Gray cast iron remains a preferred material for automotive components like engine blocks due to its excellent castability, wear resistance, and vibration damping. With increasing demands for high-power diesel engines, this study investigates how carbon equivalent (CE = C + Si/3) in the range of 3.2–3.8% affects the microstructure, mechanical properties, and thermal conductivity of gray cast iron. The findings demonstrate significant improvements in strength and thermal stability through controlled graphite morphology and pearlite refinement.
Microstructural Evolution
Graphite morphology transitions from coarse straight flakes (Type C) at CE=3.8% to fine curved structures (Type E) at CE=3.2%, as quantified below:
| CE (%) | Graphite Length (μm) | Graphite Content (%) |
|---|---|---|
| 3.2 | 55.38 | 7.0 |
| 3.4 | 78.12 | 7.5 |
| 3.6 | 94.75 | 8.0 |
| 3.8 | 111.93 | 8.5 |
Pearlite interlamellar spacing follows a similar trend:
$$ \lambda_{pearlite} = 1175 – 234.5(CE) + 12.3(CE)^2 \quad (R^2=0.98) $$
where λpearlite decreases from 1175 nm (CE=3.8%) to 397 nm (CE=3.2%).
Mechanical Performance Enhancement
The relationship between carbon equivalent and mechanical properties reveals critical trade-offs:
| CE (%) | Hardness (HB) | Tensile Strength (MPa) |
|---|---|---|
| 3.2 | 245 | 385.9 |
| 3.4 | 242.5 | 368.2 |
| 3.6 | 242.8 | 351.6 |
| 3.8 | 210.8 | 251.1 |
Second-order polynomial fits demonstrate these relationships:
$$ HB = -184(CE)^2 + 1239(CE) – 1835 \quad (R^2=0.92) $$
$$ R_m = -5175(CE)^2 + 3412(CE) – 5238 \quad (R^2=0.97) $$
Thermal Behavior Analysis
While ductile iron casting typically offers better mechanical properties, gray cast iron maintains superior thermal stability. Thermal diffusivity (α) decreases linearly with temperature:
$$ \alpha_{3.2} = -0.0097T + 0.1324 \quad (R^2=0.99) $$
$$ \alpha_{3.8} = -0.0135T + 0.1643 \quad (R^2=0.998) $$
Lower CE specimens show reduced temperature dependence, with 39.2% lower thermal sensitivity at CE=3.2% compared to CE=3.8%.
Quality Indices for Casting Optimization
Four key parameters guide material selection for ductile iron casting alternatives:
| Parameter | Formula | Optimal Range |
|---|---|---|
| Eutectic Saturation (Sc) | $$ S_c = \frac{C}{4.265 – \frac{Si+P}{3}} $$ | 0.7–1.0 |
| Maturity (Rc) | $$ R_c = \frac{R_m}{1000 – 800S_c} $$ | 0.9–1.2 |
| Hardening Coefficient (Hc) | $$ H_c = \frac{HBW}{530 – 344S_c} $$ | 0.8–1.2 |
| Quality Index (Qi) | $$ Q_i = \frac{R_c}{H_c} $$ | 1.0–1.4 |
Fractographic Observations
Ductile iron casting typically exhibits spherical graphite nodules, while low-CE gray iron shows characteristic cleavage fractures with reduced void density:
- CE=3.2%: 12 voids/mm²
- CE=3.8%: 34 voids/mm²
This microstructural refinement explains the 27% tensile strength improvement at lower carbon equivalents, making it competitive with premium ductile iron casting grades.
Industrial Implications
For applications requiring both thermal stability and mechanical performance, low-CE gray cast iron (CE=3.2–3.4%) offers:
- 15–20% higher strength than standard gray iron
- 30% lower thermal sensitivity versus high-CE counterparts
- Machinability indices (Qi=1.05) comparable to ductile iron casting
The optimized balance between graphite morphology control and pearlite refinement enables this material system to meet evolving demands in heavy-duty engine applications while maintaining the inherent advantages of gray cast iron production economics.