Method for improving liquid intake in casting to relieve shrinkage cavity and shrinkage porosity of ductile cast iron

Abstract

The application provides a method for relieving spheroidal graphite cast iron shrinkage cavity and porosity based on improving liquid inflow in casting, which relieves spheroidal graphite cast iron shrinkage cavity and porosity by improving metal liquid shrinkage when the inner gate is not closed, and specifically comprises the following steps: step S1, reducing liquid shrinkage volume when spheroidal graphite cast iron solidifies; step S2, reducing eutectic expansion volume when spheroidal graphite cast iron solidifies in a paste state; step S3, selecting corresponding cold iron specifications according to the size of the castings to be cast, calculating the position of the shrinkage cavity and porosity generated in the spheroidal graphite cast iron during pouring by simulation software, and then placing cold iron or prolonging pouring time at the position of the shrinkage cavity and porosity. The application can solve the problem that the castings are prone to defects of shrinkage cavity and porosity, thereby causing rejection, and can realize quantitative analysis for large and complex cast iron castings, so that the scheme of promoting simultaneous solidification of each part of the castings can be implemented.

Description

Technical Field

[0001] This invention relates to the field of cast iron technology, and in particular to a method for alleviating shrinkage cavities and porosity in ductile iron by increasing the amount of liquid injected during casting. Background Technology

[0002] With the rapid development of computer technology, numerous numerical simulation techniques have been applied to casting research and their scientific validity has been proven. Numerical simulation of the entire casting process provides a high tolerance for errors in casting development, enabling a shift from heuristic techniques and experimental methods to more scientific simulation. It presents the dynamic behavior of the entire casting process and system under corresponding working conditions, visualizing the distribution of casting flow fields, temperature fields, and stress fields. This allows for the reasonable prediction of the type, distribution, and size of casting defects, guiding production practice and positively impacting the optimization of casting processes, improvement of casting product quality and production efficiency, and reduction of production costs. Solidification simulation, primarily based on casting temperature field calculations and supplemented by stress and strain calculations, predicts defects such as shrinkage cavities and porosity in castings, and is a core component of simulation technology in casting production applications. JMatPro possesses fast and accurate calculation capabilities, relying on powerful and stable thermodynamic models and data as its core technology and computational foundation. All physical models have undergone extensive validation to ensure the accuracy of material property calculations. Using JMatPro to calculate the properties of metallic materials allows for a better analysis of the impact of material composition on the material itself during solidification.

[0003] Since its introduction in 1947, ductile iron has experienced rapid development due to its high strength, good toughness, and low production cost. However, ductile iron's characteristics of pasty solidification and graphitization expansion make castings prone to defects such as shrinkage cavities and porosity, leading to scrap. This is especially true for large and complex cast iron parts, where the modulus varies significantly across different parts. Without quantitative analysis, relying solely on experience to design processes makes it difficult to achieve simultaneous solidification. Summary of the Invention

[0004] This invention proposes a method to alleviate shrinkage cavities and porosity in ductile iron by increasing the liquid flow rate during casting. This method can solve the problem of castings being prone to defects such as shrinkage cavities and porosity, which can lead to scrapping. At the same time, it can achieve quantitative analysis for large and complex cast iron parts to implement a scheme to promote the simultaneous solidification of various parts of the casting.

[0005] The present invention adopts the following technical solution.

[0006] A method for alleviating shrinkage cavities and porosity in ductile iron by increasing the molten metal flow rate during casting, wherein the method alleviates shrinkage cavities and porosity in ductile iron by increasing the amount of molten metal shrinkage before the ingate is closed, specifically including the following steps:

[0007] Step S1: Reduce the liquid shrinkage volume during solidification of ductile iron;

[0008] Step S2: Reduce the eutectic expansion volume during the pasty solidification period of ductile iron.

[0009] Step S3: Select the appropriate chill specifications according to the size of the casting to be cast, use simulation software to calculate the shrinkage cavities and porosity locations generated during the pouring process of ductile iron, and then place chills at the shrinkage cavities and porosity locations or extend the pouring time.

[0010] The volume change curve of ductile iron during the solidification period of the casting process is divided into liquid shrinkage period (corresponding to...) in sequence over time. Figure 2 (The position to the right of the line on the right), the solid-liquid mixture's paste-like solidification period (corresponding to) Figure 2 The position between the two lines), the solid contraction period (corresponding to) Figure 2 (The position to the left of the line on the upper left).

[0011] In step S1, the volume change curve of ductile iron during the liquid solidification period is calculated using JMatPro.

[0012] In step S1, the time period corresponding to the liquid shrinkage period is located according to the volume change curve. This time period is when the molten iron has good fluidity and the ingate and the internal feeding channel of the casting are not closed. Then, the shrinkage speed during this time period is accelerated so that the casting produces volume voids as early as possible, thereby increasing the material content in the mold by increasing the amount of molten iron flowing in during the casting process.

[0013] In step S2, the volume change curve of ductile iron during the liquid solidification period is calculated using JMatPro.

[0014] In step S2, the time period corresponding to the liquid solidification period is located based on the volume change curve. By comparing the curves of ductile iron with different component ratios, the alloy composition of ductile iron with smaller shrinkage during the liquid stage and smaller expansion during the eutectic reaction stage is set to reduce the tendency of ductile iron to produce shrinkage cavities and porosity under a reasonable gating system.

[0015] In step S2, by increasing the proportion of silicon in the ductile iron composition, the shrinkage of ductile iron during the liquid phase and the expansion of ductile iron during graphite expansion are reduced.

[0016] In steps S1 and S2, the expansion amount of ductile iron liquid shrinkage and eutectic expansion is reduced by increasing the silicon content ratio in the ductile iron alloy composition, thereby promoting the amount of liquid entering the casting process and alleviating shrinkage cavities and porosity in the casting. Specifically:

[0017] When the ingate is in the pasty solidification stage, the blockage of the ingate is reduced by decreasing the eutectic expansion of ductile iron, allowing the mold to have more liquid feeding to alleviate shrinkage cavities and porosity in ductile iron.

[0018] When the ingate is completely sealed and there is no more liquid feeding, the gap caused by insufficient self-feeding of ductile iron is alleviated by reducing liquid shrinkage, so as to alleviate shrinkage porosity of ductile iron and enable ductile iron to achieve feeding by relying on its self-feeding mechanism.

[0019] In step S3, ProCast is used to simulate the shrinkage cavities and porosity of ductile iron during the pouring process. Chips are placed at these locations to increase the liquid shrinkage of the ductile iron. A corresponding pouring time is set to increase the shrinkage of the molten metal before the ingate is closed, creating more voids and allowing more molten metal to enter the mold, thus forming liquid feeding. In other words, the shrinkage cavities and porosity of the ductile iron are alleviated by increasing the amount of liquid entering the mold during the pouring process.

[0020] In step S3, the ductile iron casting is a complex ductile iron gear with a main wall thickness of 166mm and a height of 98mm. The gating system has a cylindrical sprue, a sprue composed of rectangles and rings, and an ingate composed of multiple rectangles. Specifically, the sprue is a cylinder with a length of 176mm and a radius of 16mm, the sprue is composed of a rectangle with a length of 480mm, a width of 46mm, and a height of 24mm and a ring with the same height, an inner radius of 437mm, and an outer radius of 503mm, and the ingate is composed of 10 rectangles with a length of 50mm, a width of 50mm, and a height of 23mm.

[0021] This invention belongs to the field of cast iron technology and discloses a method for alleviating shrinkage cavities and porosity in ductile iron by increasing the amount of molten metal entering the casting process. Theoretically, this method is applicable to all ductile iron castings and includes the following steps: Step S1, reducing the liquid shrinkage volume during solidification of ductile iron; Step S2, reducing the eutectic expansion volume during the pasty solidification period of ductile iron; Step S3, if necessary, increasing the amount of molten metal shrinkage before the ingate is closed by, for example, selecting appropriate chill specifications according to the size of the casting to be cast, placing chills at the shrinkage cavities and porosity locations, and appropriately extending the pouring time. This invention differs from traditional methods that increase the eutectic expansion of ductile iron to achieve self-compensation and offset shrinkage cavities and porosity. This method effectively increases the amount of molten metal entering the casting during pouring, thereby alleviating or even eliminating shrinkage cavities and porosity in ductile iron, and also has a significant effect on complex ductile iron castings.

[0022] This invention can reduce the amount of liquid shrinkage and eutectic expansion in ductile iron by increasing the silicon content, thereby promoting the amount of liquid entering the casting process and significantly alleviating the shrinkage porosity of ductile iron. Attached Figure Description

[0023] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0024] Appendix Figure 1 This is a schematic diagram of the simulated casting results of the casting system for complex ductile iron gear parts with thick and thin walls under different grades;

[0025] Appendix Figure 2 This is a schematic diagram of the volume change curves of ductile iron with different compositions;

[0026] Appendix Figure 3 This is a schematic diagram of the volume change curves of QT500-7 with different silicon contents;

[0027] Appendix Figure 4 This is a schematic diagram of the volume change curves of QT500-14 with different silicon contents;

[0028] Appendix Figure 5 This is a schematic diagram of the simulated casting results of a complex ductile iron gear casting system with thick and thin walls under different samples.

[0029] Appendix Figure 6 This is a schematic diagram showing the required chill dimensions for the example casting in the case of QT500-7 ductile iron with a silicon content of 2.8%.

[0030] Appendix Figure 7 This is a schematic diagram showing the placement of chills in the case of QT500-7 ductile iron with a silicon content of 2.8% (the left part of the figure is a top view of the three-dimensional casting shown on the right).

[0031] Appendix Figure 8 This is a schematic diagram of the simulated casting results after placing chills on the castings in the QT500-7 ductile iron with a silicon content of 2.8%. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] As shown in the figure, a method for alleviating shrinkage cavities and porosity in ductile iron by increasing the molten metal flow rate during casting is described. This method alleviates shrinkage cavities and porosity in ductile iron by increasing the amount of molten metal shrinkage before the ingate is closed. Specifically, the method includes the following steps:

[0034] Step S1: Reduce the liquid shrinkage volume during solidification of ductile iron;

[0035] Step S2: Reduce the eutectic expansion volume during the pasty solidification period of ductile iron.

[0036] Step S3: Select the appropriate chill specifications according to the size of the casting to be cast, use simulation software to calculate the shrinkage cavities and porosity locations generated during the pouring process of ductile iron, and then place chills at the shrinkage cavities and porosity locations or extend the pouring time.

[0037] The volume change curve of ductile iron during the solidification period of the casting process is divided into liquid shrinkage period (corresponding to...) in sequence over time. Figure 2 (The position to the right of the line on the right), the solid-liquid mixture's paste-like solidification period (corresponding to) Figure 2 The position between the two lines), the solid contraction period (corresponding to) Figure 2 (The position to the left of the line on the upper left).

[0038] In step S1, the volume change curve of ductile iron during the liquid solidification period is calculated using JMatPro.

[0039] In step S1, the time period corresponding to the liquid shrinkage period is located according to the volume change curve. This time period is when the molten iron has good fluidity and the ingate and the internal feeding channel of the casting are not closed. Then, the shrinkage speed during this time period is accelerated so that the casting produces volume voids as early as possible, thereby increasing the material content in the mold by increasing the amount of molten iron flowing in during the casting process.

[0040] In step S2, the volume change curve of ductile iron during the liquid solidification period is calculated using JMatPro.

[0041] In step S2, the time period corresponding to the liquid solidification period is located based on the volume change curve. By comparing the curves of ductile iron with different component ratios, the alloy composition of ductile iron with smaller shrinkage during the liquid stage and smaller expansion during the eutectic reaction stage is set to reduce the tendency of ductile iron to produce shrinkage cavities and porosity under a reasonable gating system.

[0042] In step S2, by increasing the proportion of silicon in the ductile iron composition, the shrinkage of ductile iron during the liquid phase and the expansion of ductile iron during graphite expansion are reduced.

[0043] In steps S1 and S2, the expansion amount of ductile iron liquid shrinkage and eutectic expansion is reduced by increasing the silicon content ratio in the ductile iron alloy composition, thereby promoting the amount of liquid entering the casting process and alleviating shrinkage cavities and porosity in the casting. Specifically:

[0044] When the ingate is in the pasty solidification stage, the blockage of the ingate is reduced by decreasing the eutectic expansion of ductile iron, allowing the mold to have more liquid feeding to alleviate shrinkage cavities and porosity in ductile iron.

[0045] When the ingate is completely sealed and there is no more liquid feeding, the gap caused by insufficient self-feeding of ductile iron is alleviated by reducing liquid shrinkage, so as to alleviate shrinkage porosity of ductile iron and enable ductile iron to achieve feeding by relying on its self-feeding mechanism.

[0046] In step S3, ProCast is used to simulate the shrinkage cavities and porosity of ductile iron during the pouring process. Chips are placed at these locations to increase the liquid shrinkage of the ductile iron. A corresponding pouring time is set to increase the shrinkage of the molten metal before the ingate is closed, creating more voids and allowing more molten metal to enter the mold, thus forming liquid feeding. In other words, the shrinkage cavities and porosity of the ductile iron are alleviated by increasing the amount of liquid entering the mold during the pouring process.

[0047] In step S3, the ductile iron casting is a complex ductile iron gear with a main wall thickness of 166mm and a height of 98mm. The gating system has a cylindrical sprue, a sprue composed of rectangles and rings, and an ingate composed of multiple rectangles. Specifically, the sprue is a cylinder with a length of 176mm and a radius of 16mm, the sprue is composed of a rectangle with a length of 480mm, a width of 46mm, and a height of 24mm and a ring with the same height, an inner radius of 437mm, and an outer radius of 503mm, and the ingate is composed of 10 rectangles with a length of 50mm, a width of 50mm, and a height of 23mm.

[0048] Example:

[0049] The mold used in this example is theoretically applicable to all ductile iron castings. The ductile iron casting used in this example is a complex ductile iron gear with a main wall thickness of 166mm and a height of 98mm. The gating system has a sprue that is 176mm long and 16mm in radius. The gating system consists of a rectangle that is 480mm long, 46mm wide, and 24mm high, and a ring with the same height, an inner radius of 437mm, and an outer radius of 503mm. The ingate consists of 10 rectangles that are 50mm long, 50mm wide, and 23mm high.

[0050] This example describes a method for alleviating shrinkage cavities and porosity in ductile iron by increasing the amount of liquid entering the casting process. The method includes the following steps: Step S1, reducing the liquid shrinkage volume during solidification of ductile iron; Step S2, reducing the eutectic expansion volume during the pasty solidification period of ductile iron; Step S3, if necessary, the amount of molten metal shrinkage before the ingate is closed can be increased by, for example, selecting the appropriate chill size according to the size of the casting to be cast, placing chills at the shrinkage cavity and porosity location, or appropriately extending the casting time.

[0051] Step S1: Calculate the volume change curve of ductile iron during the liquid solidification period using JMatPro;

[0052] Step S2: Calculate the volume change curve of eutectic expansion of ductile iron during the pasty solidification period using JMatPro;

[0053] Step S3: Simulate the shrinkage cavities and porosity of ductile iron during the casting process using ProCast. The shrinkage cavities and porosity of ductile iron can be further alleviated by placing chills at the shrinkage cavities and porosity locations or appropriately extending the casting time.

[0054] In step S1, the line to the right of the JMatPro calculation result graph corresponds to the position to the right of the liquidus line.

[0055] In step S2, the position corresponding to the middle of the two lines in the JMatPro calculation result graph is the position between the liquidus line and the solidus line.

[0056] In step S3, the amount of molten metal shrinkage is increased when the ingate is not yet closed by placing chills at the shrinkage and porosity locations and appropriately extending the pouring time. This creates more voids, allowing more molten metal to enter the mold and form liquid feeding, thereby alleviating the shrinkage and porosity of ductile iron.

[0057] In this example, ProCAST was used to simulate the solidification temperature field of castings of different materials, and the shrinkage cavities that existed when the castings cooled to a solid state were observed. Figure 1 As shown in the figure, the areas circled by lines represent shrinkage cavities and porosity present in the casting. Figure 1 It can be seen that shrinkage cavities and porosity in castings mainly occur at the thick-walled locations where thick and thin walls meet. The main form is a large number of concentrated shrinkage cavities and porosity. This is because the thin-walled areas solidify first compared to the thick-walled areas, and the resulting shrinkage cavities and porosity are compensated by the unsolidified liquid metal of the thick-walled areas. At the same time, the solidification of the thin-walled areas blocks the flow of molten iron inside the casting, resulting in insufficient compensation between the thick-walled areas.

[0058] In this example, the volume change curve of the material reflects the change in volume per unit volume relative to the original volume at different temperatures during solidification. During casting, the liquid metal cools and solidifies, resulting in shrinkage, which is reflected as a negative value on the volume change curve. The smaller the value of the volume change curve, the greater the shrinkage during solidification, making shrinkage cavities more likely. Simultaneously, an upward abrupt change appears on the curve, indicating expansion of the ductile iron within the temperature range of the abrupt change. During casting, the ductile iron expands as the liquid metal cools and solidifies. This abrupt change occurs during graphitization precipitation in the solidification stage. The expansion caused by graphitization precipitation exceeds the shrinkage of the liquid phase, causing the volume change curve to rise sharply. The greater the rise in the volume change curve at this point, the greater the expansion caused by graphitization precipitation. Generally, this stage corresponds to a temperature range of 1090–1190℃.

[0059] In this example, during the solidification process of ductile iron, it is initially in a liquid shrinkage state, at which point the volume of the ductile iron decreases. Figure 2 The position is to the right of the right-hand line. Afterwards, ductile iron will be in a solid-liquid mixed, pasty solidification state, corresponding to the middle position of the two lines in the graph. At this point, the curve will show a sudden change, because the expansion caused by graphitization precipitation is greater than the contraction of the liquid phase, resulting in a larger volume. The greater the rise in the volume change curve, the greater the expansion caused by graphitization precipitation. Finally, ductile iron will enter a solid-state contraction state, corresponding to the left of the left-hand line in the graph above, but this is not significantly related to the shrinkage cavities and porosity issues during the solidification process of ductile iron.

[0060] Traditional viewpoints suggest that the self-feeding ability of ductile iron can resolve shrinkage cavities and porosity defects. However, simply increasing the expansion capacity of ductile iron is not a sufficient solution to these problems. Premature and excessive expansion not only leads to molten iron outflow but also, due to a significant increase in the number of graphite nodules and eutectic clusters, rapid expansion of the solid-liquid zone and a more pronounced tendency for "pasty solidification" make liquid channels prone to blockage. This hinders the feeding of molten metal, thus increasing the tendency for shrinkage cavities and porosity. In fact, before ensuring good molten iron flow and before the ingate and internal feeding channels of the casting are closed, expansion within the casting should be minimized, and shrinkage should even be accelerated as much as possible to create volume voids as early as possible. This allows more molten iron to flow in, increasing the material content within the mold. Increasing the graphite expansion in ductile iron during the early stages can prevent the molten iron from entering the mold cavity for feeding. For complex castings, it may also cause thin-walled sections to block the flow of molten iron in thick-walled sections, resulting in insufficient feeding between thick-walled sections. These factors hinder self-feeding and molten metal feeding, leading to insufficient expansion to offset shrinkage after the ingate and feeding channels are closed, ultimately resulting in shrinkage cavities and porosity.

[0061] In this example, under a reasonable gating system, to reduce the tendency of ductile iron to produce shrinkage cavities and porosity, ductile iron with smaller shrinkage in the liquid phase and smaller expansion during the eutectic reaction is selected. If necessary, chills or other methods can be used to accelerate the shrinkage of the molten metal before the ingate is closed, creating more voids and allowing more molten metal to enter the mold, thus forming liquid feeding.

[0062] By comparison Figure 2 Among the samples, QT400-18, which has the largest shrinkage volume in the shrinkage cavity, exhibits the largest liquid shrinkage, approaching -2.5%. In contrast, QT500-14 and QT500-7, with the smallest shrinkage volumes, both show a change of -2.08%. This indicates that after the ingate of the gating system solidifies, the liquid shrinkage within the mold is smaller, which is more conducive to self-compensation and thus better alleviates or even solves the shrinkage cavity and porosity problem. QT500-14, with the same liquid shrinkage, performs better than QT500-7 in terms of shrinkage cavity and porosity. This is because the precipitation of graphite spheres during the eutectic stage causes expansion. The volume change curve of QT500-14 during the eutectic stage increases by 0.29%, which is less than the 0.6% increase of QT500-7. In other words, the expansion produced by QT500-14 is less than that of QT500-7. The ingate in the gating system is one of the first parts to solidify. Excessive expansion can block or even completely shut off the ingate, causing molten iron to flow out of the mold. This results in insufficient compensation for shrinkage within the mold, increasing the tendency for shrinkage cavities and porosity. Figure 2 It can be seen that QT500-14 exhibits the smallest increase in volume change during the eutectic stage, at only 0.29%, less than half that of QT500-7. This results in minimal eutectic expansion, leading to an increase in the amount of molten iron entering the casting and thus increasing the material content of the casting. Simultaneously, QT500-14 exhibits relatively small liquid shrinkage, so the relatively small self-compensating shrinkage is sufficient to offset the shrinkage generated during solidification.

[0063] By comparison Figure 2 As shown in Table 1, the ductile iron with the smaller shrinkage during the liquid phase and the smaller expansion during the eutectic reaction phase has a higher silicon content.

[0064] Table 1 Composition of ductile iron

[0065]

[0066] In this example, by comparing the effects of different silicon contents on shrinkage cavities and porosity in ductile iron, the conclusion is that silicon plays a crucial role in the feeding capacity of ductile iron. This study shows that carbon and silicon are the two main elements in ductile iron. The bonding force between silicon and iron atoms is greater than that between carbon and iron atoms. When silicon dissolves in molten iron or iron solid solutions, it weakens the bonding force between iron and carbon atoms, thus promoting graphitization. Simultaneously, increasing the silicon content leads to a wider eutectic temperature range for both stable and metastable systems. Specifically, the eutectic temperature range for austenite plus graphite is higher than that for austenite plus cementite, thus making graphite formation more favorable. Therefore, high-silicon ferritic ductile iron, due to its higher silicon content, produces smaller, more uniform, and more numerous graphite spheres, thereby enhancing the self-feeding capacity of ductile iron.

[0067] In this example, after Figure 2 It can be observed that the higher the silicon content, the smaller the shrinkage of ductile iron in the liquid state, and the smaller the expansion during graphite expansion. Tables 2 and 3 show the control groups for QT500-7 and QT500-14, where the silicon content was varied only within the grade range, while keeping all other components constant. Figures 3-5 It can be observed that as the silicon content increases, both the liquid shrinkage and eutectic expansion of ductile iron decrease. When the silicon content reaches QT500-14, the main matrix of ductile iron becomes ferrite, and the expansion amounts of both liquid shrinkage and eutectic expansion reach quite small values. At this point, although increasing the silicon content in ductile iron will also affect the expansion amounts of liquid shrinkage and eutectic expansion, the effect is minimal. High-silicon ductile iron, with its smaller shrinkage capacity in the liquid phase and smaller graphite expansion in the early stage, can lessen the blockage of the ingate during pouring, thus retaining relatively more molten iron in the mold. This provides sufficient material for subsequent feeding. Furthermore, after the ingate is closed, the liquid ductile iron shrinks even less, allowing self-feeding to better offset the shrinkage and alleviate shrinkage cavities and porosity.

[0068] Table 2. Composition of ductile iron in the control experiment.

[0069]

[0070] Table 3 Volume Change

[0071]

[0072] In summary, as the silicon content increases, both the liquid shrinkage and eutectic expansion of ductile iron decrease. Smaller liquid shrinkage and eutectic expansion are more conducive to alleviating shrinkage cavities and porosity. When the ingate is in the pasty solidification stage, the smaller the eutectic expansion of ductile iron, the less it blocks the ingate. This allows for more liquid feeding to alleviate shrinkage cavities and porosity in the ductile iron. When the ingate is completely sealed, without liquid feeding, the ductile iron must rely on its self-feeding mechanism. This requires less liquid shrinkage to compensate for the gaps caused by insufficient self-feeding, thus mitigating shrinkage cavities and porosity in the ductile iron.

[0073] In this example, increasing the silicon content in ductile iron reduces the amount of liquid shrinkage and eutectic expansion, thereby promoting the amount of liquid entering the casting process and significantly alleviating shrinkage porosity in ductile iron.

[0074] In this example, if chills are required, their specifications depend on the dimensions of the casting. Taking QT500-7 ductile iron with a silicon content of 2.8% as an example, chills are placed at shrinkage cavities and porosity. The locations of these shrinkage cavities and porosity are shown in the image. Figure 1 (b) The markings, chills specifications are shown below. Figure 6 See the location of the cold iron. Figure 7 Pouring for 90 seconds, simulation results are shown below. Figure 8 Through with Figure 1 Compared with QT500-7, shrinkage cavities and porosity are eliminated. In this invention, by placing chills, the liquid shrinkage of ductile iron is increased, thereby promoting the liquid inflow during the casting process, which can significantly alleviate shrinkage cavities and porosity in ductile iron.

[0075] The embodiments described in this example, which reduce the amount of liquid shrinkage and eutectic expansion of ductile iron by increasing the silicon content, and by placing chills on the casting and appropriately delaying the pouring time to promote the amount of liquid entering the casting process, are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. That is, all equivalent changes and modifications made within the scope of the claims of this invention should be considered within the technical scope of this invention.

Claims

1. A method for alleviating shrinkage cavities and porosity in ductile iron by increasing the liquid flow rate during casting, characterized in that: The method alleviates shrinkage cavities and porosity in ductile iron by increasing the shrinkage of the molten metal before the ingate is closed, and specifically includes the following steps: Step S1: Reduce the liquid shrinkage volume during solidification of ductile iron; Step S2: Reduce the eutectic expansion volume during the pasty solidification period of ductile iron. Step S3: Select the appropriate chill specifications according to the size of the casting to be cast, use simulation software to calculate the shrinkage cavities and porosity locations generated during the pouring process of ductile iron, and then place chills at the shrinkage cavities and porosity locations or extend the pouring time. The volume change curve of ductile iron during the solidification period of the casting process is divided into three stages according to time: liquid shrinkage period, solid-liquid mixed paste solidification period, and solid shrinkage period. In step S1, the volume change curve of ductile iron during the liquid shrinkage period is calculated using JMatPro, and the time period corresponding to the liquid shrinkage period is located based on the volume change curve. In step S2, the volume change curve of ductile iron during the pasty solidification period is calculated using JMatPro, and the time period corresponding to the pasty solidification period is located based on the volume change curve. By comparing curves of ductile iron with different component ratios, an alloy composition for ductile iron with smaller shrinkage in the liquid phase and smaller expansion in the eutectic reaction phase can be selected to reduce the tendency of ductile iron to produce shrinkage cavities and porosity under a reasonable gating system.

2. The method for alleviating shrinkage cavities and porosity in ductile iron based on increasing the liquid flow rate during casting, as described in claim 1, is characterized in that: In steps S1 and S2, the amount of shrinkage of ductile iron during liquid shrinkage and the amount of expansion of ductile iron during graphite expansion are reduced by increasing the proportion of silicon content in the ductile iron composition.

3. The method for alleviating shrinkage cavities and porosity in ductile iron based on increasing the liquid flow rate during casting, as described in claim 2, is characterized in that: In steps S1 and S2, the expansion amount of ductile iron liquid shrinkage and eutectic expansion is reduced by increasing the silicon content ratio in the ductile iron alloy composition, thereby promoting the amount of liquid entering the casting process and alleviating shrinkage cavities and porosity in the casting. Specifically: When the ingate is in the pasty solidification stage, the blockage of the ingate is reduced by decreasing the eutectic expansion of ductile iron, allowing the mold to have more liquid feeding to alleviate shrinkage cavities and porosity in ductile iron. When the ingate is completely sealed and there is no more liquid feeding, the gap caused by insufficient self-feeding of ductile iron is alleviated by reducing liquid shrinkage, so as to alleviate shrinkage porosity of ductile iron and enable ductile iron to achieve feeding by relying on its self-feeding mechanism.

4. The method for alleviating shrinkage cavities and porosity in ductile iron based on increasing the liquid flow rate during casting, as described in claim 2, is characterized in that: In step S3, ProCast is used to simulate the shrinkage cavities and porosity of ductile iron during the pouring process. Chips are placed at these locations to increase the liquid shrinkage of the ductile iron. A corresponding pouring time is set to increase the shrinkage of the molten metal before the ingate is closed, creating more voids and allowing more molten metal to enter the mold, thus forming liquid feeding. In other words, the shrinkage cavities and porosity of the ductile iron are alleviated by increasing the amount of liquid entering the mold during the pouring process.

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