Among the numerous casting process methods, sand casting finds the most extensive application due to its simple technology and low production cost, accounting for approximately 80-90% of all castings produced. Within this domain, green sand molding, utilizing clay-bonded sands, is predominant. However, in practical production with clay-bonded green sand, castings are prone to defects such as scabs, burn-on, and sand inclusions. To mitigate these defects and obtain higher quality castings, foundries commonly adopt the practice of adding pulverized coal (coal dust) to the clay green sand. The underlying mechanism is that the added coal dust, upon heating, combusts and generates a large volume of reducing gases. These gases form a protective film at the mold-metal interface. Furthermore, the thermal decomposition of coal dust leads to the precipitation of a lustrous carbon film at this interface. This dual action helps prevent the penetration of molten metal into the mold, thereby effectively countering defects like burn-on. Nevertheless, the incorporation of coal dust introduces significant issues. On one hand, the toxic and carcinogenic substances released from coal dust during casting severely impact both the ecological environment and worker health. On the other hand, at high temperatures, coal dust generates sulfur-containing gaseous compounds that can infiltrate the molten iron. In the case of ductile cast iron (nodular iron) castings, these sulfur species may react with magnesium or rare earth elements present in the surface layer of the iron melt. This reaction can deplete the effective, graphitizing-promoting elements like magnesium at the casting surface, potentially leading to degraded graphite nodularization (poor spheroidization) and adversely affecting the final quality of the ductile cast iron casting.
To gain a deeper understanding of the specific effects of traditional coal-dust-bonded sand on the microstructure of castings, this investigation focuses on the graphite morphology and matrix structure at both the surface and interior regions of trapezoidal ductile cast iron castings with varying wall thicknesses, produced using conventional coal dust green sand. The study of the relationship between the microstructure of ductile iron castings and their wall thickness holds practical significance for guiding actual production.
1. Experimental Methodology
1.1 Materials
Molding Material: Conventional coal dust green sand was sourced from an operating foundry.
Cast Material: The grade of the ductile cast iron was QT450-10. Its chemical composition range is provided in Table 1.
Auxiliary Materials: Nodulizing agent (e.g., FeSiMg), inoculant, and other standard founding additives were used.
| Element | C | Si | Mn | P | S | Mgres | REres |
|---|---|---|---|---|---|---|---|
| Content | 3.70-4.00 | 2.15-2.93 | 0.46-0.66 | 0.010-0.016 | 0.027-0.035 | 0.027-0.050 | 0.026-0.043 |
1.2 Casting Procedure and Specimen Design
To investigate the influence of varying wall thickness on the microstructure of ductile cast iron castings, a trapezoidal test casting was designed along with its gating system. The parting line was set at the broader base of the trapezoid. A simple one-pattern-per-mold layout was employed for pouring. The trapezoidal casting featured successive step thicknesses of 5 mm, 10 mm, 20 mm, 30 mm, and 40 mm. Each step had a length of 40 mm, and the overall width of the casting was 40 mm. The configuration is illustrated in Figure 1. Molds were prepared by hand using the conventional coal dust green sand. After pouring, the castings were shaken out, shot-blasted, and the gating systems were removed to obtain the required trapezoidal test specimens.
1.3 Metallographic Sample Preparation and Analysis
Sampling for metallographic analysis involved sectioning the trapezoidal casting transversely at the midpoint along the width of each thickness step using wire electrical discharge machining (EDM). For each step, four specific locations from the edge to the center were examined, labeled as V1 (sub-surface), V2, V3, and V4 (near-center). The sampling scheme is depicted in Figure 2. The metallographic specimens were ground, polished, and cleaned with alcohol. Graphite morphology was observed directly on the polished surface. To reveal the matrix microstructure, specimens were etched with a 5% nital solution, followed by alcohol cleaning. Observation was conducted using a ZEISS inverted optical microscope. Key parameters such as nodularity (graphite shape factor), pearlite content, and graphite nodule size were quantitatively evaluated using image analysis software.
2. Results and Analysis
2.1 Microstructural Characteristics of the Trapezoidal Castings
2.1.1 Graphite Structure from Surface to Center for Different Wall Thicknesses
The graphite structures observed at locations V1 through V4 for castings of different wall thicknesses are summarized. Overall, the graphite distribution was relatively sparse, and the roundness (nodularity) of the graphite particles was not optimal. Representative observations are synthesized in the data below.
2.1.2 Graphite Structure at the Extreme Surface Layer
A critical finding was that at the immediate surface layer (within the first ~100-200 µm) of all wall thicknesses, the presence of well-formed spheroidal graphite was scarcely detectable. This indicates a pronounced surface effect in ductile cast iron castings produced with coal-dust-bonded sand.
2.1.3 Matrix Structure from Surface to Center for Different Wall Thicknesses
The matrix microstructure, after etching, primarily consisted of ferrite and pearlite. A clear trend was observed: with increasing wall thickness, the proportion of ferrite in the matrix increased significantly, while the pearlite content decreased.
2.2 Quantitative Analysis and Discussion
The measured nodularity percentages for locations V1 to V4 across the different wall thicknesses are compiled in Table 2. The data reveals a general trend of decreasing nodularity from the interior (V4) towards the sub-surface (V1) regions. Most notably, the extreme surface consistently showed a near absence of nodular graphite. This provides strong evidence supporting the hypothesis that decomposition products from coal dust at high temperatures detrimentally affect graphite nodularization in ductile cast iron. The sulfur-containing gases generated are likely to infiltrate the casting surface, where they react with and consume the nodulizing elements (Mg, RE) in that region, impairing the formation of spherical graphite.
| Wall Thickness (mm) | 5 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| Extreme Surface | Nearly no nodular graphite observed | ||||
| V1 (Sub-surface) | 64.33 | 66.06 | 65.29 | 67.52 | 65.55 |
| V2 | 65.88 | 69.25 | 67.53 | 66.12 | 68.26 |
| V3 | 66.13 | 68.72 | 68.26 | 69.25 | 70.89 |
| V4 (Near-center) | 68.59 | 71.52 | 69.80 | 69.63 | 71.54 |
The matrix structure, characterized by pearlite content, is summarized in Table 3. The trend is unequivocal: as the wall thickness decreases (i.e., cooling rate increases), the pearlite content (grade) increases. This is a direct consequence of solid-state transformation kinetics. Faster cooling rates associated with thinner sections suppress the diffusion-controlled growth of ferrite during the eutectoid transformation, favoring the formation of the lamellar pearlite structure. The relationship between cooling rate ($\dot{T}$), transformation temperature, and resulting microstructure can be conceptually described by the Continuous Cooling Transformation (CCT) behavior for ductile cast iron. A higher cooling rate shifts the transformation to lower temperatures, promoting pearlite. This can be approximated for the final microstructure by:
$$ P\% = f(\dot{T}, C_{eq}) $$
where $P\%$ is the pearlite percentage, $\dot{T}$ is the average cooling rate through the eutectoid range, and $C_{eq}$ is the carbon equivalent. For a given composition, $P\%$ increases monotonically with $\dot{T}$. The cooling rate itself is inversely proportional to the square of the wall thickness ($d$) for a plate-like geometry, following the Chvorinov-type relation for solidification, which also influences the subsequent cooling in the solid state:
$$ \dot{T} \propto \frac{1}{d^n} $$
where $n$ is an exponent typically between 1 and 2. Therefore, thinner sections (smaller $d$) experience higher $\dot{T}$, leading to higher $P\%$, as observed.
| Wall Thickness (mm) | 5 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| V1 | Pearlite ~35% | Pearlite ~35% | Pearlite ~25% | Pearlite ~25% | Pearlite ~15% |
| V2 | Pearlite ~35% | Pearlite ~35% | Pearlite ~25% | Pearlite ~25% | Pearlite ~15% |
| V3 | Pearlite ~35% | Pearlite ~35% | Pearlite ~25% | Pearlite ~25% | Pearlite ~15% |
| V4 | Pearlite ~35% | Pearlite ~35% | Pearlite ~25% | Pearlite ~25% | Pearlite ~15% |
Mechanism of Surface Degradation: The surface deterioration of graphite structure in ductile cast iron when using coal-dust sand can be modeled as a sulfur pick-up reaction at the interface. The reaction between dissolved magnesium in the iron and sulfur from the mold atmosphere can be simplified as:
$$ [Mg] + [S] \rightarrow MgS_{(s)} $$
This reaction depletes the effective magnesium concentration $[Mg]_{eff}$ at the casting surface. The nodularity ($N$) is a strong function of $[Mg]_{eff}$, requiring it to be above a critical threshold $[Mg]_{crit}$. The sulfur flux ($J_S$) from the mold contributes to lowering $[Mg]_{eff}$ at the surface ($x=0$):
$$ [Mg]_{eff}(x=0) = [Mg]_{bulk} – \alpha \int J_S \, dt $$
where $\alpha$ is a stoichiometric factor. When $[Mg]_{eff}(x=0) < [Mg]_{crit}$, graphite nodularity fails at the surface, leading to the observed layer of degenerate graphite. This sulfur-induced impairment is a key drawback of conventional coal dust binders for producing high-integrity ductile cast iron components, especially for surfaces requiring good machinability or fatigue performance derived from a sound graphite structure.
3. Extended Implications and Summary
The investigation into the microstructure of trapezoidal ductile cast iron castings produced with traditional coal dust green sand clarifies the relationship between wall thickness, cooling rate, mold-metal interaction, and the final microstructure. The findings are summarized as follows:
- Overall Graphite Characteristics: Ductile cast iron castings produced with coal dust green sand exhibited a generally sparse distribution of graphite with sub-optimal nodularity. A consistent gradient was observed, with nodularity decreasing from the interior regions towards the sub-surface layers of the casting.
- Severe Surface Effect: The most striking observation was the near-complete absence of well-formed nodular graphite at the immediate surface layer of the ductile cast iron, regardless of wall thickness. This confirms that decomposition products from coal dust at high temperatures—specifically sulfur-bearing gases—are detrimental to the nodulization process. These gases infiltrate the casting surface and react with nodulizing elements, effectively poisoning the surface layer and preventing proper spheroidal graphite formation.
- Wall Thickness and Matrix Structure: A clear correlation exists between wall thickness and the matrix microstructure of the ductile cast iron. As the wall thickness decreases, the associated increase in cooling rate results in a higher percentage of pearlite and a corresponding lower percentage of ferrite. This is governed by the kinetics of the eutectoid transformation, where faster cooling suppresses ferrite growth, favoring the pearlite phase. The relationship can be quantified by the dependence of pearlite fraction on cooling rate.
- Environmental and Quality Concerns: This study underscores a dual challenge associated with coal dust green sand. While effective in preventing certain casting defects, its use poses environmental pollution through the emission of toxic compounds and presents a direct quality compromise for ductile cast iron by inducing surface and near-surface graphite degeneration. Consequently, research and development into environmentally friendly sand binder systems that do not adversely affect the microstructure of sensitive alloys like ductile cast iron is a critical and necessary direction for the modern foundry industry.
In conclusion, the production of high-quality ductile cast iron requires careful consideration of the molding materials. The traditional use of coal dust, while solving one set of problems, introduces significant microstructural defects, particularly at the surface, and environmental hazards. Advancing towards sustainable foundry practices necessitates the adoption of alternative binder technologies that ensure both ecological safety and the reliable production of sound ductile iron castings with consistent microstructure throughout the entire cross-section.