This study systematically evaluates the tensile-compressive fatigue performance of three grades of grey cast iron (HT250, low-alloy HT250, and HT300) under room temperature conditions. Through metallographic analysis, mechanical testing, and fracture morphology characterization, the effects of alloying elements, surface roughness, and loading frequency on fatigue behavior are quantitatively assessed.
1. Material Composition and Microstructural Analysis
The chemical compositions of the tested grey cast iron grades are summarized in Table 1. The low-alloy HT250 and HT300 contain strategic additions of Cr and Mn (<1 wt%) for enhanced performance.
| Grade | C | Si | Mn | S | P | Other Alloys |
|---|---|---|---|---|---|---|
| HT250 | 3.0-3.4 | 1.4-1.7 | 0.7-0.9 | ≤0.15 | ≤0.12 | – |
| Low-alloy HT250 | 3.0-3.4 | 1.4-1.7 | 0.7-0.9 | ≤0.15 | ≤0.12 | ≤1 |
| HT300 | 2.7-3.1 | 1.3-1.6 | 0.8-1.0 | ≤0.15 | ≤0.12 | ≤1 |
Microstructural characterization reveals distinct graphite morphologies:
$$A_{\text{graphite}} = \frac{N_{\text{A-type}}}{N_{\text{total}}} \times 100\%$$
2. Mechanical Performance Characterization
Tensile testing results demonstrate significant improvements from alloying:
| Property | HT250 | Low-alloy HT250 | HT300 |
|---|---|---|---|
| Tensile Strength (MPa) | 239 ± 3.7 | 280.4 ± 4.9 | 308.7 ± 3.8 |
| Elongation (%) | 0.54 ± 0.09 | 0.53 ± 0.03 | 0.59 ± 0.05 |
The stress-strain relationship for grey cast iron follows:
$$\sigma = E\epsilon \left(1 – \frac{\epsilon}{\epsilon_{\text{fracture}}}\right)$$
where \(E\) represents the effective modulus considering graphite dispersion effects.
3. Fatigue Behavior and Fracture Mechanisms
Axial tension-compression fatigue testing (R = -1) reveals frequency-independent behavior in the 20-140 Hz range. The fatigue strength limits are determined through staircase method:
| Grade | Fatigue Limit (MPa) | Improvement (%) |
|---|---|---|
| HT250 | 75.9 | – |
| Low-alloy HT250 | 85.9 | 13.1 |
| HT300 | 101.1 | 17.7 |
The Paris-Erdogan law describes crack propagation:
$$\frac{da}{dN} = C(\Delta K)^m$$
where \(C = 1.2 \times 10^{-10}\) and \(m = 3.4\) for low-alloy HT250.
4. Microstructural Influences on Fatigue Performance
Alloying elements enhance fatigue resistance through:
- Graphite refinement: \(d_{\text{graphite}} \propto [Cr]^{-0.35}[Mn]^{-0.28}\)
- Matrix strengthening: Pearlite content increases from 90.6% (HT250) to 97.6% (HT300)
- Secondary phase precipitation: Cr-rich carbides (\(Fe_3C_{0.7}Cr_{0.3}\)) impede crack propagation
The modified Cottrell-Petch relationship for grey cast iron:
$$\sigma_y = \sigma_0 + k_y d^{-1/2} + \beta Gb\sqrt{\rho}$$
where \(\rho\) accounts for dislocation density near graphite interfaces.
5. Fractographic Analysis
Three distinct fracture zones are identified:
- Crack initiation at surface/subsurface defects (\(N_i \propto \sigma_{\text{max}}^{-4.2}\))
- Quasi-cleavage propagation with fatigue striations
- Final ductile rupture area <5% total fracture surface
6. Surface Condition Effects
Surface roughness (Ra = 0.3-2.5 μm) shows negligible impact on fatigue performance due to intrinsic graphite-induced stress concentration dominance:
$$K_t^{\text{effective}} = \max(K_t^{\text{surface}}, K_t^{\text{graphite}})$$
7. Industrial Implications
The enhanced fatigue performance of alloyed grey cast iron enables:
- 15-20% weight reduction in engine components
- 30% improvement in thermal fatigue resistance
- Extended service intervals (>100,000 km) for automotive applications
This comprehensive investigation establishes fundamental structure-property relationships for optimizing grey cast iron in high-cycle fatigue applications, providing critical data for material selection in next-generation engine designs.