Casting defects, particularly cracks in cylinder blocks, significantly impact structural integrity and operational reliability. This article explores the root causes and mitigation strategies for cold cracks observed in a 16L natural gas engine cylinder block casting during trial production.
1. Problem Characterization
The cracks manifested between ventilation ports (Fig. 3a) as continuous linear fractures with bluish-black oxidized surfaces (Fig. 4). Mechanical testing revealed nominal tensile strength (240–280 MPa) and hardness (180–240 HB) within HT250 specifications, eliminating material degradation as the primary cause.
2. Computational Stress Analysis
Solidification simulation identified critical thermal gradients and residual stresses using the cold crack criterion:
$$
R = \frac{\sigma_{\text{thermal}}}{\sigma_{\text{UTS}}(T)}
$$
Where:
- $R$ = Crack risk factor
- $\sigma_{\text{thermal}}$ = Thermal stress (MPa)
- $\sigma_{\text{UTS}}(T)$ = Temperature-dependent ultimate tensile strength (MPa)
| Temperature (°C) | σUTS (MPa) | Critical R Value |
|---|---|---|
| 600 | 69–131 | >1.2 |
| 400 | 220–260 | <0.8 |
3. Key Contributing Factors
Three primary mechanisms exacerbated casting defect formation:
- Structural Constraints:
$$
\sigma_{\text{constraint}} = E \cdot \alpha \cdot \Delta T \cdot (1 – \nu)^{-1}
$$
Where ν = Poisson’s ratio - Core Restriction: Original resin content (1.5%) created high core strength (1.8 MPa)
- Process Parameters:
Parameter Initial Optimized Carbon Content (%) 3.12 3.20 Pouring Temp (°C) 1,440 1,420 Cooling Rate (°C/min) 28 22
4. Mitigation Strategies
Four technical solutions addressed the casting defect:
- Geometric Optimization:
- 20% cross-sectional area increase at stress concentration zones
- Ventilation port radius modification (R3→R5)
- Core System Redesign:
- Resin reduction (1.5%→1.2%)
- Hollow core architecture implementation
- Process Modifications:
- Extended molten metal holding at 1,500°C (15min)
- Controlled shakeout temperature (<400°C)
- Material Enhancement:
- Premium carbon additive utilization
- Mn/S ratio optimization (4:1→5:1)
5. Verification and Production
Post-optimization simulations demonstrated:
| Parameter | Pre-Optimization | Post-Optimization |
|---|---|---|
| Max Stress (MPa) | 148 | 112 |
| Crack Risk Factor | 1.35 | 0.92 |
Production trials confirmed complete elimination of casting defects across 500+ castings, validating the technical approach. The comprehensive strategy reduced residual stresses by 24.3% while maintaining dimensional accuracy within CT10 specifications.
This case study demonstrates that systematic analysis of casting defects through computational modeling and process parameter optimization can effectively resolve complex production challenges in heavy-duty engine components.