In our production of Y-series four-cylinder lightweight diesel engine gray iron integrated cylinder heads, we encountered significant leakage issues during wet-type air tightness testing after machining. The gray iron casting material was HT280, with each raw part weighing 45 kg and an annual output exceeding 40,000 units. These gray iron components are characterized by thin walls and light weight, manufactured using medium-frequency electric furnace melting with a pouring temperature of 1410–1430°C. The molding process employed green sand molding, with four pieces per mold. The upper cover and base plate cores were produced using the triethylamine cold box process, while the airway and water jacket cores utilized shell molding with coated sand. The gating system was bottom-poured. Post-processing leakage defects accounted for over 90% of total scrap, primarily concentrated at the injector holes. Scanning electron microscopy confirmed that shrinkage porosity was the root cause of leakage in these gray iron castings.
The leakage was predominantly located at the injector holes of cylinders 2 and 3, representing more than 80% of cases. These holes, with a diameter of 30 mm, were not cast, creating structural hot spots. After drilling, gray honeycomb-like pores were visible on the inner walls. At 100x magnification, dendritic structures with localized porous areas were observed, confirming shrinkage porosity. The average wall thickness of the cylinder head is 5 mm, but the injector hole region forms a substantial hot spot due to its 30 mm diameter. During cooling, uneven solidification occurs, with the hot spot solidifying last. Liquid, eutectic, and solid shrinkage cannot compensate, leading to voids as the thin walls solidify first and lack liquid metal replenishment. The fundamental issue revolves around the solidification dynamics in gray iron, where inadequate feeding promotes shrinkage porosity in isolated thick sections.
To address this, we designed and implemented internal chills. Internal chills are effective for gray iron castings with thin walls and isolated hot spots, as they control solidification sequence and adjust cooling rates to promote directional sequential or simultaneous solidification. This is particularly useful when the gating system cannot directly feed the hot spot via risers. For material selection, we chose tin-plated steel sheets over the base gray iron material due to cost-effectiveness and widespread use in diesel engine foundries. The plating prevents oxidation and rust. The chill shape was a hollow, slotted cylinder with a thickness of 0.8 mm, diameter slightly larger than the core hole, and length sufficient to avoid exposure after machining. The slot allows for core venting, and a taper facilitates insertion. The chill is fixed by elastic expansion against the core inner surface.
The mechanism of internal chills involves accelerating cooling at the hot spot, shifting shrinkage porosity toward the center or eliminating it, so that drilling removes the defective area. Metallographic examination showed a clear steel profile with a 0.3 mm transition layer, indicating good fusion without micro-cracks or pores. Hardness testing revealed the chill region at 270–279 HV, gray iron at 206–230 HV, and the transition at 240–260 HV. The denser chill and transition layers also aid in preventing leakage. However, risks include poor fusion if positioning is inaccurate, leading to hard spots during machining or micro-cracks from stress imbalances.
| C | Si | Mn | P | S | Cr | Ni | Cu | Sn |
|---|---|---|---|---|---|---|---|---|
| 0.11 | 0.1 | 0.523 | 0.016 | 0.008 | 0.032 | 0.016 | 0.017 | 0.006 |
In gray iron casting, controlling melt composition is crucial for minimizing shrinkage defects. Lower carbon equivalent (CE) increases shrinkage tendency in gray iron. We adjusted the composition by increasing silicon content and replacing nickel with tin to stabilize pearlite and promote graphitization. The CE can be calculated using the formula: $$CE = C + \frac{1}{3}(Si + P)$$ For HT280 gray iron, maintaining an optimal CE is essential to balance strength and shrinkage resistance. Secondary inoculation was applied to counteract fading, using a zirconium-silicon inoculant added during pouring at 0.06–0.08%. Excessive inoculation can exacerbate shrinkage, while insufficient amounts reduce self-feeding capacity.
We explored additional processes to mitigate leakage in gray iron castings. Tellurium-based chill coatings were tested, as tellurium is a strong anti-graphitizing element that accelerates cooling and refines grains. However, application on fine-grained coated sand (50/100) proved ineffective due to poor penetration and thin coating layers, resulting only in undercooled D and E type graphite without carbides. In contrast, this method worked well on cold box cores in other products. Modifications to the gating and riser system were also considered. Since the defect location is central, top risers were impractical, and side risers showed limited feeding due to improper neck design or insufficient modulus. Bottom gating with gates at injector bosses was avoided to prevent cleaning issues and adverse thermal effects on gray iron properties.
| Location | Count | Percentage (%) | Cumulative % |
|---|---|---|---|
| Injector Hole Cylinder 3 | 332 | 39.3 | 39.3 |
| Injector Hole Cylinders 2/3 | 271 | 32.1 | 71.4 |
| Injector Hole Cylinder 2 | 84 | 9.9 | 81.3 |
| Injector Hole Cylinder 4 | 72 | 8.5 | 89.8 |
| Injector Hole Cylinders 2/3/4 | 31 | 3.7 | 93.5 |
| Injector Hole Cylinder 1 | 17 | 2.0 | 95.5 |
| Injector Hole Cylinders 3/4 | 13 | 1.5 | 97.0 |
| Bolt Hole | 9 | 1.1 | 98.1 |
| Injector Hole Cylinders 2/4 | 7 | 0.8 | 98.9 |
| Airway | 7 | 0.8 | 99.7 |
| Tappet | 2 | 0.2 | 100.0 |
The implementation of internal chills, combined with optimized composition and inoculation, reduced leakage rates by 80% in our gray iron cylinder heads. This demonstrates the effectiveness of internal chills for addressing shrinkage porosity in gray iron castings with isolated hot spots. Key design principles include selecting compatible materials, simple shapes for cost and fusion reliability, and ensuring proper positioning to avoid machining issues. Additionally, chills should be used promptly after core assembly to prevent moisture accumulation and gas defects. Regular monitoring through dissections is necessary to assess fusion quality, especially after process changes.
For gray iron casting defects like shrinkage-induced leakage, multiple strategies exist, such as adjusting melt chemistry, adding risers, optimizing gating, or applying chill coatings. The choice depends on product geometry, defect location, gating design, and mold materials. In gray iron, the solidification behavior is governed by graphitization expansion, which can offset shrinkage if properly controlled. The modulus method for riser design can be applied, where the modulus M is given by $$M = \frac{V}{A}$$ with V as volume and A as cooling surface area. Ensuring M_riser > M_casting promotes feeding. For gray iron, the cooling rate influences graphite formation, and the solidification time t can be approximated by $$t = k \cdot M^2$$ where k is a constant dependent on the gray iron composition.
In summary, internal chills are a powerful tool for enhancing the integrity of gray iron castings, but they require careful design and monitoring. Our experience highlights the importance of a holistic approach, integrating material science and process engineering to achieve high-quality gray iron components. Future work could focus on computational modeling of solidification to predict hot spots and optimize chill placement in gray iron casting processes.