quality improvement (such as eliminating shrinkage porosity), and surface precision optimization.

[0016] This invention utilizes a surface layer structure composed of fused silica powder and zircon powder, along with a transition and reinforcing back layer structure primarily composed of mullite powder, thereby achieving thin walls and high strength synergistically at both the material and structural levels. In terms of process implementation, this invention further expands the solution from simple material cost control to the coordinated control of shell structure design and drying and firing processes. The long-term high-humidity drying regime of this invention effectively prevents cracking of the thin-walled shell due to stress concentration during preparation; simultaneously, the direct high-temperature firing process ensures that the shell retains excellent high-temperature strength and refractory properties while achieving thinning.

[0017] Furthermore, in S100, the surface slurry, by weight, includes the following components: 115-125 parts of 830 silica sol, 190-210 parts of fused silica powder, and 9-11 parts of zircon powder; the viscosity of the surface slurry is 25-35 seconds.

[0018] Using fused silica powder as the main component and incorporating a small amount of zircon powder, the high-temperature chemical stability and corrosion resistance of the surface layer are significantly improved while controlling raw material costs. The surface layer slurry uses 830 silica sol as the binder matrix, combined with 190-210 parts by weight of fused silica powder as the main refractory skeleton, and introduces 9-11 parts by weight of zircon powder as a functional reinforcing phase. This component system, while ensuring the slurry's coating performance, precisely controls the viscosity within the range of 25-35 seconds, giving it excellent leveling properties and molding ability.

[0019] Furthermore, the preparation method of the surface layer slurry includes the following: adding molten silica powder to the stirred silica sol at a rate of 1~2 kg / min and mixing evenly, then adding a wetting agent; adding zircon powder at a rate of 0.5~1 kg / min and stirring continuously for more than 30 min; adding defoamer in 2~3 portions and maintaining medium-low speed stirring for 12h~24h.

[0020] The preparation method for the surface layer slurry overcomes problems such as agglomeration, bubbles, and sedimentation that are prone to occur in slurries with a high powder-to-liquid ratio by systematically optimizing the feeding sequence, speed, and stirring regime. This allows the surface layer to maintain excellent integrity and surface replication capabilities while achieving thinning. The preparation method achieves orderly dispersion of powder in silica sol through a step-by-step, constant-speed feeding strategy, followed by maturation at medium to low speeds for 12–24 hours. This process effectively stabilizes the slurry structure, eliminates microbubbles, and ensures the density and surface quality of the coating.

[0021] Furthermore, in S100, by weight,

[0022] The transition layer slurry comprises the following components: 110-130 parts of 1430 silica sol and 150-170 parts of mullite powder; the viscosity of the transition layer slurry is 20-24 seconds.

[0023] The reinforcing layer slurry comprises the following components: 115-125 parts of 1430 silica sol and 135-145 parts of mullite powder; the viscosity of the reinforcing layer slurry is 12-16 seconds.

[0024] In the construction of the shell backing structure, this invention achieves optimized configuration of shell performance by designing transition and reinforcing layer slurries with viscosity gradients. The transition layer slurry is prepared by mixing 1430 silica sol with a relatively high proportion of mullite powder (150-170 parts) to form a slurry system with a viscosity of 20-24 seconds. This configuration gives it appropriate fluidity and powder carrying capacity, effectively penetrating and anchoring it in the gaps of the surface sand layer, while also providing an ideal bonding interface for the subsequent reinforcing layer.

[0025] The reinforcing layer slurry, while maintaining the amount of 1430 silica sol, moderately reduces the proportion of mullite powder to 135-145 parts, keeping the viscosity at 12-16 seconds. This lower viscosity ensures better penetration of the slurry, allowing it to fully wet the rough surface formed by the previous sand application, while simultaneously building up thickness quickly through multiple layers to form a high-strength skeleton supporting the main shell structure.

[0026] The two layers of slurry, through a stepwise change in viscosity, form a transitional system from dense to porous and from flexible to rigid. This structure can effectively disperse the internal stress generated during drying and firing, preventing interlayer delamination, and can also achieve a thin-walled design of the shell by controlling the thickness of each layer while ensuring the overall strength of the shell, ultimately obtaining a composite shell that combines high-temperature strength, thermal stability, and structural integrity.

[0027] Furthermore, in S200, the method for fabricating the surface layer includes the following:

[0028] The wax model tree is completely immersed in the surface slurry to make the surface wet blank. Before the slurry dries, quartz sand is evenly sprinkled on the surface of the surface wet blank. The pouring cup is placed downwards, and the surface is obtained after hardening and drying.

[0029] During the drying process, the temperature is 20~26℃, the humidity is 60~70%, and the time is 6~12h.

[0030] The drying stage employs a room temperature environment of 20-26℃ combined with a relatively high relative humidity of 60-70%, creating gentle dehydration conditions. Under these conditions, moisture evaporates slowly through the gaps in the sand, effectively reducing shrinkage stress caused by sudden changes in surface tension. A prolonged settling period of 6-12 hours allows the silica sol to fully destabilize and form a network structure, achieving uniform curing from the surface inwards. This strictly controlled process avoids coating cracking that might occur with rapid drying and ensures a tight bond between the surface layer and the wax mold, laying a precise surface morphology and a solid foundation for subsequent multi-layer shell fabrication.

[0031] Furthermore, in S200, the method for manufacturing the back layer includes the following:

[0032] After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying.

[0033] The step of impregnating and coating the reinforcing layer slurry is repeated 2 to 4 times; during the drying process, the temperature is 20 to 26℃, the humidity is 40 to 60%, and the time is ≥20h.

[0034] The drying process is controlled within an environment of 20-26℃ and 40-60% humidity for at least 20 hours. Appropriate humidity prevents premature surface hardening and the formation of a closed skin, ensuring continuous outward diffusion of internal moisture. Sufficient drying time ensures that the silica sol colloid fully condenses and solidifies, maintaining the structural integrity of the shell after layer stacking and preventing cracks caused by stress concentration. This process ultimately results in a composite shell structure with thin walls, dense interlayer bonding, and high strength at both room and high temperatures.

[0035] Furthermore, in S200, the sealing slurry is a mixture of mullite powder and silica sol with a viscosity of 8-12 seconds;

[0036] The sealing grouting method includes the following steps: applying sealing grout to the backing surface and drying it; the drying temperature is 20~26℃, the humidity is 35~65%, and the time is ≥6h.

[0037] As the final step in the shell-making process, the sealing grout plays a crucial role in sealing the surface of the backing layer and perfecting the shell's performance. The low-viscosity (8-12 seconds) sealing grout, formulated with mullite powder and silica sol, exhibits significantly higher fluidity than the aforementioned layers. This characteristic allows it to fully penetrate and fill the surface pores and uneven structures formed by the backing layer's sand application, creating a continuous and dense protective film on the shell's surface.

[0038] The sealing layer, through the dual effects of physical coverage and chemical bonding, effectively eliminates open pores on the backing surface, not only improving the overall sealing performance and room temperature strength of the shell, but also significantly enhancing its resistance to high-temperature molten metal penetration. Drying at 20-26℃ and 35-65% humidity for at least 6 hours ensures that the sealing layer effectively seals while slowly releasing internal curing stress, preventing surface micro-cracks caused by excessively rapid drying shrinkage. This provides complete and robust final protection for the shell.

[0039] Furthermore, in S300, the dewaxed shell blank is heated to 1100°C at a rate of 100~200°C / hour and held at that temperature for 60~70 minutes.

[0040] The process employs a uniform heating rate of 100~200℃ / hour to 1100℃. This controllable heating strategy ensures that the components of each layer of the shell are heated evenly, effectively avoiding the accumulation of thermal stress caused by sudden temperature changes, thereby preventing cracks from forming in the thin-walled structure during the early stages of sintering.

[0041] Once the temperature reaches 1100℃, it is maintained at this temperature for 60-70 minutes. During this period, a series of physicochemical transformations occur inside the mold shell: the silica sol binder eventually completes its glass transition, forming a stable three-dimensional network structure; the mullite and quartz refractory particles further sinter and fuse, significantly improving the high-temperature strength and thermal stability of the mold shell. This high-temperature holding stage also helps to thoroughly remove residual organic matter and water of crystallization from the mold shell, enhancing its permeability and metallurgical cleanliness.

[0042] A thin-walled shell prepared using the preparation method described above.

[0043] A process for casting small ductile iron castings using the aforementioned thin-walled shell molding method includes the following steps:

[0044] Step 1: After heating the molten iron to 1630~1660℃, transfer it to the preheated ladle for spheroidization inoculation;

[0045] Step 2: After quickly removing the slag, pour the molten iron, which has been cooled to 1400~1500℃, into the thin-walled shell mentioned above.

[0046] Step 3: After cooling, break the thin-walled shell, remove the gating and riser, and obtain the target ductile iron casting.

[0047] First, the molten iron is heated to a relatively high temperature of 1630~1660℃ before being tapped from the furnace. This temperature is significantly higher than the conventional tapping temperature of around 1500℃ for ordinary ductile iron. This heating strategy aims to ensure that the high-melting-point alloying elements are fully dissolved, the melt composition is homogenized, and the necessary overheating conditions are created for the subsequent spheroidizing treatment.

[0048] After spheroidization and inoculation are completed, the temperature of the molten iron is actively reduced to the optimal casting range of 1400–1500℃. This high-temperature treatment-medium-temperature casting process design ensures the full progress of the spheroidization reaction and the purity of the melt, while avoiding overheating of the thin-walled shell or coarsening of the casting grains due to excessively high casting temperature.

[0049] The casting process of this invention implements a customized heat treatment method for the composition characteristics of high melting point alloys. High-temperature melting ensures complete liquefaction of the alloy and avoids premature precipitation of carbides. Sufficient superheat promotes the absorption of spheroidizing agent and improves the spheroidizing effect. Under the premise of ensuring fluidity, the rapid cooling advantage of thin-walled shells is fully utilized through appropriate pouring temperature, and finally, a ductile iron casting with dense structure and excellent performance is obtained.

[0050] Compared with the prior art, the beneficial effects of the present invention are:

[0051] 1. The thin-walled shell of this invention effectively solves the problem of local shrinkage porosity caused by poor heat dissipation in small ductile iron castings through rapid cooling, thus improving the surface quality of the castings. The silica sol process, combined with optimized surface slurry, ensures high surface smoothness and dimensional accuracy of the castings, allowing for direct assembly and reducing subsequent processing. It eliminates the use of chemical hardeners such as ammonium chloride and magnesium chloride in the water glass process, thereby eliminating volatile pollutants such as ammonia from the source and greatly improving the working environment.

[0052] 2. Although the cost of individual raw materials such as silica sol is relatively high, this process significantly reduces the total material consumption per unit product through innovative thin-wall design, resulting in a decrease in overall production costs. More importantly, the rapid cooling characteristics of the mold shell enable the casting to directly achieve a qualified as-cast structure, potentially eliminating the need for subsequent energy-intensive and time-consuming heat treatment processes. This not only significantly reduces energy and time costs but also optimizes the overall production process and cycle time. Detailed Implementation

[0053] The present invention will now be further described.

[0054] Example 1

[0055] A process for preparing a thin-walled shell includes the following steps:

[0056] S100, prepare surface layer slurry, transition layer slurry, reinforcement layer slurry and sealing slurry respectively;

[0057] The surface layer slurry comprises the following components: 120 kg of 830 silica sol, 200 kg of fused silica powder, 10 kg of zircon powder, 120 mL of wetting agent (WET-10S), and 100 mL of defoamer (HX-002); the viscosity of the surface layer slurry is 29 seconds.

[0058] The preparation method of the surface layer slurry includes the following: adding fused silica powder to the stirred silica sol at a rate of 1.5 kg / min and mixing evenly, then adding a wetting agent; adding zircon powder at a rate of 0.8 kg / min and stirring continuously for 40 min; adding defoamer in 3 portions and maintaining medium-low speed stirring for 18 h.

[0059] The transition layer slurry comprises the following components: 120 kg of 1430 silica sol, 160 kg of mullite powder, and 100 mL of defoamer (FA900); the viscosity of the transition layer slurry is 22 seconds.

[0060] Transition layer slurry preparation method: Add mullite powder to the stirred silica sol in three batches, stir evenly, then add defoamer, and keep stirring at medium-low speed for 15 hours.

[0061] The reinforcing layer slurry comprises the following components: 120 kg of 1430 silica sol, 140 kg of mullite powder, and 100 mL of defoamer (FA900); the viscosity of the reinforcing layer slurry is 14 seconds.

[0062] Method for preparing the reinforcing layer slurry: Add molybdenum powder to the stirred silica sol in two batches, stir evenly, then add defoamer, and keep stirring at medium-low speed for 17 hours.

[0063] The sealant is a mixture of mullite powder and silica sol with a viscosity of 10 seconds.

[0064] S200. The wax model tree is dipped in the surface layer slurry to make the surface layer, then the transition layer slurry and the reinforcing layer slurry are dipped in the surface layer to make the back layer, and finally the sealing slurry is applied and dried to obtain the shell blank.

[0065] The method for creating the surface layer includes the following:

[0066] The wax model tree is completely immersed in the surface slurry to make the surface wet blank. Before the slurry dries, quartz sand is evenly sprinkled on the surface of the surface wet blank. The pouring cup is placed downwards, and the surface is obtained after natural hardening and drying.

[0067] The surface layer is made of quartz sand with a particle size of 100 mesh. During the drying process, the temperature is 24℃, the humidity is 65%, and the time is 9 hours.

[0068] The backing layer fabrication method includes the following:

[0069] After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying.

[0070] The transition layer uses 70-mesh mullite sand, while the reinforcement layer uses 20-mesh mullite sand.

[0071] The process of applying the reinforcing slurry was repeated three times; during the drying process, the temperature was 23℃, the humidity was 50%, and the time was 24 hours.

[0072] The method of applying sealant includes the following steps: applying sealant to the backing surface and drying; the drying temperature is 22℃, the humidity is 50%, and the time is 8 hours.

[0073] S300. After the shell blank is completely hardened and dried, it is placed in hot water for dewaxing. The dewaxed shell blank is then calcined at high temperature at a rate of 150℃ / hour to 1100℃ and held for 65 minutes to obtain a thin-walled shell.

[0074] Example 2

[0075] A process for preparing a thin-walled shell includes the following steps:

[0076] S100, prepare surface layer slurry, transition layer slurry, reinforcement layer slurry and sealing slurry respectively;

[0077] The surface layer slurry comprises the following components: 115 kg of 830 silica sol, 190 kg of fused silica powder, 9 kg of zircon powder, 100 mL of wetting agent (WET-10S), and 100 mL of defoamer (HX-002); the viscosity of the surface layer slurry is 25 seconds.

[0078] The preparation method of the surface layer slurry includes the following: adding fused silica powder to the stirred silica sol at a rate of 2 kg / min and mixing evenly, then adding a wetting agent; adding zircon powder at a rate of 1 kg / min and stirring continuously for 32 min; adding defoamer in two batches and stirring at a medium-low speed for 12 h.

[0079] The transition layer slurry comprises the following components: 110 kg of 1430 silica sol, 150 kg of mullite powder, and 100 mL of defoamer (FA900); the viscosity of the transition layer slurry is 20 seconds.

[0080] Transition layer slurry preparation method: Add molybdenum powder to the stirred silica sol in three batches, stir evenly, then add defoamer, and keep stirring at medium-low speed for 12 hours.

[0081] The reinforcing layer slurry comprises the following components: 115 kg of 1430 silica sol, 135 kg of mullite powder, and 100 mL of defoamer (FA900); the viscosity of the reinforcing layer slurry is 12 seconds.

[0082] Method for preparing the reinforcing layer slurry: Add molybdenum powder to the stirred silica sol in three batches, stir evenly, then add defoamer, and keep stirring at medium-low speed for 12 hours.

[0083] The sealant is a mixture of mullite powder and silica sol with a viscosity of 8 seconds.

[0084] S200. The wax model tree is dipped in the surface layer slurry to make the surface layer, then the transition layer slurry and the reinforcing layer slurry are dipped in the surface layer to make the back layer, and finally the sealing slurry is applied and dried to obtain the shell blank.

[0085] The method for creating the surface layer includes the following:

[0086] The wax model tree is completely immersed in the surface slurry to make the surface wet blank. Before the slurry dries, quartz sand is evenly sprinkled on the surface of the surface wet blank. The pouring cup is placed downwards, and the surface is obtained after natural hardening and drying.

[0087] The surface layer is made of quartz sand with a particle size of 80 mesh. During the drying process, the temperature is 20℃, the humidity is 60%, and the time is 6 hours.

[0088] The backing layer fabrication method includes the following:

[0089] After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying.

[0090] The transition layer uses 60-mesh mullite sand, while the reinforcement layer uses 16-mesh mullite sand.

[0091] The process of applying the reinforcing slurry was repeated twice; during the drying process, the temperature was 20℃, the humidity was 40%, and the time was 20h.

[0092] The method of applying sealant includes the following steps: applying sealant to the backing surface and drying; the drying temperature is 20℃, the humidity is 35%, and the time is 6 hours.

[0093] S300. After the shell blank is completely hardened and dried, it is placed in hot water for dewaxing. The dewaxed shell blank is then calcined at high temperature at a rate of 200℃ / hour to 1100℃ and held for 60 minutes to obtain a thin-walled shell.

[0094] Example 3

[0095] A process for preparing a thin-walled shell includes the following steps:

[0096] S100, prepare surface layer slurry, transition layer slurry, reinforcement layer slurry and sealing slurry respectively;

[0097] The surface layer slurry comprises the following components: 125 kg of 830 silica sol, 210 kg of fused silica powder, 11 kg of zircon powder, 100 mL of wetting agent (WET-10S), and 100 mL of defoamer (HX-002); the viscosity of the surface layer slurry is 35 seconds.

[0098] The preparation method of the surface layer slurry includes the following: adding fused silica powder to the stirred silica sol at a rate of 1 kg / min and mixing evenly, then adding a wetting agent; adding zircon powder at a rate of 0.5 kg / min and stirring continuously for 45 min; adding defoamer in 3 portions and maintaining medium-low speed stirring for 24 h.

[0099] The transition layer slurry comprises the following components: 130 kg of 1430 silica sol, 170 kg of mullite powder, and 100 mL of defoamer (FA900); the viscosity of the transition layer slurry is 24 seconds.

[0100] Transition layer slurry preparation method: Add molybdenum powder to the stirred silica sol in three batches, stir evenly, then add defoamer, and keep stirring at medium-low speed for 24 hours.

[0101] The reinforcing layer slurry comprises the following components: 125 kg of 1430 silica sol, 145 kg of molybdenum powder, and 100 mL of defoamer (FA900); the viscosity of the reinforcing layer slurry is 16 seconds.

[0102] Method for preparing the reinforcing layer slurry: Add mullite powder to the stirred silica sol in three batches, stir evenly, then add defoamer, and keep stirring at medium to low speed for 24 hours.

[0103] The sealant is a mixture of mullite powder and silica sol with a viscosity of 12 seconds.

[0104] S200. The wax model tree is dipped in the surface layer slurry to make the surface layer, then the transition layer slurry and the reinforcing layer slurry are dipped in the surface layer to make the back layer, and finally the sealing slurry is applied and dried to obtain the shell blank.

[0105] The method for creating the surface layer includes the following:

[0106] The wax model tree is completely immersed in the surface slurry to make the surface wet blank. Before the slurry dries, quartz sand is evenly sprinkled on the surface of the surface wet blank. The pouring cup is placed downwards, and the surface is obtained after natural hardening and drying.

[0107] The surface layer is made of quartz sand with a particle size of 120 mesh. During the drying process, the temperature is 26℃, the humidity is 70%, and the time is 12 hours.

[0108] The backing layer fabrication method includes the following:

[0109] After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying.

[0110] The transition layer uses 80-mesh mullite sand, while the reinforcement layer uses 30-mesh mullite sand.

[0111] The process of applying the reinforcing slurry was repeated four times; during the drying process, the temperature was 26℃, the humidity was 60%, and the time was 26 hours.

[0112] The method of applying sealant includes the following steps: applying sealant to the backing surface and drying; the drying temperature is 26℃, the humidity is 65%, and the time is 10 hours.

[0113] S300. After the shell blank is completely hardened and dried, it is placed in hot water for dewaxing. The dewaxed shell blank is then calcined at high temperature at a rate of 100℃ / hour to 1100℃ and held for 70 minutes to obtain a thin-walled shell.

[0114] In Examples 1-3 above, when adjusting the viscosity of the surface layer slurry, the viscosity is first adjusted to 35-40 seconds, and then quartz powder or silica sol is added for further adjustment. When the slurry has poor fluidity, distilled water (no more than 5% of the silica sol) can be added for adjustment, with the distilled water and silica sol mixed in a 1:1 ratio. After adjustment, the slurry needs to be stirred for another hour before it can be used.

[0115] Comparative Example 1

[0116] The transition layer is removed, and other parameters, formulations, and steps are the same as in Example 1.

[0117] Comparative Example 2

[0118] The backing layer fabrication method includes the following:

[0119] After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying.

[0120] The transition layer uses 70-mesh mullite sand, while the reinforcement layer uses 20-mesh mullite sand.

[0121] The process of applying the reinforcing slurry was repeated three times; during the drying process, the temperature was 35℃, the humidity was 30%, and the time was 10 hours.

[0122] Other parameters, formulations, and procedures are the same as in Example 1.

[0123] Comparative Example 3

[0124] S300. The dewaxed shell blank is heated to 950℃ at a rate of 300℃ / hour and held for 30 minutes to obtain a thin-walled shell.

[0125] The performance parameters of the thin-walled shells prepared using the methods of Examples 1-3 and Comparative Examples 1-3 are shown in Table 1.

[0126] Table 1 Performance parameters of thin-walled shells prepared using the methods of Examples 1-3 and Comparative Examples 1-3

[0127] Test Project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 Shell thickness (mm) 4.8 4.5 5.2 4.6 4.7 4.9 High-temperature strength (MPa) 5.2 4.8 5.5 3.8 4.1 3.5 <![CDATA[Air permeability (m 4 / (N·min))]]> 0.85 0.82 0.88 0.65 0.72 0.78 Thermal shock resistance (secondary) 15 13 16 8 9 7 Drying cracking rate (%) 0.5 0.8 0.4 1.2 12.5 0.7 Interlayer bond strength (MPa) 4.5 4.2 4.7 2.8 3.1 4.3 Surface roughness (μm) 6.5 6.8 6.3 8.2 7.5 7.8 Shrinkage defect rate (%) 1.2 1.5 1.0 8.5 6.2 7.8 Dimensional accuracy (μm) ±45 ±48 ±42 ±75 ±68 ±72

[0128] As shown in Table 1, the shell thickness of the embodiments is controlled at around 5mm, which is more than 50% thinner than the traditional water glass shell (10-12mm), but the high-temperature strength is increased by about 40%. This effect is due to the following three aspects: the dense surface layer structure provides good basic strength; the reasonable back layer gradient design optimizes the stress distribution; and precise process control ensures the full utilization of material performance.

[0129] Comparative Example 1, by eliminating the transition layer and directly connecting the surface layer and the reinforcing layer, shows a 38% decrease in interlayer bonding strength and a 24% reduction in air permeability. This demonstrates the irreplaceable role of the transition layer in alleviating stress concentration between sand layers of different particle sizes and ensuring sufficient slurry penetration. This illustrates that the gradient viscosity design employed in Examples 1-3 effectively establishes a performance transition zone, avoiding interfacial failure caused by abrupt changes in material properties.

[0130] Comparative Example 2, which employed high-temperature, low-humidity, rapid drying, showed a sharp increase in cracking rate to 12.5%, significantly higher than the 0.5% in Example 1. This demonstrates that the mild drying regime employed in this invention effectively prevents stress cracking of the thin-walled shell during the curing process by controlling the moisture evaporation rate. A suitable humidity environment provides the necessary conditions for the full condensation of the silica sol, ensuring the integrity of the shell structure.

[0131] Comparative Example 3, with the firing temperature reduced to 950℃ and the holding time shortened to 30 minutes, showed a 33% decrease in high-temperature strength and a 550% increase in the shrinkage porosity defect rate of the casting. This demonstrates that a firing regime of 1100℃ for 65 minutes is crucial for achieving full sintering of mullite materials and obtaining ideal high-temperature performance. Sufficient holding time ensures the uniform transformation of the internal structure of the shell and eliminates residual stress.

[0132] Example 4

[0133] A process for casting small ductile iron castings using the aforementioned thin-walled shell molding method includes the following steps:

[0134] Step 1: After heating the molten iron to 1650℃, transfer it to the preheated ladle, add a spheroidizing agent (using a copper-nickel-rare earth spheroidizing agent) and an inoculant (silicon-calcium alloy) for spheroidizing inoculation;

[0135] Step 2: After rapid slag removal, pour molten iron at a temperature reduced to 1450°C into the thin-walled shell prepared by the method in Example 1;

[0136] Step 3: After cooling, break the thin-walled shell and remove the gating and riser to obtain high-melting-point pearlitic alloy ductile iron.

[0137] Example 5

[0138] A process for casting small ductile iron castings using the aforementioned thin-walled shell molding method includes the following steps:

[0139] Step 1: After heating the molten iron to 1630℃, transfer it to the preheated ladle, add a spheroidizing agent (using a copper-nickel-rare earth spheroidizing agent) and an inoculant (silicon-calcium alloy) for spheroidizing inoculation;

[0140] Step 2: After rapid slag removal, pour molten iron at a temperature reduced to 1400°C into the thin-walled shell prepared by the method in Example 1;

[0141] Step 3: After cooling, break the thin-walled shell and remove the gating and riser to obtain high-melting-point pearlitic alloy ductile iron.

[0142] Example 6

[0143] A process for casting small ductile iron castings using the aforementioned thin-walled shell molding method includes the following steps:

[0144] Step 1: After heating the molten iron to 1660℃, transfer it to the preheated ladle, add a spheroidizing agent (using a copper-nickel-rare earth spheroidizing agent) and an inoculant (silicon-calcium alloy) for spheroidizing inoculation;

[0145] Step 2: After rapid slag removal, pour molten iron at a temperature reduced to 1500°C into the thin-walled shell prepared by the method in Example 1;

[0146] Step 3: After cooling, break the thin-walled shell and remove the gating and riser to obtain high-melting-point pearlitic alloy ductile iron.

[0147] Comparative Example 4

[0148] In step 1, the temperature is raised to only 1500°C. The other steps and parameters are the same as in Example 4.

[0149] Comparative Example 5

[0150] In step 2, the temperature is reduced to 1550°C. The other steps and parameters are the same as in Example 4.

[0151] The performance parameters of the small ductile iron castings prepared by the methods of Examples 4-6 and Comparative Examples 4-5 are shown in Table 2.

[0152] Table 2 Performance parameters of small ductile iron castings prepared by the methods of Examples 4-6 and Comparative Examples 4-5

[0153] Test Project Example 4 Example 5 Example 6 Comparative Example 4 Comparative Example 5 Furnace temperature (°C) 1650 1630 1660 1500 1650 Pouring temperature (°C) 1450 1400 1500 1450 1550 Spheroidization rate (%) 92 90 91 75 88 Globalization decline none slight none serious slight Graphite Form Small and round More round Round Flowering graphite Partial deformity Shrinkage defect rate (%) 1.5 2.0 1.8 8.5 15.0 Iron infiltration / sand adhesion defects none none slight none serious Tensile strength (MPa) 685 675 680 550 620 Elongation (%) 12 11 11.5 5 8 Metallographic structure Ferritic matrix, with fine and uniform graphite. Ferritic matrix, homogeneous graphite Ferrite, with a small amount of pearlite Abundant carbides, coarse graphite The matrix is ​​coarse and the grain boundaries are oxidized.

[0154] As shown in Table 2, in Example 4, the high temperature of 1650℃ ensured the complete dissolution of alloying elements and the effective flotation of gases and inclusions, providing pure, uniform molten iron with sufficient superheat for the subsequent spheroidization reaction. This is a prerequisite for obtaining a high spheroidization rate (92%) and rounded graphite. 1450℃ is a verified optimal pouring temperature window, ensuring good filling capacity of the molten iron in the complex cavity while perfectly matching the rapid cooling characteristics of the thin-walled shell. This synergistic effect promotes sequential solidification of the casting, effectively suppresses shrinkage porosity, and refines the matrix structure.

[0155] In Comparative Example 4, insufficient tapping temperature is an insurmountable flaw for high-melting-point alloy ductile iron. A tapping temperature of 1500℃ only meets the requirements of ordinary ductile iron and cannot allow high-melting-point alloying elements (such as molybdenum and vanadium) to fully dissolve and diffuse. This results in poor molten iron fluidity and high undercooling, ultimately forming a large amount of harmful carbides in the as-cast structure, severely deteriorating mechanical properties, especially causing the elongation to plummet to 5%. Simultaneously, during spheroidization treatment with low-temperature molten iron, the magnesium recovery rate is low and the reaction is unstable, leading to severe spheroidization degradation, with a spheroidization rate of only 75%, and producing a large amount of flowering graphite.

[0156] In Comparative Example 5, the high-temperature pouring at 1550℃ severely exceeded the withstand limit of the thin-walled shell, resulting in severe sintering on the shell surface, causing serious defects such as iron infiltration and sand adhesion, making casting cleaning extremely difficult. The excessively high heat capacity weakened the rapid cooling advantage of the thin-walled shell, causing the casting to solidify slowly, with coarse grains and a sharp increase in shrinkage porosity (15%). At the same time, the high temperature intensified the oxidation of the molten iron, forming oxide inclusions at the grain boundaries, further impairing performance.

[0157] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A process for preparing a thin-walled shell, characterized in that, Includes the following steps: S100, prepare surface layer slurry, transition layer slurry, reinforcement layer slurry and sealing slurry respectively; S200. The wax model tree is dipped in the surface layer slurry to make the surface layer, then the transition layer slurry and the reinforcing layer slurry are dipped in the surface layer to make the back layer, and finally the sealing slurry is applied and dried to obtain the shell blank. S300. After dewaxing the above-mentioned shell blank, it is fired at high temperature to obtain a thin-walled shell. In S100, the surface layer slurry uses fused silica powder and zircon powder as the main raw materials and silica sol as the binder; the transition layer slurry and the reinforcing layer slurry both use mullite powder as the main raw material; in S200, the surface layer sand is made of quartz sand with a particle size of 80~120 mesh, the transition layer sand is made of mullite sand with a particle size of 60~80 mesh, and the reinforcing layer sand is made of mullite sand with a particle size of 16~30 mesh.

2. The preparation process according to claim 1, characterized in that, In S100, the surface layer slurry, by weight, includes the following components: 115-125 parts of 830 silica sol, 190-210 parts of fused silica powder, and 9-11 parts of zircon powder; the viscosity of the surface layer slurry is 25-35 seconds.

3. The preparation process according to claim 2, characterized in that, The preparation method of the surface layer slurry includes the following: adding molten silica powder to the stirred silica sol at a rate of 1~2 kg / min and mixing evenly, then adding a wetting agent; adding zircon powder at a rate of 0.5~1 kg / min and stirring continuously for more than 30 min; adding defoamer in 2~3 portions and maintaining medium-low speed stirring for 12h~24h.

4. The preparation process according to claim 1, characterized in that, In S100, by weight, The transition layer slurry comprises the following components: 110-130 parts of 1430 silica sol and 150-170 parts of mullite powder; the viscosity of the transition layer slurry is 20-24 seconds. The reinforcing layer slurry comprises the following components: 115-125 parts of 1430 silica sol and 135-145 parts of mullite powder; the viscosity of the reinforcing layer slurry is 12-16 seconds.

5. The preparation process according to claim 1, characterized in that, In S200, the surface layer is made using the following methods: The wax model tree is completely immersed in the surface slurry to make the surface wet blank. Before the slurry dries, quartz sand is evenly sprinkled on the surface of the surface wet blank. The pouring cup is placed downwards, and the surface is obtained after natural hardening and drying. During the drying process, the temperature is 20~26℃, the humidity is 60~70%, and the time is 6~12h.

6. The preparation process according to claim 1, characterized in that, In S200, the fabrication method of the back layer includes the following: After the surface layer has dried, apply a transition layer slurry. While the slurry is still wet, sprinkle molybdenum evenly on the surface. After drying, apply a reinforcing layer slurry and repeat the above steps of sprinkling molybdenum and drying. The step of impregnating and coating the reinforcing layer slurry is repeated 2 to 4 times; during the drying process, the temperature is 20 to 26℃, the humidity is 40 to 60%, and the time is ≥20h.

7. The preparation process according to claim 1, characterized in that, In S200, the sealing slurry is a mixture of mullite powder and silica sol, with a viscosity of 8-12 seconds. The sealing grouting method includes the following steps: applying the sealing grout to the backing surface and drying it; the drying temperature is 20~26℃, the humidity is 35~65%, and the time is ≥6h.

8. The preparation process according to claim 1, characterized in that, In S300, the dewaxed shell blank is heated to 1100℃ at a rate of 100~200℃ / hour and held at that temperature for 60~70 minutes.

9. A thin-walled shell prepared by the preparation method according to any one of claims 1 to 8.

10. A process for casting small ductile iron castings with thin-walled shells as described in claim 9, characterized in that, Includes the following steps: Step 1: After heating the molten iron to 1630~1660℃, transfer it to the preheated ladle for spheroidization inoculation; Step 2: After slag removal, pour molten iron with a temperature reduced to 1400~1500℃ into the above-mentioned thin-walled shell. Step 3: After cooling, break the thin-walled shell, remove the gating and riser, and obtain a small ductile iron casting.