In the field of metal casting, various processes are employed, but sand casting stands out due to its simplicity and cost-effectiveness, accounting for approximately 80% to 90% of all castings produced. Among these, green sand mould casting is widely utilized, particularly with the addition of coal dust to enhance surface quality and prevent defects like burning-on and sand adhesion. However, the use of coal dust introduces significant challenges, including environmental pollution from toxic gases and potential adverse effects on the microstructure of ductile iron castings. This study focuses on investigating the microstructure of ductile iron castings produced using coal dust-based green sand moulds, with an emphasis on how wall thickness influences graphite morphology and matrix composition. The findings aim to provide insights into optimizing casting processes for better quality and environmental sustainability.
The primary mechanism by which coal dust improves casting quality involves its decomposition at high temperatures, generating reducing gases and a lustrous carbon film that acts as a barrier against metal penetration. Despite these benefits, coal dust releases sulfur-containing gases and carcinogenic substances, posing risks to both worker health and the ecosystem. Moreover, in ductile iron castings, these sulfur compounds can infiltrate the molten metal, reacting with magnesium or rare earth elements essential for spheroidization. This reaction may reduce the effectiveness of nodularizing agents, leading to imperfect graphite structures and compromised mechanical properties. Therefore, understanding the microstructural characteristics of ductile iron castings under these conditions is crucial for advancing foundry practices.
In this research, we designed and produced trapezoidal test castings with varying wall thicknesses to simulate real-world scenarios. The specimens were analyzed for graphite distribution, nodularity, and matrix phases from the surface to the core. Our approach involved detailed metallographic examination, supported by quantitative measurements of球化率 and pearlite content. The results reveal distinct patterns related to wall thickness and the presence of coal dust, highlighting the need for alternative, eco-friendly moulding materials. Throughout this article, we will delve into the experimental procedures, present data through tables and formulas, and discuss the implications for ductile iron casting production. The key terms ‘ductile iron castings’ and ‘ductile iron casting’ will be frequently referenced to underscore their relevance in this context.
Experimental Materials and Methods
To conduct this study, we selected coal dust-based green sand from an industrial foundry, which is commonly used for its anti-sticking properties. The base material for the castings was ductile iron with a grade of QT450-10, whose chemical composition is summarized in Table 1. This composition ensures a balance of strength and ductility, typical for applications requiring high performance. Additional materials included nodularizing agents, inoculants, and other auxiliary substances to facilitate the casting process.
| Element | Content Range (wt%) |
|---|---|
| C | 3.70–4.00 |
| Si | 2.15–2.93 |
| Mn | 0.46–0.66 |
| P | 0.010–0.016 |
| S | 0.027–0.035 |
| Mg residual | 0.027–0.050 |
| Re residual | 0.026–0.043 |
The trapezoidal casting design was employed to evaluate the effect of wall thickness on microstructure, with dimensions including thicknesses of 5 mm, 10 mm, 20 mm, 30 mm, and 40 mm, each segment being 40 mm in length and 40 mm in width. The mould was prepared manually using the coal dust sand, and the casting was poured with a single-pattern, one-box system. After solidification, the castings were subjected to standard post-processing steps such as shakeout, shot blasting, and removal of the gating system to obtain clean specimens for analysis.
For metallographic analysis, samples were extracted from each wall thickness segment using wire cutting, focusing on cross-sectional areas labeled V1, V2, V3, and V4 from the edge to the center. These locations allowed for a comprehensive examination of microstructural variations. The specimens were ground, polished, and etched with 5% nitric alcohol to reveal the matrix structure, while graphite morphology was observed without etching. A ZEISS inverted optical microscope was utilized for imaging, and image analysis software was employed to quantify球化率, pearlite content, and graphite size. The球化率, for instance, was calculated using the formula: $$ \text{Nodularity} = \frac{N_{\text{nodular}}}{N_{\text{total}}} \times 100\% $$ where \( N_{\text{nodular}} \) is the number of nodular graphite particles and \( N_{\text{total}} \) is the total number of graphite particles. Similarly, pearlite percentage was determined as: $$ P\% = \frac{A_{\text{pearlite}}}{A_{\text{total}}} \times 100\% $$ with \( A_{\text{pearlite}} \) representing the area of pearlite and \( A_{\text{total}} \) the total area analyzed.
Results and Analysis of Graphite Morphology
The examination of graphite structures across different wall thicknesses and depths revealed several key trends. In general, the distribution of graphite in ductile iron castings produced with coal dust green sand was relatively sparse, and the roundness of graphite spheres was suboptimal. Figures 3 to 6 in the original study illustrate these characteristics, showing that graphite nodules became less frequent and more irregular towards the surface regions. For instance, at the V1 position (closest to the surface), the nodularity values decreased significantly compared to the core areas, as summarized in Table 2.
| Wall Thickness (mm) | V1 (%) | V2 (%) | V3 (%) | V4 (%) |
|---|---|---|---|---|
| 5 | 64.33 | 65.88 | 66.13 | 68.59 |
| 10 | 66.06 | 69.25 | 68.72 | 71.52 |
| 20 | 65.29 | 67.53 | 68.26 | 69.80 |
| 30 | 67.52 | 66.12 | 69.25 | 69.63 |
| 40 | 65.55 | 68.26 | 70.89 | 71.54 |
Notably, at the outermost surface of all wall thicknesses, virtually no spherical graphite was observed, indicating severe degradation in nodularization. This phenomenon can be attributed to the infiltration of sulfur-containing gases from coal dust decomposition, which react with magnesium in the molten iron, reducing its effectiveness for spheroidization. The relationship between sulfur content and nodularity can be expressed as: $$ \text{Nodularity} \propto \frac{1}{[S]} $$ where [S] is the concentration of sulfur at the interface. This inverse proportionality highlights the detrimental impact of sulfur on the quality of ductile iron castings.
Furthermore, the cooling rate, which varies with wall thickness, plays a critical role in graphite formation. Thinner sections cool faster, leading to finer microstructures but potentially worse nodularity due to limited time for diffusion processes. The cooling rate \( v_c \) can be approximated by: $$ v_c = \frac{\Delta T}{t} $$ where \( \Delta T \) is the temperature drop and \( t \) is time. In thinner walls, higher \( v_c \) values exacerbate the loss of nodularizing elements, compounding the issues caused by sulfur.
Matrix Structure and Pearlite Formation
The matrix composition of the ductile iron castings showed a clear dependence on wall thickness, with pearlite content increasing as thickness decreased. This trend is evident from the metallographic images and quantitative data presented in Table 3, which summarizes the pearlite grades at different depths. For example, in 5 mm thick sections, pearlite dominated the matrix, whereas in 40 mm sections, ferrite was more prevalent.
| Wall Thickness (mm) | V1 | V2 | V3 | V4 |
|---|---|---|---|---|
| 5 | Pearlite 35 | Pearlite 35 | Pearlite 35 | Pearlite 35 |
| 10 | Pearlite 35 | Pearlite 35 | Pearlite 35 | Pearlite 35 |
| 20 | Pearlite 25 | Pearlite 25 | Pearlite 25 | Pearlite 25 |
| 30 | Pearlite 25 | Pearlite 25 | Pearlite 25 | Pearlite 25 |
| 40 | Pearlite 15 | Pearlite 15 | Pearlite 15 | Pearlite 15 |
The increase in pearlite with decreasing wall thickness is primarily due to accelerated cooling rates, which favor the formation of pearlite over ferrite during the eutectoid transformation. The kinetics of this transformation can be described by the Avrami equation: $$ f = 1 – \exp(-k t^n) $$ where \( f \) is the fraction transformed, \( k \) is a rate constant, \( t \) is time, and \( n \) is an exponent. For faster cooling, \( k \) increases, leading to higher pearlite fractions. This relationship underscores how process parameters directly affect the mechanical properties of ductile iron castings, as pearlite contributes to hardness and strength but reduces ductility.
In addition, the presence of coal dust residues in the mould may introduce impurities that act as nucleation sites for pearlite, further enhancing its formation. However, the primary factor remains the thermal dynamics during solidification and cooling. For ductile iron casting applications, controlling these variables is essential to achieve desired microstructures and performance.
Discussion on Environmental and Quality Implications
The use of coal dust in green sand moulds for producing ductile iron castings presents a dual challenge: it improves surface quality but compromises microstructural integrity and environmental health. The emission of toxic gases, such as sulfur dioxide and polycyclic aromatic hydrocarbons, during decomposition poses significant risks. From a microstructural perspective, the infiltration of sulfur into the metal surface reduces the effective concentration of nodularizing elements, leading to poor graphite spheroidization. This effect can be modeled as: $$ [Mg]_{\text{effective}} = [Mg]_{\text{initial}} – \alpha [S] $$ where \( [Mg]_{\text{effective}} \) is the available magnesium for spheroidization, \( [Mg]_{\text{initial}} \) is the initial content, and \( \alpha \) is a reaction coefficient. As \( [Mg]_{\text{effective}} \) decreases, nodularity drops, adversely affecting the ductility and toughness of ductile iron castings.
Moreover, the variation in cooling rates with wall thickness exacerbates microstructural inhomogeneities. Thinner sections, with higher cooling rates, not only promote pearlite but also limit the time for corrective diffusion processes, resulting in a gradient of properties from surface to core. This is particularly critical in applications where consistent performance is required across different sections of a casting. Therefore, alternative moulding materials that do not rely on coal dust—such as organic binders or synthetic sands—should be explored to mitigate these issues while maintaining the benefits of green sand casting.
In summary, the study of ductile iron castings in coal dust-based moulds reveals intricate interactions between process parameters and microstructure. The frequent reference to ‘ductile iron castings’ in this analysis emphasizes their central role in foundry industries and the need for continuous improvement in production techniques.
Conclusion
This investigation into the microstructure of ductile iron castings produced with coal dust green sand moulds highlights several critical findings. First, the graphite distribution is generally sparse, with reduced nodularity, especially near the surface where spherical graphite is almost absent. Second, wall thickness significantly influences the matrix structure, with thinner sections exhibiting higher pearlite content due to faster cooling rates. These observations underscore the detrimental effects of coal dust-derived sulfur on spheroidization and the importance of thermal management during casting.
For future work, research should focus on developing environmentally friendly moulding sands that eliminate the need for coal dust while preserving casting quality. By addressing these challenges, the production of high-integrity ductile iron castings can be optimized, benefiting both industry and the environment. The insights gained from this study contribute to a deeper understanding of how material choices and process conditions shape the properties of ductile iron casting products.