1. Introduction
Ductile iron has become a major metal material in industrial production due to its excellent comprehensive properties. The performance requirements for ductile iron are increasing with the diverse service environments of castings. High-silicon ductile iron, especially silicon solid solution strengthened ferritic ductile iron, has attracted more and more attention because of its higher strength, toughness, and machinability compared with mixed matrix ferritic ductile iron. However, the application of high-silicon ductile iron in thin-walled large pipe fittings is difficult due to the brittle fracture caused by excessive silicon content. The lost foam casting process is different from the traditional sand casting process, and the control of pouring temperature is crucial. The research on obtaining high-silicon ductile iron with a full ferritic structure by lost foam casting is of great significance. This article studies the microstructure and mechanical properties of high-silicon ductile iron with a silicon content of 2.9%-4.6% prepared by the lost foam casting process, and analyzes the mechanism of the action of silicon on the matrix structure and mechanical properties of high-silicon ductile iron, providing a theoretical basis for the process of lost foam casting large high-silicon ductile iron pipe fittings.
2. Experimental Materials and Methods
2.1 Experimental Design
A group of QT450-10 ductile iron samples were designed as the control group, and three groups of high-silicon ductile iron samples with a silicon content in the range of 3.5%-4.5% were designed as the experimental group. The carbon equivalent was controlled within the range of 4.0%-4.4% by adjusting the silicon and carbon contents while keeping the carbon equivalent basically unchanged.
2.2 Melting and Casting
The samples were melted in a 1 t medium-frequency induction furnace and cast by the lost foam casting process. The foam white pattern was made of EPS material, the coating was mainly composed of bauxite, and the molding sand was dry silica sand. The pouring temperature was 1496 °C, the negative pressure was 0.05 MPa, the pouring time was 30 s, the nodularization method was wire feeding nodularization, the nodularizer addition amount was 1.3%, and the inoculant was 75 ferrosilicon with an addition amount of 1.5%-2.5%.
2.3 Sample Preparation and Testing
The as-cast samples were machined into 10 mm×10 mm×10 mm metallographic samples, which were then ground, polished, and corroded with a 4% nitric acid alcohol solution. The microstructure was observed by an Olympus-DSX500 optical microscope (OM) and a ZEISS Ultra Plus field emission scanning electron microscope (SEM), and the relative content and size of ferrite and graphite were statistically measured by ipp-6.0 and OLYCIA.m3 image analysis software. The graphite morphology and size were classified and evaluated according to GB/T 9441-2021 “Metallographic Inspection of Ductile Iron”. The macro hardness of the samples was detected by a digital Brinell hardness tester and a Vickers hardness tester, and the mechanical properties were tested by an AGXPLUS universal testing machine.
3. Experimental Results and Discussion
3.1 Influence of Silicon Content on the Microstructure Characteristics of High-Silicon Ductile Iron
3.1.1 Graphite Morphology
The graphitization rate of the ductile iron samples with different silicon contents was all above 90%, and the roundness of the graphite basically remained unchanged. With the increase of silicon content, the size of the graphite balls became finer, and the number of graphite balls per unit area increased. This is because silicon can act as the core of graphite nucleation, increasing the range of the eutectic temperature of the stable and metastable systems, which is beneficial to the formation of graphite, resulting in an increase in the number of graphite nucleation cores. At the same time, with the increase of silicon content and the decrease of carbon content under the condition of keeping the carbon equivalent basically unchanged, the volume of graphite decreased.
| Sample No. (Si) (%) | Graphite Ball Quantity /number·mm-2 | Graphite Ball Diameter /μm |
|---|---|---|
| A (2.92) | 151 | 35.6 |
| B(3.68) | 165 | 30.8 |
| C(4.22) | 208 | 27.7 |
| D (4.59) | 223 | 26 |
3.1.2 Matrix Structure
The matrix structure of the ductile iron samples with different silicon contents was all composed of ferrite. From the thermodynamic point of view, the increase of silicon content raised the eutectoid transformation temperature and enlarged the eutectoid temperature range of the stable and metastable systems, providing favorable conditions for the formation of ferrite. From the kinetic point of view, during the solidification process of ductile iron, the austenite generated at high temperature underwent a solid-state phase transformation when the temperature dropped to the eutectoid temperature. Due to the dense distribution of graphite in high-silicon ductile iron, the distance between austenite and graphite was shortened, and the carbon in austenite was easily dissolved and diffused to the eutectic graphite. After the carbon in austenite diffused out, the core of ferrite was easily precipitated on the austenite interface, which was beneficial to the formation of ferrite.
| Sample No. (Si)(%) | Ferrite Content (%) | Ferrite Grain Size /μm |
|---|---|---|
| A (2.92) | 88.8 | 43.7662 |
| B(3.68) | 89.9 | 42.0642 |
| C(4.22) | 90 | 38.6304 |
| D(4.59) | 91.7 | 35.3811 |
3.2 Influence of Silicon Content on the Mechanical Properties of High-Silicon Ferritic Ductile Iron
3.2.1 Tensile Strength and Hardness
The tensile strength and Brinell hardness of high-silicon ductile iron increased with the increase of silicon content. When the silicon content was 2.92%, the tensile strength of QT450-10 ductile iron was 441 MPa, and the hardness was 128 HBW; when the silicon content was 4.59%, the tensile strength of high-silicon ductile iron was 683 MPa, and the hardness was 186.1 HBW. This is because the increase of silicon content led to the refinement of graphite and ferrite grains, the increase of the solid solution strengthening effect of silicon on ferrite, and the increase of the dislocation resistance, resulting in the improvement of the strength and hardness of high-silicon ductile iron.
| Sample No.(Si)(%) | Tensile Strength/MPa | Yield Strength/MPa | Yield Ratio | Elongation(%) | Hardness(HBW) |
|---|---|---|---|---|---|
| A (2.92) | 441 | 325 | 0.737 | 17 | 128 |
| B(3.68) | 527 | 425 | 0.806 | 18 | 149 |
| C(4.22) | 601 | 512 | 0.852 | 17 | 174.6 |
| D(4.59) | 683 | 588 | 0.861 | 17.5 | 186.1 |
3.2.2 Fracture Morphology
The fracture morphology of the ductile iron samples with different silicon contents was observed by a scanning electron microscope. When the silicon content was 2.92%, the fracture morphology of the QT450-10 sample was mainly composed of dimple morphology, belonging to ductile fracture. When the silicon content was 3.68%, the number of dimples in the high-silicon ductile iron sample decreased but the depth increased, and there was local transgranular fracture. When the silicon content was 4.22%, the fracture morphology of the high-silicon ductile iron sample was a large area of cleavage plane, belonging to brittle fracture. When the silicon content was 4.59%, the number of dimples in the high-silicon ductile iron sample was less, there was a cleavage plane, and there was intergranular fracture. With the increase of silicon content, the fracture of ductile iron changed from ductile fracture to brittle fracture, but due to the full ferritic matrix and fine and uniform graphite, high-silicon ductile iron still showed good macroscopic plasticity.
3.3 Influence Mechanism of Silicon Element on the Microstructure and Properties of High-Silicon Ductile Iron
3.3.1 Solid Solution Strengthening
Silicon can dissolve in the ferrite matrix to form a substitutional solid solution. With the increase of silicon content, the solid solution strengthening effect of silicon on ferrite increased, and the microhardness of ferrite gradually increased. The average hardness of ferrite in QT450-10 ductile iron with a silicon content of 2.92% was 186.16 HV, and that in high-silicon ductile iron with a silicon content of 4.59% was 253.17 HV.
| Sample No. (Si) (%) | Average Value (HV) |
|---|---|
| A (2.92) | 186.16 |
| B(3.68) | 223.24 |
| C(4.22) | 238.28 |
| D (4.59) | 253.17 |
3.3.2 Segregation
Silicon not only can completely dissolve in the ferrite matrix, but also can produce segregation in the ferrite grains and at the grain boundaries. With the increase of silicon content, the degree of segregation of silicon at the ferrite grain boundaries increased, resulting in an increase in the lattice distortion of ferrite, which improved the strength and hardness of high-silicon ductile iron, but decreased its plasticity and toughness. However, due to the fine and uniform graphite morphology and the full ferritic matrix of high-silicon ductile iron, the decrease in elongation was relatively small, and it was basically unchanged macroscopically.
| Fe | C | Si | Ksi | |
|---|---|---|---|---|
| Spectrum 1 | 82.79 | 15.11 | 2.10 | 0.72 |
| Spectrum 2 | 76.32 | 20.74 | 2.94 | 1.01 |
| Spectrum 3 | 79.04 | 18.96 | 2.00 | 0.68 |
| Spectrum 4 | 76.41 | 20.56 | 3.04 | 1.04 |
| Spectrum 5 | 69.87 | 25.00 | 5.13 | 1.12 |
| Spectrum 6 | 74.54 | 21.66 | 3.80 | 0.83 |
| Spectrum 7 | 73.42 | 21.56 | 5.02 | 1.09 |
| Spectrum 8 | 74.95 | 20.86 | 4.18 | 0.91 |
3.3.3 Lattice Distortion
With the increase of silicon content, the lattice constant of ferrite gradually decreased, and the lattice distortion degree increased, which increased the dislocation resistance and improved the strength and hardness of ferrite. When the silicon content was 2.92%, the minimum lattice distortion degree was 0.06%, and when the silicon content was 4.59%, the maximum lattice distortion degree was 0.22%.
| (Si)(%) | a(110)/nm | a (200)/nm | a (211)/nm | a(AVG)/nm | Distortion Degree(%) |
|---|---|---|---|---|---|
| 2.92 | 0.28628 | 0.28655 | 0.28659 | 0.28647 | 0.06 |
| 3.68 | 0.28642 | 0.28612 | 0.28620 | 0.28625 | 0.14 |
| 4.22 | 0.28607 | 0.28638 | 0.28608 | 0.28618 | 0.16 |
| 4.59 | 0.28606 | 0.28600 | 0.28600 | 0.28602 | 0.22 |
4. Conclusions
In this study, the microstructure and mechanical properties of high-silicon ductile iron with a silicon content of 2.9%-4.6% prepared by the lost foam casting process were investigated. The main conclusions are as follows:
- The matrix structure of the high-silicon ductile iron samples is ferrite. With the increase of silicon content, the size of the graphite balls becomes finer, and the number of graphite balls per unit area increases; the average size of the ferrite grains decreases.
- The tensile strength, hardness, and comprehensive mechanical properties of high-silicon ductile iron increase with the increase of silicon content. When the silicon content is 2.92%, the tensile strength is 441 MPa, the hardness is 128 HBW, and the elongation is 17%; when the silicon content is 4.59%, the tensile strength is 683 MPa, the hardness is 186.1 HBW, and the elongation is 17.5%.
- The increase of silicon content makes the graphite balls finer and more uniform, reduces the 割裂作用 of the spherical graphite on the metal matrix, enhances the fine grain strengthening effect of silicon on ferrite, and forms solid solution strengthening by the substitutional solid solution of silicon in the ferrite matrix. However, with the increase of silicon content, the segregation of silicon at the ferrite grain boundaries becomes more serious, resulting in a decrease in the lattice constant and an increase in the lattice distortion degree of ferrite.
This study provides a theoretical basis for the application of high-silicon ductile iron in the lost foam casting of large pipe fittings. However, further research is needed to optimize the process parameters and improve the performance of high-silicon ductile iron to meet the requirements of different engineering applications.
5. Discussion on the Application and Prospect of High-Silicon Ductile Iron
5.1 Current Application Status
High-silicon ductile iron has been widely used in various fields due to its excellent mechanical properties. For example, in the automotive industry, it has been used to manufacture car hubs, engine blocks, and other components, which can improve the performance and fuel efficiency of vehicles. In the machinery manufacturing industry, it has been used to produce gears, shafts, and other parts, which can enhance the strength and wear resistance of the equipment. In addition, high-silicon ductile iron has also been applied in the construction, energy, and other industries.
5.2 Limitations and Challenges
Although high-silicon ductile iron has many advantages, it also has some limitations and challenges. For example, the brittleness of high-silicon ductile iron increases with the increase of silicon content, which limits its application in some fields that require high toughness. In addition, the casting process of high-silicon ductile iron is relatively complex, and it is difficult to control the quality of the casting. Therefore, it is necessary to further study and improve the casting process of high-silicon ductile iron to improve its quality and performance.
5.3 Future Research Directions
In order to further improve the performance and application range of high-silicon ductile iron, the following research directions can be considered in the future:
- Optimization of alloy composition: By adjusting the alloy composition of high-silicon ductile iron, such as adding other alloying elements, the microstructure and mechanical properties of high-silicon ductile iron can be further improved.
- Improvement of casting process: By improving the casting process of high-silicon ductile iron, such as optimizing the pouring temperature, negative pressure, and other parameters, the quality and performance of high-silicon ductile iron can be improved.
- Study on heat treatment process: By studying the heat treatment process of high-silicon ductile iron, such as annealing, normalizing, and quenching and tempering, the microstructure and mechanical properties of high-silicon ductile iron can be further optimized.
- Application research: By conducting in-depth research on the application of high-silicon ductile iron in different fields, such as automotive, machinery, construction, and energy, the application range and market demand of high-silicon ductile iron can be further expanded.
6. Conclusion
In summary, the silicon content has a significant impact on the microstructure and properties of high-silicon ductile iron in lost foam casting. The increase of silicon content can refine the graphite and ferrite grains, enhance the solid solution strengthening and fine grain strengthening effects, and improve the strength and hardness of high-silicon ductile iron. However, it also leads to an increase in brittleness. Therefore, in the actual application, it is necessary to select the appropriate silicon content according to the specific requirements to ensure the performance and quality of high-silicon ductile iron. At the same time, further research is needed to overcome the limitations and challenges of high-silicon ductile iron and expand its application range.