A method for controlling porosity and shrinkage of a large-size equiaxed crystal high-temperature alloy casting

Abstract

This invention discloses a method for controlling porosity and shrinkage cavities in large-size equiaxed crystal high-temperature alloy castings, belonging to the field of investment casting technology. The method includes: (1) wrapping the large-size casting shell with asbestos, then placing the asbestos-wrapped shell into a sand box, wherein the sand box is modified according to the asbestos-wrapping molding process of the casting; (2) smoothly loading the sand box containing the shell into a muffle furnace for shell pre-firing; (3) using a shell transfer cart to transfer the shell to the ingot mold chamber of a vacuum smelting equipment for smelting, and then casting to obtain a large-size equiaxed crystal high-temperature alloy casting. This method reduces or eliminates the tendency to form porosity and shrinkage cavities by controlling the temperature field of the sand box and the shell transfer time.

Description

Technical Field

[0001] This invention relates to the field of investment casting technology, specifically to a method for controlling porosity and shrinkage cavities in large-size equiaxed high-temperature alloy castings. Background Technology

[0002] Compared to aero-engine turbine blades, the hot-end components of high-power industrial gas turbines (including turbine blades and retaining rings) are larger and heavier. This significant difference has a major impact on the manufacturing process of large-size high-temperature alloy castings, especially on the control of metallurgical defects. In the manufacture of large-size equiaxed superalloy castings, traditional equiaxed superalloy casting processes are highly susceptible to metallurgical defects such as porosity and shrinkage cavities, leading to a substantial decrease in the yield rate. This significantly increases the manufacturing cost of large-size superalloy hot-end components and limits their widespread application in industrial gas turbines.

[0003] The basic conditions for the formation of metallurgical defects such as porosity and shrinkage cavities in castings include: (1) a wide range of alloy crystallization temperatures; (2) small temperature differences between different parts of the casting during solidification, i.e., a small solidification temperature gradient, a wide solidification area, and a tendency to solidify in a volumetric (pasty) manner, resulting in a greater tendency for the formation of metallurgical defects such as porosity and shrinkage cavities. Generally, the basic principle of controlling metallurgical defects such as porosity and shrinkage cavities is to establish a temperature gradient between different parts of the casting to achieve sequential solidification, thereby transferring metallurgical defects such as porosity and shrinkage cavities to the feeding riser. Large-sized castings, especially solid castings with equal or large wall thickness structures (such as the thick tenon parts of retaining rings and large-sized blades), are very prone to developing metallurgical defects such as porosity and shrinkage cavities during solidification. Establishing a temperature gradient through external means is the main method to achieve sequential solidification and reduce or eliminate porosity and shrinkage cavities. In industrial production, the heat insulation effect of asbestos or heat-conducting materials such as chills are usually used to shape the mold shell in a targeted manner, thereby forming gradient heat dissipation conditions and temperature gradients. However, the temperature gradient created solely through molding processes such as asbestos or the installation of chills has limited effectiveness in controlling porosity and shrinkage defects in high-temperature alloy castings with equal or large wall thicknesses. Summary of the Invention

[0004] In view of the above-mentioned shortcomings in the preparation of large-size high-temperature alloy castings for heavy-duty gas turbines, the purpose of this invention is to provide a method for controlling porosity and shrinkage cavities in large-size equiaxed high-temperature alloy castings. This method reduces or eliminates the tendency of castings to form porosity and shrinkage cavities by further increasing the temperature gradient, thereby improving the pass rate and metallurgical quality of large-size high-temperature alloy castings.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A method for controlling porosity and shrinkage cavities in large-size equiaxed superalloy castings. This method reduces or eliminates the tendency for porosity and shrinkage cavities to form during the casting process by controlling the temperature field of the sand box and the shell transfer time. The method includes the following steps:

[0007] (1) Large-size casting shells are wrapped with asbestos and then placed in a sand box.

[0008] (2) Place the sand box with the mold shell into the muffle furnace steadily to avoid shaking; pre-fire the mold shell.

[0009] (3) After the preheating of the mold shell reaches the specified time requirement, the mold shell is transferred to the ingot mold chamber of the vacuum smelting equipment by a mold shell transfer car for smelting, and then large-size equiaxed crystal high temperature alloy castings are obtained after casting.

[0010] In step (1) above, the sand box is modified in a targeted manner according to the cotton-coating molding process of the casting. Specifically, the sand box is modified by cutting and opening windows in areas where the casting needs to be cooled quickly (such as areas without cotton coating and areas with chills) to increase the cooling rate of the casting in these areas. At the same time, the sand box is modified by insulation in areas where the casting needs to be cooled slowly (such as areas where the mold shell is cotton-coated, especially areas where feeding risers are located), and the inner and outer walls of the sand box are treated with cotton insulation.

[0011] In step (3) above, the time for transferring the shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment is controlled between 2.5 and 4 minutes. During the transfer of the shell, it is required to handle it gently and avoid shaking.

[0012] The design mechanism and beneficial effects of this invention are as follows:

[0013] 1. This invention improves the temperature gradient of the casting during the sequential solidification of high-temperature alloy steel liquid by modifying the sand box, which effectively improves the sequential solidification feeding ability of the casting and reduces or eliminates the tendency of large-size castings to form porosity and shrinkage cavities.

[0014] 2. The purpose of controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to be more than 2.5 minutes is to ensure that the areas of the casting that require rapid cooling (such as the areas without insulation and the chilled areas) are sufficiently cooled. The insulation areas of the mold shell (especially the insulation areas with thick feeding risers) lose much less heat than the mold shell material due to the good insulation effect of the asbestos material. Therefore, controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to be more than 2.5 minutes can increase the temperature gradient between the areas that require rapid cooling and the insulation areas of the mold shell (especially the insulation areas with thick feeding risers), effectively improving the sequential solidification and feeding ability of the casting, and reducing or eliminating the tendency of large-sized castings to form porosity and shrinkage cavities. The purpose of controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to within 4 minutes is to prevent a significant drop in the temperature of the mold shell and excessive heat loss from the insulation cotton of the mold shell, which would cause the high-temperature alloy steel liquid to solidify too quickly and lead to metallurgical and dimensional deformation problems in the casting. Attached Figure Description

[0015] Figure 1 These are common metallurgical defects in large-size high-temperature alloy retaining ring castings; (a) and (b) are defects in different locations.

[0016] Figure 2 This is a schematic diagram of the sand box temperature field modification of the present invention.

[0017] Figure 3 This is a casting of the third-stage retaining ring for a heavy-duty gas turbine, prepared using the process of this invention.

[0018] Figure 4 This is an X-ray film of a heavy-duty gas turbine third-stage retaining ring casting prepared using the process of this invention.

[0019] Figure 5 This is a fluorescence detection image of the third-stage retaining ring of a heavy-duty gas turbine prepared using the process of this invention. Detailed Implementation

[0020] To further understand the present invention, the present invention is described below with reference to examples. However, the examples are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0021] Example 1:

[0022] This embodiment describes a method for controlling porosity and shrinkage cavities in the third-stage retaining ring casting of heavy-duty gas turbines, aiming to solve problems such as surface porosity and shrinkage cavities in large-size high-temperature alloy retaining ring castings. Figure 1 This is a typical metallurgical defect in the retaining ring before the treatment described in this embodiment was applied.

[0023] This embodiment describes how, during the preparation of high-temperature alloy castings, the tendency to form porosity and shrinkage cavities is reduced or eliminated by controlling the temperature field of the sand box and the shell transfer time; specifically as follows:

[0024] 1. Based on the structural characteristics of the large-size third-stage retaining ring casting and the cotton-wrapping molding process, the structure and temperature field of the sand box were modified (see...). Figure 2 In areas of the casting requiring rapid solidification (the lower section of the casting without insulation), the sand box is modified by cutting and opening windows to increase heat dissipation efficiency and improve the solidification rate of the molten steel in these areas. Thick insulation is applied to the inner and outer walls of the sand box corresponding to the riser location, with each layer having a 20mm thickness (40mm total). These sand box modifications increase the temperature gradient in the sequential solidification areas of the casting during the solidification process of the high-temperature alloy steel, effectively improving the sequential solidification and feeding capacity of the casting and reducing or eliminating the tendency for large-sized castings to develop porosity, shrinkage cavities, and shrinkage cracks.

[0025] Among them: based on the structure and solidification characteristics of the third-level retaining ring casting, a gradient insulation molding process was carried out. The specific molding process is as follows: the lower section of the casting is exposed shell, the middle section is wrapped with 10mm thick insulation asbestos, the upper section is wrapped with 20mm thick insulation asbestos, and the riser area is wrapped with 40mm thick insulation asbestos.

[0026] 2. After the asbestos-wrapped molding of the large-size casting shell is completed according to the process specifications, the shell is placed in a sand box, and the top of the sand box is covered with 40mm of asbestos. The sand box with the shell is then carefully placed into the muffle furnace to avoid shaking.

[0027] 3. After the pre-firing of the shell reaches the specified time requirement, the pre-firing shell is transferred to the casting process using a shell transfer car. The time for transferring the shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment is controlled between 2.5 and 3.5 minutes. During the shell transfer process, it is required to handle it gently and avoid shaking. The purpose of controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to more than 2.5 minutes is to ensure that the areas of the casting that require rapid solidification (such as the exposed mold shell area and the corresponding sand box opening area) are sufficiently cooled. The areas covered with cotton in the mold shell (the thick cotton-covered areas corresponding to the feeding risers and the cotton-lined areas in the sand box) lose much less heat than the areas covered with cotton due to the good insulation effect of the asbestos material. Therefore, controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to more than 2.5 minutes can increase the temperature gradient between the areas requiring rapid cooling and the cotton-covered areas in the mold shell (especially the cotton-covered areas with thick feeding risers), effectively improving the sequential solidification and feeding capacity of the casting, and reducing or eliminating the tendency for the formation of metallurgical defects such as porosity, shrinkage cavities, and shrinkage cracks in large-sized castings (especially riser porosity and shrinkage cavities). The purpose of controlling the transfer time of the mold shell from the muffle furnace to the ingot mold chamber of the vacuum smelting equipment to within 3.5 minutes is to prevent a significant drop in the temperature of the mold shell and excessive heat loss from the insulation cotton part of the mold shell, which would greatly reduce the temperature gradient and cause the high-temperature alloy steel liquid to solidify too quickly, resulting in metallurgical and dimensional deformation problems in the casting.

[0028] The pouring temperature, pouring rate, and other process parameters shall be in accordance with the specific process requirements for casting. After pouring, the vacuum equipment shall be broken after 4 minutes, and then the pouring cart shall be removed. The casting shall be placed on the pouring cart for 10 minutes before being removed. After pouring, the casting shall be placed in the casting placement area for at least 4 hours. After completion, the shell cleaning, riser cutting, and sandblasting processes shall be carried out.

[0029] Visual, X-ray, and fluorescence inspections were conducted on castings prepared using the above-mentioned process. The results showed that the above process completely resolved surface porosity, shrinkage cavities, and shrinkage cracks on the casting surface (especially in the transition area between the casting and the feeding riser). The internal porosity of the casting was significantly reduced, and surface defect control was significantly improved. Figure 3 , 4 As shown in Figure 5.

Claims

1. A method for controlling porosity and shrinkage cavities in large-size equiaxed superalloy castings, characterized in that: This method involves controlling the temperature field of the sand box and the shell transfer time during the preparation of high-temperature alloy castings to reduce or eliminate the tendency for the formation of metallurgical defects such as porosity and shrinkage cavities. The method includes the following steps: (1) The large-size casting shell is wrapped with asbestos for molding, and then the asbestos-wrapped shell is placed in the sand box; according to the asbestos-wrapped molding process of the casting, the sand box is modified accordingly, specifically: the sand box is cut and opened in the area where the casting needs to be cooled quickly, so as to increase the cooling rate of the casting in that area; at the same time, the sand box is insulated in the area where the casting needs to be cooled slowly, and the inner and outer walls of the sand box are lined with cotton for insulation. (2) Place the sand box with the mold shell into the muffle furnace smoothly to avoid shaking; pre-fire the mold shell. (3) After the preheating of the mold shell reaches the specified time requirement, the mold shell is transferred to the ingot mold chamber of the vacuum smelting equipment by a mold shell transfer car for smelting, and then cast to obtain a large-size equiaxed crystal high temperature alloy casting; the time for transferring the mold shell to the ingot mold chamber of the vacuum smelting equipment is controlled at 2.5~4 minutes. During the transfer of the mold shell, it is required to handle it gently and avoid shaking.

2. The method for controlling porosity and shrinkage cavities in large-size equiaxed superalloy castings according to claim 1, characterized in that: The areas of the casting that require rapid cooling include the uninsulated areas and the chilled areas; the areas of the casting that require slow cooling control include the mold shell insulation areas and the feeding riser areas.

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