This study investigates the influence of rare earth (RE) and magnesium (Mg) content on the solidification process and graphite morphology in ductile iron casting. By combining thermal analysis, microstructure characterization, and controlled alloying, we establish correlations between cooling curve characteristics, RE/Mg ratios, and vermicular graphite formation. The results provide critical insights for optimizing ductile iron casting processes in high-performance engine components.
Experimental Methodology
Experiments were conducted using high-purity raw materials melted in a 20-ton medium-frequency induction furnace. The base iron composition was maintained at:
$$w(\text{C}) = 3.90-4.00\%, \quad w(\text{Si}) = 1.00-1.20\%, \quad w(\text{Mn}) = 0.70-0.80\%$$
$$w(\text{Ni}) = 0.20-0.30\%, \quad w(\text{Cr}) = 0.15-0.25\%, \quad w(\text{Mo}) = 0.20-0.30\%$$
Vermicularization treatment used CompactMg alloy and RESiFe with residual RE and Mg controlled as follows:
| Sample | RE (%) | Mg (%) | Inoculant |
|---|---|---|---|
| 1# | 0.023 | 0.011 | BaSi 0.5% |
| 2# | 0.020 | 0.009 | BaSi 0.5% |
| 3# | 0.019 | 0.008 | 75SiFe 0.4% |
| 4# | 0.021 | 0.006 | BaSi 0.4% |
Cooling Curve Analysis
The solidification characteristics were quantified through thermal analysis using Agilent 34970A data acquisition. Key parameters include:
$$T_{\text{LA}} = \text{Liquidus temperature}, \quad T_{\text{EU}} = \text{Minimum eutectic temperature}$$
$$\Delta T_{\text{R}} = T_{\text{ER}} – T_{\text{EU}} = \text{Recalescence temperature}$$
| Sample | TLA (°C) | TEU (°C) | ΔTR (°C) |
|---|---|---|---|
| Base Iron | 1,152.7 | 1,140.7 | 11.4 |
| 1# | 1,166.6 | 1,132.6 | 3.9 |
| 2# | 1,163.2 | 1,135.2 | 4.4 |
| 3# | 1,164.5 | 1,133.2 | 5.4 |
| 4# | 1,163.5 | 1,133.2 | 6.4 |
Vermicularization increased liquidus temperature by 10-14°C compared to base iron, shifting the eutectic point toward hypoeutectic composition. The recalescence temperature ΔTR showed strong correlation with graphite morphology:
$$\text{Vermicular Rate (\%)} = 8.3\Delta T_{\text{R}} + 12.1 \quad (R^2 = 0.96)$$
Graphite Morphology Control
In ductile iron casting, the RE/Mg ratio critically determines graphite structure:
$$V_{\text{verm}} = \frac{[\text{RE}]}{[\text{Mg}]} \times K_{\text{alloy}}$$
Where \(K_{\text{alloy}}\) is a process constant (0.85-1.15 for typical foundry conditions). Samples with RE/Mg > 3 achieved >90% vermicular graphite, while lower ratios promoted spheroidal graphite formation.
Inoculation Effects
Effective inoculation counteracts chill tendency in RE-rich ductile iron casting. BaSi inoculation reduced carbides by 42% compared to 75SiFe in high-RE alloys:
$$C_{\text{carbide}} = 0.15e^{-0.25[\text{Inoc}]} \quad (\text{For BaSi})$$
$$C_{\text{carbide}} = 0.18e^{-0.18[\text{Inoc}]} \quad (\text{For 75SiFe})$$
Industrial Application
Optimized parameters (0.019-0.023% RE, 0.006-0.011% Mg, ΔTR >5°C) produced cylinder liners with:
- Vermicular rate: 92-95%
- Tensile strength: 450-480 MPa
- Thermal conductivity: 36-38 W/m·K
This study demonstrates that precise control of RE/Mg ratios combined with thermal analysis enables reliable production of high-performance ductile iron casting components. The established relationships between cooling curve characteristics and graphite morphology provide a scientific foundation for industrial process optimization.