As a key component in internal combustion engines, crankshafts are subjected to significant bending and torsional loads during operation, with primary failure modes being fatigue fractures due to these stresses. To meet functional and operational requirements, crankshafts must exhibit high rigidity, superior fatigue strength, and excellent wear resistance. The iron mold sand-coated casting process is particularly well-suited for producing ductile iron castings like crankshafts, due to its rapid cooling rates and high mold rigidity. This process promotes the formation of pearlite and refines grain structure, leveraging graphite expansion for self-feeding to achieve dense, high-strength, and high-toughness castings. Therefore, iron mold sand-coated casting is considered an ideal production method for ductile iron crankshafts.
However, slag inclusion defects are a common issue in the production of ductile iron castings for crankshafts. When present in critical stress-bearing areas such as the connecting rod journals and main journals, these inclusions act as potential crack initiation sites, disrupting microstructural continuity and reducing plasticity, toughness, and fatigue strength. The higher the required strength of the crankshaft, the more pronounced the detrimental effect of slag inclusions on mechanical integrity, increasing the risk of fatigue failure. In our production of high-grade ductile iron castings (QT850-5) for a four-cylinder engine crankshaft using the iron mold sand-coated process, we encountered slag inclusion defects on the outer cylindrical surfaces of the journals. These defects appeared as fine dots or irregular lines, often undetectable before machining and only visible under magnetic particle inspection or bright light after processing, leading to delayed detection and significant economic losses.
The defect rate was unstable, typically ranging from 1.2% to 2%, but in one batch, the scrap rate surged to 4.9%, affecting nearly 5% of the produced crankshafts and posing substantial quality risks. To address this, we conducted a thorough analysis of the slag inclusion formation mechanisms and implemented corrective measures across technical design, raw material management, process control, and quality inspection. This article details our approach, focusing on reducing slag sources and optimizing the gating system to mitigate slag inclusions in ductile iron castings.
The production process for ductile iron crankshafts via iron mold sand-coated casting involves several key stages: melting, nodularization, molding, pouring, and cleaning. Among these, melting, nodularization, and pouring are critical for controlling slag inclusion defects. Below, we outline the process flow and parameters, emphasizing factors influencing slag formation.
| Stage | Description | Key Control Parameters |
|---|---|---|
| Melting | Iron melting in a 1-ton medium-frequency induction furnace using raw materials like pig iron, steel scrap, returns, and alloying elements (e.g., copper, manganese). | Chemical composition, material purity, heating temperature. |
| Nodularization and Inoculation | Treatment via covered ladle method with nodularizer, ferrosilicon, and iron chips to promote graphite nodule formation. | Nodularizer addition rate, inoculation amount, reaction time. |
| Molding | Iron mold sand-coated process using compressed air to shoot resin-coated sand into cavities, cured by heated patterns. | Sand thickness, curing temperature, mold rigidity. |
| Pouring | Pouring treated iron into molds with secondary inoculation (instantaneous stream inoculation) to enhance graphite refinement. | Pouring temperature, pouring speed, inoculation grain size. |
| Cleaning | Removal of flash, burrs, and residual sand from castings. | Deburring efficiency, surface finish. |
Slag inclusions in ductile iron castings originate from two primary aspects: slag sources and entrapment pathways. Slag sources include impurities from raw materials, oxidation during melting, excessive residual magnesium from nodularization forming sulfides (e.g., MgS) and oxides (e.g., MgO), and improper inoculation practices. Entrapment pathways involve inadequate slag trapping by the gating system, allowing slag to enter the mold cavity with the molten iron. To analyze this, we performed microstructural and energy-dispersive spectroscopy (EDS) on defective samples, revealing high magnesium concentrations at inclusion sites, indicating excessive nodularizer usage. The formation of slag particles can be described by Stokes’ law for particle flotation in molten iron:
$$v = \frac{2}{9} \frac{(\rho_f – \rho_s) g r^2}{\mu}$$
where \(v\) is the flotation velocity of slag particles, \(\rho_f\) is the density of molten iron (approximately 7,000 kg/m³ for ductile iron castings), \(\rho_s\) is the density of slag (typically 2,500–3,500 kg/m³), \(g\) is gravitational acceleration (9.81 m/s²), \(r\) is the radius of slag particles, and \(\mu\) is the dynamic viscosity of molten iron (around 0.005 Pa·s). This equation highlights that smaller slag particles or higher viscosity can reduce flotation, increasing the likelihood of entrapment in ductile iron castings.
Our investigation identified several issues in the production process. First, raw materials such as pig iron and steel scrap showed rust contamination, and returns (gating systems and risers) had adhered sand without prior treatment, introducing impurities into the melt. Second, excessive nodularizer was added to ensure optimal nodularization, leading to high residual magnesium that reacted with sulfur and oxygen to form slag. EDS analysis confirmed magnesium-rich inclusions, with magnesium atomic concentrations up to 0.67 in defect areas. Third, the secondary inoculation used FeSi75 inoculant with uneven grain size distribution (70% in 0.2–0.4 mm range), where fine particles oxidized before dissolving, forming oxide slag. Fourth, the gating system design was suboptimal for slag trapping; when the total cross-sectional area of ingates exceeded that of the sprue, slag floated into the cavity, and when it was smaller, turbulent flow caused splashing and oxide film formation.
| Slag Source | Description | Impact on Ductile Iron Castings |
|---|---|---|
| Raw Material Impurities | Rust, adhered sand, and contaminants in pig iron, scrap, and returns. | Introduces oxides and non-metallic inclusions, reducing iron purity. |
| Excessive Nodularizer | High magnesium residual from over-addition of nodularizing agents. | Forms MgS and MgO slag compounds, increasing inclusion risk. |
| Inoculant Issues | Fine-grained inoculant (0.2–0.4 mm) with poor dissolution and oxidation. | Generates oxide slag particles that aggregate during solidification. |
| Gating System Design | Inadequate slag trapping due to improper area ratios and flow dynamics. | Allows slag entrainment into mold cavities, causing defects. |
To address these issues, we implemented a series of improvements focused on slag source reduction and gating system optimization. First, we enhanced raw material management by enforcing purity controls: returns were shot-blasted to remove adhered sand, rusty or contaminated materials were rejected, and iron chips were screened to eliminate impurities like alloy tool bits. Second, we optimized nodularizer addition based on residual magnesium levels and metallographic results, minimizing excess usage while maintaining nodularization quality. The target residual magnesium was set using the formula:
$$Mg_{residual} = k \cdot \frac{S_{initial}}{S_{final}}$$
where \(Mg_{residual}\) is the desired residual magnesium content, \(k\) is a process constant (typically 0.04–0.06 for ductile iron castings), \(S_{initial}\) is the initial sulfur content, and \(S_{final}\) is the sulfur content after treatment. Third, we adjusted the secondary inoculant grain size from 0.2–0.85 mm to 0.4–0.85 mm, ensuring better dissolution and reducing oxide formation. Fourth, we redesigned the gating system to improve slag trapping, increasing the slope from the lower to upper runner, adding a flow-stabilizing corner near the sprue, and enlarging the slag trap height by 10 mm to enhance slag flotation and adhesion. Additionally, we introduced a two-stage filtration system: a foam ceramic filter in the pouring cup combined with a fiber filter at the ingates to capture slag particles effectively. The filtration efficiency can be approximated by:
$$\eta = 1 – \exp\left(-\alpha \cdot \frac{L}{d}\right)$$
where \(\eta\) is the filtration efficiency, \(\alpha\) is a material-dependent constant, \(L\) is the filter thickness, and \(d\) is the pore size. This dual-filter approach significantly reduced slag entrainment in ductile iron castings.
We validated these measures through iterative experiments, particularly focusing on ingate dimensions to optimize flow dynamics. Initially, the ingate size was 13 mm × 14 mm × 7 mm (cross-sectional area 94.5 mm²), which we first increased to 16 mm × 17 mm × 8 mm (132 mm²) to reduce splashing. However, testing on batches of 5,520 and 4,617 castings showed scrap rates of 1.1% and 1.45%, respectively, indicating worsened slag inclusion due to incomplete ingate filling and poor slag trapping. Based on the principle of runner slag trapping, where the ingate suction zone affects slag flotation, we derived the condition for effective trapping:
$$h_{ingate} > \frac{v_{horizontal} \cdot h_{runner}}{v_{flotation}}$$
where \(h_{ingate}\) is the ingate height, \(v_{horizontal}\) is the horizontal flow velocity in the runner, \(h_{runner}\) is the runner height, and \(v_{flotation}\) is the slag flotation velocity. To meet this, we reduced the ingate height to 6 mm while adjusting other dimensions to 16.2 mm × 17 mm × 6 mm (cross-sectional area 97.2 mm²), ensuring full filling and improved slag retention. This modification lowered the scrap rate to 0.7% in a batch of 8,256 ductile iron castings, demonstrating significant improvement.
| Condition | Ingate Dimensions (mm) | Cross-Sectional Area (mm²) | Scrap Rate (%) | Notes |
|---|---|---|---|---|
| Original Design | 13 × 14 × 7 | 94.5 | 4.9 (peak) | High slag inclusion due to poor material and gating control. |
| After Material Management | 13 × 14 × 7 | 94.5 | 1.97 | Improved raw material purity and inoculant grain size. |
| First Ingate Adjustment | 16 × 17 × 8 | 132 | 1.45 | Increased area led to incomplete filling and higher slag entrainment. |
| Second Ingate Adjustment | 16.2 × 17 × 6 | 97.2 | 0.7 | Optimized height for better slag trapping and flow stability. |
| Final Implementation | 16.2 × 17 × 6 | 97.2 | ~0.5 | Combined with gating redesign and dual filtration, stable low scrap rate. |
The success of these measures underscores the importance of a holistic approach to quality control in ductile iron castings production. Reducing slag sources at the melting stage is the most direct and effective strategy, as once slag forms in the molten iron, it is challenging to remove completely. Our experience shows that strict raw material standards, controlled nodularizer addition, and appropriate inoculant selection are critical for minimizing slag generation. Furthermore, optimizing the gating system design enhances slag trapping capacity, with proper ingate sizing and filtration playing key roles. The final scrap rate stabilized around 0.5%, meeting customer requirements and reducing quality risks significantly.
In conclusion, addressing slag inclusions in ductile iron castings, such as crankshafts, requires a multifaceted strategy that integrates material management, process optimization, and design improvements. By focusing on slag source reduction and enhancing the gating system’s ability to capture inclusions, we achieved a substantial decrease in defect rates, improving the reliability and performance of our ductile iron castings. Future work may involve advanced simulation techniques to model flow dynamics and slag behavior, further refining the production process for high-integrity ductile iron components.