In the production of high-grade ductile iron crankshafts, surface inclusion defects often emerge as a critical issue after machining, particularly in key stress areas such as oil hole edges and fillet radii. These defects, if left unaddressed, can lead to catastrophic failures in engine components. As a manufacturer specializing in ductile iron casting, I have encountered recurring inclusion problems in our QT800-2 grade ductile cast iron crankshafts. Through a systematic investigation of raw materials and production processes, we identified the root causes and implemented effective countermeasures. This article details our comprehensive approach to analyzing and resolving surface inclusion defects in ductile iron components, emphasizing the importance of process control in ductile iron casting.
The manufacturing process for high-grade ductile iron crankshafts involves several critical stages, each contributing to the final material properties. Our production begins with molding using alkaline phenolic resin self-setting sand, which provides adequate dimensional stability for the complex crankshaft geometry. The mold consists of upper and lower sections, with additional chill sand cores employed for intricate features that cannot be formed directly. This approach ensures consistent mold quality while maintaining production efficiency through automated line operations.
Melting constitutes the most crucial phase in ductile iron production. We utilize medium-frequency induction furnaces to melt the charge materials, which include pig iron, steel scrap, crankshaft/vermicular iron returns, and various alloys such as ferrosilicon and ferromanganese. Precise control of base iron composition is paramount for achieving high-quality ductile cast iron, particularly regarding trace elements that can promote inclusion formation. The base iron composition is strictly maintained within the following ranges:
| Element | C | Si | Mn | S | P |
|---|---|---|---|---|---|
| Content (%) | 3.6-3.8 | 1.2-1.5 | ≤0.6 | ≤0.022 | ≤0.06 |
Following tapping, we employ a wire feeding process for nodularization treatment using magnesium-containing wire (20% Mg) combined with inoculating wire to promote graphite spheroidization. Copper and antimony additions facilitate pearlite formation, enhancing the mechanical properties of the final ductile iron casting. The target chemical composition for the finished crankshaft is maintained as follows:
| Element | Si | Cu | Sb | Mg | RE |
|---|---|---|---|---|---|
| Content (%) | 2.0-2.4 | 0.5-0.6 | 0.01-0.02 | 0.030-0.045 | 0.005-0.020 |
Pouring is conducted using automatic pouring machines with teapot ladles to minimize slag entrainment. The pouring temperature is carefully controlled between 1380°C and 1400°C to ensure proper fluidity while avoiding excessive oxidation. To combat inoculation fading and refine graphite structure, we implement a secondary inoculation process using silicon-zirconium wire added during pouring at 0.08%-0.12% by mass. The casting orientation employs a horizontal pouring with vertical cooling approach, where molds are rotated to a vertical position within 100 seconds after pouring to facilitate directional solidification through the riser.
Post-casting operations include a minimum 24-hour cooling period before shakeout, followed by basic cleaning to remove residual sand and excess gating systems. Heat treatment is essential for developing the desired microstructure in high-grade ductile iron. The crankshafts are heated to approximately 900°C, transforming the ferritic-pearlitic matrix to austenite with partial dissolution of spherical graphite into the austenitic phase. After sufficient soaking, air cooling transforms the austenite to fine pearlite, significantly enhancing the comprehensive strength of the ductile iron crankshaft.
The surface inclusion defects observed in our ductile iron crankshafts typically manifest as punctate or elongated crack-like imperfections after machining and polishing. Through scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis of ultrasonically cleaned defect sites, we identified these inclusions as primarily consisting of oxides and sulfides with elevated concentrations of oxygen, silicon, magnesium, and aluminum. The chemical signature suggests the presence of MgO, SiO₂, Al₂O₃, and MgS compounds, characteristic of secondary slag formation in ductile iron casting.
Inclusions in ductile iron can be categorized as primary or secondary slag. Primary slag originates from inadequate slag removal after treatment or insufficient gating system design that allows nodularization reaction products to enter the mold cavity. While primary slag causes are relatively straightforward to identify and address, the majority of our defects were attributed to secondary slag formation. Secondary slag develops during mold filling through oxidation of the iron or chemical reactions between alloying elements and mold/core materials. The fundamental reactions governing secondary slag formation in ductile iron can be represented as:
$$ \text{2Mg} + \text{O}_2 \rightarrow \text{2MgO} $$
$$ \text{Si} + \text{O}_2 \rightarrow \text{SiO}_2 $$
$$ \text{4Al} + \text{3O}_2 \rightarrow \text{2Al}_2\text{O}_3 $$
$$ \text{Mg} + \text{S} \rightarrow \text{MgS} $$
These reaction products, being less dense than the molten ductile iron, tend to float to the surface but can become entrapped at the metal-mold interface or within the casting section if solidification occurs too rapidly.
Our investigation revealed that sulfur content and residual magnesium levels significantly influence inclusion formation in ductile iron. Higher sulfur concentrations necessitate increased magnesium additions for effective nodularization, subsequently elevating the residual magnesium content. Excessive residual magnesium promotes the formation of magnesium-based compounds, increasing inclusion risk. Our initial process maintained residual magnesium between 0.03% and 0.045%, typically toward the upper limit. By optimizing the wire feeding process to minimize magnesium additions while maintaining satisfactory nodularization, we achieved a substantial reduction in inclusion formation. The comparative distribution of residual magnesium content before and after process optimization is summarized below:
| Process Phase | Minimum Mg (%) | Maximum Mg (%) | Average Mg (%) | Standard Deviation |
|---|---|---|---|---|
| Before Optimization | 0.030 | 0.045 | 0.040 | 0.0042 |
| After Optimization | 0.030 | 0.040 | 0.035 | 0.0028 |
Microstructural examination confirmed that the optimized process maintained graphite nodularity above grade 3 with nodule size between grades 5 and 7, meeting all specification requirements for high-grade ductile iron. The relationship between residual magnesium content and inclusion propensity can be modeled using the following empirical equation derived from our production data:
$$ P_i = k_1 \cdot [\text{Mg}]^2 + k_2 \cdot [\text{S}] + k_3 \cdot [\text{Mg}][\text{O}] $$
Where $P_i$ represents the inclusion propensity index, $[\text{Mg}]$, $[\text{S}]$, and $[\text{O}]$ denote the concentrations of magnesium, sulfur, and oxygen in percent, respectively, and $k_1$, $k_2$, $k_3$ are process-dependent constants.
Raw material quality profoundly impacts inclusion formation in ductile iron casting. We transitioned from standard Q10 pig iron to high-purity, low-phosphorus, low-titanium pig iron to minimize tramp element introduction. Strict batch management ensures consistent chemical composition between material lots, preventing significant fluctuations that could destabilize the process. Returns are thoroughly cleaned through shot blasting to eliminate adhering sand, and cross-contamination with other foundry returns like filters and risers is strictly prohibited. Ideally, only gating systems and risers from crankshaft castings themselves should be recycled to maintain composition consistency in ductile iron production.
Crankshaft bending during heat treatment and subsequent cooling emerged as another significant contributor to surface inclusion defects after machining. When crankshafts exhibit excessive curvature, the machining allowance becomes unevenly distributed. The convex side receives sufficient machining to remove surface imperfections, while the concave side may retain subsurface inclusions that become exposed after final polishing. The relationship between crankshaft bending and effective machining depth can be described as:
$$ d_e = d_n – \delta \cdot \sin(\theta) $$
Where $d_e$ is the effective machining depth, $d_n$ is the nominal machining depth, $\delta$ is the deflection amplitude, and $\theta$ is the angular position along the crankshaft.
To address this issue, we modified our process sequence by relocating the riser removal operation from post-heat treatment to pre-heat treatment. This change eliminated the unbalanced weight distribution during thermal processing that previously contributed to bending. Additionally, we implemented specialized gauges to measure crankshaft deflection systematically, with corrective straightening applied to components exceeding tolerance limits. These measures significantly reduced the incidence of inclusion-related defects attributable to insufficient machining of curved surfaces.
The comprehensive implementation of these corrective actions—optimized magnesium control, enhanced raw material management, and crankshaft straightness control—yielded a 60% reduction in surface inclusion defects in our high-grade ductile iron crankshafts. This improvement substantially enhanced product quality and customer satisfaction while demonstrating the critical importance of integrated process control in ductile iron casting.
In conclusion, surface inclusion defects in high-grade ductile iron crankshafts stem from multiple interrelated factors within the manufacturing process. Through systematic analysis and targeted interventions, we successfully mitigated these defects by implementing three key strategies: First, stringent control of raw material quality minimizes the introduction of inclusion-forming elements. Second, optimization of residual magnesium content reduces the formation of magnesium-based compounds without compromising nodularization in ductile cast iron. Third, comprehensive management of crankshaft geometry throughout processing ensures adequate machining allowance for surface defect removal. The successful resolution of these inclusion issues underscores the fundamental principle that excellence in ductile iron casting requires holistic attention to all process variables, from raw material selection to final machining. Continuous monitoring and refinement of these parameters remain essential for maintaining the high quality standards demanded of critical components like crankshafts in demanding applications.