In our production facility, we utilize a furan resin sand line to manufacture various components, with a primary focus on wind power castings and injection molding machine parts. During the production of bearing seat castings, we observed the occurrence of white spots at the outer edges of slant neck risers. These white spots are situated between the casting surface and the coating layer, and upon grinding inspection, they are found to extend up to approximately 2 mm deep from the surface. Even after shot blasting, these spots persist, necessitating additional grinding operations. This issue has led to increased labor hours, higher production costs, and potential delays in customer deliveries, highlighting the need for a thorough investigation into the root causes and solutions for these defects in ductile iron castings.
From a macroscopic examination, the white spot material does not appear to be caused by mold sand particles, as evidenced by its distinct granular morphology. Similar white spots are also present in the coating layer at the same locations. Given that standard production processes—including molding, melting, pouring, and operator actions—were conducted without notable deviations, we hypothesize that the white spots arise from interactions between the molten iron and the sand mold. The structure of the slant neck riser creates a localized high-temperature zone of 1200–1400°C at the riser neck’s outer edge, facilitating surface reactions that result in the formation of a silicon-rich layer on the ductile iron casting surface, manifesting as white spots. Since direct sampling from the castings was impractical, we designed experiments to replicate the phenomenon using actual production parameters such as wall thickness, molding conditions, pouring temperature, and iron composition, enabling detailed analysis of the white spot regions.
Problem Analysis and Experimental Approach
The formation of white spots in ductile iron castings is closely linked to the thermal conditions provided by the slant neck riser. At elevated temperatures, chemical reactions occur at the interface between the molten iron and the coating, leading to the deposition of silicon-based compounds. To quantify this, we consider the temperature-dependent reaction kinetics, which can be modeled using the Arrhenius equation for reaction rates: $$ k = A e^{-E_a / RT} $$ where \( k \) is the rate constant, \( A \) is the pre-exponential factor, \( E_a \) is the activation energy, \( R \) is the gas constant, and \( T \) is the temperature in Kelvin. In the context of ductile iron castings, this equation helps explain how temperature gradients influence the formation of white spots, particularly in regions like the riser neck edges where heat accumulation is pronounced.
Our experimental setup aimed to simulate the conditions under which white spots form in ductile iron castings. We prepared materials including 20 kg of a specific coating (e.g., Narumi CQ607), one 150 slant neck riser, a test block mold of dimensions 600 mm × 300 mm × 75 mm, several ϕ40 mm ceramic tubes for gating, and sand boxes of 700 mm × 700 mm × 200 mm and 700 mm × 700 mm × 300 mm. The molten iron composition was rigorously controlled, as summarized in Table 1, to mirror typical production parameters for ductile iron castings.
| Element | Range | Notes |
|---|---|---|
| Carbon (base iron) | 3.40–3.45% | Critical for graphite formation |
| Silicon (base iron) | 2.83–2.93% | Influences fluidity and reaction kinetics |
| Silicon (casting) | 3.50–3.60% | Final content after inoculation |
| Manganese | < 0.025% | Minimized to prevent segregation |
| Phosphorus | ≤ 0.040% | Controlled for soundness |
| Sulfur | ≤ 0.025% | Reduced to avoid adverse effects |
| Sulfur (post-treatment) | 0.005–0.015% | After spheroidization |
| Magnesium | 0.035–0.055% | Essential for nodular graphite |
| Rare Earth | < 0.010% | Aids in microstructure control |
| Carbon Equivalent | 4.56–4.65 | Calculated as CE = %C + %Si/3 |
| Antimony (addition) | 0.005% | Used for trace hardening |
| Nickel (addition) | 5% | Enhances mechanical properties |
The experimental procedure involved several key steps: first, selecting and positioning the slant neck riser on the test block mold; second, assembling the gating system with ceramic tubes to ensure proper fluid flow; third, applying the coating to the sand mold with controlled viscosity; fourth, pouring molten iron at a temperature of 1340–1350°C to replicate the thermal conditions of actual ductile iron casting production; and finally, shaking out and cleaning the test blocks for analysis. This method allowed us to consistently reproduce the white spots, facilitating further investigation into their composition and origin in ductile iron castings.
Detailed Experimental Design and Material Considerations
To ensure the reliability of our findings, we meticulously designed the experiment to account for variables that could influence white spot formation in ductile iron castings. The test block dimensions of 600 mm × 300 mm × 75 mm were chosen to represent typical section thicknesses in bearing seat castings, while the slant neck riser was positioned to emulate the thermal dynamics encountered in production. The pouring temperature range of 1340–1350°C was selected based on historical data indicating that this interval maximizes fluidity while minimizing defects in ductile iron castings. Additionally, the coating application process was standardized, with multiple layers applied at specific Baume degrees to assess its impact on white spot development.
The iron treatment process played a crucial role in this study, as the composition directly affects the reactivity at the mold-metal interface. Spheroidization was carried out using a blend of nodularizing agents (70% N1 and 30% N2), with a total addition of 0.90–1.20%, while inoculation involved BS-1A added at the ladle bottom in amounts ranging from 0.10% to 0.70%. These parameters are critical for achieving the desired microstructure in ductile iron castings and were monitored closely to maintain consistency. The carbon equivalent (CE) was calculated using the formula: $$ \text{CE} = \%\text{C} + \frac{\%\text{Si}}{3} $$ which yielded values between 4.56 and 4.65, ensuring adequate graphitization potential in the ductile iron castings.
| Treatment Type | Agent Composition | Addition Rate | Purpose |
|---|---|---|---|
| Spheroidization | N1 (70%), N2 (30%) | 0.90–1.20% | Promote nodular graphite formation |
| Inoculation | BS-1A | 0.10–0.70% | Enhance graphite distribution and reduce chilling |
During the experiment, we also recorded environmental factors such as mold humidity and ambient temperature, as these can influence coating performance and reaction rates. The use of ceramic tubes in the gating system helped minimize turbulence and oxidation, which are common concerns in the production of high-integrity ductile iron castings. After pouring, the test blocks were allowed to cool naturally before shakeout, and samples were extracted from the white spot regions for microscopic and chemical analysis. This comprehensive approach enabled us to isolate the factors contributing to white spot formation and develop targeted solutions for ductile iron castings.
Microscopic and Chemical Analysis of White Spots
Upon examination of the test samples, the white spots exhibited a distinct morphology under scanning electron microscopy (SEM). At 25x magnification, the surface showed irregular, granular deposits that aligned with the macroscopic observations. Elemental mapping via energy-dispersive X-ray spectroscopy (EDS) revealed significant concentrations of silicon and oxygen in these areas, with iron present in adjacent regions. This suggests that the white material consists primarily of silicon oxides, which form as a result of high-temperature reactions between the coating and the molten iron in ductile iron castings.
We conducted point analysis at multiple locations within the white spot regions to quantify the elemental composition. The results, summarized in Tables 3 through 6, indicate that silicon and oxygen are the dominant elements, consistent with the formation of silica (SiO₂) or related compounds. For instance, at one analysis point, silicon accounted for over 30% of the mass fraction, while oxygen levels exceeded 45%. Trace elements such as titanium, sulfur, and manganese were also detected, likely originating from impurities in the coating or alloying additions in the ductile iron castings. The presence of carbon in some spectra may be attributed to residual organic components from the coating or environmental contamination.
| Element | Mass % | Atomic % |
|---|---|---|
| C | 23.46 | 34.07 |
| O | 46.92 | 51.17 |
| Si | 17.57 | 10.92 |
| S | 0.35 | 0.19 |
| Fe | 11.70 | 3.66 |
| Element | Mass % | Atomic % |
|---|---|---|
| C | 10.81 | 17.45 |
| O | 45.94 | 55.66 |
| Si | 34.40 | 23.74 |
| S | 0.24 | 0.14 |
| Ti | 0.18 | 0.07 |
| Mn | 0.34 | 0.12 |
| Fe | 8.09 | 2.81 |
| Element | Mass % | Atomic % |
|---|---|---|
| C | 11.27 | 17.73 |
| O | 47.59 | 56.22 |
| Si | 36.15 | 24.33 |
| Ti | 0.55 | 0.22 |
| Fe | 4.43 | 1.50 |
| Element | Mass % | Atomic % |
|---|---|---|
| C | 19.56 | 29.69 |
| O | 42.27 | 48.17 |
| Si | 29.75 | 19.31 |
| S | 0.45 | 0.26 |
| Ti | 0.23 | 0.09 |
| Mn | 0.20 | 0.07 |
| Fe | 7.29 | 2.38 |
| In | 0.26 | 0.04 |
The thickness of the white spot layer was estimated to range from tens of micrometers up to 2 mm, based on cross-sectional measurements and grinding tests. This variation can be attributed to localized temperature fluctuations and coating inhomogeneities. To understand the reaction mechanisms, we referred to unpublished data suggesting that at temperatures between 1200°C and 1400°C, silicon dioxide in the coating can undergo reduction and re-oxidation processes. The key reactions involved are: $$ \text{SiO}_2 + (\ast) \rightarrow \text{SiO}_{\text{gas}} $$ where (\ast) represents reducing agents such as magnesium, carbon, silicon, aluminum, or hydrogen present in ductile iron castings, followed by: $$ 2\text{SiO}_{\text{gas}} + \text{O}_2 \rightarrow \text{SiO}_2 $$ and $$ 2\text{SiO}_{\text{gas}} \rightarrow \text{SiO}_2 + \text{Si} $$ These equations explain the deposition of silicon-rich layers, which manifest as white spots on the surface of ductile iron castings. The Gibbs free energy change for these reactions, given by \( \Delta G = \Delta H – T\Delta S \), where \( \Delta H \) is enthalpy change and \( \Delta S \) is entropy change, becomes negative at high temperatures, favoring spontaneous formation of silicon oxides in ductile iron castings.
Improvement Strategies and Implementation
To address the white spot issue in ductile iron castings, we implemented a multi-faceted approach focusing on coating modifications and operational refinements. Firstly, we increased the zircon powder content in the coating formulation to 30% by mass, which enhanced the refractory properties and reduced the availability of reactive silicon. This adjustment improved the coating’s ability to withstand prolonged exposure to high temperatures, thereby minimizing the formation of silicon-rich layers in ductile iron castings. Additionally, we optimized the particle size distribution of the coating aggregates to achieve a penetration depth of 3–5 mm into the sand mold, ensuring better adhesion and barrier effectiveness. The target coating thickness was set between 0.35 mm and 0.50 mm, as calculated using the formula: $$ t_c = \frac{V_c}{A_s} $$ where \( t_c \) is the coating thickness, \( V_c \) is the coating volume, and \( A_s \) is the surface area, which proved critical for shielding the mold from metal penetration in ductile iron castings.
On the operational front, we standardized the coating application process by specifying Baume degrees for each layer: the first coat at 38–40°Be, the second at 55–60°Be, and the third at 45–50°Be. This gradation ensured adequate viscosity control and layer integrity. Furthermore, we mandated the use of blowtorches to bake the riser neck edges post-coating, which elevated the coating strength by reducing moisture content. The drying process was quantified by monitoring the internal mold humidity using temperature-humidity sensors, with a target relative humidity below 5% to prevent steam-related defects in ductile iron castings. Personnel training was intensified through detailed work instructions and on-site supervision, emphasizing consistent coating practices and timely interventions.
| Coating Layer | Baume Degree (°Be) | Purpose | Drying Method |
|---|---|---|---|
| First | 38–40 | Base adhesion and coverage | Air drying |
| Second | 55–60 | Build thickness and refractory layer | Blowtorch baking |
| Third | 45–50 | Final smoothing and sealing | Blowtorch baking |
The effectiveness of these measures was evaluated through repeated production trials on bearing seat castings. Post-implementation, the incidence of white spots at slant neck riser edges decreased significantly, with over 95% of castings exhibiting no visible defects after shot blasting. This reduction translated into estimated savings of 15–20% in grinding time and a corresponding decrease in production costs for ductile iron castings. The improved coating stability also contributed to better surface finish and dimensional accuracy, enhancing the overall quality of ductile iron castings. Continuous monitoring via periodic sampling and SEM analysis confirmed the sustained absence of silicon-rich layers, validating the long-term reliability of our approach for ductile iron castings.
Conclusion and Future Perspectives
In summary, our investigation into the white spots at slant neck riser edges in ductile iron castings revealed that the phenomenon is primarily driven by high-temperature reactions between the coating and molten iron, leading to the formation of silicon oxide layers. Through controlled experiments and detailed analysis, we identified key factors such as coating composition, application parameters, and thermal management as critical influencers. The implementation of optimized coatings with higher zircon content, along with standardized operational procedures, successfully mitigated the issue, resulting in improved productivity and cost efficiency for ductile iron castings.
Looking ahead, further research could explore the role of alternative coating materials or advanced temperature control systems in preventing similar defects in ductile iron castings. Additionally, computational modeling of heat transfer and reaction dynamics could provide deeper insights into optimizing riser designs for ductile iron castings. By continuing to refine these aspects, we aim to enhance the reliability and performance of ductile iron castings in demanding applications, ensuring they meet the evolving standards of industries such as wind energy and heavy machinery.