Grey cast iron is widely used in automotive engine components, such as cylinder heads, due to its excellent thermal conductivity and damping capacity. Surface hardness control remains critical for balancing machining performance and service reliability. This article systematically analyzes key factors affecting surface hardness through metallurgical mechanisms and experimental verification.
1. Formation Mechanism of Surface Ferrite Layer
The surface ferrite layer (1–2 mm thickness) significantly reduces hardness values. Two primary mechanisms govern its formation:
$$ a_C = \gamma_C \cdot [C] $$
Where $a_C$ represents carbon activity, $\gamma_C$ is the activity coefficient, and [C] is carbon concentration. High silicon content increases carbon activity, promoting graphite precipitation and ferrite formation during solidification.
| Molding Sand Moisture (%) | Surface Hardness (HB) | Ferrite Layer Thickness (mm) |
|---|---|---|
| 3.0–3.4 | 213 | 0 |
| 3.6–4.0 | 189 | 1–2 |
2. Critical Process Parameters
2.1 Pig Iron Addition Ratio
The relationship between pig iron content and mechanical properties follows:
$$ \sigma_b = 250 – 3.5P $$
Where $\sigma_b$ is tensile strength (MPa) and $P$ is pig iron percentage. Experimental data demonstrate:
| Pig Iron (%) | Surface Hardness (HB) | Tensile Strength (MPa) |
|---|---|---|
| 5 | 210 | 279 |
| 10 | 205 | 263 |
| 15 | 193 | 248 |
| 20 | 187 | 235 |
2.2 Aging Treatment Temperature
The hardness-temperature relationship follows Arrhenius behavior:
$$ H = H_0 \cdot e^{-\frac{Q}{RT}} $$
Where $H$ is hardness, $Q$ is activation energy (85–100 kJ/mol for grey cast iron), and $T$ is absolute temperature. Temperatures above 550°C cause significant hardness reduction.
2.3 Inoculation Practice
Optimal silicon-barium inoculant addition ranges from 0.2–0.4%:
$$ N = N_0(1 – e^{-k\Delta t}) $$
Where $N$ is effective nucleation sites, $k$ is fading rate constant (0.35–0.45 min⁻¹), and $\Delta t$ is inoculation-to-pouring time.
2.4 Superheating Temperature
Melting temperature significantly affects graphite morphology:
| Temperature (°C) | Graphite Type | Pearlite (%) |
|---|---|---|
| 1,350 | A | 95 |
| 1,450 | A+D | 90 |
| 1,550 | D+E | 85 |
3. Process Optimization Strategy
For consistent surface hardness (HB 200–220) in grey cast iron cylinder heads:
- Maintain molding sand moisture ≤3.4%
- Limit pig iron addition to 5–10%
- Control aging temperature ≤550°C
- Use 0.3% Si-Ba inoculant with ≤5 min holding time
- Keep superheating temperature ≤1,500°C
$$ \text{Hardness Control Index} = \frac{[C]^{0.7} \cdot [Si]^{-0.3}}{[Mn]^{0.2} \cdot [S]^{0.1}} $$
This empirical formula helps predict hardness variations during grey cast iron production.