In the production of high-performance engine components, such as cylinder liners for marine diesel engines, the material properties of cast iron play a critical role. Ductile iron castings, particularly those with compacted graphite morphology, offer superior tensile strength, thermal conductivity, and fatigue resistance compared to traditional gray iron. However, achieving a high vermicularity in ductile iron castings is challenging due to the narrow processing window for vermicularizing elements like rare earth (RE) and magnesium (Mg). This study investigates how the concentrations of RE and Mg influence the solidification process and graphite morphology in ductile iron castings, using thermal analysis and microstructural characterization. The findings provide insights into optimizing processing parameters for enhanced performance in demanding applications.
The solidification of ductile iron castings involves complex phase transformations, where the cooling curve characteristics—such as primary crystallization temperature (TLA), eutectic undercooling temperature (TEU), and eutectic recalescence temperature (TER)—serve as key indicators of graphite formation. The recalescence temperature difference, ΔTR = TER – TEU, is particularly significant as it reflects the rate of latent heat release during eutectic solidification, which correlates with graphite morphology. In this work, we conducted experiments on iron melts treated with varying RE and Mg contents, along with different inoculants, to analyze their effects on these thermal parameters and the resulting graphite structures. Our goal is to establish predictive relationships that can aid in controlling the quality of ductile iron castings.
We prepared the base iron using high-purity pig iron, scrap steel, and intermediate alloys such as ferrochromium and ferromolybdenum, with copper and tin added as pure metals. The melting process was carried out in a 20-ton medium-frequency induction furnace, where the temperature was raised to 1520°C and held for 10 minutes before tapping. The chemical composition of the base iron was controlled within the following ranges: 3.90–4.00% C, 1.00–1.20% Si, 0.70–0.80% Mn, ≤0.025% P, ≤0.012% S, 0.20–0.30% Ni, 0.15–0.25% Cr, 0.20–0.30% Mo, 1.60–1.80% Cu, and 0.04–0.06% Sb. This composition ensures a near-eutectic base suitable for producing ductile iron castings with consistent properties.
Vermicularization was achieved through a composite treatment using CompactMg alloy and RESiFe alloy, added via the pouring method. Each treatment batch involved 2500 kg of molten iron, with residual RE and Mg contents adjusted as per experimental design. Inoculation was performed using either BaSi or 75SiFe inoculants at specified addition rates to mitigate chilling tendencies and promote graphite nucleation. The specific treatment conditions are summarized in Table 1, which outlines the residual element contents and inoculant types used in this study.
| Condition | Residual RE (%) | Residual Mg (%) | Inoculant Type | Inoculant Addition (%) |
|---|---|---|---|---|
| 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 |
Thermal analysis was conducted by sampling the treated molten iron and pouring it into resin-sand molds equipped with K-type thermocouples. Temperature data were recorded at a frequency of 70 measurements per minute using an Agilent 34970A data acquisition system. The cooling curves (temperature vs. time) were processed to derive first and second derivatives, enabling the determination of characteristic temperatures: TLA (primary crystallization temperature), TEU (minimum eutectic temperature), and TER (maximum eutectic temperature). The recalescence temperature ΔTR was calculated as ΔTR = TER – TEU. For each condition, three replicates were measured to ensure statistical reliability. Microstructural analysis was performed on samples extracted from both thermal analysis specimens and actual cylinder liner castings, which were polished and examined to assess graphite morphology and vermicularity.
The cooling curves for the base iron and treated conditions revealed distinct solidification behaviors. The base iron, with a near-eutectic composition, exhibited a low TLA of 1152.7°C, indicative of limited primary austenite formation. Its eutectic transformation showed a high ΔTR of 11.4°C, corresponding to the rapid growth of flake graphite typical of gray iron. In contrast, vermicularization treatments shifted the eutectic point to the right, increasing TLA to a range of 1163.2–1166.6°C due to the hypo-eutectic shift promoted by RE and Mg. This is expressed mathematically by the change in primary solidification temperature:
$$ T_{LA} = T_0 + \Delta T_{LA} $$
where \( T_0 \) is the base iron primary temperature and \( \Delta T_{LA} \) is the increase due to vermicularization. For instance, Condition 1 with the highest RE and Mg residuals (0.023% RE, 0.011% Mg) achieved the highest TLA of 1166.6°C, while Conditions 3 and 4 with lower residuals showed TLA around 1163.5°C. The eutectic temperatures TEU and TER decreased post-treatment, with ΔTR values ranging from 3.9°C to 6.4°C, suggesting a transition from flake to compacted graphite growth. Table 2 summarizes these thermal characteristics, highlighting the correlation between element content and solidification parameters.
| Condition | TLA (°C) | TEU (°C) | TER (°C) | ΔTR (°C) |
|---|---|---|---|---|
| Base Iron | 1152.7 | 1140.7 | 1152.1 | 11.4 |
| 1 | 1166.6 | 1132.6 | 1136.5 | 3.9 |
| 2 | 1163.2 | 1135.2 | 1139.6 | 4.4 |
| 3 | 1164.5 | 1133.2 | 1138.6 | 5.4 |
| 4 | 1163.5 | 1133.2 | 1139.6 | 6.4 |
Graphite morphology analysis confirmed that the base iron consisted entirely of coarse flake graphite, whereas treated conditions displayed varying proportions of vermicular and spheroidal graphite. Condition 1, with high RE and Mg, resulted in approximately 30% vermicularity, dominated by spheroidal graphite. As residual element contents decreased, vermicularity increased—Condition 2 reached about 50%, and Conditions 3 and 4 exceeded 90% vermicularity. Notably, Condition 4, with a higher RE/Mg ratio of 3.5 compared to 2.4 in Condition 3, achieved the highest vermicularity, underscoring the importance of element balance. The relationship between ΔTR and vermicularity can be modeled as:
$$ V = k \cdot \Delta T_R + C $$
where \( V \) is vermicularity, \( k \) is a proportionality constant, and \( C \) is an intercept. For ΔTR values above 5°C, vermicularity consistently exceeded 50%, indicating a threshold for effective compacted graphite formation in ductile iron castings.
The mechanism behind these observations lies in the solidification kinetics. In gray iron, flake graphite grows rapidly with carbon diffusion in the melt, releasing substantial latent heat and yielding high ΔTR. For ductile iron castings with spheroidal graphite, the formation of an austenite shell around graphite nodules impedes carbon diffusion, reducing growth rates and ΔTR. Vermicular graphite, with its branched structure, exhibits intermediate growth dynamics, leading to ΔTR values between those of gray and ductile iron. This is quantified by the latent heat release rate \( \dot{Q} \):
$$ \dot{Q} = m \cdot L \cdot \frac{d\phi}{dt} $$
where \( m \) is mass, \( L \) is latent heat, and \( \frac{d\phi}{dt} \) is the phase transformation rate. Higher \( \dot{Q} \) in flake graphite results in larger ΔTR, while lower \( \dot{Q} \) in vermicular graphite corresponds to moderate ΔTR. The influence of RE and Mg on graphite morphology stems from their ability to modify the interfacial energy between graphite and the melt, promoting compacted growth. Specifically, RE elements enhance graphite branching, whereas Mg favors spheroidization; thus, an optimal RE/Mg ratio is crucial for high vermicularity in ductile iron castings.
Inoculation played a vital role in suppressing chill tendencies, especially at higher RE contents. Conditions 3 and 4, which employed BaSi and 75SiFe inoculants, respectively, showed no evidence of ledeburite or free cementite in the matrix, indicating effective inoculation. The choice of inoculant—BaSi for high RE/Mg ratios and 75SiFe for lower ratios—ensured minimal white iron formation while maintaining high vermicularity. This highlights the synergy between vermicularization and inoculation in producing sound ductile iron castings. The overall solidification process can be described by the following kinetic equation for eutectic transformation:
$$ \frac{dT}{dt} = -\alpha (T – T_{\infty}) + \beta \dot{Q} $$
where \( \frac{dT}{dt} \) is the cooling rate, \( \alpha \) and \( \beta \) are constants, \( T_{\infty} \) is the ambient temperature, and \( \dot{Q} \) is as defined earlier. This model accounts for the observed temperature shifts and recalescence behavior in ductile iron castings.
In practical terms, these findings emphasize the need for precise control of RE and Mg levels in the production of ductile iron castings for critical components. For instance, in marine engine cylinder liners, where high vermicularity is desired to combat thermal fatigue and wear, maintaining a RE/Mg ratio above 3.0 and ΔTR above 5°C can yield vermicularity over 90%. Additionally, using efficient inoculants like BaSi mitigates the risk of carbides, ensuring a pearlitic matrix with minimal ferrite. This approach enhances the mechanical and thermal properties of ductile iron castings, extending service life in harsh environments.
To summarize, the solidification of ductile iron castings is highly sensitive to RE and Mg contents, which alter primary and eutectic temperatures, recalescence behavior, and graphite morphology. The recalescence temperature ΔTR serves as a reliable indicator of vermicularity, with higher values favoring compacted graphite. Optimizing the RE/Mg ratio and selecting appropriate inoculants are key strategies for achieving high-performance ductile iron castings. Future work could explore the effects of other alloying elements and cooling rates on these relationships, further refining processing guidelines for industrial applications.