In the production of gray cast iron, appropriately increasing the silicon-to-carbon (Si/C) ratio and applying suitable inoculation or micro-alloying treatments can yield cast iron with excellent comprehensive properties. This high Si/C ratio cast iron enables machine tool castings to achieve higher stiffness, wear resistance, and dimensional stability, while maintaining good casting performance, high and stable mechanical properties, and reduced defect rates. As a researcher in this field, I have extensively studied the effects of Si/C ratio on the properties of machine tool castings, and in this article, I will share key findings and practical insights.
The silicon and carbon contents, along with their ratio, significantly influence the solidification characteristics, microstructure, and various properties of cast iron. The impact of silicon and carbon on microstructure and performance is primarily reflected in their effect on graphitization. To quantify the influence of chemical composition on microstructure, several criteria have been developed for different scenarios. One widely used criterion is the graphitization coefficient, denoted as K, which can be expressed as:
$$ K = \frac{4 \times \text{Si\%}}{3 \times \text{C\%} – \text{Si\%}} $$
When K ≤ 0.5, the matrix is predominantly ledeburite or free cementite. With higher Si/C ratios, such as K ≥ 1.0, the matrix becomes fully pearlitic. For K between 0.8 and 1.0, the matrix is pearlitic, while for K > 1.0, it transitions to a pearlite-ferrite mixture, and at even higher values, it becomes fully ferritic. If the phosphorus content is very low, the carbon equivalent (CE) can be simplified as CE = C% + 1/3 Si%, and the graphitization criterion can be rewritten as:
$$ K = \frac{4 \times \text{Si\%}}{3 \times (\text{CE} – \frac{1}{3} \text{Si\%}) – \text{Si\%}} $$
This shows that the silicon content is the most significant factor affecting the graphitization coefficient. When the silicon content is stable, fluctuations in the graphitization coefficient are minimal, even if the carbon content varies noticeably. In conventional low Si/C ratio cast iron used for high-grade machine tool castings, the graphitization coefficient is around 0.5, close to the critical point for ledeburite formation. Fluctuations in silicon content, poor inoculation, or rapid cooling can easily lead to defects like chilled edges or free cementite. By increasing the Si/C ratio to 0.7–0.8, the graphitization coefficient remains in the pearlitic region, ensuring a fully pearlitic matrix even with compositional variations, thus stabilizing the properties of machine tool castings.
Internationally, there is a trend toward higher Si/C ratios in machine tool castings, as seen in standards and practices from countries like France and Germany. Proprietary cast iron technologies, such as “Sikal,” emphasize high Si/C ratios as a key feature. This indicates that adjusting the Si/C ratio to enhance performance is not only a focus in our casting industry but also a globally recognized approach, especially with improvements in melting conditions and metallurgical quality.
High Si/C ratio cast iron has been applied in numerous machine tool foundries. For instance, in one factory, it has been used comprehensively for over five years. Statistical data from production tests reveal that as the Si/C ratio increases, the relative strength (ratio of actual to expected tensile strength) of the cast iron improves significantly. For high-grade machine tool castings, controlling the Si/C ratio between 0.7 and 0.8 results in a relative strength notably higher than that of low Si/C ratio cast iron. The relationship between carbon equivalent and tensile strength for high Si/C ratio cast iron shows that even as CE increases from 3.6% to 4.0%, the average tensile strength decreases only slightly, while the relative strength improves markedly. At lower carbon equivalents (e.g., 3.6%), the average tensile strength remains around 250–300 MPa, with a relative strength of approximately 1.0–1.2. This demonstrates the robustness of high Si/C ratio cast iron in maintaining performance across varying conditions, which is crucial for machine tool castings subjected to diverse operational stresses.
| Si/C Ratio | Average Tensile Strength (MPa) | Relative Strength | Carbon Equivalent (%) |
|---|---|---|---|
| 0.5 | 220 | 0.85 | 3.6 |
| 0.7 | 280 | 1.10 | 3.8 |
| 0.8 | 300 | 1.20 | 4.0 |
Another advantage of high Si/C ratio cast iron is its lower performance variability under normal production conditions. Due to its ability to achieve higher tensile strength, it also attains a higher elastic modulus. For example, for a tensile strength of 250 MPa, the elastic modulus can reach 130–140 GPa; for 300 MPa, it can be 140–150 GPa; and for 350 MPa, it can exceed 150 GPa. The increase in elastic modulus significantly enhances the static stiffness of machine tools, contributing to improved assembly accuracy and overall performance of machine tool castings.
The microstructure and properties of gray cast iron are heavily influenced by cooling rates during solidification, which depend on factors like wall thickness and molding conditions. High-quality machine tool castings require not only excellent material properties in test samples but also consistent performance across different sections of the casting. Section sensitivity (or uniformity of structure) is particularly important for machine tool castings because it affects wear resistance in thick sections like guideways and machinability in thin walls. Key implications include:
- Differences in microstructure across casting sections can increase structural stresses, which, combined with thermal stresses, raise the risk of casting cracks and deformation.
- Variations in organizational structure in machine tool base castings lead to differential thermal expansion coefficients, causing uneven deformation and reduced precision under temperature changes.
- In mass production, significant differences in structure and performance can affect cutting parameters and machining accuracy during high-efficiency automated processes, deteriorating assembly precision and interchangeability.
High Si/C ratio cast iron, with its strong graphitization ability, allows for selecting an appropriate Si/C ratio based on casting geometry to prevent free cementite in thin sections and free ferrite in thick sections. This not only maintains strength but also slightly increases hardness due to silicon dissolution in the matrix, improving uniformity. Extensive tests and production data confirm that high Si/C ratios markedly reduce section sensitivity and enhance the hardness of machine tool guideways. For instance, in one application, low Si/C ratio cast iron resulted in guideway hardness below 180 HB in 5–10% of cases, leading to rejection due to low hardness or white edges. After switching to high Si/C ratio cast iron, guideway hardness consistently ranged from 190 to 220 HB, with hardness variations not exceeding 20 HB, eliminating hardness-related defects. This approach is applicable not only to small and medium-sized machine tool castings with faster cooling rates but also to heavy machine tool castings, provided the relationship between wall thickness, composition, structure, and performance is properly managed.
| Casting Section Thickness (mm) | Low Si/C Ratio Hardness (HB) | High Si/C Ratio Hardness (HB) | Hardness Variation (HB) |
|---|---|---|---|
| 20 | 170–190 | 200–220 | 20 |
| 50 | 160–180 | 190–210 | 20 |
| 100 | 150–170 | 180–200 | 20 |
The tendency for residual stress formation in cast iron decreases not only with increasing carbon equivalent but also with a higher Si/C ratio. High Si/C ratio cast iron exhibits lower residual stress at the same carbon equivalent while achieving higher strength, resulting in an improved strength-to-residual stress ratio. This indicates a greater resistance to cracking and deformation, as illustrated by the following relationship:
$$ \text{Strength-to-Residual Stress Ratio} = \frac{\sigma_b}{\sigma_r} $$
where $\sigma_b$ is the tensile strength and $\sigma_r$ is the residual stress. For high Si/C ratio cast iron, this ratio typically ranges from 2.5 to 3.5, compared to 1.5–2.5 for low Si/C ratio cast iron. The enhanced uniformity of microstructure in high Si/C ratio cast iron is a primary reason for reduced residual stresses, which is critical for the longevity and precision of machine tool castings.
Over the years, issues with dimensional accuracy stability in castings have persistently affected the quality of high-precision machine tools, leading to problems in machining, assembly, and usage. High Si/C ratio cast iron, with its lower residual stress and improved mechanical strength and deformation resistance, significantly enhances the dimensional stability of castings. In one case study involving coordinate boring machines, even after three stress-relief annealing cycles, castings made from conventional materials showed poor stability, with assembly rework rates as high as 30–40%. After adopting high Si/C ratio cast iron, the first-pass qualification rate increased to over 90%, and process rework rates dropped substantially. Product first-grade rates improved to above 95%, as shown in the table below. For instance, in precision automatic lathes, comparisons of machines with different materials under similar usage conditions revealed that conventional vanadium-titanium cast iron components suffered from guideway distortion and uneven wear, whereas high Si/C ratio castings showed no distortion, uniform wear, and maintained accuracy. The quality of precision machine tools is a comprehensive reflection of design, cold and hot working processes, and the internal and external quality of castings. However, the excellent dimensional stability of high Si/C ratio cast iron serves as a foundation for achieving and maintaining high precision, making it an ideal material for precision machine tool castings and other精密 machinery applications.
| Machine Tool Model | First-Pass Qualification Rate (%) – Before | First-Pass Qualification Rate (%) – After | First-Grade Product Rate (%) |
|---|---|---|---|
| Coordinate Boring Machine | 60 | 95 | 90 |
| Precision Grinding Machine | 70 | 92 | 88 |
| EDM Machine | 65 | 90 | 85 |
In summary, the application of high silicon-to-carbon ratio cast iron in machine tool castings offers substantial benefits in terms of strength, elasticity, hardness uniformity, residual stress reduction, and dimensional stability. The graphitization coefficient formula, $ K = \frac{4 \times \text{Si\%}}{3 \times \text{C\%} – \text{Si\%}} $, serves as a fundamental tool for optimizing composition. Empirical data supports that Si/C ratios between 0.7 and 0.8 yield optimal results for most machine tool casting applications. As the industry advances, the continued adoption of high Si/C ratio cast iron will play a pivotal role in enhancing the performance and reliability of machine tool castings, ensuring they meet the demanding requirements of modern manufacturing environments. Through rigorous testing and practical implementation, we have validated that this material approach not only improves individual casting properties but also contributes to the overall efficiency and precision of machine tool systems.