In steel casting production, orange peel defects remain a persistent challenge for large castings (>10 tons), particularly in high-alloy components requiring pouring temperatures of 1,540–1,580°C. These defects manifest as irregular 1–2 mm deep dimples (Figure 1) that compromise surface quality and ultrasonic testing reliability. Our research identifies the root cause as chemical interactions between alumina-based coatings (Al2O3 ≥98%) and molten steel, quantified through elemental analysis:
| Element | Normal Surface (%) | Defective Area (%) |
|---|---|---|
| Al | 0.08–0.12 | 5.7–6.3 |
| Fe | Balance | Balance |
| Cr | 1.2–2.5 | 1.1–2.3 |
The chemical reaction mechanism at high temperatures can be expressed as:
$$3\text{FeO} + 2\text{Al}_2\text{O}_3 \rightarrow 3\text{FeAl}_2\text{O}_4 + \text{O}_2↑$$
To address this, we developed a multi-layer coating system combining silica sand (SiO2), chromite sand (FeCr2O4), and zircon flour (ZrSiO4). The coating stack configuration follows:
$$T_{\text{total}} = \sum_{i=1}^{n} t_i \cdot \rho_i$$
Where Ttotal = total thermal resistance, ti = layer thickness, and ρi = thermal resistivity.
| Coating Type | Thickness (mm) | Thermal Resistivity (m·K/W) | Cost Index |
|---|---|---|---|
| Zircon (3 layers) | 0.6–0.9 | 2.8×10-3 | 1.5 |
| Zircon (2 layers) | 0.4–0.6 | 1.9×10-3 | 1.0 |
| Alumina (2 layers) | 0.4–0.6 | 0.7×10-3 | 0.8 |
Experimental results demonstrate that triple-layer zircon coatings eliminate >95% of orange peel defects in steel castings, achieving surface roughness (Ra) improvements:
$$R_{a,\text{new}} = 0.82 \cdot e^{-0.35n}$$
Where n = number of zircon layers (1 ≤ n ≤3). Post-treatment costs decreased by 42% through reduced grinding requirements.
For cost-sensitive applications, dual-layer zircon coatings provide sufficient protection when combined with chromite sand backing:
$$C_{\text{total}} = C_{\text{material}} + k \cdot C_{\text{post-processing}}$$
Where k = 0.58 for dual-layer vs. 0.22 for triple-layer systems.
Implementation in production-scale steel casting shows:
- 98.7% defect-free surfaces with triple-layer zircon
- 87.4% improvement in UT qualification rates
- 31% reduction in finishing labor hours
This coating strategy enhances steel casting quality while maintaining economic viability through adaptable layer configurations. Future work will optimize zircon content for specific alloy systems using machine learning models.