In my recent investigation, I encountered a perplexing issue in a production facility where cylinder blocks made of grey cast iron, specifically HT250 grade, developed sudden rust spots during storage prior to machining. These rust spots were primarily observed on the crankcase side near the ingate, appearing as scattered, diffuse patches that worsened over time. Even after re-blasting to remove the rust, the spots reappeared within days, indicating a subsurface defect. This problem affected approximately 30% of the cylinder blocks, severely disrupting supply to the main engine manufacturer. As part of the technical team, I was tasked with analyzing the root cause, focusing on metallurgical and process factors. Grey cast iron is widely used for such components due to its good castability and mechanical properties, but this defect posed a significant quality challenge.
The rust spots were macroscopically characterized as discrete points ranging from 1 mm×1 mm to 4 mm×3 mm, localized on the upper surface of the crankcase adjacent to the ingate. To understand the underlying mechanism, I conducted a series of microstructural and compositional analyses. Initial visual inspection confirmed that the defect was consistent across three different types of cylinder blocks, while one variant remained unaffected. The key difference lay in the ingate design: the unaffected blocks had ingates at the bearing seat, whereas the affected ones had ingates at the crankcase flange. This hinted at fluid flow dynamics during mold filling playing a role in defect formation.
Microscopic examination of cross-sections taken perpendicular to the rust spots revealed a critical finding: a grayish slag inclusion embedded shallowly beneath the surface, at depths varying between 0.06 mm and 0.22 mm depending on the block type. This inclusion was porous and loosely adherent, explaining why blasting could not entirely eliminate it. To identify the slag composition, I performed energy-dispersive spectroscopy (EDS) and chemical analysis. The EDS results consistently showed prominent peaks for sulfur (S), calcium (Ca), aluminum (Al), silicon (Si), and iron (Fe). A representative EDS spectrum from a slag area highlighted the presence of Ca and S, suggesting compounds like calcium oxide (CaO) and calcium sulfide (CaS). The chemical analysis of the rust spot regions further confirmed these elements, aligning with the hypothesis that exogenous impurities were involved.
Given that Ca and Al are not typical constituents of grey cast iron in significant amounts, I traced potential sources through the casting process. The focus turned to auxiliary materials such as binders, coatings, and inoculants. After systematic排查, the stream inoculant used during pouring emerged as the prime suspect. The facility employed a high-efficiency stream inoculant with fine granules (0.1–0.7 mm size) for the grey cast iron treatment. Microscopic inspection of this inoculant revealed grayish phases within its structure, which EDS analysis identified as rich in Al, Ca, Fe, and Si—matching the slag composition in the cylinder blocks. Chemical analysis of the inoculants provided quantitative data, as summarized in Table 1.
| Inoculant Type | Ca | Al | Mn | S | Other Elements |
|---|---|---|---|---|---|
| High-Efficiency Stream Inoculant | 3.32 | 0.90 | 0.21 | <0.0045 | Balance (Si, Fe) |
| Ordinary Stream Inoculant | 0.88 | 1.12 | 0.21 | 0.013 | Balance (Si, Fe) |
The high-calcium content (w(Ca) > 1%) in the efficient inoculant was particularly noteworthy. In grey cast iron metallurgy, calcium has a strong affinity for oxygen and sulfur, especially at elevated temperatures. During pouring, the stream inoculant is introduced into the molten iron to promote graphite nucleation. However, if not fully dissolved or if introduced prematurely, it can form slag particles. I hypothesized that for the affected cylinder blocks, the ingate design at the crankcase flange resulted in a relatively smooth mold filling with reduced turbulence. When the inoculant was added, a slight lead time in dosing meant that some granules entered the sprue before the iron flow, becoming carried into the mold cavity. In thin-walled sections like the crankcase, the molten iron velocity drops rapidly, allowing viscous slag—comprising CaO and CaS from reactions between Ca, atmospheric oxygen, and sulfur in the iron—to float and adhere to the upper surface of the cavity. This formed shallow, porous inclusions, as observed.
To mathematically model the slag formation tendency, consider the reaction kinetics for calcium compounds in grey cast iron. The formation of CaO and CaS can be described by the following equations:
$$ \text{Ca (from inoculant)} + \frac{1}{2} \text{O}_2 \rightarrow \text{CaO} $$
$$ \text{Ca} + \text{S (in iron)} \rightarrow \text{CaS} $$
The free energy change for these reactions at casting temperatures (around 1400°C) is highly negative, favoring slag formation. The amount of slag generated depends on factors like calcium content, oxygen potential, and sulfur activity. Using thermodynamic data, the propensity for inclusion formation can be approximated. For instance, the stability of CaS in grey cast iron is influenced by sulfur concentration, which typically ranges from 0.05% to 0.12% in HT250. The reaction quotient Q for CaS formation is given by:
$$ Q = \frac{a_{\text{CaS}}}{a_{\text{Ca}} \cdot a_{\text{S}}} $$
where \( a \) denotes activity. When Q exceeds the equilibrium constant K, precipitation occurs. In practice, high-calcium inoculants increase \( a_{\text{Ca}} \), shifting the equilibrium toward slag formation.
The rusting mechanism on the grey cast iron surface is an electrochemical process exacerbated by the hygroscopic nature of CaO and CaS. These compounds absorb moisture from the air during storage, creating localized wet cells on the cylinder block surface. The standard rusting reactions involve iron oxidation and oxygen reduction, summarized below:
Anodic reaction: $$ \text{Fe} – 2e^- \rightarrow \text{Fe}^{2+} $$
Cathodic reaction: $$ \text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^- $$
Overall reaction: $$ 2\text{Fe} + 2\text{H}_2\text{O} + \text{O}_2 \rightarrow 2\text{Fe(OH)}_2 $$
Followed by: $$ 4\text{Fe(OH)}_2 + 2\text{H}_2\text{O} + \text{O}_2 \rightarrow 4\text{Fe(OH)}_3 $$
And dehydration: $$ 2\text{Fe(OH)}_3 \rightarrow \text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O} \text{ (rust)} $$
The presence of CaO and CaS inclusions accelerates this by providing a porous, moisture-retentive site. The slag’s depth (0.1–0.2 mm) means it is not fully removed by shot blasting, leaving a reservoir for continuous corrosion. I confirmed this by placing polished samples in a desiccator; even in controlled humidity, the rust spots expanded, indicating ongoing electrochemical activity.
To further validate the source, I compared the microstructures of the inoculant and the slag. Table 2 summarizes the EDS data from key areas, highlighting the compositional match.
| Sample Area | Ca | S | Al | Si | Fe | O |
|---|---|---|---|---|---|---|
| Rust Spot Slag (Cylinder Block) | 18.5 | 12.3 | 8.7 | 15.2 | 35.1 | 10.2 |
| Gray Phase in High-Efficiency Inoculant | 20.1 | 10.8 | 9.5 | 16.4 | 33.0 | 10.2 |
| Normal Grey Cast Iron Matrix | 0.1 | 0.3 | 0.5 | 25.0 | 73.0 | 1.1 |
The data clearly shows elevated Ca, S, and Al in both the slag and the inoculant’s gray phase, confirming the transfer of impurities. For grey cast iron, such inclusions are detrimental not only for corrosion but also for mechanical properties, as they can act as stress concentrators.
In the broader context of grey cast iron casting, the choice of inoculant is critical. Inoculation aims to improve graphite morphology and reduce chilling, but high-calcium formulations, while effective for nucleation, pose slagging risks. The fluid dynamics during mold filling also play a role. For the unaffected cylinder blocks with ingates at the bearing seat, the molten grey cast iron enters with higher turbulence, dispersing any slag particles and preventing localized adhesion. This aligns with Bernoulli’s principle for fluid flow, where velocity changes affect particle settlement. The pressure difference \( \Delta P \) in a thin section can be expressed as:
$$ \Delta P = \frac{1}{2} \rho (v_1^2 – v_2^2) $$
where \( \rho \) is the density of grey cast iron, and \( v_1 \) and \( v_2 \) are velocities at the ingate and crankcase, respectively. A rapid drop in velocity (high \( \Delta P \)) in thin walls promotes slag deposition.
Based on these findings, I recommended switching from the high-calcium inoculant to an ordinary stream inoculant with lower calcium content (w(Ca) ≈ 0.88%). After implementation, the rust spots on the grey cast iron cylinder blocks were eliminated, confirming the hypothesis. This case underscores the importance of tailoring inoculation practices to the casting geometry. For thin-walled grey cast iron components where mold filling is平稳, high-calcium inoculants should be avoided to prevent slag-related defects.
To generalize, the quality of grey cast iron castings depends on a balance of inoculation efficiency and slag minimization. The following equation can guide inoculant selection for grey cast iron:
$$ \text{Slag Risk Index} = k \cdot \frac{[Ca]_{\text{inoculant}} \cdot [S]_{\text{iron}}}{T_{\text{pouring}} \cdot v_{\text{flow}}} $$
where \( k \) is a material constant, \( [Ca]_{\text{inoculant}} \) is the calcium content in the inoculant, \( [S]_{\text{iron}} \) is the sulfur content in the grey cast iron, \( T_{\text{pouring}} \) is the pouring temperature, and \( v_{\text{flow}} \) is the flow velocity in critical sections. A higher index indicates greater slagging potential.
In conclusion, my analysis demonstrated that rust spots on grey cast iron cylinder blocks originated from CaO and CaS slag inclusions derived from a high-calcium stream inoculant. These inclusions, embedded shallowly beneath the surface, absorbed moisture and initiated electrochemical corrosion during storage. The problem was specific to ingate designs that reduced turbulence in thin-walled areas, allowing slag accumulation. By opting for a low-calcium inoculant, the issue was resolved. This experience highlights the need for careful process optimization in grey cast iron foundries, particularly for components prone to surface defects. Future work could involve real-time monitoring of inoculant dissolution and computational fluid dynamics simulations to predict slag formation in grey cast iron castings.
Throughout this investigation, the repeated focus on grey cast iron properties and processing parameters was essential. Grey cast iron remains a versatile material, but its performance can be compromised by minor impurities. Ensuring clean melt practices and appropriate inoculation strategies is paramount for high-quality grey cast iron components in automotive and industrial applications.