In our foundry, during the experimental implementation of two novel processes—pneumatic desulfurization and in-mold spheroidization—we encountered distinctive and recurring surface defects on cast nodular iron crankshafts. These casting defects, not commonly observed with other conventional molding techniques, manifested as white and black discolorations on the machined surfaces, primarily on the upper cope-side sections of the main bearing journals and connecting rod journals. The frequent, batch-wide occurrence of these flaws severely compromised product quality and yield, necessitating a thorough investigation into their nature and root cause.
The white spots, with dimensions ranging from approximately 2 to 10 mm, were characterized by a layer of white, powdery substance that could be easily dislodged with a needle, leaving behind a pit. The black spots were identified as dense areas of micro-shrinkage porosity and slag inclusions. To decipher the fundamental characteristics of these casting defects, a systematic analytical campaign involving macro-examination, hot acid etching, optical emission spectroscopy (OES), and metallography was undertaken.
1. Characterization of the Casting Defects
1.1 Hot Acid Etching Analysis
Samples were subjected to etching in a 50% hydrochloric acid aqueous solution heated to 70±5°C for 30 minutes. This treatment revealed critical subsurface features of the casting defects:
- White Spot Samples: Portions of the white powder were dissolved by the acid, but the underlying areas exhibited severe porosity and cavities, indicating that the defect was more than just a surface deposit.
- Black Spot Samples: The number and areal extent of the shrinkage porosity regions increased significantly, confirming the subsurface nature of the degradation.
1.2 Spectroscopic (OES) Analysis
Point analysis on the defect zones provided elemental clues:
| Defect Type | Location Analyzed | Key Observation | Inference |
|---|---|---|---|
| White Spot | On the white powder | Intense Mg spectral lines | Severe magnesium enrichment. |
| Away from defect | Mg ~0.02-0.04%, other elements (Ce, La) normal. | Base matrix composition is nominal. | |
| Black Spot | On the black spot | Weak Mg lines, high Ce content. | Presence of rare-earth (Ce) rich slag. |
| Away from defect | Mg ~0.03-0.04%, Ce ~0.02%, La ~0.01%. | Nominal spheroidizing element levels. |
Based on spectroscopy, the white powder was inferred to be primarily magnesium oxide (MgO), while the black spots consisted of slag, predominantly rich in rare-earth oxides (e.g., Ce2O3).
1.3 Metallographic Analysis
Microscopic examination of polished and etched samples provided the definitive microstructural narrative for these casting defects.
White Spots: After etching, angular, octahedral-shaped white particles (MgO) remained in the pits. The pits and their immediate surroundings were filled with dark gray oxide slag. Furthermore, numerous silvery-gray sulfide and oxide inclusions were present. Notably, the matrix adjacent to these defects showed significant graphite flotation, characterized by exploded, dendritic, and thick/flakey graphite forms. A pronounced decarburized layer was observed alongside oxide films, indicating a reaction with the mold atmosphere.
Black Spots: The etched samples revealed a network of grayish rare-earth oxides and titanium nitride inclusions bordering the shrinkage porosity zones. The surrounding areas were again dominated by severe graphite flotation regions, containing exploded, dendritic, flake, and large, irregular spheroidal graphite. Complex magnesium sulfide-based inclusions were also identified within these zones.
The consistent association of both defect types with graphite flotation and various inclusions (oxides, sulfides, nitrides) pointed towards a common origin related to slag formation and flotation dynamics during solidification.
2. Thermodynamic and Kinetic Root Cause Analysis
The integration of findings leads to the conclusion that both white and black spot casting defects are primary slag defects. The slag originates from the in-mold spheroidization reaction itself. The sequence of events can be modeled as follows:
2.1 Slag Formation Thermodynamics
The in-mold process involves a highly exothermic reaction between the Mg/RE-containing alloy and the molten iron. The principal reactions leading to slag (inclusion) formation are:
$$ \text{[Mg]}_{in\,Fe} + \frac{1}{2}\text{O}_2 (atm/mold) \rightarrow \text{MgO}_{(s)} \quad \Delta G^\circ < 0 $$
$$ 2\text{[Ce]}_{in\,Fe} + \frac{3}{2}\text{O}_2 \rightarrow \text{Ce}_2\text{O}_{3(s)} \quad \Delta G^\circ < 0 $$
$$ \text{[Mg]}_{in\,Fe} + [S]_{in\,Fe} \rightarrow \text{MgS}_{(s)} \quad \Delta G^\circ < 0 $$
Where [ ] denotes elements dissolved in iron. The very low sulfur content (“double-zero” grade) achieved by prior pneumatic desulfurization, while beneficial for graphite morphology, inadvertently reduces the quantity of MgS formed. MgS, when present in sufficient volume, can act as a collector for finer oxide inclusions, aiding their coalescence and flotation. Its scarcity leaves the fine, particulate MgO and RE oxides dispersed.
2.2 Flotation Kinetics and Defect Formation
The critical factor is the limited time available in the in-mold process. The sequence from reaction completion to casting solidification is extremely short. According to Stokes’ law, the flotation velocity of a spherical inclusion is:
$$ v = \frac{2}{9} \cdot \frac{(\rho_{Fe} – \rho_{slag}) g r^2}{\eta} $$
Where:
- $v$ = terminal rising velocity
- $\rho_{Fe}, \rho_{slag}$ = densities of iron and slag particle
- $g$ = gravitational acceleration
- $r$ = radius of the slag particle
- $\eta$ = dynamic viscosity of molten iron
The fine, powder-like slag particles (small $r$) generated have a very low flotation velocity ($v \propto r^2$). Consequently, they lack sufficient time to float to the upper surface of the molten metal in the mold cavity before being trapped by the advancing solidification front. They are consequently pushed to the last-freezing regions, typically the thermal centers of sections or, critically, to the upper surfaces and undersides of cores where metal flow is stagnant. This explains the exclusive appearance of these casting defects on the cope surfaces and core-related dead zones.
The subsequent mechanism is summarized in the table below:
| Stage | Process | Outcome for White Spot | Outcome for Black Spot |
|---|---|---|---|
| 1. Solidification | Fine MgO/RE-oxide particles are trapped at the solid-liquid interface, creating local solute enrichment (Mg, Ce) and turbulence. | Localized Mg-rich zone inhibits graphite spheroidization, promotes flotation and degenerate graphite. | RE-rich oxides and other complex inclusions act as nucleation sites for gas bubbles and shrinkage pores. |
| 2. Machining | Surface layer is removed, exposing the sub-surface defect zone. | Brittle, loosely-bonded MgO powder is exposed, appearing as a white spot. Underlying porosity may be revealed. | Clusters of micro-porosity and embedded slag inclusions are exposed, appearing as a black, dense spot. |
| 3. Performance Impact | Defect acts as a stress concentrator and micro-crack initiator. | Drastically reduces fatigue strength and impact toughness. | Drastically reduces fatigue strength and impact toughness. |
3. Development and Implementation of the Corrective Methodology
The analysis clearly indicated that eliminating these casting defects required a fundamental change in slag behavior: from fine, dispersed powder to a coalesced, viscous liquid agglomerate that could be effectively separated and trapped before entering the casting cavity. The developed solution focused on modifying the slag chemistry and physics within the reaction chamber (runner system) of the in-mold process.
3.1 Slag Modifier Additives
A specific blend of additives was introduced into the reaction chamber alongside the spheroidizing alloy:
| Additive | Chemical Formula | Primary Function | Mechanism |
|---|---|---|---|
| Fluorspar | CaF2 | Fluxing Agent | Lowers the melting point and viscosity of the forming slag, promoting coalescence. Reacts with oxides: $\text{SiO}_2 + 2\text{CaF}_2 \rightarrow \text{SiF}_4 \uparrow + 2\text{CaO}$. |
| Sodium Fluorosilicate | Na2SiF6 | Fluxing & Coalescing Agent | Decomposes to provide active fluoride ions, further reducing slag surface tension and aiding particle aggregation. |
| Glass Powder / Fiber | Mainly SiO2 | Slag Thickener & Collector | Melts to form a viscous, silicate-based glassy network that physically entraps fine oxide particles, dramatically increasing the effective slag agglomerate size ($r$). |
The combined effect can be described by a modification of the slag viscosity and interfacial energy. The additives reduce the slag-melt interfacial tension ($\gamma_{sl}$) and increase the slag phase volume, enhancing the probability of collision and coalescence. The coalescence rate can be conceptually related to Smoluchowski’s coagulation theory, where the rate constant for two particles aggregating increases with their size and decreases with viscosity.
3.2 Optimized Process Flow and Slag Separation
Merely changing the slag’s physical state was insufficient; a means to actively separate it from the clean metal was essential. The in-mold gating system was designed to leverage centrifugal force. As the reacted metal flows through a specially designed “mixing chamber” or runner vortex, the denser, clean iron is forced outward, while the less dense, coalesced viscous slag agglomerate is concentrated towards the center of the flow rotation.
A strategic slag trap or dam was placed at the exit of this chamber. The large, viscous slag mass is retained behind this barrier, while the purified molten iron proceeds into the casting cavity. This process ensures that the casting defects stemming from primary slag are physically prevented from entering the mold.
4. Results and Confirmation
The implementation of this comprehensive solution—slag modification coupled with mechanical separation—completely eliminated the occurrence of both white and black spot casting defects in subsequent production batches of crankshafts. The improvement was validated through:
- Macroscopic Inspection: No visual evidence of white or black discolorations on machined surfaces.
- Radiographic Testing (Optional): Confirmed the absence of sub-surface shrinkage porosity clusters typical of the black spot defect.
- Mechanical Property Consistency: The significant scatter and lower bounds in fatigue and impact test results, previously dragged down by defective castings, were eliminated. Product quality and casting yield improved substantially.
This case underscores that advanced foundry processes, while offering benefits like efficiency and magnesium recovery, can introduce unique failure modes. A deep, physics-based understanding of the slag formation and transport kinetics is paramount for diagnosing and eradicating such specialized casting defects. The successful resolution hinged on shifting the paradigm from merely generating less slag to actively controlling its physical state and ensuring its complete removal from the metal stream prior to solidification.