This technical analysis examines the premature fracture of a QT800-3 ductile iron casting crankshaft during full-load bench testing. The study employs macroscopic observation, scanning electron microscopy (SEM), and metallurgical analysis to identify the root cause of failure and propose corrective measures.
1. Macroscopic Fracture Analysis
The fracture initiates at the transition zone between the connecting rod journal and rollover groove, propagating at 45° to the crankshaft axis. Three distinct zones are identified:
| Zone | Characteristics | Width |
|---|---|---|
| Crack Initiation | Radial patterns, 4.4mm below surface | 0.8-1.2mm |
| Propagation | Cleavage features with secondary cracks | 12-15mm |
| Final Fracture | Dimpled rupture morphology | 5-7mm |
The oxidation pattern suggests progressive seizure between journal and bearing, with calculated interface temperature reaching:
$$ T_{interface} = T_{ambient} + \frac{\mu P v}{k} \sqrt{\frac{\alpha t}{\pi}} $$
Where μ=0.12 (friction coefficient), P=85MPa (contact pressure), v=8.3m/s (sliding velocity), k=46W/m·K (thermal conductivity), α=1.2×10⁻⁵m²/s (thermal diffusivity).
2. Microstructural Characterization
Metallographic examination reveals critical variations in the ductile iron casting structure:
| Region | Graphite Morphology | Nodularity | Defects |
|---|---|---|---|
| Zone I | Type II (30μm), 85% nodularity | 2级 | 0.25mm micro-shrinkage |
| Zone IV | Type VI flake graphite | – | Surface cracks (≤0.4mm) |
The residual stress distribution in induction-hardened regions follows:
$$ \sigma_{res}(x) = \sigma_{max} \left[1 – \left(\frac{x}{d}\right)^2\right] $$
Where σmax = 650MPa (surface compressive stress), d = 3.2mm (case depth).
3. Failure Mechanism
The sequential failure process in ductile iron casting components includes:
- Improper induction hardening extending into rollover groove
- Quenched microstructure embrittlement (Martensite + 15-20% retained austenite)
- Surface cracking during roll forming:
$$ \varepsilon_{critical} = \frac{HV}{3E} \approx 0.0025 $$ - Fatigue crack propagation rate:
$$ \frac{da}{dN} = C(\Delta K)^m $$
C=1.8×10⁻¹³, m=3.2 (for QT800-3)
4. Process Optimization
Modified parameters for ductile iron casting crankshaft manufacturing:
| Parameter | Original | Optimized |
|---|---|---|
| Inductor Width | 28mm | 22mm |
| Quench Delay | 1.2s | 0.8s |
| Roll Force | 12kN | 9.5kN |
The improved stress concentration factor:
$$ K_t = 1 + 2\sqrt{\frac{a}{\rho}} $$
Where a=0.4mm (defect size), ρ=0.15mm (root radius), reduced from 3.8 to 2.1.
5. Conclusion
This analysis demonstrates that proper process control in ductile iron casting components can increase fatigue resistance by 40-60%. Key improvements include:
- Induction hardening zone control: ±0.5mm positional accuracy
- Surface compressive stress enhancement: >750MPa
- Nodularity maintenance: >80% in critical areas
The optimized process achieves:
$$ N_f = \frac{1}{C} \left(\frac{\Delta \sigma}{\sigma_f’}\right)^{-b} $$
Where σf‘=1250MPa (fatigue strength coefficient), b=-0.095 (exponent), resulting in 3.8×10⁶ cycles at 650MPa stress amplitude.