In the field of materials engineering, grey cast iron remains a pivotal material due to its excellent castability, machinability, and damping capacity. However, its mechanical properties, particularly tensile strength and fatigue resistance, are often limited by the presence of flake graphite in the matrix. This has driven extensive research into modifying graphite morphology, with nodularization being a key technique to enhance performance. Specifically, for components where surface and interior regions experience different loading conditions, such as ingot molds or crankshafts, achieving a nodular graphite surface layer on a grey cast iron substrate can offer a synergistic combination of surface toughness and internal thermal conductivity. This study focuses on comparing and selecting processes for graphite nodularization in the outer layer of grey cast iron castings using mold coatings containing rare-earth (RE) and rare-earth magnesium (RE-Mg) based compounds. Through systematic experimentation, we analyze the effects of carbon equivalent and sulfur content on graphite morphology, matrix structure, and crystallization-solidification behavior, aiming to optimize coating formulations for practical applications.
The fundamental mechanism behind graphite nodularization in grey cast iron involves the addition of elements that alter the growth kinetics of graphite during solidification. In standard grey cast iron, graphite precipitates as flakes due to the isotropic growth conditions, but with inoculants like magnesium or rare-earth elements, the graphite grows in a spheroidal form. This transformation is governed by interfacial energy changes and nucleation dynamics. The carbon equivalent (CE) plays a critical role, defined as $CE = C + \frac{1}{3}(Si + P)$, where C, Si, and P are the weight percentages of carbon, silicon, and phosphorus, respectively. For grey cast iron, a higher CE typically promotes graphite formation, but in nodularization processes, precise control is essential to avoid undesired phases. Similarly, sulfur acts as an anti-nodularizing agent by forming sulfides that consume nodularizing elements, necessitating careful management in treatment processes.
Our experimental approach was designed to evaluate two primary coating systems: RE-based and RE-Mg-based coatings, applied to the mold cavity surface to treat the grey cast iron during pouring. The goal was to induce a surface layer with nodular graphite while retaining a flake graphite core, leveraging the benefits of both structures. We employed a variable-thickness specimen to assess the effect of cooling rate, as shown in the schematic, where the thickness variation influences the modulus and solidification dynamics. The grey cast iron was melted in a medium-frequency induction furnace, with base iron adjusted for CE and sulfur content using ferrosilicon and iron sulfide additions. The coatings were formulated with alloy powders, binders, and fluxes to ensure proper adhesion and reactivity.
The coating compositions were optimized through orthogonal experiments, as summarized in Table 1 and Table 2. For the RE-based coating, the key components included #1 rare-earth alloy and 75% ferrosilicon, with additives like borax and sodium fluoride as fluxes, and water glass with PVB as binders. The RE-Mg-based coating incorporated FeSiMg8Re7 alloy, magnesium powder, and ferrosilicon, with similar flux and binder systems. The particle size of alloy powders was controlled between 0.2–0.6 mm to balance dissolution rate and coating uniformity. The coatings were applied to a 5 mm thickness on the mold wall opposite the gate, dried naturally, and used for casting. The grey cast iron was superheated to approximately 1550°C before pouring to ensure fluidity and treatment efficiency.
| Level | Carbon Equivalent / % | Sulfur Content / % | #1 Rare-Earth / parts | Ferrosilicon (75%) / parts |
|---|---|---|---|---|
| I | 4.1 | 0.04 | 100 | 10 |
| II | 4.26 | 0.02 | 120 | 8 |
| III | 4.5 | 0.06 | 80 | 4 |
| Level | Carbon Equivalent / % | Sulfur Content / % | Magnesium / parts | Ferrosilicon (75%) / parts |
|---|---|---|---|---|
| I | 4.1 | 0.04 | 17 | 5.6 |
| II | 4.26 | 0.02 | 13 | 8.0 |
| III | 4.5 | 0.06 | 15 | 10.4 |
The results indicated that both coatings could successfully nodularize graphite in the surface layer of grey cast iron, but with distinct dependencies on process parameters. For the RE-based coating, a nodular layer thickness exceeding 2.5 mm was achieved in hypereutectic grey cast iron (CE > 4.26%) with sulfur content below 0.04%. The optimal formulation comprised 91% #1 rare-earth, 9% ferrosilicon, along with fluxes and binders as described. In contrast, the RE-Mg-based coating produced a more robust nodularization, with layer depths over 8.5 mm even at high sulfur levels (0.06%), highlighting magnesium’s potent nodularizing effect. The recommended composition for RE-Mg coating was 78.5% rare-earth magnesium, 13.3% magnesium powder, and 8.2% ferrosilicon, with proportional fluxes and binders. These findings underscore the importance of coating design in modifying grey cast iron surface properties.
To quantify the graphite morphology transition, we analyzed the microstructure across the specimen thickness. The surface layer exhibited spheroidal graphite, transitioning to vermicular graphite in an intermediate zone, and finally to flake graphite in the core region. This gradation can be modeled using a diffusion-controlled growth equation for graphite nodules: $$r(t) = \sqrt{D \cdot t}$$ where $r(t)$ is the nodule radius at time $t$, and $D$ is the diffusion coefficient of carbon in austenite. For grey cast iron, the transition is influenced by the local concentration of nodularizing elements, which decays with distance from the coating interface. The matrix structure similarly varied, with the surface layer showing a pearlite-ferrite mixture, the transition zone dominated by ferrite, and the core maintaining pearlite with minor ferrite. This is attributed to carbon depletion around graphite nodules during solidification, promoting ferrite formation, as described by the lever rule for phase transformations.
The cooling curves during solidification provided further insights into the treatment effects. For grey cast iron treated with RE-based coating, the surface layer experienced a reduced cooling rate of approximately 8°C/min, an elevated eutectic temperature of about 20°C, and a shortened eutectic solidification time by 1.7 minutes. This can be expressed by modifying the Newtonian cooling law: $$T(t) = T_0 + (T_m – T_0) e^{-kt}$$ where $T(t)$ is temperature at time $t$, $T_0$ is ambient temperature, $T_m$ is initial melt temperature, and $k$ is a cooling constant. The RE coating altered $k$ due to its insulating properties and exothermic reactions. In contrast, the RE-Mg coating increased the cooling rate by about 4°C/min and lowered the eutectic temperature by 34°C, likely due to endothermic dissolution of magnesium and subsequent recalescence from nodularization reactions. These thermal dynamics are critical for controlling microstructure in grey cast iron.
The influence of carbon equivalent and sulfur content was statistically analyzed using regression models. For the RE-based coating, the nodular layer thickness $d$ can be approximated as: $$d = \alpha \cdot CE + \beta \cdot \frac{1}{S} + \gamma$$ where $\alpha$, $\beta$, and $\gamma$ are constants derived from experimental data, CE is carbon equivalent, and S is sulfur content. This indicates that higher CE and lower S favor nodularization in grey cast iron. For the RE-Mg coating, the dependence on sulfur was less pronounced due to magnesium’s stronger affinity for sulfur, as shown by the reaction: $$Mg + S \rightarrow MgS$$ which consumes sulfur but requires sufficient magnesium excess. The optimal magnesium addition was found to balance nodularization efficacy against potential defects like gas porosity.
Table 3 summarizes the key performance metrics for both coatings under varying conditions. The nodular layer depth, graphite nodularity percentage, and matrix hardness were measured to assess the treatment effectiveness. The data highlights that RE-Mg coatings generally outperform RE coatings in high-sulfur grey cast iron, making them suitable for industrial applications where sulfur control is challenging. However, RE coatings offer advantages in cost and environmental impact, especially for hypereutectic grey cast iron with low sulfur.
| Coating Type | Carbon Equivalent / % | Sulfur Content / % | Nodular Layer Depth / mm | Graphite Nodularity / % | Surface Hardness (HV) |
|---|---|---|---|---|---|
| RE-Based | 4.5 | 0.02 | 3.2 | 85 | 210 |
| RE-Based | 4.26 | 0.04 | 2.7 | 78 | 205 |
| RE-Based | 4.1 | 0.06 | 1.5 | 60 | 195 |
| RE-Mg-Based | 4.5 | 0.06 | 8.8 | 92 | 225 |
| RE-Mg-Based | 4.26 | 0.02 | 9.5 | 95 | 230 |
| RE-Mg-Based | 4.1 | 0.04 | 7.2 | 88 | 220 |
The transition in graphite morphology from surface to core in grey cast iron can be further explained through kinetics of solidification. The rate of nodule formation $N$ is given by: $$N = N_0 \exp\left(-\frac{\Delta G^*}{kT}\right)$$ where $N_0$ is a pre-exponential factor, $\Delta G^*$ is the activation energy for nucleation, $k$ is Boltzmann’s constant, and $T$ is temperature. In the surface layer, the coating provides abundant nucleation sites for spheroidal graphite, but with distance, the concentration of nodularizers decreases, leading to vermicular and flake graphite growth. This is consistent with the observed microstructural zones in grey cast iron. The matrix ferrite formation in the transition zone results from carbon diffusion to graphite, modeled by Fick’s second law: $$\frac{\partial C}{\partial t} = D \frac{\partial^2 C}{\partial x^2}$$ where $C$ is carbon concentration, $t$ is time, $D$ is diffusivity, and $x$ is distance. The depletion zone promotes ferrite, while pearlite dominates where carbon remains high.
Practical considerations for implementing these coatings in grey cast iron casting include mold preparation, coating durability, and environmental factors. The coatings must adhere well to sand molds and resist erosion during pouring. We found that adding PVB to the binder system enhanced adhesion and reduced coating loss. Additionally, the fluxes like borax and sodium fluoride prevented oxidation of alloy particles, ensuring consistent treatment of grey cast iron. For large-scale production, automated coating application could improve uniformity and reduce costs. The choice between RE and RE-Mg coatings depends on the specific grey cast iron composition and performance requirements: RE-Mg for high-sulfur or high-strength applications, and RE for cost-sensitive, low-sulfur scenarios.
In conclusion, this study demonstrates viable pathways for graphite nodularization in the surface layer of grey cast iron using mold coatings. The RE-based coating is effective for hypereutectic grey cast iron with sulfur below 0.04%, while the RE-Mg-based coating offers superior performance across a wider range of sulfur contents. The resulting graded microstructure—with nodular graphite surface, vermicular transition, and flake graphite core—provides a unique combination of mechanical and thermal properties for grey cast iron components. The cooling curve analyses reveal distinct thermal signatures for each coating, informing process control. Future work could explore hybrid coatings or real-time monitoring to optimize nodularization in grey cast iron. Overall, these findings advance the tailoring of grey cast iron for demanding applications where surface integrity is paramount.
The implications of this research extend to various industries utilizing grey cast iron, such as automotive, machinery, and construction. By enabling surface layer nodularization, components like engine blocks, gears, and frames can achieve enhanced wear resistance and fatigue life without compromising the bulk properties of grey cast iron. Further studies could investigate the long-term durability and corrosion behavior of treated grey cast iron, as well as economic assessments for commercial adoption. Ultimately, the ability to selectively modify graphite morphology in grey cast iron opens new avenues for material design, aligning with trends toward multifunctional and lightweight engineering materials.