Core bar for high-temperature knockout of heat-resistant steel casting and core pulling method

Abstract

The invention relates to a core bar for high-temperature knockout of a heat-resistant steel casting and a core pulling method, belongs to the technical field of casting of large heat-resistant steel castings, and solves the problems of high cracks and extremely low yield after casting due to the fact that a brittle phase is separated out in the solidification process of the heat-resistant steel casting (such as a 0Cr25Ni20 material) and the subsequent cooling stress is large in the prior art. According to the technical scheme, the core bar specially designed for supporting the sand core of the central cavity of the casting is composed of a core bar steel pipe with an exhaust hole, a disc-shaped lower core head with inclination, a top end hanging ring and reinforcing ribs for connecting the core bar steel pipe with the disc-shaped lower core head; a sand core comprising the core bar; the invention also provides a high-temperature mold dismantling method for integrally hoisting the sand core through a hoisting ring when the temperature of the root of a casting riser is not lower than 800 DEG C. And finally, the sand core and the pit of the casting can be removed at high temperature, cracks caused by shrinkage hindering of the sand mold are avoided, the high-temperature casting can be directly put into a furnace for solid solution heat treatment, the yield and quality of the casting are improved, and the energy consumption and the production cost are reduced.

Description

Technical Field

[0001] This invention relates to the field of casting technology for large heat-resistant steel castings, and in particular to a core material and core-removing method for high-temperature molding of heat-resistant steel castings. Background Technology

[0002] Large heat-resistant steel castings, especially key components such as the base of the pit furnace for power plant rotor heat treatment, are typically cast from austenitic heat-resistant stainless steel such as 0Cr25Ni20. These castings need to withstand huge loads for a long time in high-temperature environments above 1000℃, thus placing extremely stringent requirements on the purity of the material, the uniformity of the microstructure, and high-temperature performance.

[0003] In traditional casting processes, such castings are generally produced using sand gravity casting. However, austenitic heat-resistant steels such as 0Cr25Ni20 are prone to precipitating carbides and forming brittle phases like the σ phase at grain boundaries during solidification and subsequent cooling, leading to a decrease in high-temperature plasticity and a sharp increase in crack susceptibility. Current technologies, to avoid cracking, typically employ low-temperature molding (e.g., below 250°C) or prolonged slow cooling to allow the casting to shrink slowly under the protection of the sand mold. However, this method is not only time-consuming and energy-intensive, but also fails to fundamentally solve the problem of casting cracks caused by thermal stress concentration at thick sections because the sand mold still provides significant resistance to casting shrinkage at low temperatures. Practice shows that manufacturing such upper base castings using traditional processes yields a yield of less than 20%, resulting in serious waste of materials and resources and becoming a bottleneck restricting the manufacturing of high-end equipment. Summary of the Invention

[0004] Based on the above analysis, the present invention aims to provide a core and core-raising method for high-temperature molding of heat-resistant steel castings, in order to at least solve one of the technical problems in the prior art that heat-resistant steel castings are prone to post-casting cracks and have low yield due to low molding temperature and insufficient sand mold collapsibility.

[0005] On one hand, embodiments of the present invention provide a core for high-temperature molding of heat-resistant steel castings, used to support the sand core forming the central cavity of the casting, comprising:

[0006] The core steel pipe has a wall thickness of 8-12mm and multiple vent holes with a diameter of 15-25mm are opened on the pipe wall;

[0007] A disc-shaped lower core head is fixedly connected to the bottom end of the core steel tube, and its diameter is larger than the outer diameter of the core steel tube;

[0008] The lifting ring is fixedly connected to the top end of the core steel tube;

[0009] Several reinforcing ribs are welded between the lower core head and the core steel pipe.

[0010] Furthermore, the number of reinforcing ribs is 6 to 8, and they are evenly distributed along the circumference.

[0011] Furthermore, the bottom surface of the lower core head has an axial slope of 1:10.

[0012] Furthermore, the outer diameter of the core steel tube is 200-300 mm.

[0013] Based on the above core design, in order to realize its practical application in the casting process, this invention discloses a sand core, including the core as described above, and a sand core body formed by filling and wrapping the core with resin sand molding process.

[0014] Furthermore, the cavity surface of the sand core is coated with an alcohol-based zircon powder coating with a thickness of 1-3 mm.

[0015] Accordingly, the present invention proposes a high-temperature core-removal method, which uses the sand core described above and includes: after casting is completed, when the temperature at the root of the riser of the casting is not lower than 800°C, the sand core is vertically lifted out of the casting by connecting the lifting ring with a lifting device.

[0016] Furthermore, after removing the sand core, the entire casting with riser is lifted out.

[0017] In addition, the present invention discloses a casting mold including a sand core as described above, wherein the sand core serves as an intermediate core for forming the central cavity of the casting.

[0018] Accordingly, the present invention also discloses an upper base for a rotor heat treatment pit furnace, the base being cast from the mold described above.

[0019] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0020] 1) The core frame specially designed in this invention (especially its core frame steel pipe with vent holes, inclined lower core head, top lifting ring, and circumferential reinforcing rib structure) gives the core frame the strength and reliability to support the sand core and withstand the overall lifting in high-temperature environments above 800°C. This core frame is the core tool for safely and smoothly performing high-temperature core raising operations, and it is also the foundation and guarantee for the successful implementation of the entire casting method of this invention.

[0021] 2) This invention utilizes a specialized core-forming process to achieve high-temperature core raising, overcoming the implementation obstacles of the high-temperature mold-forming process. High-temperature mold-forming eliminates the lengthy process of slowly cooling the casting to room temperature within the mold; subsequent direct solution treatment avoids the enormous energy consumption of reheating the casting from room temperature. This continuous process significantly shortens the production cycle, reduces energy costs, and improves production efficiency.

[0022] 3) This invention employs a synergistic process of high-temperature core raising (800-850°C) based on the aforementioned core material and direct solution treatment, enabling the casting to quickly pass through the sensitive temperature range where a large amount of σ-brittle phase precipitates while still possessing good plasticity. Furthermore, the solution treatment rapidly fixes a single austenitic structure, thereby doubly suppressing the generation of post-cast cracks from both physical process and microstructure perspectives, and steadily increasing the yield from less than 20% in the traditional process to over 90%.

[0023] 4) This invention designs the composition of molten steel to control key elements such as C, Si, and N within precise ranges (e.g., C: 0.025-0.030%, N: 0.025-0.030%), and strictly limits the content of P, S, and the five major harmful elements. This not only controls the precipitation of brittle σ phase, but also uses N to stabilize austenite, partially replacing the use of the precious metal Ni. This achieves excellent high-temperature performance while optimizing costs.

[0024] 5) By adopting a two-layer slow-flow bottom return pouring system, a chill at the bottom of the support arm, and a three waist-shaped riser, this invention achieves stable and clean filling of molten steel and guides the casting to solidify sequentially in a preset direction (from bottom to top). Shrinkage defects such as shrinkage cavities and porosity are effectively driven away and concentrated in the riser, thereby obtaining a high-quality casting body with a dense interior.

[0025] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0026] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0027] Figure 1 This is a three-dimensional structural diagram of the upper base casting;

[0028] Figure 2 This is a schematic diagram of the overall layout of the casting process of the present invention;

[0029] Figure 3 This is a schematic diagram of the overall core structure of the No. 1 sand core in Embodiment 1 of the present invention;

[0030] Figure 4 for Figure 3 Top view of the lower core head and reinforcing rib;

[0031] Figure 5aThis is a schematic cross-sectional view of the sand core assembly in Embodiment 1 of the present invention;

[0032] Figure 5b This is a three-dimensional schematic diagram of the sand core assembly according to Embodiment 1 of the present invention;

[0033] Figure 6 for Figure 5b A top view of the outer core assembly (core #8) shown;

[0034] Figure 7 Photographs of the finished castings obtained using the method of the present invention (Example 1);

[0035] Figure 8 The diagram shows the process failure results of Comparative Example 1 using the traditional process.

[0036] Figure 9a A three-dimensional schematic diagram of the two-layer slow-flow bottom-return casting system. Figure 1 ;

[0037] Figure 9b A three-dimensional schematic diagram of the two-layer slow-flow bottom-return casting system. Figure 2 .

[0038] Figure label:

[0039] 1. Support body; 2. Support arm; 3. Reinforcing ring; 4. Casting; 5. Covering agent; 6. Riser; 7. Chill; 8. Intermediate core assembly; 9. Outer core assembly; 10. Core steel pipe; 11. Lower core head; 12. Reinforcing rib; 13. Lifting ring; 14. Vent hole; 15. Straight sprue; 16. Horizontal sprue; 17. Ingate; 18. Core #1; 19. Core #2; 20. Core #3; 21. Core #4; 22. Core #5; 23. Core #6; 24. Core #7; 25. Core #8; 26. Upper parting line cover core; 27. Pour cup; 28. Straight sprue; 29. ​​First layer horizontal sprue; 30. First layer bottom return gate; 31. Second layer horizontal sprue; 32. Second layer side inlet gate. Detailed Implementation

[0040] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0041] In the casting process of large heat-resistant steel castings (such as castings made of 0Cr25Ni20 material), a high-temperature mold removal process is often used to avoid post-cast cracks. This involves removing the casting from the mold while its temperature is still above 800℃ in the plastic range. However, this process faces an implementation challenge: the large sand cores that form the complex internal cavities of the casting experience a significant decrease in strength at high temperatures. Traditional core frames, due to their simple structure and insufficient strength, cannot safely and completely remove the sand cores. Forced removal can easily lead to sand core collapse, casting damage, or even safety accidents, making the high-temperature mold removal process difficult to apply practically to large and complex castings.

[0042] The present invention aims to provide a special core frame and core lifting method that combines high strength, good air ventilation and safe lifting functions, so as to overcome the obstacles in the implementation of high temperature boxing process.

[0043] A specific embodiment of the present invention discloses a special core for high-temperature (above 800°C) molding of heat-resistant steel castings. The core is pre-embedded inside the sand core that forms the central cavity of the casting, serving as a reinforcing skeleton for the sand core.

[0044] It should be noted that conventional core reinforcement only considers the strength at room temperature during core fabrication and placement, and does not address lifting conditions at temperatures above 800℃. The strength of ordinary carbon steel core reinforcement drops sharply at these temperatures, and the lack of a dedicated lifting structure makes safe lifting impossible.

[0045] like Figure 3 As shown, the core structure of this invention consists of a core steel tube, a disc-shaped lower core head, a lifting ring, and several reinforcing ribs. The core steel tube, as the core load-bearing component, is made of heat-resistant steel (preferably 20 steel or equivalent high-temperature resistant material) with a wall thickness of 8-12mm, capable of maintaining sufficient strength and rigidity in high-temperature environments above 800℃, ensuring that it does not bend or deform when supporting the entire sand core. For ease of understanding of the structure, Figure 3 The middle section of the core steel tube is partially cut to clearly show its wall thickness.

[0046] The pipe wall is evenly perforated with several vent holes of 15-25mm in diameter (e.g. Figure 3 The evenly distributed circular holes on the surface of the steel pipe (as shown) are used to promptly expel the gas generated by the heating of the resin sand during the casting process, effectively preventing porosity defects in the casting. The venting holes are preferably distributed in a spiral or matrix pattern to maximize venting efficiency.

[0047] Spiral straw ropes are wound around the outer wall of the core steel tube. These straw ropes serve as ventilation channels during core making, working in conjunction with the exhaust holes to form a continuous gas passage, further enhancing the exhaust effect.

[0048] The disc-shaped lower core head is fixedly connected to the bottom end of the core steel pipe. Its diameter is larger than the outer diameter of the steel pipe, which increases the contact area with the sand bed in the pit, providing a stable and reliable support foundation for the sand core. The bottom surface of the lower core head has an axial slope of 1:10 (larger at the bottom and smaller at the top). This slope design matches the prefabricated reference surface in the pit, playing a self-positioning and guiding role. This allows the large volume and weight of the central sand core to be easily and accurately placed in the predetermined position, improving the core placement efficiency and the accuracy of the box assembly.

[0049] The top of the core steel pipe is securely welded with a lifting ring (preferably made of round steel with a diameter of not less than 30mm, bent and welded on both sides), serving as a dedicated load-bearing component during high-temperature core raising, capable of safely supporting the overall weight of the sand core. For example... Figure 4 As shown, 6 to 8 reinforcing ribs (preferably 8) are welded circumferentially between the lower core head and the core steel pipe. They are evenly distributed along the circumference, which significantly enhances the tensile and shear resistance of the connection part and avoids structural tearing due to stress concentration during lifting, thereby ensuring the overall reliability of the core under high temperature conditions.

[0050] Furthermore, in order to achieve rational use of materials and facilitate venting while ensuring structural strength, the outer diameter of the core steel tube is 200-300mm (e.g., 200mm, 220mm, 250mm, 280mm, 300mm). This size range matches the central cavity size of the large upper base, providing sufficient support strength without being too bulky.

[0051] The specialized core assembly provided in this embodiment is the material guarantee for achieving the crucial step of first lifting out the central sand core in the aforementioned high-temperature core-removal method. It makes it possible to safely and reliably remove large sand cores under extreme working conditions.

[0052] Based on the above core design, a special sand core needs to be prepared to realize its practical application in the casting process. This sand core mainly includes the aforementioned high-temperature resistant core and a sand core body formed by filling and wrapping the core with resin sand molding technology.

[0053] This sand core serves as the functional carrier for the specialized core skeleton. In its preparation, molding materials such as furan resin self-hardening sand are used to fill and compact the core skeleton within the core box. After hardening, this forms a sand core assembly with precise dimensions and necessary strength. This sand core is specifically designed to construct the cavity structure corresponding to the complex central cavity of the upper base in the mold. After the molten metal is poured, filled, and solidified, the central cavity of the upper base is ultimately formed.

[0054] Compared to conventional sand cores, the sand cores in this solution benefit from their unique internal core design, which allows them to maintain structural stability at high temperatures and possess the strength required for overall lifting. They are the supporting components for the successful implementation of the innovative process of high-temperature boxing and subsequent direct solution treatment.

[0055] Furthermore, to improve the surface quality of the casting's inner cavity and prevent molten metal from penetrating and adhering to sand, the forming surface of the sand core (the side in contact with the molten metal) is sprayed with an alcohol-based zircon powder coating. The coating thickness is 1–3 mm, preferably 2 mm. The zircon powder coating has the advantages of high refractoriness and a low coefficient of thermal expansion, effectively resisting the high-temperature corrosion of 0Cr25Ni20 steel. After spraying, ignition and combustion allow the coating to dry rapidly, forming a robust refractory coating. This measure ensures a smooth surface on the casting's inner cavity, eliminating the need for arduous cleaning work and avoiding mechanical sand adhesion defects.

[0056] Based on the aforementioned core and sand core, this invention proposes a high-temperature core-raising method, which uses the aforementioned sand core.

[0057] The method includes: after the casting is completed, first remove the upper sand box, then use an insertion thermocouple to measure the temperature at the root of the riser of the billet. When it is confirmed that the temperature is not lower than 800°C, connect the lifting ring at the top of the core with the connecting hook of the lifting equipment (such as a crane) and lift the sand core vertically and smoothly out of the mold cavity (after it is separated from the inner wall of the formed billet).

[0058] This invention utilizes a specialized core skeleton to achieve high-temperature core removal, overcoming implementation obstacles in the high-temperature casting removal process. Compared to existing methods, this method coordinates the timing of casting removal with specialized tools. A specific temperature window of 800℃ to 850℃ is selected, at which point the cast billet is still in the high-temperature plastic stage, internal thermal stress is easily released, and the sand core has not yet reached its sintering temperature, effectively avoiding removal difficulties caused by excessive fusion between the sand core and the billet surface. With the aid of a specially designed high-temperature core skeleton structure, utilizing the lifting ring at its top, the sand core can be safely and smoothly removed under high-temperature conditions, creating conditions for the subsequent smooth removal of the cast billet and direct solution treatment.

[0059] Furthermore, after the sand core is lifted out, the entire cast billet with riser is lifted out of the pit using a crane.

[0060] It should be noted that after the central sand core is removed, the space originally occupied by the sand core is released, and the thermal constraint inside the billet is relieved. At this time, because the billet temperature is still high and its strength is lower than room temperature, the lifting operation must be smooth and slow, using special lifting equipment to ensure that the billet is subjected to uniform force and to avoid deformation or cracks. The billet after being lifted out is then transferred to the heat treatment process.

[0061] In addition, the present invention proposes a casting mold including the sand core described above, wherein the sand core serves as an intermediate core for forming the central cavity of the casting.

[0062] For example, such as Figure 5a , 5b and Figure 6As shown, the mold is a complete sand mold assembled for casting the upper base. The mold consists of an intermediate core group 8 and an outer core group 9. The intermediate core group 8 includes a central main core (i.e., core #1 18), inside which a high-temperature resistant core is embedded. The core is completely encased in resin sand to form the sand core body. The intermediate core group 8 also includes auxiliary inner cores (core #3 20 and core #6 23). Core #3 20 and core #6 23 are both symmetrically split structures, arranged around the central main core 1 #18. The mating surface between core #18 and core #3 20 is designed with a 1:20 slope. Combined with the enclosing structure of core #6 23, core #18 can be easily pulled out. These auxiliary inner cores are used together to form the complex cavity structure inside the casting.

[0063] The outer core group 9 consists of core #2 (19), core #4 (21), core #5 (22), core #7 (24), and core #8 (25). Each core is symmetrically designed, and each number represents a pair of identical sand cores arranged symmetrically.

[0064] Figure 5b for Figure 5a The corresponding three-dimensional diagram of the sand core assembly, Figure 6 for Figure 5b The top view, specifically the top view of the outer core group 9 (corresponding to core 8# 25), shows the relative positional relationship between the outer core group 9 and the middle core group 8. Core 8# 25 is arranged around the outer edge of the middle core group 8, and its inner contour matches the outer contour of the middle core group 8. A gap is reserved between core 8# 25 and core 6# 23. Together, they enclose the cavity area with the main wall thickness of the casting. Cores 4# 21, 5# 22, 7# 24, and 2# 19 have different dimensions to control the balance of local sand intake (if the dimensions were uniform, the lower sand core would be too thick). They are located at specific positions on core 8# 25 and are used to form structural features such as local bosses, flanges, and mounting holes in the casting.

[0065] All sand core components are equipped with positioning core heads and mating surfaces. During assembly, the cores are installed layer by layer in sequence (1# core 18 → 3# core 20 → 2# core 19 → 4# core 21 → 6# core 23 → 5# core 22 → 7# core 24 → 8# core 25). After each layer of cores is installed, the dimensions are checked using a template or measuring tape to ensure that the relative positions of each core are accurate and the cavity dimensions meet the process requirements.

[0066] The key feature of this casting structure is that the central core employs a reinforced structure with a dedicated core skeleton, enabling it to be lifted out as a whole using lifting rings at high temperatures (riseer root temperature ≥800℃), thus achieving "high-temperature mold removal" and preventing billet cracks caused by uneven cooling. Simultaneously, the outer core group 9, through a design of "differentiated size control of sand intake," combined with symmetrical layout and strict positioning, ensures both the accuracy of the casting contour and the uniformity of solidification, while simplifying the process adaptability of the sand core, further synergistically improving the internal quality and performance consistency of the casting.

[0067] Compared to existing molds, this mold integrates the aforementioned dedicated central sand core. The presence of this sand core enables the entire mold to perform high-temperature mold-making processes, thereby determining the quality of the final casting.

[0068] Based on the above-mentioned core frame, sand core, core raising method, and casting mold, a specific embodiment of the present invention also discloses an upper base for a rotor heat treatment pit furnace and its casting method. The upper base is made of 0Cr25Ni20 austenitic heat-resistant steel, and the method includes the following steps:

[0069] S1. Pouring: Molten steel is poured into the mold using a two-layer slow-flow bottom-return pouring system;

[0070] S2. Feeding and Cooling: An external chill is installed at the bottom of the support arm of the casting, and three waist-shaped risers are used for feeding;

[0071] S3. High-temperature mold opening: After casting is completed, the mold opening is carried out when the temperature at the root of the riser of the casting is 800℃~850℃ (e.g., 800℃, 810℃, 820℃, 830℃, 840℃, 850℃).

[0072] S4. Heat treatment: The castings that have been removed from the high-temperature mold are directly placed into a heat treatment furnace for solution treatment without cooling to room temperature.

[0073] This invention, through the synergistic effect of high-temperature molding and direct solution treatment, enables castings to quickly bypass the precipitation-sensitive zone of the brittle σ phase, thereby fundamentally suppressing the formation of crack initiation sites. This technical effect lies in the synergistic effect of the following steps:

[0074] S1. Pouring Procedure: A two-layer stepped slow-flow bottom-return pouring system is used. This step employs an open-type two-layer stepped slow-flow bottom-return pouring system, such as... Figure 9a and Figure 9b As shown, the core consists of a straight gating system, two independent horizontal gating systems at the top and bottom, and corresponding ingates forming a complete flow path. Molten steel is precisely injected into the mold through the straight gating system, the layered horizontal gating systems, and the ingates in sequence, forming a collaborative filling mode of "stable flow at the bottom layer and continuous flow in the middle layer".

[0075] The entire gating system is arranged in a stepped, layered manner along the height of the upper base, with the two gating units connected sequentially from top to bottom and precisely aligned with the key parts of the mold.

[0076] The lower pouring unit consists of a first-layer horizontal sprue and a first-layer ingate. The first-layer horizontal sprue is horizontally arranged in the bottom area of ​​the mold, with one end vertically connected to the sprue and the other end evenly distributed with multiple first-layer ingates. The ingate adopts a bottom-return structure design, with its end fitting into the cavity inlet at the lowest end of the upper base support arm. After molten steel flows into the first-layer horizontal sprue through the sprue, it gently flows upward along the bottom of the cavity through the bottom-return ingate, achieving impact-free filling, avoiding damage to the sand core and reducing the risk of air entrapment.

[0077] Upper pouring unit: Composed of a second horizontal sprue and a second ingate. The second horizontal sprue is arranged parallel to the first horizontal sprue and is fixedly connected to the sprue via a vertical connecting section, forming a stepped structure with a height difference. The second ingate adopts a side-entry design, evenly distributed on the side of the second horizontal sprue facing the mold. Its port is aligned with the cavity where the reinforcing ring in the middle of the upper base is located. Molten steel enters the cavity horizontally through this ingate, supplementing the molten steel flow in the upper and middle areas, forming continuous filling with the lower unit, and maintaining a steady rise in the molten steel level inside the cavity.

[0078] The direct sprue, as the main channel for molten steel, is designed with the same diameter as the two horizontal sprues, both with a diameter of 140mm, to ensure sufficient molten steel flow and balanced flow distribution between the two units.

[0079] A key aspect of this invention is that the design of the sprue system strictly adheres to the "open" principle of casting steel parts, meaning that the total cross-sectional area of ​​the sprue is greater than that of the runner, and the total cross-sectional area of ​​the runner is greater than that of the sprue, in order to achieve stable and low-turbulence filling of the molten steel. Under this principle, the sprues should preferably be arranged in a "small and evenly distributed" manner.

[0080] When the number of ingates cannot be increased due to casting structure limitations, the cross-sectional area of ​​a single ingate needs to be increased accordingly to meet the aforementioned proportional requirements and ensure smooth filling. This method preferably uses ceramic tubes with uniform specifications to form the ingates, with an inner diameter controlled within the range of 70mm to 90mm. This size range is set under the aforementioned constraints to balance filling smoothness and flow efficiency: if the inner diameter of a single ingate is too small, the total cross-sectional area will be insufficient with a limited number, making it impossible to form an effectively open gating system. This can easily lead to excessively fast steel filling speed, damaging the sand mold, causing slag inclusions and air entrapment defects, and failing to meet the requirements of high flow rate and low velocity; if the inner diameter is greater than 90mm, a hot spot is easily formed at the sprue of the casting, hindering casting shrinkage and affecting the sequential solidification of the casting.

[0081] Preferably, setting the diameter of the ingate to 80mm can achieve uniform and stable filling of the mold with molten metal while meeting the steel flow requirements, effectively improving the density of the internal structure and surface integrity of the billet.

[0082] In the S2 feeding and cooling steps, such as Figure 2As shown, an external chill is installed on the lower part of the support arm, fitting against the cavity surface, and three waist-shaped risers are arranged at intervals along the axial direction of the support arm to create sequential solidification conditions. The chilling effect of the external chill causes the lower part of the support arm to solidify first, forming a solidification front. The waist-shaped risers provide liquid metal feeding channels for the thicker middle and upper parts, and directionally migrate shrinkage cavities and porosity defects generated during solidification to the interior of the risers, thereby ensuring that the casting body is dense and defect-free (e.g., meeting the HB 6570I standard).

[0083] Furthermore, the height-to-diameter ratio of the waist-shaped riser is set to 1.1–1.3 (e.g., 1.1, 1.15, 1.2, 1.25, 1.3). This parameter range is crucial for balancing feeding efficiency and material economy: when the height-to-diameter ratio is below 1.1, the static pressure of the molten metal inside the riser is insufficient, the feeding flow is weak, and it is difficult to effectively overcome the resistance of the solidification channel; when it is above 1.3, although the feeding pressure increases, the heat dissipation of the top surface of the riser accelerates, which will solidify prematurely to form a solid shell, thus blocking the feeding channel and increasing the amount of molten steel used and cutting costs.

[0084] Preferably, the height-to-diameter ratio is set to 1.2, which optimizes the process yield while ensuring sufficient feeding pressure and liquid time, thus achieving the best overall performance in terms of technical performance and manufacturing cost.

[0085] In the S3 high-temperature mold-making process, after casting, the mold-making operation is initiated when the temperature at the root of the riser of the billet drops to the range of 800℃ to 850℃. The sand core is first lifted out as a whole using a lifting ring attached to the top of the central sand core support. This operational sequence is based on the design of the billet-sand core system under high-temperature conditions: utilizing the high-temperature strength maintained by the dedicated core support above 800℃, the constraint of the sand core on the central area of ​​the billet is first released, releasing its shrinkage stress, while simultaneously creating operational space for lifting the billet itself. If this sequence is violated, the weight of the billet will crush the weakened sand core, leading to billet jamming or structural damage. This method, combined with the dedicated core support, is the core of achieving the feasibility and safety of the high-temperature mold-making process.

[0086] This invention deviates from conventional methods. Approximately 24 hours after casting, when the temperature at the riser root is still as high as 800℃~850℃ (at which point the overall billet temperature is far above the lower limit of the σ phase precipitation peak), the billet is unpacked. Subsequently, the still-high-temperature billet is immediately placed directly into a heat treatment furnace for solution treatment (S4). The solution treatment temperature is 1090℃~1110℃ (e.g., 1090℃, 1095℃, 1100℃, 1105℃, 1110℃), and after holding at this temperature, it is cooled by water quenching.

[0087] This process completely bypasses the sensitive temperature window for the large-scale precipitation of brittle phases during the entire cooling-heating process from casting to solution treatment. Solution treatment (preferably 1100℃±10℃) can fully dissolve the precipitated trace amounts of brittle phases, and stabilize them to room temperature through rapid water quenching, ultimately obtaining a single austenitic structure with uniform composition and excellent toughness.

[0088] This embodiment eliminates the main problem leading to scrap (precipitation of σ brittle phase) by precisely controlling the cooling path of the casting during the high-temperature stage, thereby achieving a qualitative leap in yield from less than 20% to over 90%.

[0089] Furthermore, precise control of chemical composition is one of the key factors ensuring the successful casting of large 0Cr25Ni20 austenitic heat-resistant steel castings. This invention has determined the following optimized internal control composition ranges (mass percentage): C: 0.025%–0.030%, Si: 1.6%–1.7%, Mn: 1.4%–1.5%, P≤0.005%, S≤0.005%, Cr: 24.00%–25.00%, Ni: 19.00%–19.50%, N: 0.025%–0.030%, and strictly controls residual elements, requiring Sn+As+Pb+Sb+Bi≤0.02%.

[0090] This composition control scheme differs significantly from the composition range of standard 0Cr25Ni20, and is based on synergistic optimization of the material's precipitated phase behavior and high-temperature performance:

[0091] On the one hand, this invention strictly controls the C content to an extremely low level of 0.025%-0.030%, aiming to minimize grain boundary chromium carbides (mainly Cr). 23 The precipitation of C6 not only serves as a crack initiation source but also causes chromium depletion at grain boundaries, impairing corrosion resistance. Simultaneously, precisely controlling nitrogen (N) at 0.025%-0.030%, leveraging its role as a strong austenite-forming element, increases the Ni equivalent, stabilizing the austenite structure and reducing the amount of precious metal Ni added. This ultra-low C and N combination can control the harmful σ-brittle phase equilibrium precipitation below 2%, reducing high-temperature crack susceptibility.

[0092] On the other hand, the present invention controls the P and S contents to ≤0.005% respectively, and limits the total of the five harmful elements (Sn, As, Pb, Sb, Bi) to no more than 0.02%. This requirement, which far exceeds the conventional standard, reduces the segregation of low-melting-point eutectic phases at grain boundaries, improves the bonding strength of grain boundaries at high temperatures and the overall purity of molten steel, and further enhances the casting's resistance to hot cracking.

[0093] To obtain a uniform, single-phase austenitic structure and eliminate precipitated phases, step S4 involves heating the casting to 1090℃~1110℃ for solution treatment. This temperature range ensures that any carbides and σ phases that may precipitate in the as-cast structure are fully dissolved back into the austenitic matrix. The holding time is calculated as 3-4 hours / 100mm based on the maximum wall thickness of the casting to ensure sufficient dissolution. After holding, the casting is immediately subjected to rapid water quenching to fix the high-temperature single-phase austenite to room temperature, preventing harmful phases from precipitating again along grain boundaries and ensuring that the casting achieves optimal room-temperature toughness, high-temperature strength, and creep resistance.

[0094] Furthermore, to avoid damage to the casting's performance from subsequent processes, this invention employs cold working methods (such as abrasive wheel cutting or band saw cutting) to remove the riser after the solution treatment in step S4. This aims to completely avoid the localized thermal effects caused by traditional flame cutting (hot cutting). Hot cutting reheats the cutting area to the temperature range sensitive to σ-phase precipitation, leading to microcracks near the cut surface and deteriorating the microstructure. Cold working technology completely eliminates the risk of secondary damage introduced by this thermal process, ensuring the performance integrity of the casting.

[0095] The base for the rotor heat treatment pit furnace prepared by the aforementioned method is a large hollow frame structure: the top is a thickened annular hollow bearing frame, which serves as the direct bearing surface for the hundred-ton rotor. A large area of ​​hollow space is left in the middle of the bearing frame to accommodate the space inside the furnace. Several symmetrically distributed vertical supports are connected below the bearing frame, which are the core load-bearing components of the structure. Adjacent supports are connected by at least two layers of horizontal annular support arms. Annular reinforcing rings are also provided at the joints between the support arms and the supports to enhance the stability of the overall frame and avoid stress concentration.

[0096] The upper base is obtained through the aforementioned casting method. Due to the precise control of solidification sequence, chemical composition, and heat treatment regime during its manufacturing process, this upper base possesses unique intrinsic properties that distinguish it from similar products manufactured using traditional methods. Compared to existing technologies, the core advantage of this upper base lies in the optimization of its internal structure and the significant improvement in performance, which directly translates into its long lifespan and high reliability under extreme operating conditions.

[0097] Furthermore, the content of the σ-brittle phase in the metallographic structure of the upper base is less than 2%. The σ-brittle phase is a hard and brittle intermetallic compound, and its excessive precipitation is the root cause of cracking and eventual scrapping of castings produced by conventional processes. This invention, through the combined effect of the aforementioned chemical composition design and high-temperature molding and direct solution heat treatment processes, successfully suppresses the σ-phase content to below 2%, far lower than the content that may be produced by conventional processes (typically >5%). This gives the product excellent resistance to crack initiation and propagation when subjected to high temperatures and loads.

[0098] Furthermore, the wall thickness of the upper base support arm continuously increases from bottom to top. By designing the portion of the mold cavity corresponding to the upper base support arm to be thinner at the bottom and thicker at the top, and combining this with chills pre-embedded in the lower part of the support arm for rapid cooling, it is ensured that the casting solidifies sequentially from the far end (lower part) away from the riser to the near end (upper part) closer to the riser. Ultimately, shrinkage defects caused by solidification shrinkage are concentrated and guided to the top riser, resulting in a dense and defect-free interior of the casting. This structural feature is crucial for ensuring the load-bearing capacity of the casting.

[0099] The present invention will be described in more detail below through specific embodiments. These embodiments are merely descriptions of the best implementation of the invention and do not limit the scope of the invention in any way.

[0100] Example 1

[0101] This embodiment manufactures an upper base for a Φ3.5×14m pit furnace, with dimensions of Φ2990 / Φ2100×4130mm, a net weight of 48.177 tons, and is made of 0Cr25Ni20 austenitic heat-resistant steel.

[0102] The casting method steps are as follows:

[0103] Process design and model making: based on Figure 1 The casting process was designed based on the three-dimensional structure shown, and the final scheme was determined after optimization using numerical simulation software, such as... Figure 2 As shown. Manufacturing the main prototype model and core box.

[0104] Core making and shaping:

[0105] When making #1 sand core (intermediate core), the special core material of this invention is used, such as... Figure 3 As shown. The core steel tube has an outer diameter of 240mm and a wall thickness of 10mm, with 20mm diameter vent holes all around. The lower core head is an 850mm diameter disc with a 1:10 axial slope machined on the bottom surface, and is welded to the steel tube by eight evenly distributed reinforcing ribs. A 20mm diameter lifting ring is welded to the top.

[0106] Using this specialized core as a framework, furan resin self-hardening sand molding was employed to form core #1 (the core body). All sand cores (including...) Figure 4 The cavity surfaces of the intermediate core group 6 and the outer peripheral core group shown are both coated with alcohol-based zircon powder coating, and the coating thickness is controlled at 2mm.

[0107] The mold designed according to the process (as shown in Figure 5) Figure 6 As shown, core assembly is performed by creating a reference surface at the bottom of the casting pit. First, a No. 1 sand core (intermediate core) with a special core skeleton is lowered in, followed by outer sand cores No. 3, No. 6, No. 2, No. 4, No. 5, No. 7, and No. 8 in sequence to form a complete casting cavity.

[0108] Lower core assembly: Create a reference surface on the bottom of the casting pit, according to... Figure 4 Insert the cores sequentially, ensuring accurate positioning of core #1. After assembling the mold, dry it in a hot air furnace at 190℃ for at least 24 hours. One hour before pouring, blow argon gas into the mold cavity and check its quality using an oxygen analyzer.

[0109] Smelting and composition control: The process of electric furnace roughing followed by VOD refining is adopted. The chemical composition of the molten steel is strictly controlled within the internal control range, with the following specific mass percentages: C: 0.028%, Si: 1.65%, Mn: 1.45%, P: 0.004%, S: 0.003%, Cr: 24.50%, Ni: 19.25%, N: 0.028%, and the total amounts of Co, B, Nb+Ta, and the five major harmful elements all meet the internal control requirements.

[0110] Pouring: using methods such as Figure 9a , Figure 9b The two-layer slow-flow bottom-return gating system shown is used for pouring (this system needs to be connected). Figure 5a , Figure 5b (The casting cavity is formed by assembling sand cores). The diameter of the ingate is 80mm. The pouring temperature is 1490℃. Argon ring protection is used for pouring.

[0111] Feeding and cooling: such as Figure 2 As shown ( Figure 2 This presents the structural form of the upper base corresponding to the casting cavity. In actual production, it is necessary to... Figure 5a , Figure 5b The sand cores are assembled into a cavity of this shape, and then fitted with... Figure 9a , Figure 9b The gating system, together forming a complete casting process layout, has an external chill 4 installed at the lower position of each support arm 2 corresponding to the cavity. Three waist-shaped risers 3 are used for feeding, with a riser height-to-diameter ratio designed to be 1.2.

[0112] High-temperature core removal and mold removal: 24 hours after casting, the mold box on the parting surface (referring to the upper part of the sand box and the corresponding sand mold structure separated along the parting surface (the opening and closing surface of the sand mold), which is the forming component surrounding the upper part of the mold cavity to be cast) is lifted away. When the temperature at the root of the riser is measured to be 830°C using an insertion thermocouple, the high-temperature core removal method of this invention is executed by connecting the pre-set lifting ring 10 at the top of the core through the crane hook: keeping the lifting direction perpendicular to the sand core axis, the No. 1 sand core is lifted vertically out of the mold cavity; then, with the help of a special lifting tool, the billet to be treated, still with the riser, is lifted out of the casting pit.

[0113] Heat treatment: The high-temperature castings removed after high-temperature core raising are directly placed into a heat treatment furnace for solution treatment at a temperature of 1100℃. After holding at this temperature for a sufficient time, they are then water quenched.

[0114] Post-processing: The riser is removed by cold working (grinding wheel cutting). Minor defects found are repaired by welding after PT and UT non-destructive testing (preheated to 300℃, using special welding rods). The weld repair area is then subjected to local heat treatment.

[0115] The final product is a rotor heat treatment pit furnace base casting with a dense internal structure and no casting cracks.

[0116] Example 2

[0117] This embodiment manufactures an upper base for a Φ2.5×12m pit furnace, with dimensions of Φ2600 / Φ1920×4200mm, a net weight of 42.50 tons, and a material of 0Cr25Ni20.

[0118] The casting method in this embodiment is basically the same as that in Embodiment 1, and the core parameters are selected as follows:

[0119] Special core material: The core steel tube has an outer diameter of 220mm, a wall thickness of 9mm, a lower core head diameter of 800mm, and 6 reinforcing ribs.

[0120] Sand core and casting mold: The same core assembly method and coating process as in Example 1 are used.

[0121] Gating system: Two-layer slow-flow bottom return type, with an inner gate diameter of 70mm.

[0122] Riser: Three waist-shaped risers with a height-to-diameter ratio of 1.1.

[0123] Chemical composition: C: 0.025%, Si: 1.60%, Ni: 19.00%, N: 0.025%.

[0124] High-temperature packaging temperature: 800℃.

[0125] Solution treatment temperature: 1090℃.

[0126] Finally, a qualified upper base casting was successfully obtained.

[0127] Example 3

[0128] This embodiment manufactures an upper base for a Φ2.5×18m pit furnace, with dimensions of Φ2500 / Φ1750×1420mm, a net weight of 25.10 tons, and a material of 0Cr25Ni20.

[0129] The casting method in this embodiment is basically the same as that in Embodiment 1, and the core parameters are selected as follows:

[0130] Special core material: The core steel tube has an outer diameter of 260mm, a wall thickness of 11mm, a lower core head diameter of 900mm, and 8 reinforcing ribs.

[0131] Sand core and casting mold: The same core assembly method and coating process as in Example 1 are used.

[0132] Gating system: Two-layer slow-flow bottom return type, with an inner gate diameter of 90mm.

[0133] Riser: Three waist-shaped risers with a height-to-diameter ratio of 1.3.

[0134] Chemical composition: C: 0.030%, Si: 1.70%, Ni: 19.50%, N: 0.030%.

[0135] High-temperature baking temperature: 850℃.

[0136] Solution treatment temperature: 1110℃.

[0137] Finally, a qualified upper base casting was successfully obtained.

[0138] Comparative Example 1

[0139] This comparative example uses a conventional manufacturing process to produce an upper base of the same specifications as Example 1. Its main difference from the present invention is:

[0140] Core and Sand Core: The sand core is made using a conventional core with a simple structure, instead of the high-temperature special core structure of this invention.

[0141] Core removal and core raising process: Low-temperature core removal is adopted. After casting, the billet is slowly cooled in the mold to below 250°C before core removal. Because the sand core bonds more tightly with the billet at low temperatures, and there are no special lifting rings for the core, core raising is difficult, resulting in damage to the billet.

[0142] Heat treatment: After the billet is removed from the mold, it is cooled to room temperature and then reloaded into the furnace for solution treatment.

[0143] Riser design and cutting: Six independent circular risers are used, and the risers are removed using flame cutting (thermal cutting).

[0144] Composition control: Chemical composition is only controlled within standard ranges (e.g., C≤0.08%, Si:1.50-2.50%), without strict internal control.

[0145] Comparative Example 2

[0146] The only difference between this comparative example and Example 1 is that:

[0147] Core structure: The No. 1 sand core was manufactured using a traditional, simple core frame without specially designed lifting rings and reinforcing ribs. During attempts to perform high-temperature core lifting, the core frame structure was found to be insufficiently strong, unable to be reliably hung, or failed during lifting, making it impossible to safely lift the central sand core at temperatures above 800°C. This prevented subsequent processes from being executed, ultimately forcing the adoption of the low-temperature core lifting solution from Comparative Example 1.

[0148] Characterization results and analysis

[0149] The characterization results of the above-described embodiments and comparative examples are shown in Table 1 below.

[0150] Table 1. Comparison of quality and performance of castings from the examples and comparative examples.

[0151] project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Is the finished product qualified? yes yes yes No (crack) No (process interrupted) σ Brittle phase content (%) <1.5 <1.8 <1.7 >8 - Grain size (mm) 5.5 5.7 5.6 Not detected - Tensile strength (MPa) 555 548 552 - - Yield strength (MPa) 245 241 243 - - Elongation (%) 43 41 42 - - Non-destructive testing results No cracks No cracks No cracks Macroscopic cracks exist -

[0152] Table 1 clearly demonstrates the effectiveness of the present invention. Examples 1-3 using the present invention successfully yielded qualified castings: non-destructive testing confirmed the absence of cracks. Figure 7 The finished product photos visually demonstrate its intact macroscopic morphology; the content of σ brittle phase is stably below 2% (1.5%-1.8%); the grain size is fine and uniform (5.5-5.7mm); the mechanical properties are excellent, with tensile strength of 548-555MPa, yield strength of 241-245MPa, and elongation of 41%-43%, all of which are superior to the standard requirements.

[0153] In contrast, Comparative Example 1, using traditional processes and lax composition control, resulted in the scrapping of the cast billet due to macroscopic cracks, such as... Figure 8 As shown, the surface of the cast billet exhibited clear cracks, which corroborated the test results of high σ-brittle phase content (above 8%), leading to process failure. Comparative Example 2 showed that the high-temperature core-forming step could not be performed at all due to the lack of a dedicated core, forcing the process to be interrupted, demonstrating the importance of a dedicated core.

[0154] In summary, this invention systematically solves the industry problem of low yield in large 0Cr25Ni20 castings through the synergistic effects of specialized core reinforcement, high-temperature core raising, direct solution treatment, and precise composition control. It enables the stable production of crack-free, low-brittle phase, fine-grained, high-performance castings. The yield has been increased from less than 20% to over 90%, representing a significant technological advancement.

[0155] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A core for hot box of a cast of heat-resistant steel, characterized in that, A sand core for supporting a core bone steel pipe, the pipe wall thickness is 8-12mm, a plurality of exhaust holes with a diameter of 15-25mm are arranged on the pipe wall; A disc-shaped lower core head is fixedly connected to the bottom end of the core bone steel pipe, and has a diameter larger than the outer diameter of the core bone steel pipe; A lifting ring is fixedly connected to the top end of the core bone steel pipe; A plurality of reinforcing ribs are welded between the lower core head and the core bone steel pipe. The number of the reinforcing ribs is 6-8, and the reinforcing ribs are uniformly distributed in the circumferential direction.

2. The core bone according to claim 1, characterized in that, The bottom surface of the lower core head has an axial slope of 1:

10.

3. The core bone according to claim 1, characterized in that, The outer diameter of the core bone steel pipe is 200-300mm.

4. The core bone of claim 1, wherein The sand core body formed by filling and wrapping the core bone outside through a resin sand molding process.

5. A sand core, characterized by The cavity surface of the sand core is sprayed with alcohol-based zirconium powder paint, and the paint thickness is 1-3mm.

6. The sand core of claim 5, wherein, The sand core of claim 5 or 6 is used, including: after pouring is completed, when the temperature of the root of the casting head is not lower than 800℃, the lifting ring is connected through hoisting equipment, and the sand core is vertically lifted out of the casting as a whole.

7. A high temperature core initiation method characterized by, After the sand core is lifted out, the casting with the head is lifted out as a whole.

8. The high temperature boxing method of claim 7, wherein, The sand core of claim 5 or 6 is used as an intermediate core for forming a central cavity of a casting.

9. A mold characterized in that, The base is cast by the mold of claim 9.

10. An upper base for a heat treatment of a rotor shaft type furnace, characterized in that, ​

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