In the production of ductile iron castings, the spheroidization treatment process plays a critical role in determining the final microstructure and mechanical properties. As a widely used material in various industries, ductile iron castings require precise control over graphite nodule formation and pearlite content to achieve high performance. Lost foam casting, a method that utilizes foam patterns embedded in dry sand under vacuum conditions, offers advantages such as high precision and clean production. However, traditional spheroidization techniques in lost foam casting often lead to issues like low spheroidization grades, reduced graphite nodule counts, and insufficient pearlite content, resulting in high rejection rates. This article explores an improved spheroidization and inoculation process developed through our experiments, focusing on enhancing the quality of ductile iron castings.
The conventional spheroidization process for ductile iron castings involves pouring molten iron into a ladle containing spheroidizing agents and inoculants. In lost foam casting, the high pouring temperatures accelerate the reaction of spheroidizing agents, leading to rapid magnesium loss and low absorption rates. This results in poor spheroidization, characterized by low graphite nodule counts and inadequate pearlite formation. For instance, our initial process yielded only 30% graphite nodule count and 85% pearlite content, which falls short of the desired mechanical properties for high-quality ductile iron castings. The challenges include controlling the spheroidization reaction time and minimizing magnesium burn-off, which are crucial for improving the microstructure of ductile iron castings.
To address these issues, we implemented a multi-stage inoculation approach using composite inoculants. The selection of inoculants is based on their melting points and resistance to fading. We employed a combination of ferrosilicon, barium-silicon, and barium-calcium-silicon inoculants, referred to as first, second, and third inoculants, respectively. The first inoculant, added at 0.15% of the iron weight, is introduced during the spheroidization reaction. Twenty seconds after the reaction begins, the second inoculant (0.35–0.45% of iron weight) and the third inoculant (0.2–0.4% of iron weight) are sequentially added. This multi-stage inoculation enhances graphite nucleation, reduces abnormal graphite formations, and delays inoculation fading, ultimately improving the consistency and quality of ductile iron castings. The relationship between inoculation efficiency and fading resistance can be expressed using the following formula for inoculation effectiveness: $$E_i = \sum_{k=1}^{3} \left( w_k \cdot e^{-t/\tau_k} \right)$$ where \(E_i\) is the inoculation effectiveness, \(w_k\) is the weight percentage of the k-th inoculant, \(t\) is time, and \(\tau_k\) is the fading time constant for each inoculant. This formula helps in optimizing the inoculation process for ductile iron castings.
In addition to inoculation, we modified the spheroidization ladle design to a dam-type ladle, which allows for a two-stage spheroidization reaction. The ladle is divided by a dam, with spheroidizing agents placed in two layers separated by a cover plate. The bottom layer contains 0.7–0.9% spheroidizing agent (by iron weight), covered with 0.05–0.15% covering agent and compacted. A cover plate, made of rust-free carbon steel or pre-formed ductile iron, is placed on top, followed by an upper layer of 0.55–0.65% spheroidizing agent, 0.05–0.15% first inoculant, and 0.05–0.15% covering agent. The cover plate has a planar top surface and a bottom shape matching the ladle base, with a gap of ≤5 mm from the ladle walls. When molten iron at 1560–1600°C is poured into the non-agent side of the ladle, the upper layer agents react first, allowing initial magnesium absorption. As the cover plate melts, the lower layer agents undergo a second reaction, prolonging the spheroidization time and increasing magnesium absorption efficiency. This two-stage process reduces magnesium burn-off and improves spheroidization grades for ductile iron castings.
The implementation begins with pre-treatment of the molten iron. After heating the electric furnace iron to 1560–1600°C, slag is removed, and 0.05–0.15% silicon carbide pre-treatment agent is added, followed by holding for 9–11 minutes. The iron is then poured into the dam-type ladle, triggering the sequential spheroidization reactions. The improved process significantly enhances the microstructure of ductile iron castings, as evidenced by the increase in graphite nodule count to 60% and pearlite content to 90%. The following table summarizes the key parameters and results of the improved process for ductile iron castings:
| Parameter | Original Process | Improved Process |
|---|---|---|
| Graphite Nodule Count (%) | 30 | 60 |
| Pearlite Content (%) | 85 | 90 |
| Spheroidizing Agent Usage (%) | Single layer: 1.2–1.5 | Two layers: 1.25–1.55 total |
| Inoculant Usage (%) | Single addition: 0.5–0.7 | Multi-stage: 0.55–1.0 total |
| Reaction Time (s) | ~30 | ~60 (two-stage) |
The spheroidization efficiency can be modeled using the magnesium absorption rate equation: $$\eta_{Mg} = \frac{Mg_{abs}}{Mg_{input}} = 1 – e^{-k \cdot t}$$ where \(\eta_{Mg}\) is the magnesium absorption efficiency, \(Mg_{abs}\) is the absorbed magnesium, \(Mg_{input}\) is the input magnesium from agents, \(k\) is the reaction rate constant, and \(t\) is the reaction time. The two-stage process increases \(t\), thereby improving \(\eta_{Mg}\) for ductile iron castings. Additionally, the pearlite formation is influenced by the cooling rate and inoculation, which can be described by the equation: $$P_c = P_0 + \alpha \cdot I_e$$ where \(P_c\) is the pearlite content, \(P_0\) is the base pearlite content, \(\alpha\) is a constant, and \(I_e\) is the inoculation effectiveness. The improved inoculation raises \(I_e\), leading to higher \(P_c\) in ductile iron castings.
Our experiments demonstrate that the combination of multi-stage inoculation and two-stage spheroidization in a dam-type ladle effectively addresses the limitations of traditional methods. The increased graphite nodule count and pearlite content contribute to better tensile strength, ductility, and overall reliability of ductile iron castings. However, further improvements are needed to achieve even higher pearlite levels, potentially through optimized cooling rates or additional alloying elements. The table below provides a detailed comparison of the microstructural properties before and after the process improvement for ductile iron castings:
| Property | Original Value | Improved Value | Change (%) |
|---|---|---|---|
| Graphite Nodule Count (%) | 30 | 60 | +100 |
| Pearlite Content (%) | 85 | 90 | +5.9 |
| Nodularity Grade | Low (e.g., Grade 3-4) | High (e.g., Grade 1-2) | Significant |
| Rejection Rate (%) | High (e.g., 15-20) | Reduced (e.g., 5-10) | -50 to -67 |
The economic impact of this improved process is substantial, as it reduces scrap rates and enhances the consistency of ductile iron castings. In industrial applications, the ability to produce high-quality ductile iron castings with reliable microstructures translates to longer service life and better performance in critical components. Future work will focus on refining the inoculant compositions and exploring dynamic control of the spheroidization parameters using real-time monitoring systems. For instance, the integration of sensors to measure temperature and magnesium levels could further optimize the process for ductile iron castings.
In conclusion, our exploration of spheroidization treatment for ductile iron castings in lost foam casting has led to significant advancements through multi-stage inoculation and a two-stage spheroidization reaction in a dam-type ladle. These improvements have resulted in higher graphite nodule counts and pearlite content, which are essential for superior mechanical properties in ductile iron castings. While the current process marks a step forward, ongoing research aims to achieve further enhancements in pearlite levels and overall efficiency, ensuring that ductile iron castings meet the evolving demands of modern industries.