In our foundry, we operate a self-hardening furan resin sand line. Our product mix is dominated by ductile iron castings for wind power equipment and injection molding machines. One family of bearing housing ductile iron castings uses a slant neck riser. During routine inspection, we found white spots at the outer edge of the riser neck. The white spots lie between the casting surface and the coating layer. After grinding, the deepest white spots were approximately 2 mm below the casting surface. Shot blasting could not remove them. Additional grinding was required, which increased labor hours, cleaning cost, and even affected customer delivery schedules. I led a detailed investigation into the formation mechanism and developed a set of process improvements that eliminated the defect in our production line.
1. Problem Observation and Initial Analysis
The defect occurred on bearing housing ductile iron castings. From the surface morphology, the white substance did not have the appearance of sand grains. The same condition was also observed on the coating layer at the same location. Molding, melting, pouring, and operator actions showed no unusual deviations. Therefore, I focused on the interface between the liquid iron and the sand mold. The slant neck riser supplied a local high-temperature condition to the outer edge of the riser neck. At approximately 1200–1400 °C, a reaction occurred at the surface of the ductile iron castings, forming a silicon-rich layer that appeared white.
| Feature | Observation | Engineering Meaning |
|---|---|---|
| Defect location | Outer edge of the slant neck riser neck | Local hot spot created by the riser geometry |
| Defect position | Between casting surface and coating layer | Interfacial reaction rather than bulk metal defect |
| Maximum depth | About 2 mm below the casting surface | Surface layer contamination and penetration |
| Removal method | Shot blasting ineffective; additional grinding required | Strongly bonded Si-rich layer |
| Coating condition | Similar white spots found in the coating layer | Coating participates in the reaction |
| Process abnormality | No obvious abnormality in molding, melting, pouring, or operation | Defect linked to local thermal and chemical conditions |
For ductile iron castings, the solidification and cooling history near a riser neck is different from that of the main casting body. The slant neck riser maintains a high temperature at the outer edge of the neck. This thermal condition promotes chemical reactions between the coating, the sand mold, and the liquid iron. I estimated the local thermal field using a simplified one-dimensional heat conduction model:
$$T(x,t)=T_0+(T_p-T_0)\operatorname{erfc}\left(\frac{x}{2\sqrt{\alpha t}}\right)$$
where T is temperature, x is distance from the mold-metal interface, t is time, T0 is the initial mold temperature, Tp is the pouring temperature, and α is the thermal diffusivity:
$$\alpha=\frac{k}{\rho c_p}$$
Here, k is thermal conductivity, ρ is density, and cp is specific heat capacity. The heat flux from the liquid metal to the coating and mold can be written as:
$$q=\frac{T_{metal}-T_{coating}}{R_{total}}$$
$$R_{total}=R_{metal}+R_{coating}+R_{mold}$$
Because the slant neck riser is a hot spot, Tmetal remains high for a longer time. This extends the reaction time at the coating interface. In ductile iron castings, the presence of Mg, C, Si, Al, and H can reduce silica in the coating, producing gaseous SiO. The following reactions were considered:
$$\mathrm{SiO_2(s)+M(g)\rightarrow SiO(g)+MO(g)}$$
$$\mathrm{2SiO(g)+O_2(g)\rightarrow 2SiO_2(s)}$$
$$\mathrm{2SiO(g)\rightarrow SiO_2(s)+Si(s)}$$
In these equations, M represents elements such as Mg, C, Si, Al, or H that are present in ductile iron castings or in the local atmosphere. The thermodynamic driving force for each reaction can be assessed using:
$$\Delta G=\Delta G^\circ+RT\ln Q$$
$$\Delta G^\circ=\Delta H^\circ-T\Delta S^\circ$$
When ΔG is negative, the reaction is thermodynamically favorable. At 1200–1400 °C, these reactions become possible, especially under the strong thermal influence of the slant neck riser. The result is a silicon-rich surface layer on the ductile iron castings, which appears as a white spot after cooling and cleaning.
2. Experimental Reproduction
Because the white spot could not be sampled directly from production castings without damaging the component, I designed a test that reproduced the local condition of the slant neck riser. The objective was to create the white layer on a test block, sample it, and analyze its composition.
| Item | Description | Purpose |
|---|---|---|
| Coating | Commercial foundry coating, 20 kg | Provide a controlled coating layer |
| Riser | One slant neck riser | Reproduce the hot spot at the riser neck edge |
| Test block mold | 600 mm × 300 mm × 75 mm | Represent a section thickness similar to the casting |
| Ceramic tube | 40 mm diameter | Connect the riser and test block |
| Sand boxes | 700 mm × 700 mm × 200 mm and 700 mm × 700 mm × 300 mm | Contain the furan resin sand mold |
The test block was molded with the slant neck riser placed on the side of the block. A ceramic tube connected the riser to the block from the bottom. The sand mold was coated using the same coating procedure used in production. The pouring temperature was controlled at 1340–1350 °C. The chemical composition of the molten iron was controlled according to the production specification for ductile iron castings.
| Element | Control Range | Remarks |
|---|---|---|
| C, base iron | 3.40–3.45% | Mass fraction |
| Si, base iron | 2.83–2.93% | Mass fraction |
| Si, casting | 3.50–3.60% | Final silicon in ductile iron castings |
| Mn | Less than 0.025% | Low manganese |
| P | 0.040% maximum | Mass fraction |
| S, before treatment | 0.025% maximum | Base sulfur |
| S, after treatment | 0.005–0.015% | After spheroidization |
| Mg | 0.035–0.055% | Residual magnesium |
| RE | Less than 0.010% | Rare earth |
| CE | 4.56–4.65% | Carbon equivalent |
| Sb | 0.005% | Added amount |
| Treatment Step | Type | Ratio | Addition |
|---|---|---|---|
| Spheroidization | N1 | 70% | Total 0.90–1.20% |
| Spheroidization | N2 | 30% | Total 0.90–1.20% |
| Inoculation | BS-1A | Ladle bottom | 0.10–0.70% |
The experimental procedure was as follows:
1. A slant neck riser was selected and positioned at the side of the test block mold.
2. A ceramic tube was installed from the bottom to connect the riser and the test block.
3. The sand mold was coated with the selected coating.
4. Molten iron was poured at 1340–1350 °C.
5. After solidification, the test block and riser were shaken out and cleaned.
6. The white spot area was sectioned and prepared for scanning electron microscopy and energy-dispersive spectroscopy.
The solidification time of the test block can be approximated by:
$$t_{solid}=\frac{\rho V H_f}{hA(T_m-T_0)}$$
where V is volume, A is surface area, Hf is latent heat, h is the heat transfer coefficient, Tm is the melting temperature, and T0 is the initial mold temperature. The test block was designed so that the local cooling time near the riser neck was similar to that of the production ductile iron castings.
3. Microscopic and Compositional Analysis
The sectioned test block showed a white substance on the surface. Under scanning electron microscopy, the white area was distinct from the dark metallic area. Elemental mapping showed that Si and O were concentrated in the white area, while Fe was distributed mainly outside the Si-rich region. Ti was also detected in small amounts, most likely from a marker used during sample preparation.
| Analysis Position | C (%) | O (%) | Si (%) | S (%) | Ti (%) | Mn (%) | Fe (%) | In (%) |
|---|---|---|---|---|---|---|---|---|
| Position 1 | 23.46 | 46.92 | 17.57 | 0.35 | — | — | 11.70 | — |
| Position 2 | 10.81 | 45.94 | 34.40 | 0.24 | 0.18 | 0.34 | 8.09 | — |
| Position 3 | 11.27 | 47.59 | 36.15 | — | 0.55 | — | 4.43 | — |
| Position 4 | 19.56 | 42.27 | 29.75 | 0.45 | 0.23 | 0.20 | 7.29 | 0.26 |
The point analysis results showed that the selected white regions were dominated by Si and O. Minor amounts of Ti, S, Mn, and Fe were also detected. The dark areas were rich in Fe, indicating that liquid iron had penetrated into the coating or that the coating had partially lost its isolating effect. Based on the elemental distribution, I concluded that the white substance was mainly silicon oxide. The thickness of the silicon-rich layer was estimated to be between several tens of micrometers and 2 mm, which agreed with the grinding observations on the production ductile iron castings.
The growth of the silicon-rich layer can be described qualitatively by:
$$\delta_{Si-rich}=\int_{t_0}^{t_f} v_{SiO}(T(t))dt$$
where vSiO is the deposition or reaction rate, and t0 to tf is the time interval during which the interface remains above the critical reaction temperature. A simplified estimate is:
$$d_{white}\approx \bar{v}_{growth}\Delta t_{high}$$
For example, if the average growth rate is between 0.01 and 0.05 mm/min and the high-temperature duration is between 10 and 40 min, the calculated thickness is:
$$\begin{aligned}
d_{min} &= 0.01 \times 10 = 0.10\ \text{mm} \\
d_{max} &= 0.05 \times 40 = 2.00\ \text{mm}
\end{aligned}$$
This range matches the measured depth of the white spots in the ductile iron castings.
4. Formation Mechanism
The slant neck riser provides a local thermal field that keeps the outer edge of the riser neck at a high temperature for a long time. Under this condition, the coating layer and the sand mold surface react with the liquid iron and the furnace atmosphere. The silica in the coating can be reduced or vaporized, forming gaseous SiO. The gas can migrate through pores in the coating. When it reaches an oxidizing or cooler zone, it can oxidize back to SiO2 or decompose to SiO2 and Si. These products deposit on the surface of the ductile iron castings and form a white, silicon-rich layer.
| Reaction | Role in White Spot Formation | Effect on Ductile Iron Castings |
|---|---|---|
| SiO2 + C → SiO + CO | Reduction of silica by carbon | Produces gaseous SiO |
| SiO2 + H2 → SiO + H2O | Reduction by hydrogen | Increases SiO partial pressure |
| SiO2 + Mg → SiO + MgO | Reduction by magnesium | Possible in ductile iron castings |
| 2SiO + O2 → 2SiO2 | Oxidation of SiO gas | Deposits white silica-rich phase |
| 2SiO → SiO2 + Si | Disproportionation of SiO | Forms Si-rich surface layer |
The coating acts as a barrier between the sand mold and the liquid metal. If the coating has low refractoriness, low zircon content, or insufficient thickness, it cannot resist the long high-temperature exposure created by the slant neck riser. Liquid iron can then penetrate into the coating and sand mold, and silica-based reaction products can accumulate at the casting surface. The local thermal condition is especially important because the riser neck remains hot while the main casting body begins to cool. This thermal gradient drives gas migration and surface reactions.
The activity of silica in the coating can be expressed as:
$$a_{SiO_2}=\gamma_{SiO_2}x_{SiO_2}$$
where γSiO2 is the activity coefficient and xSiO2 is the mole fraction. Increasing the zircon content reduces the mole fraction of free silica, which lowers the activity of SiO2 and suppresses the formation of gaseous SiO. This is one of the key reasons for using a high-zircon coating in ductile iron castings with hot riser necks.
5. Process Improvements
Based on the analysis, I concluded that the white spot was directly related to the coating and the local thermal field. The improvements were therefore focused on coating material, coating application, and mold drying. I implemented the following measures in our production line.
| Improvement Area | Original Condition | New Condition | Target Effect |
|---|---|---|---|
| Coating aggregate | Relatively high silica content | Zircon content increased to 30% | Lower SiO2 activity, higher refractoriness |
| Aggregate particle size | Broad and uncontrolled distribution | Optimized for penetration and surface coverage | Penetration layer of 3–5 mm |
| Coating thickness | Variable | Controlled to 0.35–0.5 mm | Better shielding and isolation |
| Baume degree | Not strictly controlled | First coat 38–40 °Be, second coat 55–60 °Be, third coat 45–50 °Be | Consistent coating viscosity and coverage |
| Drying | Natural drying only | Torch baking at riser neck outer edge plus hot air | Higher coating strength and lower moisture |
| Operator training | Experience-based | Standard work instruction and process tracking | Repeatable application quality |
| Mold atmosphere | Limited monitoring | Temperature and humidity checked inside the mold | Dry mold cavity and stable coating |
5.1 Coating Material Optimization
I increased the zircon content in the coating aggregate to 30%. Zircon has high refractoriness and low reactivity with liquid iron. It reduces the amount of free silica available for the SiO-forming reactions. The coating also needs to sinter and form a stable layer at high temperature. A stable sintered layer can isolate the sand mold from the liquid metal and prevent iron penetration. The coating thickness was controlled to 0.35–0.5 mm. The penetration layer was targeted at 3–5 mm so that the coating could bind the surface sand grains and create a protective barrier.
The relationship between coating thickness, mass, and area is:
$$t_c=\frac{m_c}{\rho_c A_c}$$
where tc is coating thickness, mc is coating mass, ρc is coating density, and Ac is coated area. For a given coating area, the mass and density must be controlled to achieve the target thickness. The Baume degree of the coating was also standardized:
$$\rho=\frac{145}{145-B}$$
where B is the Baume degree. The first coat was applied at 38–40 °Be, the second coat at 55–60 °Be, and the third coat at 45–50 °Be. This sequence provided good penetration in the first coat, high solid content in the second coat, and a smooth final layer in the third coat. The thermal resistance of the coating layer can be estimated by:
$$R_c=\frac{t_c}{k_c}$$
where kc is the thermal conductivity of the coating. A higher zircon content and a denser sintered layer increase the effective thermal resistance and reduce the heat flux to the sand mold, which lowers the reaction rate.
5.2 Coating Application and Drying
I specified that the outer edge of the riser neck must be baked with a torch after coating. This ensures that the coating is fully dried and that the binder is properly cured. The torch baking also improves the strength of the coating at the hot spot. In addition, the mold is dried with hot air, and the internal temperature and humidity of the sand mold are measured before pouring. These steps reduce the amount of moisture that can react with the liquid iron and produce gases.
The penetration depth of the coating into the sand mold can be approximated by:
$$x_p=2\sqrt{\alpha t}$$
where α is the diffusivity of the coating liquid in the sand and t is the penetration time. By controlling the coating viscosity and application time, the penetration layer was kept in the 3–5 mm range. This range is sufficient to bind the surface sand and form a protective layer without excessive penetration that could cause sand inclusion or coating spalling.
5.3 Operator Standardization
I trained the coating operators and issued a work instruction for coating application. The instruction defines the number of coats, the Baume degree for each coat, the drying method, and the required coating thickness. The coating process is now tracked on the shop floor. The mold is checked for internal dryness before pouring. These actions made the process repeatable and reduced the variation that had contributed to the white spot problem in ductile iron castings.
6. Verification and Production Results
After implementing the improvements, I tracked the bearing housing ductile iron castings over multiple production batches. The white spot at the outer edge of the slant neck riser was effectively controlled. The cleaning time was reduced, and the additional grinding operation was largely eliminated.
| Metric | Before Improvement | After Improvement | Change |
|---|---|---|---|
| White spot occurrence | Frequent at riser neck edge | Rare and localized | Significant reduction |
| Maximum white spot depth | Up to 2 mm | Less than 0.2 mm in isolated trials | Lower penetration |
| Shot blasting removal | Ineffective | Mostly effective | Less manual work |
| Additional grinding time per casting | High | Low | Reduced labor |
| Coating thickness | Variable | 0.35–0.5 mm | Controlled |
| Coating zircon content | Lower | 30% | Higher refractoriness |
| Mold dryness | Not fully controlled | Torch baking and hot air drying | Lower moisture |
The rework cost before and after the improvement can be estimated by:
$$C_{rework}=N t_g r_l$$
where N is the number of affected castings, tg is the additional grinding time per casting, and rl is the labor rate. The cost saving is:
$$\Delta C=(N t_g-N’ t’_g)r_l$$
For example, if 100 castings required 15 min of extra grinding each before the improvement, and after the improvement only 3 castings require 2 min each, with a labor rate of 50 per hour, the saving is:
$$\begin{aligned}
C_{before} &= 100 \times \frac{15}{60} \times 50 = 1250 \\
C_{after} &= 3 \times \frac{2}{60} \times 50 = 5 \\
\Delta C &= 1250-5=1245
\end{aligned}$$
This calculation shows that even a small number of white spots can create a large cleaning cost in ductile iron castings production. Controlling the defect at the coating and thermal-field level is therefore more effective than removing the white spot after casting.
7. Discussion
The white spot is not a simple sand burn-on defect. It is a silicon-rich surface layer formed by high-temperature reactions between the coating, the mold atmosphere, and the liquid iron. The slant neck riser is the key thermal trigger because it keeps the outer edge of the riser neck at 1200–1400 °C for a longer time. In this temperature range, silica in the coating can be reduced to gaseous SiO, which then oxidizes or decomposes to form SiO2 and Si on the surface of the ductile iron castings. The presence of Fe in the dark areas indicates that liquid iron can penetrate the coating when the coating is not sufficiently refractory or when the coating layer is too thin.
The heat balance of the local interface can be written as:
$$Q_{in}=Q_{casting}+Q_{coating}+Q_{mold}+Q_{loss}$$
where Qin is the heat supplied by the liquid iron, Qcasting is the heat stored in the casting, Qcoating is the heat absorbed by the coating, Qmold is the heat transferred to the sand mold, and Qloss is the heat lost to the environment. A high-zircon coating increases the thermal resistance and reduces the heat flux into the sand mold. This lowers the amount of silica that reacts and reduces the gas formation. The coating must also sinter at high temperature. The sintering kinetics can be described qualitatively by:
$$t_{sinter}\propto\exp\left(\frac{E_a}{RT}\right)$$
where Ea is the activation energy, R is the gas constant, and T is the absolute temperature. At the hot spot near the slant neck riser, the sintering rate is higher. If the coating has a well-designed aggregate size distribution and a high zircon content, it forms a dense sintered layer that resists iron penetration and gas migration.
For ductile iron castings, the problem is also influenced by the residual magnesium and the reducing atmosphere in the mold. Magnesium can reduce silica and form MgO, which may further contribute to the gas phase. The reactions are complex, but the practical conclusion is clear: the coating must be designed for the local thermal load created by the riser. A coating that works well on a thin section may fail at a hot riser neck. Therefore, the coating specification should be differentiated by location in ductile iron castings.
8. Conclusions
1. The slant neck riser provides a temperature field condition that promotes white spot formation on ductile iron castings. The white spot is directly related to the local thermal history at the riser neck outer edge.
2. The white substance is mainly a silicon-rich layer composed of Si and O. It forms through high-temperature reactions involving silica in the coating and the mold atmosphere. The depth can range from several tens of micrometers to about 2 mm.
3. Increasing the zircon content in the coating to 30% reduces the free silica content and suppresses the formation of gaseous SiO. This improves the refractoriness and high-temperature stability of the coating.
4. Optimizing the coating aggregate particle size and controlling the penetration layer to 3–5 mm allows the coating to bind the surface sand and form an effective isolating barrier.
5. Controlling the coating thickness to 0.35–0.5 mm and standardizing the Baume degree for each coat ensures consistent coating strength and coverage. Torch baking at the riser neck outer edge and hot air drying further improve the coating quality.
6. The improved coating and application process significantly reduced the white spot defect on ductile iron castings, shortened cleaning time, lowered production cost, and improved delivery reliability.
7. The root cause of the white spot is not a single material defect but a combined effect of local thermal concentration, silica reactivity, coating thickness, coating composition, and mold moisture. A systematic approach that controls both the thermal field and the coating barrier is required to solve this type of defect in ductile iron castings.
8. For future production, I recommend maintaining the high-zircon coating specification for all hot spots associated with slant neck risers, monitoring coating thickness and Baume degree, and keeping the mold cavity dry before pouring. These practices provide stable protection for ductile iron castings and prevent the recurrence of the white spot.