1. Introduction
Connectors made of KTH330 – 08 malleable cast iron are widely used in various structures such as road and railway bridges, tower masts, pipeline supports, and lifting machinery. Their excellent corrosion resistance, convenient transportation, and easy installation make them a popular choice. However, during the casting process of these connectors, issues like low production efficiency and high defect rates, including misruns and cracks, have become major concerns. This study aims to address these problems by developing an optimized one – box – two – piece casting process, using ProCAST numerical simulation software for in – depth analysis and improvement.
2. Characteristics of KTH330 – 08 Connectors
2.1 Component Structure
The KTH330 – 08 connector has a length of 157 mm, a width of 100 mm, and a height of 120 mm. It features a wall thickness of 8 mm and a bottom plate thickness of 7 mm. On one side, there are 4 identical hollow bosses, and on the end face, there is a through – hole with a diameter of 13 mm. The structure is a thin – walled one with multiple transitions from thin to thick sections, as shown in Figure 1.
| Feature | Dimension |
|---|---|
| Length | 157 mm |
| Width | 100 mm |
| Height | 120 mm |
| Wall Thickness | 8 mm |
| Bottom Plate Thickness | 7 mm |
| Number of Hollow Bosses | 4 |
| Diameter of Through – Hole | 13 mm |
2.2 Material Composition and Properties
The KTH330 – 08 material is a type of malleable cast iron. “KTH” represents black – heart malleable cast iron, “330” indicates the minimum tensile strength of 330 MPa, and “08” represents the minimum elongation after fracture of 8%. Its chemical composition is shown in Table 1.
| Element | Content (%) |
|---|---|
| C | 2.6 – 2.8 |
| Si | 1.4 – 1.6 |
| Mn | 0.4 – 0.6 |
| S | ≤ 0.18 |
| P | ≤ 0.12 |
Table 1: Chemical Composition of KTH330 – 08
The liquidus temperature of KTH330 – 08 is 1247 °C, and the solidus temperature is 1151 °C. It is melted in an intermediate – frequency induction furnace using 50% scrap steel and 50% recycled materials. A carburetant is added to adjust the carbon content, and the tapping temperature ranges from 1480 to 1520 °C. Before pouring, a bismuth – aluminum compound inoculant is added to the bottom of the ladle to promote the formation of as – cast white – mouth structure and shorten the annealing time.
3. Initial Casting Process Design
3.1 Pouring Position and Parting Surface
To simplify the molding and core – making processes, the pouring position and parting surface of the connector are designed as shown in Figure 2. The parting surface is set at the largest cross – section of the casting, and the pouring position is selected to match the parting surface.
3.2 Gating System Design
The gating system is the passage for the molten metal to enter the mold cavity. For malleable cast iron parts, the molten iron usually enters the cavity through the sprue, runner, and blind riser. The cross – sectional area of the ingate can be calculated using the following formula: \(A_{in}=\frac{x\sqrt{G_{c}}}{\sqrt{H_{p}}}\) where \(A_{in}\) is the cross – sectional area of the ingate (\(cm^2\)), \(G_{c}\) is the mass of the casting (kg), \(H_{p}\) is the average static pressure head height (mm), and x is an empirical coefficient.
To increase the production capacity of the foundry, a one – box – two – piece casting process is adopted. The total mass of the gating system and the casting is approximately 5.14 kg. According to the design principles of the closed gating system, the cross – sectional areas of the sprue, runner, and ingate are 228 \(cm^2\), 171 \(cm^2\), and 114 \(cm^2\) respectively. The initial casting process design is shown in Figure 3(a).
3.3 Core Design
The sand core is mainly used to form the inner holes, cavities, and some external shapes of the casting that are difficult to mold and remove sand. When designing the sand core, principles such as minimizing the number of sand cores, manufacturing complex sand cores in sections, and keeping the core – box surface consistent with the parting surface for easy core setting and mold closing should be considered.
The sand core of this connector needs 3 core prints for positioning and fixing in the sand mold. The size of the core prints depends on the casting process and the size of the corresponding holes and grooves. The sand core is produced by the hot – box process, and the shape of the core prints is determined by the shape of the bottom holes and cylinders, which are two ring – shaped and one approximately elliptical core prints. The length of the larger – diameter cylindrical core print is 70 mm, and the length of the smaller – diameter cylindrical core print is 35 mm. The sand core structure is shown in Figure 3(b).
4. Numerical Simulation and Initial Process Improvement
4.1 Simulation Parameters Setting
During the casting process design stage, simulation can save a lot of time and help engineers quickly identify shortcomings in the casting process. In this simulation, the pouring temperature is set to 1350 °C, the initial temperatures of the casting and the mold are both 20 °C. The mold material is silica sand, and the sand – core material is phenolic resin sand. For the heat – transfer coefficient of the solidification process mathematical model, referring to the HTC recommended values in the ProCAST user manual, the heat – transfer coefficient between the casting and the mold is set to \(600 ~W / m^{2} \cdot K\), and the heat – transfer coefficient between the mold and the air is set to \(10 ~W / m^{2} \cdot K\).
4.2 Simulation Results and Defect Analysis
The numerical simulation results show that there are macroscopic shrinkage holes at the connection between the bottom plate of the solidified connector and the ingate, as shown in Figure 4. When the gating system is removed, this defect will cause the surface of the casting to lack material, resulting in scrap products.
4.3 Process Improvement Measures
To eliminate this defect, risers are added to the casting process. The design of the risers for cast iron parts is based on the characteristics of post – feeding of the gating system and self – feeding of graphite phase – change expansion. The new casting process is shown in Figure 5.
5. Defect Analysis in Trial Production of the Improved Process
5.1 Defect Phenomenon
After trial – production of the improved casting process, misruns are found at the bosses of the connector. This defect is difficult to repair and directly leads to the scrapping of the casting. Misruns usually occur at the upper part of the casting, and the defective part is in the shape of a smooth semi – circle.
5.2 Cause Analysis
As shown in Figure 6, misruns mainly occur at the upward – facing cylindrical – wall bosses during the pouring of the connector, while the other side at the same height does not have such defects. Preliminary analysis indicates that due to the thin – walled structure of the casting, the temperature of the molten metal reaching this area is low, and its fluidity is weakened. In addition, the obstruction of the sand core at the bosses also contributes to the occurrence of misruns.
6. Optimization of New Casting Process Parameters
6.1 Residual Head
The residual head can ensure that the molten metal can fill the farthest and highest parts of the casting from the sprue, obtaining a casting with a clear outline and complete shape. Since the connector has many thin – walled structures, the temperature of the molten metal drops rapidly after flowing into the mold cavity, resulting in a decrease in fluidity. Also, there are structures at the far end of the casting that hinder the flow of the molten metal. Therefore, the original designed minimum head may be insufficient, causing misruns at the far end of the casting.
Two groups of simulation calculations are designed considering the specifications of the sand boxes used in the factory. One group uses the original sprue height, and the other group uses a sprue height 10 mm higher than the original. The simulation results of the filling time and solidification time at the boss section are shown in Figure 7 and Figure 8.
| Simulation Group | Sprue Height | Filling Time at Boss (s) | Solidification Time at Boss (s) |
|---|---|---|---|
| Original | Original height | [Value from Figure 7(a)] | [Value from Figure 7(b)] |
| Improved | Original height + 10 mm | [Value from Figure 8(a)] | [Value from Figure 8(b)] |
6.2 Pouring Temperature
In casting production, increasing the pouring temperature can ensure better fluidity of the molten metal, which is beneficial for the aggregation and floating of inclusions and reducing defects such as pores and slag inclusions in the casting. However, if the pouring temperature is too high, it will affect the surface quality of the casting.
For the thin – walled structure of the connector, four pouring – temperature control groups of 1350 °C, 1360 °C, 1370 °C, and 1380 °C are set. The relationship between the solid – phase fraction of KTH330 – 08 and temperature is shown in Figure 9.
When the temperature of the molten metal is lower than 1159 °C, the solid – phase fraction of the molten metal rises rapidly with the decrease in temperature, and its fluidity drops sharply. When the temperature is higher than 1159 °C, the solid – phase fraction is relatively small, and the fluidity is better.
By selecting a point where misruns are likely to occur in the casting and extracting the temperature of the molten metal at this point during filling at different pouring temperatures, the solid – phase fraction can be obtained to predict whether misruns will occur. The temperature distribution and solid – phase fraction at different pouring temperatures are shown in Table 2.
| Pouring Temperature (°C) | Temperature at Extraction Point (°C) | Solid – Phase Fraction at Extraction Point (%) | Filling Status |
|---|---|---|---|
| 1350 | 1158 | 53 | Almost solidified, difficult to fill |
| 1360 | 1163 | 42.3 | Can fill with certain fluidity |
| 1370 | 1173 | 39.2 | Good fluidity for filling |
| 1380 | 1186 | 35 | Good fluidity for filling |
Table 2: Temperature and Solid – Phase Fraction at Different Pouring Temperatures
The solidification – time cloud maps of the upward – facing boss section at the end of filling at different pouring temperatures are shown in Figure 11. As the pouring temperature increases, the temperature of the molten metal reaching the boss position also increases, and the solidification time is extended, which is beneficial for the molten metal to fill the mold cavity. To avoid misruns and cold shuts at the bosses of the connector, the pouring temperature is preferably selected as (1370 ± 5) °C.
6.3 Riser Position
Adding a riser near the area where misruns are likely to occur can provide sufficient filling pressure for the molten metal and reduce the tendency of misruns. However, adding new risers will reduce the casting process yield and increase production costs. Therefore, adjusting the riser position is considered.
The adjustment scheme of the riser position is shown in Figure 12. The simulation results after adjusting the riser position are shown in Figure 13. It is found that after changing the riser position, the ingate is far from the sprue, and the sprue cannot provide sufficient filling pressure for the flow of the molten metal. At the same time, the original solidification sequence of the casting is changed, and shrinkage – hole defects occur at the original ingate. Therefore, adjusting the riser position is not an appropriate measure to improve the casting process.
6.4 Comprehensive Analysis of Optimization Measures
By comprehensively analyzing the simulation results of adjusting the residual head, pouring temperature, and riser position, it is concluded that increasing the residual – head height and the pouring temperature are effective measures to eliminate the misrun defects of the connector.
7. Prediction of Shrinkage Holes and Internal Stress in the New Casting Process
7.1 Simulation Analysis of Shrinkage Holes and Porosity
Shrinkage porosity and shrinkage holes are common defects in the casting process. Shrinkage porosity occurs in the late stage of casting solidification. When the heat – transfer system design is improper, shrinkage holes are formed at the hot spots. Shrinkage holes are large and concentrated pores that occur in the final solidification process of the casting.
The ProCAST numerical – simulation software’s shrinkage – porosity and shrinkage – hole calculation module is used to analyze the new casting process with the selected process parameters. The shrinkage – porosity and shrinkage – hole distribution cloud maps are shown in Figure 14.
7.2 Simulation Analysis of Internal Stress in the Casting
During the solidification, cooling, shake – out, heat – treatment, repair welding, handling, and machining of the casting, if the casting structure design or processing technology is improper, the casting may deform or crack under the combined action of temperature, external force, and internal stress, resulting in the size of the casting not conforming to the drawing or even scrapping.
The connector is a thin – walled hollow structure with uneven wall thickness, and large internal stresses will be generated during the solidification process, which may cause deformation or cracking of the casting. Since the parts do not allow repair welding, cracked castings will be directly scrapped. Therefore, it is of great significance to simulate and analyze the stress during the solidification process of the casting before mass production.
