During the operation of a mining vehicle, a four-cylinder crankshaft fractured abruptly at the 7th and 8th crank arms after 1,269 hours of service. This study systematically investigates the root cause of the failure through macroscopic examination, microscopic analysis, and material characterization. The results reveal that invasive gas porosity—a critical casting defect—served as the primary initiation site for fatigue cracks, ultimately leading to catastrophic failure.
1. Material Characterization
The crankshaft material QT900-5 meets specified chemical requirements, as demonstrated in Table 1. The carbon equivalent (CE) is calculated using:
$$ CE = \%C + \frac{\%Si}{3} + \frac{\%P}{3} = 3.64 + \frac{2.16}{3} + \frac{0.017}{3} = 4.36 $$
This high CE value increases the risk of graphite flotation but remains within acceptable limits for heavy-section castings.
| Element | C | Si | Mn | Cu | Mg | Cr |
|---|---|---|---|---|---|---|
| Measured | 3.64 | 2.16 | 0.50 | 0.55 | 0.04 | 0.04 |
| Specification | 3.60-3.90 | 1.90-2.40 | 0.30-0.50 | 0.40-0.60 | 0.02-0.06 | ≤0.10 |
2. Fractographic Analysis
The fracture surface at the 7th crank arm exhibited three distinct zones:
- Crack initiation zone: Elliptical cavity (3×7 mm) containing oxide layers
- Propagation zone: Beach marks with crack growth rate governed by:
$$ \frac{da}{dN} = C(\Delta K)^m $$
where \( C = 1.2×10^{-10} \), \( m = 3.2 \) for QT900-5 - Instantaneous fracture zone: Cleavage facets with dimple rupture
3. Microstructural Investigation
Metallographic analysis of the defect region revealed:
- Nodular graphite (95% nodularity, size class 6)
- Matrix: 95% pearlite + 5% ferrite
- Oxide inclusions (5-10 μm thick) lining cavity walls
The gas porosity formation follows the dimensionless relationship:
$$ \frac{P_g}{P_{atm}} = 1 + \frac{\rho_m g h}{\sigma_{ys}} $$
where \( P_g \) = gas pressure, \( \rho_m \) = molten metal density, \( h \) = casting height.
| Property | Measured | Specification |
|---|---|---|
| Tensile Strength (MPa) | 916 | ≥820 |
| Yield Strength (MPa) | 521 | ≥460 |
| Elongation (%) | 7.4 | ≥4.5 |
| Hardness (HBW) | 302 | 250-320 |
4. Casting Defect Formation Mechanism
The identified casting defect originates from gas entrapment during mold filling and solidification. Key contributing factors include:
- High volatile content (2.3-2.5%) in molding sand
- Inadequate venting depth (≤30 mm)
- Residual moisture in charge materials
The gas evolution rate follows Arrhenius behavior:
$$ Q_{gas} = A e^{-E_a/(RT)} $$
where \( Q_{gas} \) = gas generation rate, \( E_a \) = activation energy (45 kJ/mol for bentonite).
5. Process Optimization
Corrective measures to prevent similar casting defects:
- Implement real-time sand volatile monitoring
- Increase venting depth to 40 mm
- Enhance melt degassing:
$$ \eta_{degas} = 1 – e^{-k\theta} $$
where \( k \) = degassing coefficient (0.25 min⁻¹), \( θ \) = holding time
6. Fatigue Life Prediction
The modified Goodman equation estimates the fatigue limit reduction due to casting defects:
$$ \frac{\sigma_a}{\sigma_e} + \frac{\sigma_m}{\sigma_{UTS}} = \frac{1}{1 – (\frac{d}{D})^{0.5}} $$
where \( d \) = defect size, \( D \) = critical defect size (8 mm for QT900-5).
7. Industrial Implementation
After implementing corrective measures, defect rates decreased by 72% as shown:
| Parameter | Pre-improvement | Post-improvement |
|---|---|---|
| Gas Porosity (%) | 4.7 | 1.3 |
| Scrap Rate (%) | 12.6 | 3.5 |
| Fatigue Life (cycles) | 2.1×10⁶ | 3.8×10⁶ |
This comprehensive analysis demonstrates that systematic control of molding parameters and melt quality effectively mitigates casting defects in high-strength ductile iron components. The methodology establishes a quantitative framework for defect prevention in heavy-duty castings subjected to cyclic loading.