In our production of large-bore marine diesel engine crankshafts using ductile iron, we encountered a severe and persistent quality issue that significantly impacted our operational efficiency and economic performance. The defect in question was the formation of slag inclusions, often referred to as “black spots,” within the casting. At its peak, the overall casting rejection rate soared to approximately 30%, with slag inclusions accounting for a staggering 70% of all rejected crankshafts. This critical situation necessitated the formation of a dedicated quality task force to scientifically analyze the root causes and mechanisms behind slag inclusion formation and to develop effective countermeasures. Through systematic investigation and process refinement, we achieved remarkable results. The rejection rate began a steady decline and was ultimately reduced to below 1% within a few years. Crucially, not a single crankshaft was scrapped due to slag inclusions during this period, fundamentally transforming the production landscape, lowering costs, and enhancing both economic and social benefits.
The primary and most detrimental defect we faced was the presence of slag inclusions. These inclusions predominantly manifested on the upper surfaces of the crankpin and main journals, on the lower surfaces of cores, and on the shoulder faces in the cope half of the mold. Their characteristic shape was crescent or “半月形,” and they were notably more severe in areas furthest from the ingates. These defects were typically discovered during machining operations. Inclusions contained within the machining allowance could be removed, but more severe cases were clearly visible as dull, cloud-like patches on the machined surface. These areas were prone to rapid oxidation upon exposure to air.
To understand the nature of these slag inclusions, we conducted a detailed metallographic and chemical analysis comparing material from defective and sound regions. Samples were taken from a main journal near the large flange at the far end of the casting.
| Observation | Sound Region | Slag Inclusion Region |
|---|---|---|
| Macrostructure | Uniform, dense matrix with well-dispersed nodular graphite. | Presence of blocky, curved, and streaky inclusions. |
| Microstructure (Dark Field) | N/A | Inclusions appear gray-black with bright surrounding halos; graphite remains nodular at inclusion edges. |
| Primary Conclusion | Healthy ductile iron microstructure. | Inclusions are primarily oxide-based slag. |
Chemical analysis of these regions revealed significant differences in key elements, as summarized below:
| Element | Sound Region (wt.%) | Slag Inclusion Region (wt.%) |
|---|---|---|
| Mg | 0.035 | 0.047 |
| RE (Rare Earth) | 0.021 | 0.029 |
| Si | 2.45 | 2.65 |
| S | 0.008 | 0.012 |
The elevated levels of Mg, RE, Si, and S in the slag inclusion zone strongly suggest that a significant portion of these elements exists in the form of compounds—namely, oxides and sulfides—rather than in solid solution within the iron matrix. This finding is central to understanding the mechanism of slag inclusion formation.
Mechanism of Slag Inclusion Formation
The formation of slag inclusions in ductile iron is a complex process involving chemical reactions during treatment and physical phenomena during pouring and solidification. Our analysis identifies two main sources: primary and secondary slag.
1. Primary Slag Formation (During Treatment):
During the nodularization process, elements like Magnesium (Mg) and Rare Earth (RE) from the treatment alloy react vigorously with oxygen and sulfur present in the base iron.
$$2Mg + O_2 \rightarrow 2MgO$$
$$2Mg + S \rightarrow Mg_2S$$
$$2RE + 3O_2 \rightarrow RE_2O_3$$
$$2RE + 3S \rightarrow RE_2S_3$$
Compounds like MgO and Mg2S have densities lower than that of ductile iron melt and typically float to the surface for removal. However, oxides of certain rare earth elements (e.g., RE2O3) have densities much closer to that of the iron melt, making their buoyant rise difficult. A substantial amount of these fine, dense oxide particles can remain suspended in the melt, forming the core of primary slag inclusions.
2. Secondary Slag Formation (During Pouring and Molding):
This stage is critical for the formation of macroscopic slag inclusion defects. As the treated iron is poured, fresh metal surfaces are continuously exposed to air and moisture, leading to re-oxidation.
$$Si + O_2 \rightarrow SiO_2$$
$$2Mg + O_2 \rightarrow 2MgO \quad \text{(re-occurring)}$$
This forms a surface oxide film on the flowing metal. Turbulence and splashing during mold filling inevitably tear and entrain this film into the bulk liquid. These entrapped oxide films, being large and lightweight, can rise rapidly. During their ascent, they act as scavengers, adsorbing the primary RE oxides, sulfides, and even free graphite particles. This aggregated mass floats to the upper surfaces of the mold cavity (cope side, top of cores) and becomes trapped by the advancing solidification front, resulting in the characteristic crescent-shaped slag inclusion. The severity is dictated by the balance between the rate of slag flotation and the rate of solidification layer formation, which is heavily influenced by pouring temperature.
The final slag inclusion is therefore a conglomerate of primary (dense RE oxides) and secondary (lighter silicate/magnesia films) oxidation products. The factors influencing the severity of slag inclusion can be summarized by the following relationship:
$$S_{slag} \propto \frac{[O]_{total} \cdot [RE]_{res} \cdot [S]_{base}}{T_{pour} \cdot t_{flotation}}$$
Where:
$S_{slag}$ = Severity of slag inclusion defect.
$[O]_{total}$ = Total oxygen content (dissolved + from re-oxidation).
$[RE]_{res}$ = Residual rare earth content post-treatment.
$[S]_{base}$ = Sulfur content in base iron.
$T_{pour}$ = Pouring temperature.
$t_{flotation}$ = Effective time available for slag flotation before solidification.
Root Cause Analysis and Key Factors
Our investigation pinpointed several interconnected root causes, with the original gating system design being a major contributor.
Ineffective Gating System: The initial design used a single, heavy ingate at the riser end. For a long, complex crankshaft casting with multiple off-set crankpins, this resulted in severe turbulence, metal “waterfalling,” and extended flow paths. The iron at the flow front cooled excessively, increasing oxide film formation. Areas farthest from the ingate suffered the most from both temperature loss and prolonged exposure to turbulence, explaining the severe slag inclusions at the remote end.
Beyond gating, four key metallurgical and process factors were identified:
| Factor | Role in Slag Inclusion Formation | Critical Control Point |
|---|---|---|
| Pouring Temperature ($T_{pour}$) | The most critical factor. Low temperature increases melt viscosity, reduces slag buoyancy, and promotes rapid formation of a thick solid “shell” that traps slag. It also thickens the oxide film on the flow front. | Primary controlling variable. Directly impacts $t_{flotation}$ in the severity equation. |
| Base Iron Sulfur Content ($[S]_{base}$) | High S content consumes more Mg/RE to form sulfides, potentially leaving less for effective nodularization and increasing the volume of primary slag products. | Should be minimized. A high S level exacerbates slag formation at any given temperature. |
| Residual Rare Earth Content ($[RE]_{res}$) | Excessive RE increases the amount of dense RE oxides that are difficult to float out, providing more nuclei for secondary slag to aggregate around. | Must be optimized. Sufficient for nodularization but minimal to avoid excessive primary oxides. |
| Residual Magnesium Content ($[Mg]_{res}$) | While necessary for nodularization, high residual Mg increases the tendency for surface oxidation and dross formation during pouring. However, its effect is less pronounced than S or RE. | Kept at the minimum level required for consistent nodularization. |
The interaction is clear: While $[S]_{base}$, $[RE]_{res}$, and $[Mg]_{res}$ are important, their detrimental impact is profoundly amplified by low $T_{pour}$. At high pouring temperatures, the system is more forgiving of minor variances in these chemical factors.
Implemented Countermeasures and Solutions
Based on the above analysis, we implemented a comprehensive set of corrective actions targeting every stage of the process.
1. Redesign of the Gating System:
We replaced the problematic single-ingate system with a dispersed, multiple-ingate system. This design significantly shortens the flow path for iron to reach all sections of the crankshaft, minimizes temperature drop and thermal gradients, and drastically reduces turbulence, waterfalling, and metal front oxidation. This was the single most impactful change in eliminating slag inclusions, particularly in remote sections.
2. Maximizing Melt Temperature and Quality:
A high, consistent base iron temperature is foundational. We implemented:
- Computer control for the cupola to optimize combustion and stability.
- Strict charge management: using best-quality charge materials during the middle of the melt, pre-heating, and adding a buffer coke layer.
- Using a small amount of calcium carbide ($CaC_2$) early in the melt for desulfurization.
- Thorough slagging of the receiver ladle before holding ductile iron.
- Targeting a spout temperature above 1500°C.
The goal is to maximize $T_{pour}$ and minimize $[S]_{base}$.
3. Optimizing Treatment Parameters:
- Carbon Equivalent (CE): Controlled near the eutectic point ($CE \approx 4.3$). Eutectic composition provides the maximum fluidity and longest “liquid window” for a given superheat, maximizing $t_{flotation}$.
$$ CE = \%C + \frac{1}{3}(\%Si + \%P) $$ - Treatment Alloy Addition: Minimized to the lowest effective level to achieve consistent nodularity without chill. We adopted multi-component alloys (e.g., Si-Ca, RE-Ca-Si-Fe) which promote the formation of lower-melting-point, lower-density complex silicates that help carry dense RE oxides to the surface.
4. Effective Slag Management and Covering:
- Cryolite ($Na_3AlF_6$) Usage: Employed strategically in two stages. A small amount (0.1%) added to the treatment ladle helps form a fluid slag. After transfer and skimming, another 0.1% is plunged into the pouring ladle to further cleanse the melt, before a final cover is applied. Cryolite decomposes, releasing gases that capture inclusions, and forms a thin, protective fluid layer that dissolves oxides and prevents re-oxidation. Fume extraction is mandatory.
- Perlite as a Covering Agent: We replaced part of the cryolite cover with expanded perlite. It forms an excellent, glassy, impermeable barrier over the metal surface, effectively agglomerating slag, preventing air ingress, and minimizing temperature loss and magnesium fade.
5. Strict Process Control Timings and Temperature:
- Treatment-to-Pour Time: The entire process from treatment ladle filling to final skimming is completed within 12 minutes, followed by a 5-minute quiet holding period to allow final slag flotation.
- Pouring Temperature Control: This is critically enforced. The target range is 1360°C to 1380°C. Below 1350°C, the risk of slag inclusion increases exponentially. We define a critical pouring temperature threshold:
$$ T_{pour}^{critical} = T_{liquidus} + \Delta T_{superheat} + \Delta T_{loss}$$
Where $\Delta T_{loss}$ accounts for ladle and gating system losses, and $\Delta T_{superheat}$ must be sufficient to allow slag flotation before the $T_{liquidus}$ is reached at the casting surface. - Mold and Core Drying/Gas Venting: All cores are baked to a residual moisture content below 0.5%. Extensive venting is provided via the mold parting line, multiple small vents on heavy sections, and a vent on top of the spherical riser to ensure smooth gas escape and minimize back-pressure that can cause turbulence.
Consolidated Production Control Parameters
The following table summarizes the key control parameters established to prevent slag inclusion:
| Process Stage | Parameter | Target/Control Limit |
|---|---|---|
| Base Iron | Sulfur Content ($[S]_{base}$) | < 0.02% (Aim for ≤ 0.015%) |
| Cupola Spout Temperature | > 1500°C | |
| Treatment | Carbon Equivalent (CE) | ~4.3 (Eutectic) |
| Residual Rare Earth ($[RE]_{res}$) | 0.02% – 0.03% (Do not exceed 0.04%) | |
| Residual Magnesium ($[Mg]_{res}$) | Minimal for consistent nodularization | |
| Post-Treatment | Total Process Time (Treatment to Skim) | < 12 minutes |
| Quiet Holding Time | ~5 minutes | |
| Cryolite Addition | 0.1% in ladle + 0.1% cover (with fume extraction) | |
| Pouring | Pouring Temperature ($T_{pour}$) | 1360°C – 1380°C (Never below 1350°C) |
| Gating System Design | Dispersed, multiple ingates to minimize flow length & turbulence | |
| Mold/Core | Core Moisture | < 0.5% |
Conclusion
The successful resolution of the chronic slag inclusion problem in large-face ductile iron crankshafts was achieved through a methodical, science-based approach. The core conclusions are:
- The slag inclusion defect is a composite of primary (dense RE oxides) and secondary (entrained silicate/magnesia films) oxidation products formed during treatment and molding, respectively.
- While chemical factors like sulfur and residual rare earth content are significant, the pouring temperature is the dominant controlling variable. Its effect on melt fluidity, slag buoyancy, and solidification kinetics outweighs others. The relationship can be conceptualized by the severity function $S_{slag}$.
- Gating system design is paramount. A system that minimizes flow length, temperature drop, and turbulence is essential to prevent the conditions that foster severe re-oxidation and slag entrapment, especially in complex, long castings.
- A holistic set of countermeasures, targeting melt quality, treatment optimization, active slag management, and strict thermal and temporal process controls, is required for consistent, slag-free production. The implementation of a dispersed gating system, coupled with rigorous enforcement of high pouring temperatures and optimized treatment practices, proved to be the decisive factors in eliminating slag inclusions and achieving high-quality, economical crankshaft production.