Abstract: This paper focuses on the issue of low production efficiency of malleable cast iron connectors in a foundry and aims to develop a casting process of one box with two pieces. Based on the original casting process of one box with one piece, combined with the design method of cast iron casting process, the ProCAST numerical simulation software is used to simulate and optimize the preliminary casting process of the connector, and the production trial is carried out. Considering the insufficient casting defects exposed in the trial production, the causes are analyzed through the visual results obtained by simulation. By adjusting the height of the residual indenter, the pouring temperature, and the riser position, the casting process of the malleable iron connector is finally determined, which improves the production capacity of the connector in the foundry and makes the qualified rate of the casting reach more than 90%.
1. Introduction
Connectors are widely used in facilities such as highway and railway bridges, tower structures, pipeline supports, and lifting machinery. Compared with traditional welded railings, pipe-connected railings using connectors have superior corrosion resistance. For remote areas, pipe-connected railings are more convenient in terms of transportation and installation. Due to their strong connection, quick disassembly and installation, and strong adaptability to different application scenarios, they are widely used in the industrial field.
The material of the connector is KTH330 – 08, where KTH represents blackheart malleable cast iron, 330 represents the minimum tensile strength of 330 MPa, and 08 represents the minimum elongation after fracture of 8%. Its mechanical properties are similar to ASTMA47 – 32510. The graphite in malleable cast iron is in the form of clusters, flocs, and sometimes a small amount of spheroids, which has less damage to the matrix than flake graphite. Compared with gray cast iron, malleable cast iron has better strength, plasticity, toughness, and elongation. According to the matrix structure, malleable cast iron can be divided into ferritic malleable cast iron and pearlitic malleable cast iron, which is usually used to cast thin-walled parts with complex shapes and in environments that need to withstand vibration and shock loads. However, in terms of casting performance, due to the poor fluidity of the molten iron, it is easy to cause insufficient pouring and shrinkage holes, and the tendency of hot cracking is serious, making it difficult for malleable cast iron to obtain thin-walled castings with complete contours. At the same time, due to the rapid solidification of thin-walled castings, deformation and cracking caused by internal stress are common defects in casting production.
The malleable cast iron connector is cast using sand molds, the shell core material is phenolic resin, and the molding sand is silica sand. During the casting production, it is found that the casting has defects such as insufficient pouring and cracks, resulting in a low yield rate. Therefore, improving the casting process of the connector and increasing the yield rate of the casting is the focus of this study. In this paper, the ProCAST numerical simulation software is used to simulate and analyze the casting process of the connector. By visualizing the filling and solidification process of the casting, the changes in the flow field and temperature field inside the casting during this process are obtained, and the causes of insufficient pouring of the casting are analyzed. By studying the influence of the residual indenter, pouring temperature, and riser position on the filling and solidification process of the casting, a set of the best casting process design scheme is finally formed. At the same time, the stress and deformation of the casting during the solidification process are simulated and analyzed to predict that the casting will not produce large stress concentration and deformation under the conditions of this production process, ensuring that the casting is not easy to crack due to internal stress during the subsequent heat treatment and correction process.
2.Design of the Connector Casting Process
2.1 Part Structure and Material
The connector is 157 mm long, 100 mm wide, 120 mm high, with a wall thickness of 8 mm and a bottom plate thickness of 7 mm. There are four hollow bosses of the same size on one side, and a through hole with a diameter of 13 mm on the end face. The main body is a thin-walled structure with multiple mutations from thin walls to thick walls, and the part structure is shown in Figure 1.
The material of the connector is KTH330 – 08, and the chemical composition is shown in Table 1. Its liquidus temperature is 1,247 °C, and the solidus temperature is 1,151 °C. The molten metal is smelted by a medium-frequency induction furnace using 50% scrap steel and 50% recycled materials, and a carburizer is added to adjust the carbon content. The tapping temperature is about 1,480 – 1,520 °C. Before tilting the furnace to pour the steel, bismuth – aluminum is added to the bottom of the ladle for composite inoculation treatment to promote the formation of as-cast white-mouth structure and shorten the annealing time.
Table 1: Chemical Composition of KTH330 – 08 (w/%)
| Element | C | Si | Mn | S | P |
|---|---|---|---|---|---|
| Content | 2.6 – 2.8 | 1.4 – 1.6 | 0.4 – 0.6 | ≤0.18 | ≤0.12 |
2.2 Preliminary Design of the Casting Process
In order to facilitate molding and core making, the pouring position and parting surface of the connector are shown in Figure 2. The parting surface is placed at the largest cross-section of the casting, and the pouring position is selected to match the parting surface.
The pouring system is the channel through which the molten metal enters the mold cavity, and its design depends on the structure of the casting, technical characteristics, alloy type, structure type of the pouring system, and the introduction position of the molten metal. For malleable cast iron castings, the molten iron mostly enters the mold cavity through the sprue, runner, and blind riser. For the cross-sectional area of the inner gate in the pouring system, based on the simplified and combined fluid mechanics calculation formula, combined with practical experience and actual production, the following formula is proposed: AN = x√Gc / √Hp (1)
where AN is the cross-sectional area of the inner gate, cm²; Gc is the mass of the casting, kg; Hp is the average static pressure head height, mm; x is the empirical coefficient.
In order to increase the production capacity of the connector in this foundry, a casting process of one box with two pieces is adopted, so the total mass of the pouring system and the casting is about 5.14 kg. According to the design principle of the closed pouring system, the cross-sectional areas of the sprue, runner, and inner gate are 228, 171, and 114 cm², respectively. The preliminary design scheme of the casting process is shown in Figure 3(a).
The sand core is mainly used to form the inner holes, cavities, and some external shapes of the casting that are not easy to mold and remove the sand. The design should consider minimizing the number of sand cores, reasonably designing the shape of the complex sand core, and keeping the parting surface of the sand core consistent with the parting surface of the mold to facilitate core setting and mold closing. The sand core is positioned and fixed in the sand mold by the core print. For this sand core, the center of gravity of the sand core and the line of action of the buoyancy need to be set on the edge of the core print, so three core prints need to be designed. The size of the core print depends on the size of the corresponding hole and groove. The sand core is produced using the hot box process, and the shape of the core print is determined by the shape of the bottom hole and the cylinder, which is two annular and one approximately oval core print. According to the process design principle, the length of the larger cylindrical core print is 70 mm, and the length of the smaller cylindrical core print is 35 mm. The structure of the sand core is shown in Figure 3(b).
2.3 Numerical Simulation and Process Improvement
Simulating and analyzing the casting process in the design stage can save a lot of time costs and help casting engineers quickly find the deficiencies in the casting process. It is of great significance to reduce the number of production trials and quickly formulate a reasonable casting process. The pouring temperature is set at 1,350 °C, and the initial temperatures of the casting and the mold are set at 20 °C. The mold material is set as silica sand, and the sand core material is phenolic resin sand. For the heat transfer coefficient of the mathematical model of the solidification process, referring to the recommended value of HTC in the ProCAST user manual, the heat transfer coefficient between the casting and the mold is set at 600 W/m²·K, and the heat transfer coefficient between the mold and the air is set at 10 W/m²·K.
The numerical simulation results show that there is a macroscopic shrinkage hole at the connection between the bottom plate of the solidified connector and the inner gate, as shown in Figure 4. When the pouring system is removed, this defect will cause the surface of the casting to be missing, resulting in scrap. Therefore, the casting process needs to be improved by adding a riser to eliminate the defect. The design of the riser for cast iron parts needs to be based on the characteristics of the post-feeding of the pouring system and the self-feeding of the graphite phase transformation expansion. By adding a riser to compensate for the insufficient molten metal in the post-feeding and self-feeding, the new casting process is shown in Figure 5.
2.4 Analysis of Defects in the New Casting Process Trial Production
After the trial production of the above casting process, it is found that there is an insufficient pouring defect at the boss of the connector. This defect is difficult to remedy, resulting in the direct scrapping of the casting. The insufficient pouring usually appears at the upper part of the casting, and the incomplete part is a smooth semicircle. The cause is usually that the amount of molten metal in the ladle is insufficient or the pouring speed is too fast, causing the molten metal to overflow from the sprue or riser, resulting in an early stop of pouring. This kind of defect usually appears at the far end of the casting from the gate.
As shown in Figure 6, the insufficient pouring defect mainly occurs at the cylindrical boss on the upper surface of the connector during pouring, and there is no such defect on the other side at the same height. The preliminary analysis is that due to the thin-walled structure of the casting, the temperature of the molten metal reaching this point is lower, the fluidity is weakened, and the obstruction of the sand core at this point makes the casting appear insufficient pouring at this point.
3.Optimization of New Casting Process Parameters
In the following, three measures, including increasing the residual indenter, increasing the pouring temperature, and adjusting the position of the blind riser, will be used to solve the insufficient pouring defect, and the best solution will be selected.
3.1 Residual Indenter
The residual indenter can ensure that the molten metal can fill the farthest and highest part of the casting from the sprue, obtaining a casting with a clear outline and complete shape. The height from the highest point of the casting to the liquid level in the pouring cup must be greater than or equal to the minimum residual indenter height hM, that is, the sprue must have a sufficient height. Due to the many thin-walled structures of the connector, the temperature of the molten metal will rapidly decrease after it flows into the mold cavity, resulting in a decrease in fluidity. At the same time, there are structures that hinder the flow of the molten metal at the distal end of the casting. Therefore, the originally designed minimum indenter may be insufficient, causing insufficient pouring at the distal end of the casting, and it is necessary to extend the length of the sprue. Considering the specifications of the sand box used in the factory, two sets of simulation calculations are designed. One group uses the original sprue height, as shown in Figure 7; the other group uses a height 10 mm higher than the original sprue height, as shown in Figure 8.
By simulating and analyzing the filling process of the connector, the distribution cloud map of the filling time at different parts of the casting is obtained, as shown in Figures 7(a) and 8(a). The color from purple to red in the color card represents the time when the molten metal fills this position, and red is the position where the molten metal finally reaches. After the molten metal enters the mold cavity, due to the structure of the casting and the decrease in the temperature of the molten metal, the flow speed of the molten metal decreases, and the pressure provided by the pressure head causes the liquid level in the mold cavity to rise slowly. The boss on the upper surface of the casting is the last part to be filled by the molten metal. In the actual pouring process, due to the continuous decrease in the temperature of the molten metal, the molten metal stops flowing before reaching the upper surface boss, resulting in an insufficient pouring defect at this point. When the indenter is increased, the molten metal has sufficient filling pressure after entering the mold cavity, and the liquid level in the mold rises rapidly. By intercepting the section of the upward-facing connector boss, the solidification time at this position after lengthening the sprue is later than the solidification time of the original sprue, as shown in Figures 7(b) and 8(b). Compared with the original indenter height, the lengthened sprue will make the molten metal fill the distal end of the casting earlier, ensuring a clear outline and complete shape of the casting.
3.2 Pouring Temperature
In casting production, increasing the pouring temperature can ensure that the molten metal has better fluidity, which is beneficial to the aggregation and floating of inclusions, reducing the defects of pores and slag inclusions in the casting. However, if the pouring temperature is too high, the surface quality of the casting will be affected. Too high a pouring temperature will intensify the evaporation of moisture and the combustion of organic matter in the mold, generating a large amount of gas. At the same time, the molten metal is also more likely to react with the mold material to form compounds, and finally, there will be pores or sand inclusions on the surface of the casting. Considering the thin-walled structure of the casting and the production situation of the foundry, four pouring temperature control groups of 1,350, 1,360, 1,370, and 1,380 °C are set.
Due to the thin-walled structure of the connector, the molten metal cools rapidly after entering the mold cavity. As the temperature of the molten metal decreases, its solid fraction increases, and the filling ability of the molten metal decreases, making it difficult to fill the mold cavity, which is prone to cause insufficient pouring and cold shut defects. The relationship between the solid fraction of KTH330 – 08 and the temperature is shown in Figure 9. When the temperature of the molten metal is lower than 1,159 °C, as the temperature decreases, the solid fraction of the molten metal increases rapidly, and its fluidity will decrease sharply. When the temperature of the molten metal is higher than 1,159 °C, the solid fraction of the molten metal is relatively small, and the fluidity of the molten metal is better. When the temperature of the molten metal is lower than 1,159 °C, it is approximately considered that the molten metal loses its fluidity. From the perspective of the casting structure, the structures on both sides of the upper surface of the casting are different. One side is a hollow boss structure, and the other side is a smooth surface. When the solid – liquid phase fraction of the molten metal increases, the fluidity decreases. At the same time, the filling of the molten metal is hindered by the sand core, and it is not easy to obtain a casting with a complete outline.
A point where the casting is prone to insufficient pouring is selected (Figure 10(a)), and by extracting the temperature of this point during filling at different pouring temperatures, the solid fraction of the molten metal at this moment is obtained to predict whether there will be an insufficient pouring defect at this point. When the pouring temperature is 1,350 °C, when the molten metal fills the distal end of the upper surface of the casting, as shown in Figure 10(a), the temperature extracted at this point is 1,158 °C. Comparing with the solid fraction change curve, the solid fraction at this time is 53%, and the molten metal is mostly solidified, and it basically loses its fluidity. When the transfer or continuous pouring time in the casting production process is too long, the actual pouring temperature will drop, and it is easy to make the fluidity of the molten metal decrease when it reaches the distal end, and it cannot fill the mold cavity. When the pouring temperature is increased to 1,360 °C, as shown in Figure 10(b), the temperature of the molten metal in all parts of the casting is increased to a certain extent. At this time, the temperature at the extraction point of the molten metal is 1,163 °C, and the solid fraction is 42.3%. The molten metal has a certain fluidity and can smoothly fill the mold cavity under the action of the indenter. When the filling temperature is 1,370 °C, as shown in Figure 10(c), the temperature at the extraction point is 1,173 °C, and the solid fraction is 39.2%. When the pouring temperature is increased to 1,380 °C, as shown in Figure 10(d), the temperature of all parts of the casting is significantly increased, and the temperature at the extraction point is 1,186 °C, and the solid fraction is 35%.
Figure 11 is the cloud map of the solidification time at the cross-section of the upward boss of the connector at the end of filling at different pouring temperatures. By analyzing the solidification process of the molten metal after entering the mold cavity, it can be used as a basis for improving the process. In Figure 11, when the pouring temperature of the molten metal is 1,350 °C, the molten metal at the boss almost solidifies immediately after filling, and the metal solidified earlier on the mold wall will hinder the continuous filling of the molten metal into the mold cavity, especially at this time, the temperature of the molten metal is already very low, the solid fraction is high, and the fluidity is seriously reduced, which is extremely prone to insufficient pouring and cold shut defects. With the increase of the pouring temperature, the temperature of the molten metal reaching this position also increases, the solidification time is prolonged, which is conducive to the continuous filling of the molten metal into the mold cavity.
As can be seen from Figure 9, when the temperature of the molten metal is greater than 1,160 °C, the change curve of the solid fraction with the temperature tends to be flat. Therefore, it is necessary to ensure that the solid fraction of the molten metal when filling to the distal end of the casting is lower than 0.5%, that is, the temperature of the molten metal at this point is higher than 1,160 °C. Therefore, in order to avoid the insufficient pouring and cold shut defects at the boss of the connector, the pouring temperature should be selected as (1,370 ± 5) °C.
3.3 Riser Position
Adding a riser near the place where the casting is prone to insufficient pouring will provide sufficient filling pressure for the molten metal and reduce the tendency of insufficient pouring. However, adding a new riser will reduce the process yield of the casting and increase the production cost. Therefore, considering achieving the same purpose by adjusting the position of the riser, the adjustment scheme of the riser position is shown in Figure 12.
As shown in Figure 13(a), after adjusting the position of the riser, the distribution cloud map of the filling time of different parts of the casting is found. After changing the position of the riser, the inner gate is far away from the sprue, and the sprue cannot provide sufficient filling pressure for the flow of the molten metal. At the same time, as shown in Figure 13(b), after adjusting the position of the riser, the original solidification sequence of the casting will be changed, and shrinkage porosity defects will be generated at the original inner gate. Compared with the original casting process, it is more detrimental to the quality of the casting. Therefore, adjusting the position of the riser is an improper casting process improvement measure.
Through the comprehensive analysis of the simulation results of adjusting the residual indenter, pouring temperature, and riser position, increasing the height of the residual indenter and increasing the pouring temperature are effective ways to eliminate the insufficient pouring defect of the connector.
4. Prediction of Shrinkage Porosity and Internal Stress in the New Casting Process
4.1 Simulation Analysis of Shrinkage Porosity and Shrinkage of the Casting
During the solidification process of the casting, if the feeding system is not designed properly, shrinkage porosity will be formed at the hot spots. According to its distribution characteristics, it can be divided into concentrated shrinkage porosity and dispersed shrinkage porosity, and the fine and dispersed shrinkage porosity is called shrinkage. Castings with serious shrinkage porosity are prone to cracks during solidification and heat treatment. Shrinkage is formed in the later stage of casting solidification. Due to the low temperature gradient of the residual molten metal at this time, the molten metal will solidify in a simultaneous solidification manner. A large number of fine equiaxed crystals nucleated in the molten metal will divide the residual molten metal into many closed small molten pools, and these small molten pools cannot be compensated by the molten iron during the further cooling and solidification process, and finally a large number of dispersed micropores will be generated in this area. Shrinkage porosity is a large and concentrated pore generated during the final solidification of the casting. Alloys with eutectic composition, pure metals, and castings solidified in a layer-by-layer solidification manner are prone to such defects. The fundamental reason for the formation of shrinkage porosity is that the sum of the liquid shrinkage and solidification shrinkage of the molten metal during the solidification process of the casting is much greater than its solid shrinkage. Using the shrinkage porosity calculation module in the ProCAST numerical simulation software, the new casting process under the selected process parameters is simulated and analyzed, and the distribution cloud map of the shrinkage porosity is shown in Figure 14.
4.2 Simulation Analysis of Internal Stress of the Casting
During the solidification cooling, shakeout cleaning, heat treatment, repair welding, handling, and machining of the casting, when the structural design or processing technology of the casting is improper, the casting will deform or crack under the combined action of temperature, external force, and internal stress, resulting in the casting dimensions not conforming to the drawings or the casting being scrapped. During the solidification cooling stage of the casting, due to the different cooling rates of different parts of the casting and the resulting differences in tissue transformation, the shrinkage of the casting is hindered, and finally, internal stress is generated inside the casting. In thin-walled castings with complex structures, due to the rapid solidification of the casting as a whole, the solidification and phase transformation time of each part of the casting is short, and a large internal stress will be generated inside the casting. When the internal stress exceeds the tensile strength of the material at the current temperature, cracks will be generated inside the casting.
The connector is generally a thin-walled hollow structure with uneven wall thickness. A large internal stress will be generated inside the casting during the solidification process, which may cause deformation or cracking of the casting. Since the part is not allowed to be repaired by welding, the cracked casting will be directly scrapped. Therefore, before mass production, the stress simulation analysis of the solidification process of the casting is required to explore the stress and strain situation of the new casting process under the selected process parameters, which is of great significance for the quality control of the casting in mass production.
During the solidification process of the casting, the distribution of its internal stress will continuously change with temperature. Two time nodes, 60 and 1,000 s after filling, are selected to analyze its stress distribution, as shown in Figures 15 – 16. When 60 s after filling, the overall solid volume fraction of the casting is close to 88%, and the casting has basically solidified, with only some molten metal remaining at the riser and the pouring basin. As shown in Figure 15, the internal stress of the casting at this time is small, and the maximum stress is located at the inner gate, with a value of 60 MPa. When 1,000 s after filling, the casting has cooled to about 200 °C, and the internal stress at the connection of each sprue is the largest, with a value of about 200 MPa, as shown in Figure 16. Most of the stress of the casting is lower than 40 MPa, and the internal stress at the connection between the cylinder wall and the bottom plate is relatively large, with a value of about 106 MPa. At the inner gate, the stress value reaches 160 MPa. The material of the casting is KTH330 – 08, and its tensile strength change curve with temperature is higher than the maximum value of the internal stress during the solidification process of the casting. Therefore, the casting will not crack due to internal stress during the production process. The deformation simulation analysis of the connector is shown in Figure 17. The deformation of the cylinder wall is relatively large, and the deformation at the mouth of the cylinder is about 2 mm. Since the casting is made of malleable cast iron and has certain plasticity, the deformation can be corrected in the subsequent process.
5. Conclusion
(1) When the height of the sprue is increased by 10 mm, the filling process of the molten metal is more smooth, and the speed at which the molten metal flows into the cylindrical boss of the connector is faster.
(2) When the pouring temperature is (1,370 ± 5) °C, the fluidity of the molten metal during filling is better, and it can still smoothly fill the mold cavity when reaching the place prone to insufficient pouring defects.
(3) The shrinkage porosity, stress, and strain of the new casting process are simulated and analyzed. After the casting is solidified, most of the stress values are lower than 40 MPa, and the internal stress at the connection between the cylinder wall and the bottom plate is relatively large, with a value of about 106 MPa.