In the automotive industry, ductile iron castings play a critical role in producing high-strength components, such as steering knuckles, which connect the wheel and suspension systems. These parts must withstand various loads under constrained conditions, demanding excellent mechanical properties. However, ductile iron castings often face challenges like shrinkage porosity and cavities due to their mushy solidification characteristics, especially in multi-hot-spot geometries like steering knuckles. Traditional methods involve using risers for feeding, which complicates process design and reduces yield rates. This study explores a novel approach to eliminate shrinkage defects in ductile iron castings by leveraging graphitization expansion and precise melt quality control, focusing on eutectic degree adjustment and multi-stage inoculation. Through thermal analysis and experimental validation, we demonstrate how optimized parameters can produce sound, shrinkage-free ductile iron castings without risers, significantly improving productivity and cost-efficiency.
Ductile iron castings, particularly for automotive steering knuckles, exhibit complex geometries with uneven wall thicknesses, leading to multiple hot spots that are prone to shrinkage defects. These defects not only compromise mechanical integrity but also increase rejection rates in mass production. Common solutions, such as adding暗冒口 or using chilled molds, add complexity and cost. Instead, this research emphasizes melt quality management through chemical composition control and enhanced inoculation. By adjusting the eutectic degree close to 1 and minimizing undercooling in solidification, we harness the self-expansion of graphite to compensate for shrinkage. This paper details the methodology, including structural analysis, process design, and results discussion, supported by thermal analysis curves, tables, and formulas to illustrate key findings.
The steering knuckle, a typical ductile iron casting, features an irregular claw or yoke-like structure with varying cross-sections, resulting in primary, secondary, and isolated hot spots. These areas are susceptible to shrinkage porosity if not properly addressed. In this study, we aimed to produce ductile iron castings meeting the QT450-10 standard, with chemical composition requirements including 3.6–3.7% C, 1.7–1.8% Si, Mn ≤ 0.3%, P ≤ 0.05%, S ≤ 0.02%, 0.03–0.045% residual Mg, and 0.02–0.03% residual RE. Mechanical properties must satisfy tensile strength σb > 450 MPa, elongation δ > 10%, and hardness 160–210 HB, with a microstructure comprising over 85% ferrite and pearlite, graphite spherulites of grades 1–3, and sizes 5–8. Crucially, the castings must be free of cracks, cold shuts, shrinkage cavities, and porosity.
To achieve this, we employed a charge mix of 30% pig iron, 40% scrap steel, and 30% returns, with graphite-based carburizers. Silicon carbide (SiC) pretreatment at 0.2% was applied during melting, and the tapping temperature was controlled at 1,550°C without holding. The ductile iron castings were produced using a wire-feeding process for nodularization over 60 seconds, with a batch size of 1 ton per heat. Inoculation involved multiple stages: 0.2% ladle inoculation during tapping, followed by 0.1% stream inoculation with 75% ferrosilicon during pouring. The pouring temperature ranged from 1,380 to 1,420°C, with undercooling maintained below 3°C, and total pouring time limited to 10 minutes. Key to this process is the use of high-quality raw materials, precise temperature control, and robust inoculation to enhance graphite nucleation, thereby maximizing self-expansion for shrinkage compensation.
Testing methods included chemical analysis using a full-spectrum direct reading spectrometer, thermal analysis with a dedicated instrument to capture cooling curves, metallographic examination via an inverted metallurgical microscope, tensile testing on a universal testing machine, and hardness measurement with a digital Brinell hardness tester. These tools allowed us to correlate process parameters with microstructure and mechanical properties in ductile iron castings.
The eutectic degree (Sc) is a critical parameter in ductile iron castings, defined by the formula:
$$ S_c = \frac{\%C}{4.26 – 0.31 \times (\%Si) – 0.3 \times (\%P) – 0.40 \times (\%S) + 0.027 \times (\%Mn)} $$
where Sc = 1 indicates ideal eutectic composition. By adjusting Sc close to 1, we promote abundant graphite precipitation during solidification, enhancing self-expansion to counteract shrinkage. Table 1 summarizes the effects of different eutectic degrees on microstructure and shrinkage in ductile iron castings, based on our experimental data.
| Eutectic Degree (Sc) | Graphite Spherulite Count (per mm²) | Spherulite Size Grade | Shrinkage Porosity Level | Remarks |
|---|---|---|---|---|
| 0.92 | 150–200 | 6–7 | High | Large shrinkage areas in hot spots |
| 0.95 | 250–300 | 7 | Moderate | Reduced shrinkage but still visible |
| 0.98 | 350–400 | 7–8 | Low | Minor shrinkage in isolated regions |
| 1.00 | 400–500 | 8 | None | Sound castings with uniform structure |
As shown in Table 1, when Sc approaches 1, the graphite spherulite count increases significantly, leading to finer and more uniform structures that minimize shrinkage. This is because near-eutectic compositions reduce the liquidus-solidus range, facilitating better feeding through graphitization expansion. In contrast, lower Sc values result in larger graphite spherulites and higher shrinkage tendencies due to increased solidification contraction. For ductile iron castings, controlling residual magnesium is also vital; we maintained it at 0.035–0.045% to balance nodularization and shrinkage prevention.
Inoculation plays a pivotal role in enhancing the quality of ductile iron castings by providing nucleation sites for graphite, refining eutectic cells, and reducing undercooling. We evaluated various inoculation combinations, including SiC pretreatment, floating silicon inoculation, wire-feeding inoculation, ladle inoculation, and stream inoculation. The thermal analysis curves, characterized by parameters like liquidus temperature (tAL), eutectic temperature (tEU), and undercooling degree (ΔT = tEG – tEU), where tEG is the theoretical eutectic temperature, provided insights into melt quality. Table 2 compares the effects of different inoculation methods on thermal analysis features and shrinkage in ductile iron castings.
| Inoculation Combination | Liquidus Temperature, tAL (°C) | Eutectic Temperature, tEU (°C) | Undercooling Degree, ΔT (°C) | Graphite Spherulite Count (per mm²) | Shrinkage Porosity Observation |
|---|---|---|---|---|---|
| SiC + Floating Silicon | 1,164 | 1,123 | 6.7 | 100–200 | Extensive shrinkage in hot spots |
| SiC + Floating Silicon + Wire-Feeding | 1,161 | 1,122 | 5.6 | 250–350 | Moderate shrinkage, improved structure |
| SiC + Floating Silicon + Wire-Feeding + Ladle | 1,155 | 1,123 | 1.7 | 400–450 | Minor shrinkage, mostly eliminated |
| SiC + Floating Silicon + Wire-Feeding + Ladle + Stream | 1,154 | 1,123 | 0.7 | 500–600 | No shrinkage, sound castings |
From Table 2, it is evident that multi-stage inoculation progressively reduces undercooling and increases graphite spherulite counts. For instance, the combination of SiC pretreatment, floating silicon, wire-feeding, ladle, and stream inoculation resulted in the lowest ΔT of 0.7°C and the highest spherulite density, effectively eliminating shrinkage in ductile iron castings. This is because strong inoculation provides numerous nucleation cores, accelerating graphite precipitation and releasing latent heat during eutectic solidification. The relationship between undercooling and shrinkage can be expressed as:
$$ \Delta T = t_{EG} – t_{EU} $$
where a smaller ΔT indicates better nucleation, leading to enhanced self-expansion. In ductile iron castings, this translates to reduced shrinkage risks. Additionally, the nodularization grade improved from 84% to 91% with comprehensive inoculation, as confirmed by metallographic analysis.
The mechanical properties of ductile iron castings were evaluated to validate the process efficacy. Table 3 compares the tensile strength, yield strength, elongation, and hardness for castings with and without shrinkage defects, demonstrating the superiority of the optimized approach.
| Shrinkage Condition | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|---|
| With Shrinkage | 463 | 328 | 12.6 | 174 |
| Shrinkage-Free | 508 | 360 | 20.0 | 188 |
As shown in Table 3, shrinkage-free ductile iron castings exhibit an 8.4% increase in tensile strength, a 9.7% increase in yield strength, a 58% improvement in elongation, and an 8% rise in hardness compared to those with shrinkage. This underscores the importance of eliminating defects to achieve high-performance ductile iron castings for automotive applications. The enhanced properties are attributed to the refined microstructure and effective graphitization expansion, which compensates for solidification shrinkage without external feeding.
In conclusion, this study demonstrates that by precisely controlling the eutectic degree close to 1 and minimizing undercooling through multi-stage inoculation, it is possible to produce sound, shrinkage-free ductile iron castings for automotive steering knuckles without using risers or chills. The optimized process, involving SiC pretreatment, wire-feeding, floating silicon, ladle, and stream inoculation, significantly increases graphite spherulite counts and self-expansion capacity, leading to improved mechanical properties and higher yield rates. This approach not only simplifies casting design but also reduces costs, making it highly applicable for mass production of high-quality ductile iron castings. Future work could explore real-time monitoring systems to further enhance process control in ductile iron castings.