In the field of mechanical engineering, casting parts serve as fundamental components in various transmission systems, often subjected to complex stress conditions, impact, wear, and corrosion during operation. To meet the demands of lightweight design, these casting parts must possess excellent mechanical properties, including sufficient strength, stiffness, and dynamic stability. High uniformity in microstructure is required, and casting defects such as porosity, slag inclusions, sand inclusions, cold shuts, and shrinkage must be avoided. Typically, wheel disc casting parts are produced using ordinary clay sand molding. Due to thermal joints at the hub and rim, risers or chills are commonly employed to prevent shrinkage defects in the hub area. This paper details the trial production of high-strength ductile iron casting parts, specifically a wheel disc, based on first-hand experience in process design and validation. The focus is on achieving defect-free casting parts through optimized casting and heat treatment processes, with repeated emphasis on the term “casting parts” to underscore their significance in industrial applications.
The wheel disc casting part under consideration was ordered for a printing equipment application, with material specifications conforming to EN-GJS-HB300 (EN1563). The casting part features uneven wall thickness, with significant variations between the thickest and thinnest sections, along with numerous holes, making the casting process challenging. The structural analysis revealed a轮廓尺寸 of 322 mm in maximum diameter and 36 mm in height, with a part weight of approximately 6.6 kg. Key sections include a rim with a wall thickness of 13 mm, a hub with 36 mm, and a web with 7 mm. Multiple holes are present, some of which are cast, while others are machined subsequently. Dimensional tolerances are set to CT11, and surface roughness requirements are stringent, ranging from Ra1.6 μm to Ra12.5 μm, necessitating precision in casting and machining. The casting weight is about 12.6 kg, including machining allowances.
From a technical perspective, the material must meet the following mechanical properties: tensile strength ≥800 MPa, yield strength ≥480 MPa, and Brinell hardness between 245 and 335 HB. These requirements dictate the need for careful control over the entire production process, from melting to heat treatment, to ensure the quality of the casting parts.
The casting process design began with the selection of the molding method. Resin sand mechanical molding was adopted, using furan resin at 1%–1.5% of the original sand mass and a low-sulfur acid catalyst at 30%–70% of the resin mass, mixed in an automatic sand mixer. Given that the wheel disc is a medium-small casting part, two pieces were arranged in a single mold box for production efficiency. This approach is common for such casting parts to optimize resource use.
Determining the parting plane and pouring position is critical for casting parts with complex geometries. For this wheel disc, the parting plane was set at the mid-height of the rim to facilitate mold assembly and core positioning, as shown in the schematic. The pouring position was chosen with the large planar surface facing downward, adhering to the principle of orienting important surfaces downward to minimize defects. The hub protrusion was placed upward, allowing risers to effectively compensate for shrinkage and vents to prevent porosity. This orientation is essential for producing sound casting parts.
The casting shrinkage rate is influenced by multiple factors, including material, structure, mold type, and riser design. Based on empirical data, a shrinkage rate of 1.0% was selected for these casting parts. This can be expressed mathematically as:
$$ \text{Shrinkage Rate} = \frac{L_{\text{pattern}} – L_{\text{casting}}}{L_{\text{pattern}}} \times 100\% = 1.0\% $$
where \( L_{\text{pattern}} \) is the pattern dimension and \( L_{\text{casting}} \) is the final casting dimension. Adjustments were made during production based on actual dimensional changes to meet drawing specifications, ensuring consistency across all casting parts.
The gating system was designed as a semi-closed system to ensure smooth metal flow, minimize oxidation, and facilitate gas expulsion from the mold cavity. Each casting part was fed by two ingates spaced 180° apart to avoid localized overheating and ensure uniform temperature distribution. The cross-sectional areas were calculated as follows: total ingate area ≈ 1,600 mm², sprue area ≈ 1,963 mm² (with a minimum diameter of 50 mm), and total runner area ≈ 2,250 mm². The ratio of these areas is a key parameter in gating design for casting parts:
$$ \sum A_{\text{sprue}} : \sum A_{\text{runner}} : \sum A_{\text{ingate}} = 1963 : 2250 : 1600 = 1.2 : 1.4 : 1 $$
This ratio promotes controlled filling and reduces turbulence, which is vital for high-quality casting parts.
Riser design focused on achieving sequential solidification to eliminate shrinkage defects. The largest thermal junction is at the hub base. According to casting principles, riser diameter should be 2.2 to 3.0 times the wall thickness or thermal node diameter. A combination of top and side risers was used: a top riser with a diameter and height of 88 mm was placed above the hub to feed the major thermal section, while side risers (50 mm in diameter and height) connected to the ingates supplemented the rim. The riser volume can be estimated using:
$$ V_{\text{riser}} = \frac{\pi d^2 h}{4} $$
where \( d \) is the riser diameter and \( h \) is the height. For the top riser, \( V \approx \frac{\pi \times 88^2 \times 88}{4} \approx 535,000 \, \text{mm}^3 \). This design ensures adequate feed metal for the casting parts, preventing internal defects.
Production validation involved meticulous control of melting, pouring, and heat treatment processes. A 1-ton medium-frequency induction furnace was used for melting. The chemical composition was tightly controlled within specified ranges, as summarized in Table 1. This control is crucial for achieving the desired microstructure and properties in ductile iron casting parts.
| Element | Target Range | Measured Values |
|---|---|---|
| C | 3.30–3.80 | 3.56, 3.56 |
| Si | 2.0–2.8 | 2.42, 2.61 |
| Mn | 0.20–0.40 | 0.28, 0.28 |
| P | ≤0.05 | 0.035, 0.035 |
| S | ≤0.03 | 0.015, 0.015 |
| Mg | 0.03–0.06 | 0.035, 0.035 |
| RE | 0.01–0.04 | 0.011, 0.011 |
| Mo | ≤0.3 | 0.18, 0.10 |
| Cu | ≤0.8 | 0.65, 0.60 |
Nodularization was performed using a rare-earth magnesium alloy via the ladle inoculation method, with an addition of 1.2%. Inoculation involved 75SiFe added in the intermediate ladle and pouring ladle at 0.2%–0.25% each to enhance graphite nucleation. The tapping temperature was around 1,550°C, and the pouring temperature was controlled between 1,360°C and 1,400°C to accommodate the thin web sections of the casting parts. The relationship between pouring temperature and fluidity can be described by:
$$ \text{Fluidity} \propto e^{-\frac{Q}{RT}} $$
where \( Q \) is the activation energy, \( R \) is the gas constant, and \( T \) is the temperature in Kelvin. Higher temperatures improve fluidity but must be balanced against oxidation and shrinkage risks for casting parts.
Heat treatment was essential to meet the high strength and hardness requirements. Austempering was employed: austenitization at 900°C for 2 hours, followed by isothermal quenching in a nitrate salt bath at 390°C for 1 hour. This process transforms the matrix into ausferrite (acicular ferrite and retained austenite), imparting high toughness and strength to the casting parts. The kinetics of austempering can be modeled using the Avrami equation:
$$ f = 1 – e^{-(kt)^n} $$
where \( f \) is the transformed fraction, \( k \) is the rate constant, \( t \) is time, and \( n \) is the Avrami exponent. For ductile iron casting parts, this treatment optimizes the microstructure for demanding applications.
The trial production yielded casting parts free from visible defects such as shrinkage, porosity, slag inclusions, or cold shuts. The physical appearance of the casting parts after heat treatment is shown below, demonstrating the success of the process design. These casting parts exemplify how meticulous engineering can produce high-integrity components.
Material testing was conducted on attached test blocks to verify compliance with specifications. The metallographic structure, presented in Table 2, shows high nodularity and the desired ausferritic matrix, which are critical for the performance of ductile iron casting parts.
| Parameter | Target | Measured Values |
|---|---|---|
| Nodularity (%) | ≥85 | 92, 93 |
| Graphite Size | 5–7 | 6, 6 |
| Matrix Structure | Ausferrite | Ausferrite + Austenite |
Mechanical properties exceeded the customer’s requirements, as detailed in Table 3. The data confirm that the casting parts possess excellent strength, ductility, and impact resistance, making them suitable for rigorous service conditions.
| Property | Target (EN-GJS-HB300) | Customer Requirement | Measured Values |
|---|---|---|---|
| Tensile Strength (MPa) | ≥800 | ≥900 | 1060, 978 |
| Yield Strength (MPa) | ≥480 | ≥600 | 670, 645 |
| Elongation (%) | ≥8 | ≥8 | 11.4, 8.2 |
| Impact Toughness (J/cm²) | – | ≥100 | 128, 127 |
| Brinell Hardness (HB) | 245–335 | 245–300 | 275, 269 |
The relationship between hardness and tensile strength for ductile iron casting parts can be approximated by empirical formulas, such as:
$$ \text{Tensile Strength (MPa)} \approx 3.45 \times \text{HB} $$
For a hardness of 275 HB, this gives approximately 949 MPa, aligning with the measured values. This correlation is useful for quality control in producing casting parts.
Further analysis involves the solidification modeling of casting parts. The Chvorinov’s rule can be applied to estimate solidification time:
$$ t = B \left( \frac{V}{A} \right)^2 $$
where \( t \) is solidification time, \( B \) is a mold constant, \( V \) is volume, and \( A \) is surface area. For the wheel disc casting parts, the modulus \( \frac{V}{A} \) varies across sections, influencing riser placement. Computational simulations could optimize this, but empirical design proved effective here.
In summary, the trial production of high-strength ductile iron casting parts was successful. The combination of a semi-closed gating system, filtered metal flow, and strategically placed risers ensured defect-free casting parts. The use of rare-earth magnesium for nodularization, coupled with secondary inoculation, achieved high nodularity and uniform microstructure. Austempering heat treatment further enhanced the mechanical properties, meeting all technical specifications. This process has been validated for batch production, with casting parts passing subsequent machining and inspection stages. The experience underscores the importance of integrated process design for complex casting parts, where every step—from mold making to heat treatment—must be meticulously controlled. Future work could explore advanced simulation tools to refine riser design and reduce material usage, further optimizing the manufacturing of such casting parts. Overall, this project demonstrates that with proper engineering, high-performance casting parts can be consistently produced for demanding applications, contributing to advancements in the field of metal casting.