Sand casting parts made of duplex stainless steels, such as A995-5A, face significant challenges in avoiding crack defects due to the inherent formation of brittle σ-phase intermetallic compounds during slow cooling. This article systematically analyzes the root causes of cracking and proposes optimized cooling strategies to enhance the reliability of heavy-section sand casting parts.
1. Mechanism of σ-Phase Formation and Crack Propagation
The A995-5A duplex stainless steel composition (Table 1) promotes σ-phase precipitation at critical temperature ranges:
$$ t_{\sigma} = A \cdot \exp\left(\frac{Q}{RT}\right) $$
Where:
\( t_{\sigma} \) = incubation time for σ-phase formation
\( Q \) = activation energy (≈250 kJ/mol for A995-5A)
\( R \) = gas constant
\( T \) = absolute temperature
| Element | Cr | Ni | Mo | N | Fe |
|---|---|---|---|---|---|
| Content (wt%) | 24-26 | 6-8 | 4-5 | 0.1-0.3 | Bal. |
For sand casting parts with section thickness exceeding 100 mm, the critical temperature ranges are:
- σ-phase formation: 600-1000°C
- 475°C embrittlement: 400-550°C
2. Cooling Rate Optimization Strategies
Four cooling methods were evaluated for sand casting parts with 170 mm maximum thickness:
| Method | Cooling Rate (°C/min) | σ-Phase Content | Crack Frequency |
|---|---|---|---|
| Traditional Cooling | 0.5-1.2 | 8-12% | High (＞30%) |
| Forced Air Cooling | 2.5-4.0 | 3-5% | Moderate (10-15%) |
| Spray Cooling | 5.0-8.0 | 1-2% | Low (＜5%) |
| Water Quenching | ＞15 | 0.5-1% | Rare (＜1%) |
The optimized cooling protocol for sand casting parts combines:
$$ \frac{dT}{dt} \geq \frac{T_{start} – T_{end}}{t_{critical}} $$
Where \( T_{start} \) = 900°C, \( T_{end} \) = 500°C, and \( t_{critical} \) ≤ 60 minutes
3. Process Implementation for Sand Casting Parts
Key parameters for successful implementation:
- Mold Breakout Temperature: 850-900°C
- Cooling Medium Selection:
- Air velocity: 15-25 m/s
- Water spray density: 2-4 L/m²·s
- Temperature Monitoring:
$$ T_{surface} = T_0 – k\sqrt{t} $$Where \( k \) = 8-12 for sand casting parts
4. Quality Validation
Post-treatment inspection results for sand casting parts:
| Test Method | Acceptance Criteria | Pass Rate |
|---|---|---|
| PT Inspection | ASTM E165 | 98.7% |
| Ferrite Content | 40-55% | 99.2% |
| Corrosion Test | ASTM G48 | 100% |
The optimized cooling strategy enables sand casting parts to achieve:
- Yield strength: ≥550 MPa
- Impact toughness: ≥100 J (-46°C)
- PREN: ≥40
5. Conclusion
For heavy-section sand casting parts made of A995-5A duplex stainless steel, controlled accelerated cooling through forced air/spray cooling after high-temperature mold breakout proves to be the optimal solution. This methodology:
- Reduces σ-phase content below 2%
- Maintains cooling rates above 5°C/min in critical ranges
- Enables production cycle time reduction by 40-50%
Further improvements for sand casting parts could involve real-time cooling rate monitoring systems and adaptive cooling pattern control based on section thickness variations.