Multi-layer composite die pipe for pouring high-temperature alloy bar

Abstract

The invention discloses a multi-layer composite die pipe for pouring a high-temperature alloy bar, which is characterized in that a riser is arranged at the top of the multi-layer composite die pipe, a base is arranged at the bottom of the multi-layer composite die pipe, and the riser is communicated with the multi-layer composite die pipe; the riser is composed of a flow guide section and a feeding section from top to bottom. The multi-layer composite die pipe is composed of a die pipe thin-wall section and a die pipe thick-wall section from top to bottom. The feeding section is inserted into the die pipe thin-wall section, and the outer wall of the feeding section is attached to the inner wall of the die pipe thin-wall section. According to the method, the surface quality of the high-temperature alloy bar can be remarkably improved, surface defects are eliminated, internal and external inclusions are greatly reduced, the purity of the high-temperature alloy bar is improved, meanwhile, the solidification process can be accurately controlled, and internal shrinkage cavities and shrinkage porosity are eliminated.

Description

Technical Field

[0001] This invention belongs to the field of high-temperature alloy casting technology, specifically relating to a multi-layer composite mold tube for casting high-temperature alloy bars. This multi-layer composite mold tube is particularly suitable for casting high-temperature alloy bars in the aerospace field, and eliminates surface and internal defects of high-temperature alloy bars through the structural design of the mold tube. Background Technology

[0002] High-temperature alloys, due to their excellent high-temperature mechanical properties and good resistance to oxidation and creep, have important applications in the aerospace field, especially in slender rod-shaped components such as aero-engine turbine shafts and gas turbine rotors. These rod-shaped components are generally cast in molds using a casting process, and their final microstructure and properties are largely affected by the quality of the mold.

[0003] Currently, existing technologies generally use steel molds for casting high-temperature alloy bars. These molds are typically made of a single metal material (such as heat-resistant steel) and are single-layered in structure, lacking layered or composite structures. In practical applications, existing molds mainly suffer from the following technical defects:

[0004] First, excessive quenching leads to poor surface quality of the high-temperature alloy bars. Due to the high thermal conductivity of steel, a single steel mold tube has a strong quenching effect on the high-temperature alloy liquid. The high-temperature alloy liquid solidifies too quickly after contacting the inner wall of the mold tube, which easily forms defects such as cold shuts and cracks on the surface of the bar. At the same time, excessively rapid cooling is also not conducive to the sequential solidification of the high-temperature alloy liquid.

[0005] Secondly, it easily introduces external inclusions. Existing technologies use ceramic fibers for risers and steel for the mold tubes. Non-metallic inclusions such as alumina and silicon dioxide in the riser materials can be introduced into the high-temperature alloy liquid. Some gases adsorbed by the mold tube can cause the high-temperature alloy liquid to generate endogenous inclusions such as alumina and magnesium oxide during the casting process. Simultaneously, the high-temperature alloy liquid undergoes a violent thermo-chemical interaction with the inner wall of the steel mold tube. This interaction leads to the melting and peeling of the mold tube's inner wall material. These eroded particles, as foreign inclusions, are drawn into the solidified high-temperature alloy liquid, forming internal inclusion defects.

[0006] Third, the solidification process cannot be effectively controlled, resulting in severe shrinkage cavities and porosity inside the bar. A single steel mold tube cannot effectively control the direction of heat flow during solidification. Its strong axial heat dissipation makes the solidification interface uneven, which is not conducive to achieving ideal directional solidification conditions. This will cause the feeding channels of the high-temperature alloy liquid to be blocked in the later stage of solidification, thus forming shrinkage cavities and porosity inside the bar, reducing the density and mechanical properties of the bar.

[0007] Therefore, there is an urgent need to develop a multi-layer composite mold tube for casting high-temperature alloy bars, which can eliminate surface and internal defects of high-temperature alloy bars through the structural design of the mold tube.

[0008] Chinese patent application CN114289684A discloses a composite mold tube and assembly method for reducing inclusion content in high-temperature alloy master alloys. The mold tube includes a main mold tube and an inner sleeve. The main mold tube comprises a thin-walled mold tube segment and a thick-walled mold tube segment. The inner sleeve is disposed within the thin-walled mold tube segment, with its outer wall tightly fitted to the inner wall of the thin-walled mold tube segment. This technical solution uses a mold tube with a single structure and a single material, and cannot effectively eliminate defects both internally and externally in the high-temperature alloy master alloy bar. Summary of the Invention

[0009] To address the problems existing in the prior art, the present invention provides a multi-layer composite mold tube for casting high-temperature alloy bars. The multi-layer composite mold tube has a riser at the top and a base at the bottom, and the riser is connected to the multi-layer composite mold tube. The riser consists of a guide section and a feeding section from top to bottom, and the multi-layer composite mold tube consists of a thin-walled section and a thick-walled section from top to bottom. The feeding section is inserted into the thin-walled section of the mold tube, and the outer wall of the feeding section is in contact with the inner wall of the thin-walled section of the mold tube.

[0010] Preferably, the multi-layer composite mold tube is cylindrical in shape, and the thin-walled section of the mold tube consists of a first insulation layer and a first steel mold layer from the inside to the outside, while the thick-walled section of the mold tube consists of a prefabricated layer, a second insulation layer and a second steel mold layer from the inside to the outside.

[0011] In any of the above embodiments, it is preferred that the prefabricated layer is integrally formed, the first insulation layer and the second insulation layer are integrally formed, and the first steel mold layer and the second steel mold layer are integrally formed.

[0012] In any of the above embodiments, it is preferred that the thickness of the precast layer gradually increases from top to bottom, the thickness of the second insulation layer gradually decreases from top to bottom, and the sum of the thickness of the precast layer and the thickness of the second insulation layer remains consistent from top to bottom; the thickness of the second steel mold layer remains consistent from top to bottom.

[0013] In any of the above embodiments, it is preferred that the thickness of the first insulation layer remains consistent from top to bottom, and the thickness of the first insulation layer is the same as the thickness of the top of the second insulation layer; the thickness of the first steel mold layer remains consistent from top to bottom, and the thickness of the first steel mold layer is the same as the thickness of the second steel mold layer.

[0014] In any of the above embodiments, preferably, the prefabricated layer is made of refractory material with a melting point of not less than 1800°C, including any one or more of alumina, magnesium oxide, and spinel; the first insulation layer and the second insulation layer are both made of ceramic fiber, which is composed of alumina and silicon oxide, and the alumina and silicon oxide account for 45-65 wt% and 35-55 wt% of the ceramic fiber by mass, respectively; the first steel mold layer and the second steel mold layer are both made of steel material, including any one or more of heat-resistant steel, stainless steel, and carbon steel.

[0015] In any of the above schemes, the preferred approach is to design the thickness matching relationship between the prefabricated layer and the second insulation layer based on the principles of metal solidification and heat transfer, while also incorporating empirical values ​​of key process parameters in engineering applications. The design model is as follows: In the formula:

[0016] ρ — density of the alloy liquid, kg / m³ 3 ;

[0017] c—Specific heat capacity of the alloy liquid, J / (kg·℃);

[0018] T p —Temperature of the pouring gate, °C;

[0019] T s —Temperature of the solid-liquid phase line, °C;

[0020] T c —Baking temperature of the mold tube, °C;

[0021] σ sp —Inner diameter of the mold tube, in meters;

[0022] λ1——The thermal conductivity of the second insulation layer material, W / (m·K);

[0023] λ2—The thermal conductivity of the prefabricated layer material, W / (m·K);

[0024] δ1—Thickness of the second insulation layer, in meters;

[0025] δ2—thickness of the precast layer, in meters;

[0026] t — Pouring time, in seconds;

[0027] Where 20mm≤δ1+δ2≤50mm, λ1=0.05-0.2W / (m·K), λ2=5-50W / (m·K).

[0028] In any of the above embodiments, it is preferred that the total height of the multi-layer composite mold tube is 800-1400mm, and the thickness of the first section of the steel mold layer and the second section of the steel mold layer is 5-10mm.

[0029] In any of the above embodiments, preferably, the riser is funnel-shaped, the ratio of the total height of the riser to the total height of the multilayer composite mold tube is 1:5-10, the height of the feeding section accounts for 50-65% of the total height of the riser; the wall thickness of the feeding section remains consistent from top to bottom, and the wall thickness of the feeding section is the same as the thickness of the top of the preform layer; the outer diameter of the guide section is larger than the outer diameter of the multilayer composite mold tube; and the riser is made of zirconium oxide material.

[0030] In any of the above embodiments, it is preferred that the thickness of the base is the same as the thickness of the bottom end of the prefabricated layer, and the material of the base is the same as the material of the prefabricated layer.

[0031] In this invention, the refractory material of the precast layer does not react with the high-temperature alloy liquid. This refractory material has a melting point of not less than 1800℃, preventing the high-temperature alloy liquid from reacting with the oxide scale of the mold tube and also preventing iron accumulation. The base material is the same as the precast layer material, ensuring overall compatibility and preventing the base from reacting with the high-temperature alloy liquid. The steel mold layer prevents the alloy liquid from seeping out during casting and also provides reinforcement. The riser is made of zirconium oxide material with a density of not less than 95% and a room temperature compressive strength of not less than 100 MPa. This material neither reacts with the high-temperature alloy liquid nor lacks excellent erosion resistance.

[0032] In this invention, based on the principles of metal solidification and heat transfer, and combined with empirical values ​​of key process parameters in engineering applications, a thickness matching relationship between the precast layer and the second insulation layer is designed. The thickness of the precast layer gradually increases from top to bottom, while the thickness of the second insulation layer gradually decreases from top to bottom, and the sum of the thicknesses of the precast layer and the second insulation layer remains consistent from top to bottom.

[0033] In the design model, once the type and material of the high-temperature alloy rod are determined, the density of the alloy liquid, the specific heat capacity of the alloy liquid, the inner diameter of the mold tube (i.e., the rod diameter), and the temperature of the solid-liquid phase line can be determined. By selecting the materials for each layer, the thermal conductivity of the precast layer material and the thermal conductivity of the second insulation layer material can be determined. Based on empirical values ​​of key process parameters in engineering applications, the temperature of the pouring gate, the baking temperature of the mold tube, and the pouring time can be determined. Through the design model, the matching relationship between the thickness of the precast layer and the thickness of the second insulation layer can be obtained.

[0034] The multi-layer composite mold tube for casting high-temperature alloy bars of the present invention has the following beneficial effects:

[0035] (1) This invention can significantly improve the surface quality of high-temperature alloy bars and eliminate surface defects. By using refractory materials with a thermal conductivity much lower than that of steel as the prefabricated layer (inner layer) and insulation layer (middle layer) of the mold tube, the quenching effect of the mold tube on the high-temperature alloy liquid is weakened. This prevents the alloy liquid from solidifying too quickly due to contact with the inner wall of the mold tube, thereby effectively preventing the generation of surface defects such as cold shuts and cracks.

[0036] (2) This invention can significantly reduce internal and external inclusions and improve the purity of high-temperature alloy bars. By using refractory materials to prepare the prefabricated layer (inner layer) of risers and mold tubes, the source of external inclusions is cut off from the source, avoiding the problem of metal inclusions being introduced due to the melting and peeling of the inner wall of the steel mold tube. The selected refractory materials have high chemical stability and are not easy to react with the alloy liquid to generate new oxide inclusions. At the same time, it reduces the secondary oxidation caused by the adsorption of gas on the surface of the mold tube.

[0037] (3) This invention can precisely control the solidification process and eliminate internal shrinkage cavities and porosity. The multi-layer composite structure of the mold tube of this invention creates conditions for achieving ideal directional solidification. By adjusting the thermal conductivity and thickness of each layer (the inner and middle layers are refractory materials, and the outer layer is steel material), the direction of heat flow during solidification can be effectively guided to form a flat and upward-progressing solidification interface. This ensures that the alloy liquid always has sufficient liquid for feeding during the solidification process, fundamentally eliminating shrinkage cavities and porosity defects, and greatly improving the density and mechanical properties of the bar. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of a preferred embodiment of a multilayer composite mold tube for casting high-temperature alloy bars according to the present invention;

[0039] Figure 2 for Figure 1 The schematic diagram of the multilayer composite mold tube in the embodiment shown is as follows;

[0040] Figure 3 for Figure 1 A schematic diagram of the riser structure in the illustrated embodiment;

[0041] Figure 4 Photographs of the surface morphology of high-temperature alloy bars cast using different mold tubes, wherein: (a) Example, (b) Comparative Example 1, (c) Comparative Example 2;

[0042] Figure 5 The images show the morphology of inclusions inside high-temperature alloy bars cast using different molds, including: (a) Example 1, (b) Comparative Example 1, and (c) Comparative Example 2.

[0043] Figure 6 These are photographs of the internal morphology of high-temperature alloy bars cast using different molds.

[0044] Explanation of annotations in the image:

[0045] 1-Multi-layer composite molded tube, A-Thin-walled section of molded tube, B-Thick-walled section of molded tube, 101-First insulation layer, 102-First steel mold layer, 103-Precast layer, 104-Second insulation layer, 105-Second steel mold layer;

[0046] 2-Riser, 201-Guide section, 202-Replenishment section;

[0047] 3-Base. Detailed Implementation

[0048] To further understand the invention, the following detailed description of the invention will be provided in conjunction with specific embodiments.

[0049] Example:

[0050] like Figures 1-3 As shown, in a preferred embodiment of the multilayer composite mold tube for casting high-temperature alloy bars according to the present invention, the multilayer composite mold tube 1 is provided with a riser 2 at the top and a base 3 at the bottom, and the riser 2 is connected to the multilayer composite mold tube 1; the riser 2 is composed of a guide section 201 and a feeding section 202 from top to bottom, and the multilayer composite mold tube 1 is composed of a thin-walled section A and a thick-walled section B from top to bottom; the feeding section 202 is inserted into the thin-walled section A of the mold tube, and the outer wall of the feeding section 202 is in contact with the inner wall of the thin-walled section A of the mold tube.

[0051] The multi-layer composite mold tube 1 is cylindrical in shape. The thin-walled section A of the mold tube consists of a first insulation layer 101 and a first steel mold layer 102 from the inside out. The thick-walled section B of the mold tube consists of a prefabricated layer 103, a second insulation layer 104 and a second steel mold layer 105 from the inside out.

[0052] The precast layer 103 is integrally formed, the first insulation layer 101 and the second insulation layer 104 are integrally formed, and the first steel mold layer 102 and the second steel mold layer 105 are integrally formed.

[0053] The thickness of the precast layer 103 gradually increases from top to bottom, the thickness of the second insulation layer 104 gradually decreases from top to bottom, and the sum of the thickness of the precast layer 103 and the thickness of the second insulation layer 104 remains consistent from top to bottom; the thickness of the second steel mold layer 105 remains consistent from top to bottom.

[0054] The thickness of the first insulation layer 101 remains consistent from top to bottom, and the thickness of the first insulation layer 101 is the same as the thickness of the top of the second insulation layer 104; the thickness of the first steel mold layer 102 remains consistent from top to bottom, and the thickness of the first steel mold layer 102 is the same as the thickness of the second steel mold layer 105.

[0055] The precast layer 103 is made of refractory material with a melting point of not less than 1800°C, including any one or more of alumina, magnesium oxide, and spinel; the first insulation layer 101 and the second insulation layer 104 are both made of ceramic fiber, which is composed of alumina and silicon oxide, and the alumina and silicon oxide account for 45-65 wt% and 35-55 wt% of the ceramic fiber by mass, respectively; the first steel mold layer 102 and the second steel mold layer 105 are both made of steel material, including any one or more of heat-resistant steel, stainless steel, and carbon steel.

[0056] Based on the principles of metal solidification and heat transfer, and combined with empirical values ​​of key process parameters in engineering applications, the thickness matching relationship between the precast layer and the second insulation layer is designed. The design model is as follows:

[0057]

[0058] In the formula:

[0059] ρ — density of the alloy liquid, kg / m³ 3 ;

[0060] c—Specific heat capacity of the alloy liquid, J / (kg·℃);

[0061] T p —Temperature of the pouring gate, °C;

[0062] T s —Temperature of the solid-liquid phase line, °C;

[0063] T c —Baking temperature of the mold tube, °C;

[0064] σ sp —— Inner diameter of the mold tube, in meters (m);

[0065] λ1——The thermal conductivity of the second insulation layer material, W / (m·K);

[0066] λ2—The thermal conductivity of the prefabricated layer material, W / (m·K);

[0067] δ1—Thickness of the second insulation layer, in meters;

[0068] δ2—thickness of the precast layer, in meters;

[0069] t — Pouring time, in seconds;

[0070] Where 20mm≤δ1+δ2≤50mm, λ1=0.05-0.2W / (m·K), λ2=5-50W / (m·K).

[0071] The total height of the multi-layer composite mold tube 1 is 800-1400mm, and the thickness of the first section of steel mold layer 102 and the second section of steel mold layer 105 is 5-10mm.

[0072] The riser 2 is funnel-shaped, and the ratio of the total height of the riser 2 to the total height of the multilayer composite mold tube 1 is 1:5-10. The height of the feeding section 202 accounts for 50-65% of the total height of the riser 2. The wall thickness of the feeding section 202 remains consistent from top to bottom, and the wall thickness of the feeding section 202 is the same as the thickness of the top of the prefabricated layer 103. The outer diameter of the guide section 201 is larger than the outer diameter of the multilayer composite mold tube 1. The riser 2 is made of zirconium oxide material.

[0073] The thickness of the base 3 is the same as the thickness of the bottom end of the prefabricated layer 103, and the material of the base 3 is the same as the material of the prefabricated layer 103.

[0074] In this embodiment, the refractory material for both the prefabricated layer and the base is alumina. In the ceramic fiber material of the first insulation layer and the second insulation layer, the mass percentages of alumina and silicon dioxide are 55wt% and 45wt%, respectively. The steel material for both the first steel mold layer and the second steel mold layer is heat-resistant steel.

[0075] The total height of the multi-layer composite mold tube is 1200mm. The ratio of the total height of the riser to the total height of the multi-layer composite mold tube is 1:8. The height of the feeding section accounts for 55% of the total height of the riser. The thickness of the first and second steel mold layers is 7mm. The sum of the thickness of the precast layer and the thickness of the second insulation layer remains consistent from top to bottom, which is 35mm.

[0076] In this embodiment, the refractory material of the precast layer does not react with the high-temperature alloy liquid. This refractory material has a melting point of not less than 1800℃, preventing the high-temperature alloy liquid from reacting with the oxide scale of the mold tube and also preventing iron accumulation. The base material is the same as the precast layer material, ensuring overall compatibility and preventing the base from reacting with the high-temperature alloy liquid. The steel mold layer prevents the alloy liquid from seeping out during casting and also provides reinforcement. The riser is made of zirconium oxide material with a density of not less than 95% and a room temperature compressive strength of not less than 100MPa. This material does not react with the high-temperature alloy liquid and has excellent erosion resistance.

[0077] In this embodiment, based on the principles of metal solidification and heat transfer, and combined with empirical values ​​of key process parameters in engineering applications, the thickness matching relationship between the prefabricated layer and the second insulation layer is designed. The thickness of the prefabricated layer gradually increases from top to bottom, while the thickness of the second insulation layer gradually decreases from top to bottom, and the sum of the thicknesses of the prefabricated layer and the second insulation layer remains consistent from top to bottom.

[0078] In the design model, once the type and material of the high-temperature alloy rod are determined, the density of the alloy liquid, the specific heat capacity of the alloy liquid, the inner diameter of the mold tube (i.e., the rod diameter), and the temperature of the solid-liquid phase line can be determined. By selecting the materials for each layer, the thermal conductivity of the precast layer material and the thermal conductivity of the second insulation layer material can be determined. Based on empirical values ​​of key process parameters in engineering applications, the temperature of the pouring gate, the baking temperature of the mold tube, and the pouring time can be determined. Through the design model, the matching relationship between the thickness of the precast layer and the thickness of the second insulation layer can be obtained.

[0079] This embodiment has the following beneficial effects: (1) It can significantly improve the surface quality of high-temperature alloy bars and eliminate surface defects. By using refractory materials with a thermal conductivity much lower than that of steel as the prefabricated layer (inner layer) and insulation layer (middle layer) of the mold tube, the cooling effect of the mold tube on the high-temperature alloy liquid is weakened, so that the alloy liquid will not solidify too quickly due to contact with the inner wall of the mold tube, thereby effectively preventing the generation of surface defects such as cold shuts and cracks. (2) It can greatly reduce internal and external inclusions and improve the purity of high-temperature alloy bars. By using refractory materials to prepare the riser and the prefabricated layer (inner layer) of the mold tube, the source of external inclusions is cut off from the source, avoiding the problem of the steel mold tube inner wall being eroded and peeled off, thus introducing metal inclusions. The selected refractory materials have high chemical stability and are not easy to react with the alloy liquid to generate new oxide inclusions. At the same time, it reduces the secondary oxidation caused by the adsorption of gas on the surface of the mold tube. (3) It can accurately control the solidification process and eliminate internal shrinkage cavities and porosity. By adjusting the thermal conductivity and thickness of each layer of material, the direction of heat flow during solidification can be effectively guided, forming a flat and upward-progressing solidification interface. This ensures that the alloy liquid always has sufficient liquid for feeding during solidification, fundamentally eliminating shrinkage cavities and porosity defects, and significantly improving the density and mechanical properties of the bar.

[0080] Comparative Example 1:

[0081] Based on the multi-layer composite material mold tube designed in the above embodiments, the first insulation layer, the first steel mold layer, the prefabricated layer, the second insulation layer, and the second steel mold layer are combined into one layer, that is, these layers are integrally molded and uniformly made of heat-resistant steel. In contrast, the mold tube in Comparative Example 1 is not layered and is directly integrally molded from heat-resistant steel. The riser structure and base structure are the same as in the embodiments, and the riser material and base material are also made of heat-resistant steel.

[0082] Comparative Example 2:

[0083] Based on the multi-layer composite material mold tube designed in the above embodiment, the matching relationship between the thickness of the prefabricated layer and the thickness of the second insulation layer is eliminated. It is not necessary to calculate the matching relationship between their thicknesses according to the design model. That is, the thicknesses of the first and second insulation layers remain consistent from top to bottom, and the thickness of the prefabricated layer also remains consistent from top to bottom. The riser structure and base are the same as in the embodiment, and the riser material, base material, and materials of each layer in the mold tube are also the same as in the embodiment.

[0084] The high-temperature alloy bars cast in the above embodiments and two comparative examples were tested and compared.

[0085] Surface morphology of high-temperature alloy bars cast using different molds, such as Figure 4 As shown in the figure, (a) is an example, (b) is a comparative example one, and (c) is a comparative example two. It can be seen from the figure that the high-temperature alloy bar cast by the multi-layer composite mold tube of this example has a smooth surface without obvious defects. However, the high-temperature alloy bar cast by the mold tubes of the two comparative examples has serious defects such as casting nodules, sand holes, and oxidation on the surface of the bar.

[0086] The morphology of internal inclusions in high-temperature alloy bars cast using different molds is as follows: Figure 5 As shown in the figure, (a) is an example, (b) is a comparative example one, and (c) is a comparative example two. It can be seen from the figure that the high-temperature alloy rods cast using the multi-layer composite mold tube of this example have very few inclusions inside the rods, while the high-temperature alloy rods cast using the mold tubes of the two comparative examples have more inclusions inside the rods.

[0087] The internal morphology of high-temperature alloy bars cast using different molds is as follows: Figure 6 As shown in the figure, when high-temperature alloy bars are cast using the multi-layer composite mold tube of this embodiment, there are very few shrinkage cavities and porosity inside the bars. However, when high-temperature alloy bars are cast using the two comparative mold tubes, there are more shrinkage cavities and porosity inside the bars.

[0088] Special Note: The technical solution of this invention involves numerous parameters, and the synergistic effects between these parameters must be comprehensively considered to achieve the beneficial effects and significant progress of this invention. Furthermore, the value ranges of each parameter in the technical solution were obtained through extensive experimentation. For each parameter and the combinations thereof, the inventors have recorded a large amount of experimental data; however, due to space limitations, the specific experimental data is not disclosed here.

[0089] It will be readily understood by those skilled in the art that this invention includes any combination of the inventive description and specific embodiments outlined in the foregoing specification, as well as the various parts shown in the accompanying drawings. Due to space limitations and for the sake of brevity, not all of these combinations have been described in detail. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A multi-layer composite mold tube for casting high-temperature alloy bars, characterized in that, The multi-layer composite mold tube has a riser at the top and a base at the bottom, and the riser is connected to the multi-layer composite mold tube. The riser consists of a guide section and a shrinkage section from top to bottom, and the multi-layer composite mold tube consists of a thin-walled section and a thick-walled section from top to bottom. The shrinkage section is inserted into the thin-walled section of the mold tube, and the outer wall of the shrinkage section is in contact with the inner wall of the thin-walled section of the mold tube.

2. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 1, characterized in that, The multi-layer composite mold tube is cylindrical in shape. The thin-walled section of the mold tube consists of a first insulation layer and a first steel mold layer from the inside out. The thick-walled section of the mold tube consists of a prefabricated layer, a second insulation layer and a second steel mold layer from the inside out.

3. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 2, characterized in that, The prefabricated layer is integrally formed, the first insulation layer and the second insulation layer are integrally formed, and the first steel mold layer and the second steel mold layer are integrally formed.

4. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 3, characterized in that, The thickness of the precast layer gradually increases from top to bottom, the thickness of the second insulation layer gradually decreases from top to bottom, and the sum of the thickness of the precast layer and the thickness of the second insulation layer remains consistent from top to bottom; the thickness of the second steel mold layer remains consistent from top to bottom.

5. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 4, characterized in that, The thickness of the first insulation layer remains consistent from top to bottom, and the thickness of the first insulation layer is the same as the thickness of the top of the second insulation layer; the thickness of the first steel mold layer remains consistent from top to bottom, and the thickness of the first steel mold layer is the same as the thickness of the second steel mold layer.

6. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 5, characterized in that, The precast layer is made of refractory material with a melting point of not less than 1800°C, including any one or more of alumina, magnesium oxide, and spinel; the first insulation layer and the second insulation layer are both made of ceramic fiber, which is composed of alumina and silicon oxide, with the alumina and silicon oxide accounting for 45-65 wt% and 35-55 wt% of the ceramic fiber by mass, respectively; the first steel mold layer and the second steel mold layer are both made of steel material, including any one or more of heat-resistant steel, stainless steel, and carbon steel.

7. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 6, characterized in that, Based on the principles of metal solidification and heat transfer, and combined with empirical values ​​of key process parameters in engineering applications, the thickness matching relationship between the precast layer and the second insulation layer is designed. The design model is as follows: In the formula: ρ —— Density of molten alloy, kg / m³ 3 ; c—Specific heat capacity of the alloy liquid, J / (kg·℃); T p —Temperature of the pouring gate, °C; T s —Temperature of the solid-liquid phase line, °C; T c —Baking temperature of the mold tube, °C; σ sp —Inner diameter of the mold tube, in meters; λ1——The thermal conductivity of the second insulation layer material, W / (m·K); λ2—The thermal conductivity of the prefabricated layer material, W / (m·K); δ1—Thickness of the second insulation layer, in meters; δ2—thickness of the precast layer, in meters; t — Pouring time, in seconds; Where 20mm≤δ1+δ2≤50mm, λ1=0.05-0.2W / (m·K), λ2=5-50W / (m·K).

8. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 7, characterized in that, The total height of the multi-layer composite mold tube is 800-1400mm, and the thickness of the first section of the steel mold layer and the second section of the steel mold layer is 5-10mm.

9. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 8, characterized in that, The riser is funnel-shaped, and the ratio of the total height of the riser to the total height of the multilayer composite mold tube is 1:5-10. The height of the feeding section accounts for 50-65% of the total height of the riser. The wall thickness of the feeding section remains consistent from top to bottom, and the wall thickness of the feeding section is the same as the thickness of the top of the preform layer. The outer diameter of the flow guiding section is larger than the outer diameter of the multilayer composite mold tube. The riser is made of zirconium oxide material.

10. The multi-layer composite mold tube for casting high-temperature alloy bars according to claim 9, characterized in that, The thickness of the base is the same as the thickness of the bottom of the prefabricated layer, and the material of the base is the same as the material of the prefabricated layer.

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