This article details the optimized production process for 16L vermicular graphite iron (VGI) engine cylinder blocks, focusing on chemical composition control, melt treatment, and quality validation. The implementation of advanced process parameters ensures stable vermicularity (>80%) and mechanical properties exceeding 420 MPa tensile strength.
1. Chemical Composition Design
The chemical composition of VGI for engine cylinder blocks is critical for achieving balanced graphite morphology and mechanical properties. Key elements are controlled within narrow ranges:
| Element | Control Range (wt%) | Function |
|---|---|---|
| C | 3.7–3.9 | Prevents shrinkage while avoiding graphite flotation |
| Si | 1.9–2.1 | Controls graphitization and machining performance |
| Mn | 0.2–0.4 | Enhances pearlite formation |
| S | 0.01–0.02 | Minimizes vermiculizer consumption |
| Cu | 1.2–1.3 | Strengthens matrix through solid solution |
| Sn | 0.09–0.11 | Promotes pearlite refinement |
The carbon equivalent (CE) is calculated using:
$$ CE = C + \frac{Si + Cu}{3} $$
Maintained at 4.45–4.55 to optimize fluidity and shrinkage resistance.
2. Melt Preparation and Pretreatment
The melt process for engine cylinder block production includes:
- Charge materials: 20–30% returns + 70–80% shredded steel scrap
- High-temperature graphitized carburizer (C ≥98%, S ≤0.03%)
- SiC pretreatment (2.5–3.0 kg/ton) for nucleation enhancement
- Superheating to 1,540–1,550°C with 20 min holding
3. Vermiculization and Inoculation
Critical parameters for engine cylinder block vermiculation:
| Process Stage | Temperature (°C) | Key Additives |
|---|---|---|
| Pre-treatment | 1,480–1,490 | Mg-FeSi (3.0–3.5 kg/ton) |
| Post-treatment | 1,420–1,440 | FeSi75 (0.8–1.2 kg/ton) |
The vermicularity (V) is calculated as:
$$ V = \frac{N_v}{N_t} \times 100\% $$
Where \( N_v \) = number of vermicular graphite particles, \( N_t \) = total graphite particles.
4. Quality Validation
Mechanical properties from 16 engine cylinder block samples:
| Location | Tensile Strength (MPa) | Hardness (HBW) | Vermicularity (%) |
|---|---|---|---|
| Head bolt area | 425–485 | 235–249 | 80–90 |
| Oil pan flange | 443–497 | 246–259 | 80–95 |
| Bearing cap | 424–471 | 230–250 | 80–95 |
The pearlite content follows the relationship:
$$ P_c = 85 + 12[\mathrm{Mn}] + 25[\mathrm{Cu}] + 50[\mathrm{Sn}] $$
Where alloy contents are in weight percent, maintaining \( P_c \) ≥85%.
5. Process Optimization
Key improvements for engine cylinder block production:
- Carbon adjustment from 3.7–3.8% to 3.8–3.85% reduced shrinkage porosity
- Strict S control (<0.02%) improved vermiculizer efficiency by 18–22%
- Cooling rate optimization achieved <1% carbides in thin-wall sections
This systematic approach ensures consistent production of high-performance vermicular graphite iron engine cylinder blocks meeting stringent automotive requirements. The combination of precise composition control, advanced melt treatment, and real-time process monitoring establishes a robust manufacturing foundation for next-generation engine components.