In the production of a filter shell for a water treatment device, I encountered severe sand casting defect issues that initially led to a scrap rate exceeding 70%. The shell is made of low-carbon steel (equivalent to ZG230-450), with a thin wall thickness of only 11 mm and a complex geometry featuring multiple local hot spots. This article presents my systematic investigation into the root causes of these sand casting defects and the successful process modifications that dramatically improved casting quality.
Original Casting Process and Defects
The original casting process is shown schematically (the thin solid line in the referenced figure). The gating system consisted of two ingates positioned at the left and right flanges, with open risers placed on top of each flange. Chills were applied at the heavy bottom section, and chromite sand was used at the ring area to enhance chilling. The mold and cores were made using VRH-CO2 sodium silicate sand vacuum hardening method, with 70/100 mesh washed sand from Inner Mongolia. The steel was melted in a 1.5-ton electric arc furnace with forced deoxidation, poured at 1560–1590 °C from a 3-ton bottom-pour ladle with a 50 mm diameter nozzle.
Despite these measures, numerous sand casting defects appeared randomly across the casting. Table 1 summarizes the observed defects and their suspected locations.
| Defect Type | Location | Frequency |
|---|---|---|
| Shrinkage porosity and cracking | Random in cope and drag | Severe |
| Cold shut, misrun, gas entrapment | Upper mold surface | Frequent |
| Large-area flow marks | Upper mold surface | Common |
| Incomplete fusion (joint flash) | Parting line | Occasional |
These sand casting defects made the castings almost unusable without extensive repair. The most critical sand casting defect was the combination of shrinkage and hot tearing at the thin-wall sections, caused by poor feeding and non-uniform solidification.
Process Analysis
I analyzed the root causes of these sand casting defects through several perspectives:
- Gating system limitations: The original ingates were too small and too concentrated. The steel could not fill the cavity quickly enough, especially after the melt rose above the parting line where the cross-section expanded rapidly. This led to cold shuts and flow marks — typical sand casting defects caused by premature solidification.
- Localized overheating: Introducing melt only through the two flanges created local hot spots at those areas, while other regions remained cooler. This non-uniform temperature distribution promoted shrinkage porosity and hot tearing — classic sand casting defects in thin-wall shells.
- Direct chills on heavy sections: Placing chills directly on the upper heavy sections caused excessive thermal shock, resulting in surface wrinkles and cracks — another sand casting defect that affected surface integrity.
- Alloy characteristics: ZG230-450 has a relatively narrow solidification temperature range (ΔT ≈ 30–40 °C), which inherently gives it a moderate hot-tearing tendency. The formation of flow marks and cold shuts is also favored by this alloy. To minimize these sand casting defects, I needed to adopt a “simultaneous solidification” principle: high pouring temperature, large flow rate, and rapid filling to ensure uniform solidification throughout the casting.
The solidification temperature range can be expressed as:
$$ \Delta T = T_{\text{liquidus}} – T_{\text{solidus}} $$
For ZG230-450, typical values are $$ T_{\text{liquidus}} \approx 1510 \,^\circ\text{C}, \quad T_{\text{solidus}} \approx 1470 \,^\circ\text{C} $$, giving ΔT ≈ 40 °C. This narrow range demands careful control to avoid sand casting defects related to mushy zone feeding.
Improved Casting Process
Based on the analysis, I redesigned the process to follow the simultaneous solidification principle. The key modifications are shown in the dashed lines in the referenced figure. Table 2 compares the original and improved process parameters.
| Parameter | Original | Improved |
|---|---|---|
| Gating system cross-section | Small, two ingates | Increased area, additional ingates at bottom heavy section |
| Risers on ring area | None | Open risers added for feeding and venting |
| Chills on upper heavy section | Direct contact | Indirect (10–15 mm sand layer between chill and mold) |
| Ear core vents | None | Three Ø15 mm vents at ear joints |
| Pouring technique | Constant flow | “Small-big-small”: initial slow, then fast, then slow |
| Pouring temperature | 1560–1590 °C | 1590 ± 30 °C (tapped at 1620 ± 30 °C) |
| Composition control | Standard | Lower limit of specification + microalloying with rare earths |
The improved gating system was designed to achieve a high filling rate. The total effective cross-sectional area increased by approximately 40% compared to the original. The flow rate during the critical mid-pouring stage can be estimated as:
$$ Q = A \cdot v $$
where A is the total ingate area and v is the melt velocity. By increasing A and maintaining a higher pouring temperature, the filling time decreased from about 45 seconds to 25 seconds, significantly reducing the risk of cold shut and flow mark sand casting defects.
To further mitigate sand casting defects, I introduced the following specific measures:
- Indirect chills: A 10–15 mm layer of molding sand was placed between the chill and the mold cavity. This reduced the thermal shock that previously caused surface cracks — a common sand casting defect at heavy sections.
- Microalloying: Rare earth elements were added to the ladle during tapping. This refined the grain structure and reduced the hot-tearing tendency, thereby suppressing shrinkage-related sand casting defects.
- Controlled pouring sequence: The “small-big-small” technique was employed: initially a slow stream to avoid turbulence, then a fast stream when the melt reached the parting line, followed by a slow stream to fill the risers. This ensured proper feeding and minimized gas entrapment — another frequent sand casting defect.
The image above illustrates typical sand casting defects found in thin-wall steel castings. Understanding each defect’s morphology helped me pinpoint the process weaknesses.
Results and Discussion
After implementing the improved process, I produced a batch of 53 shells. The results are summarized in Table 3.
| Defect Type | Original (approx. 100 units) | Improved (53 units) |
|---|---|---|
| Shrinkage porosity/crack | >50% | ~2% (minor, weldable) |
| Cold shut / misrun | ~30% | 0% |
| Flow marks | ~60% | ~5% (within acceptable limit) |
| Gas entrapment | ~20% | 0% |
| Surface cracks (heavy sections) | ~40% | ~4% (weldable) |
| Total scrap rate | >70% | < 5% |
The improvement was dramatic. The most troublesome sand casting defects — cold shuts, flow marks, and gas entrapment — were completely eliminated. Shrinkage-related sand casting defects were reduced to a few minor cases that could be repaired by welding. The surface quality of the upper mold cavity was vastly improved, with only faint flow marks visible on a small fraction of castings.
I attribute this success to the simultaneous solidification approach. The mathematical rationale can be expressed through the solidification modulus:
$$ M = \frac{V}{A} $$
where V is volume and A is surface area. For a thin-wall shell, the modulus varies significantly across the part. By ensuring rapid filling and high pouring temperature, the temperature gradient across the casting was minimized, forcing nearly simultaneous solidification. This prevented the formation of isolated liquid pools that lead to shrinkage-related sand casting defects.
Furthermore, the indirect chill approach reduced the cooling rate gradient at heavy sections. The thermal stress equation:
$$ \sigma_{\text{thermal}} = E \cdot \alpha \cdot \Delta T $$
where E is elastic modulus, α is thermal expansion coefficient, and ΔT is temperature difference. By reducing ΔT between the chill area and the surrounding mold, the thermal stress decreased, mitigating hot tearing — another type of sand casting defect that plagued the original process.
Conclusion
Through systematic analysis and modification of the sand casting process, I successfully eliminated most sand casting defects in a low-carbon steel thin-wall shell. The key was to adopt the simultaneous solidification principle: high pouring temperature, large gating system with multiple ingates, indirect chills, and controlled pouring sequence. The scrap rate dropped from over 70% to below 5%. This approach has been successfully applied to other similar thin-wall steel castings, such as large plate-like parts and box-shaped shells, consistently reducing sand casting defect incidence.
The experience underscores the importance of understanding the specific solidification behavior of each alloy and geometry. For thin-wall castings prone to sand casting defects like cold shuts and shrinkage, traditional rules of thumb (e.g., introducing metal through thin sections) may not be effective. Instead, a holistic view of filling and solidification is required to achieve defect-free castings.