In the manufacturing industry, machine tool castings serve as the foundational components for precision equipment, often referred to as the “mother of industries.” The accuracy and stability of these castings directly influence the quality of machined parts and overall assembly precision. Historically, challenges in achieving dimensional stability due to residual stress release during service have hindered the reliability of domestic high-end machine tool castings compared to international standards. To address this, we have developed an advanced grey iron material with high carbon equivalent (CE) and high silicon-to-carbon ratio (Si/C), alloyed with elements like Sn, Cr, and N. This material enhances casting performance, reduces shrinkage and residual stress, and improves mechanical properties, ultimately leading to superior machine tool castings. This article details our research and production practices, focusing on compositional optimization, experimental comparisons, and practical applications, with an emphasis on using tables and formulas to summarize key findings.
Our journey in developing high-stiffness, low-stress materials for machine tool castings involved three distinct phases. Initially, we used a low CE and low Si/C HT300 material, which provided adequate strength but suffered from higher residual stress and section sensitivity. Next, we transitioned to a high CE with low Si/C HT300 material, observing improvements in casting properties but limited gains in elasticity and stress reduction. Finally, we adopted a high CE and high Si/C HT300 material, alloyed with Sn, Cr, and N, which delivered optimal results in terms of mechanical performance, elastic modulus, and stress resistance. These phases are summarized through comparative data on chemical composition, mechanical properties, metallurgical quality, and microstructure, all critical for evaluating machine tool castings.
The carbon equivalent (CE) is a key parameter in grey iron, defined as: $$CE = C + \frac{1}{3}(Si + P)$$ where C, Si, and P are the mass percentages of carbon, silicon, and phosphorus, respectively. For machine tool castings, a high CE (typically 3.80–3.90%) promotes better fluidity, reduced shrinkage, and lower tendency for chilling. The silicon-to-carbon ratio (Si/C) is another vital factor, calculated as: $$Si/C = \frac{Si}{C}$$ A high Si/C (0.7–0.8) enhances graphitization, minimizes undercooling, and improves hardness uniformity. In our studies, we combined high CE and high Si/C with micro-alloying to achieve a pearlite matrix over 95%, which is essential for high strength and wear resistance in machine tool castings.
To quantitatively compare the three material phases, we conducted extensive testing over multiple heats. The chemical compositions, averaged from 40 to 100 heats per phase, are presented in Table 1. This table highlights the evolution in CE, Si/C, and alloying elements like Sn, Cr, and N, which are crucial for refining microstructure and enhancing performance in machine tool castings.
| Material Phase | Number of Heats | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Cu (%) | Cr (%) | Sn (%) | N (%) | CE (%) | Si/C |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Low CE, Low Si/C HT300 | 40 | 3.08 | 1.66 | 0.85 | 0.03 | 0.08 | 0.50 | – | 0.02 | 0.0057 | 3.64 | 0.54 |
| High CE, Low Si/C HT300 | 100 | 3.22 | 1.85 | 0.87 | 0.03 | 0.07 | – | 0.04 | – | 0.0084 | 3.85 | 0.58 |
| High CE, High Si/C HT300 | 100 | 3.04 | 2.33 | 0.75 | 0.03 | 0.08 | – | 0.25 | 0.06 | 0.0087 | 3.83 | 0.77 |
The mechanical properties and metallurgical quality indices were evaluated for each phase, as shown in Table 2. These include tensile strength, elastic modulus, hardness, and derived parameters like maturity (RG), hardening degree (HG), and quality coefficient (Qi), which are critical for assessing the suitability of materials for machine tool castings. The maturity RG, for instance, relates tensile strength to composition and cooling conditions, often expressed as: $$RG = \frac{\sigma_b}{1000 – 800 \times Sc}$$ where $\sigma_b$ is the tensile strength in MPa, and Sc is the eutecticity, calculated from CE. Similarly, the hardening degree HG indicates the hardness response, and Qi combines multiple factors to evaluate overall quality. The high CE, high Si/C phase demonstrated superior values across all metrics, underscoring its advantages for high-precision machine tool castings.
| Material Phase | Tensile Strength (MPa) | Elastic Modulus (GPa) | Test Bar Hardness (HBW) | Guideway Hardness (HBW) | Machinability Index (m) | Eutecticity (Sc) | Maturity (RG) | Hardening Degree (HG) | Quality Coefficient (Qi) |
|---|---|---|---|---|---|---|---|---|---|
| Low CE, Low Si/C HT300 | 332.0 | 120.6 | 203.4 | 189.4 | 1.63 | 0.83 | 0.99 | 0.65 | 1.52 |
| High CE, Low Si/C HT300 | 342.4 | 122.5 | 213.5 | 186.4 | 1.60 | 0.89 | 1.18 | 0.79 | 1.49 |
| High CE, High Si/C HT300 | 369.9 | 132.5 | 223.8 | 205.8 | 1.63 | 0.88 | 1.22 | 0.80 | 1.52 |
Microstructural analysis revealed significant differences among the phases, as summarized in Table 3. The high CE, high Si/C material exhibited over 99% Type A graphite with a size of 4–5 grade (6–25 mm at 100x magnification), and a pearlite content exceeding 99% with a fine interlamellar spacing of 0.73 μm. This refined microstructure contributes to high strength, good damping capacity, and low stress concentration, all vital for machine tool castings. The graphite morphology in this phase showed curved, blunt-tipped flakes, which reduce stress risers and enhance mechanical properties. In contrast, the low CE, low Si/C material had coarser pearlite and less optimal graphite, leading to higher residual stress.
| Material Phase | CE (%) | Si/C | Type A Graphite (%) | Graphite Size (Grade) | Pearlite Content (%) | Pearlite Spacing (μm) |
|---|---|---|---|---|---|---|
| Low CE, Low Si/C HT300 | 3.64 | 0.54 | 98 | 4 | 98 | 1.08 |
| High CE, Low Si/C HT300 | 3.85 | 0.58 | 98 | 4 | 99 | 0.79 |
| High CE, High Si/C HT300 | 3.83 | 0.77 | 99 | 4 | 99 | 0.73 |
Residual stress is a critical factor for dimensional stability in machine tool castings. We evaluated stress using a standard stress frame with dimensions as illustrated, where the central thick bar (30 mm diameter) is prone to stress concentration. The residual stresses $\sigma_1$ and $\sigma_2$ were measured at specific points using the blind-hole method, and the results are in Table 4. The high CE, high Si/C material showed lower tensile and compressive stresses, with values like 25.0 MPa and 5.8 MPa, compared to higher stresses in the other phases. This reduction is attributed to the improved graphitization and lower shrinkage tendency, which minimize internal strains during solidification. The relationship between residual stress and material properties can be approximated by: $$\sigma_r = E \cdot \epsilon$$ where $\sigma_r$ is residual stress, E is elastic modulus, and $\epsilon$ is strain. The high elastic modulus (≥130 GPa) in our optimized material further contributes to stress resistance, ensuring long-term accuracy for machine tool castings.
| Material Phase | Measurement Point | $\sigma_1$ (MPa) | $\sigma_2$ (MPa) | Tensile Strength (MPa) | Elastic Modulus (GPa) |
|---|---|---|---|---|---|
| Low CE, Low Si/C HT300 | Central Thick Bar | 32.4 | 17.2 | 332.0 | 120.6 |
| High CE, Low Si/C HT300 | Central Thick Bar | 20.2 | -6.7 | 342.4 | 122.5 |
| High CE, High Si/C HT300 | Central Thick Bar | 25.0 | 5.8 | 365.3 | 132.5 |
Based on these findings, we established comprehensive performance criteria for high-end machine tool castings, as detailed in Table 5. These criteria encompass chemical composition, microstructure, mechanical properties, and residual stress limits, ensuring that each casting meets stringent requirements for precision and durability. For instance, the carbon equivalent must range from 3.80% to 3.90%, and the silicon-to-carbon ratio from 0.7 to 0.8, with specific alloy additions like Sn (0.04–0.06%) and N (0.008–0.010%). The microstructure should feature over 90% Type A graphite, 4–5 grade size, and at least 98% pearlite. Mechanically, tensile strength should exceed 300 MPa, elastic modulus 130 GPa, and hardness 200 HBW ±10 HBW. Residual stresses are capped at 50 MPa for tension and 98 MPa for compression in the as-cast state. These standards guide our production processes to consistently deliver high-quality machine tool castings.
| Category | Parameter | Target Range |
|---|---|---|
| Chemical Composition | Carbon Equivalent (CE, %) | 3.80–3.90 |
| Silicon-to-Carbon Ratio (Si/C) | 0.7–0.8 | |
| Nitrogen (N, %) | 0.0080–0.0100 | |
| Tin (Sn, %) | 0.04–0.06 | |
| Copper (Cu, %) | 0–0.5 | |
| Chromium (Cr, %) | 0–0.25 | |
| Microstructure | Type A Graphite (%) | ≥90 |
| Graphite Size (Grade) | 4–5 | |
| Pearlite Content (%) | ≥98 | |
| Phosphide + Carbide (%) | ≤1 | |
| Mechanical Properties | Tensile Strength (MPa) | ≥300 |
| Elastic Modulus (GPa) | ≥130 | |
| Guideway Hardness (HBW) | 200 ±10 | |
| Test Bar Hardness (HBW) | 220 ±10 | |
| Metallurgical Quality | Machinability Index (m) | ≥1.0 |
| Maturity (RG) | ≥1.0 | |
| Hardening Degree (HG) | ≤1.0 | |
| Quality Coefficient (Qi) | ≥1.0 | |
| Residual Stress (As-Cast) | Tensile Stress | <50 MPa |
| Compressive Stress | <98 MPa |
To validate these criteria in practice, we applied the high CE, high Si/C material to a bed casting for a CNC machining center—a critical machine tool casting. The bed measures 1800 mm × 1000 mm × 900 mm, weighs 1300 kg, and has wall thicknesses ranging from 15 mm to 80 mm. Key requirements included HT300 grade, guideway hardness of 180–210 HBW, and a microstructure with over 90% Type A graphite and 98% pearlite. We used a furan resin sand process for molding and core-making, with single-cast test bars (Ø30 mm) prepared according to GB/T 9439-2023 standards.
The melting process involved several steps: First, we employed a medium-frequency induction furnace with a synthetic cast iron practice, using medium-temperature graphite-based carburizer (1.4–1.6% addition, 1–8 mm粒度) to achieve high carbon levels. Second, metallurgical silicon carbide (0.6–1.2% addition, 2–9 mm粒度) was added to enhance graphitization and reduce undercooling. Third, inoculation was performed with 0.4% FeSi alloy (3–10 mm粒度) during tapping, followed by 0.05–0.1% 75% Si powder for stream inoculation. Fourth, nitrogen was introduced via ferromanganese nitride (0.06–0.12%), added with the inoculant to promote nitride formation and refine the matrix. Finally, melting temperature was controlled at 1500–1540°C, pouring temperature at 1360–1390°C, and shakeout below 280°C to minimize thermal stress. This rigorous process ensures consistent quality for machine tool castings.
Chemical analysis of six heats (Samples 1–6) confirmed compliance with our targets, as shown in Table 6. The CE averaged 3.82%, and Si/C averaged 0.78, with Sn around 0.07% and N around 0.0085%. These compositions align with the high CE, high Si/C approach, optimized for machine tool castings.
| Sample | CE (%) | Si/C | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Sn (%) | N (%) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 3.80 | 0.79 | 3.00 | 2.37 | 0.785 | 0.034 | 0.083 | 0.070 | 0.0089 |
| 2 | 3.81 | 0.79 | 3.00 | 2.38 | 0.757 | 0.034 | 0.068 | 0.070 | 0.0086 |
| 3 | 3.82 | 0.77 | 3.03 | 2.33 | 0.764 | 0.037 | 0.077 | 0.070 | 0.0079 |
| 4 | 3.81 | 0.80 | 3.00 | 2.39 | 0.752 | 0.028 | 0.067 | 0.061 | 0.0090 |
| 5 | 3.83 | 0.76 | 3.05 | 2.31 | 0.771 | 0.036 | 0.083 | 0.056 | 0.0081 |
| 6 | 3.82 | 0.78 | 3.02 | 2.36 | 0.771 | 0.034 | 0.079 | 0.067 | 0.0076 |
Mechanical testing of single-cast test bars and guideway hardness measurements are summarized in Table 7. Tensile strength ranged from 356 to 382 MPa, exceeding the 300 MPa requirement. Elastic modulus values were between 130 and 137 GPa, meeting the ≥130 GPa target. Guideway hardness averaged around 205 HBW, within the 200 ±10 HBW range. These results demonstrate that the high CE, high Si/C material delivers robust performance for machine tool castings, ensuring durability and precision in service.
| Sample | Tensile Strength (MPa) | Test Bar Hardness (HBW) | Guideway Hardness (HBW) – Three Points | Elastic Modulus (GPa) |
|---|---|---|---|---|
| 1 | 357 | 218 | 209, 210, 206 | 133 |
| 2 | 366 | 223 | 206, 202, 210 | 130 |
| 3 | 382 | 225 | 199, 205, 208 | 132 |
| 4 | 356 | 220 | 208, 205, 202 | 137 |
| 5 | 374 | 220 | 201, 198, 197 | 131 |
| 6 | 382 | 232 | 204, 209, 213 | 137 |
Microstructural examination of the bed casting guideway revealed Type A graphite (over 90%) at 4–5 grade size, uniformly distributed without directionality, and a pearlite content above 95%. This aligns with our specifications and contributes to the casting’s high strength and low stress. The graphite flakes were short, curved, and blunt-tipped, which reduces stress concentration and improves fatigue resistance—a key attribute for machine tool castings subjected to cyclic loads.
Hardness uniformity across the guideways is critical for consistent machining performance. We tested four guideways, each approximately 1500 mm long, at three evenly spaced points. As shown in Table 8, the hardness values varied by less than 10 HBW, with averages around 198–201 HBW. This uniformity indicates minimal section sensitivity, a benefit of the high Si/C ratio, which promotes even cooling and graphitization throughout the machine tool casting.
| Guideway | Hardness Values (HBW) – Three Points | Average Hardness (HBW) | Maximum Difference (HBW) |
|---|---|---|---|
| 1 | 201, 194, 200 | 198.3 | 7 |
| 2 | 201, 203, 199 | 201.0 | 4 |
| 3 | 194, 194, 202 | 196.7 | 8 |
| 4 | 192, 196, 200 | 196.0 | 8 |
Residual stress measurements on the as-cast bed casting were conducted at four points on the guideways using the blind-hole method. The strains $\epsilon_1$, $\epsilon_2$, $\epsilon_3$ were recorded, and stresses $\sigma_1$ and $\sigma_2$ calculated, as presented in Table 9. All values were below the limits of 50 MPa tensile and 98 MPa compressive, with the highest tensile stress at 37.8 MPa and compressive at -46.6 MPa. This low stress level ensures dimensional stability during machining and service, a paramount requirement for high-end machine tool castings. The stress state can be analyzed using the formula: $$\sigma_1, \sigma_2 = \frac{E}{1-\nu^2} (\epsilon_1 + \nu \epsilon_2)$$ where $\nu$ is Poisson’s ratio (typically 0.26 for grey iron). Our results validate the effectiveness of the high CE, high Si/C material in minimizing residual stresses.
| Location | Point | $\epsilon_1$ (με) | $\epsilon_2$ (με) | $\epsilon_3$ (με) | $\sigma_1$ (MPa) | $\sigma_2$ (MPa) |
|---|---|---|---|---|---|---|
| Bed | 1 | 61 | -7 | -81 | 37.8 | -20.6 |
| 2 | 24 | -8 | 29 | -9.0 | -39.1 | |
| 3 | 3 | -2 | -7 | 3.9 | -0.3 | |
| 4 | 79 | 36 | -30 | 2.0 | -46.6 |
In conclusion, our production practice demonstrates that high carbon equivalent (3.80–3.90%) and high silicon-to-carbon ratio (0.7–0.8) grey iron, alloyed with Sn, Cr, and N, is highly effective for manufacturing high-performance machine tool castings. This material combination yields a microstructure with over 90% Type A graphite (4–5 grade size) and at least 98% pearlite, leading to tensile strength above 300 MPa, elastic modulus over 130 GPa, hardness of 200 HBW ±10 HBW, and low as-cast residual stresses (tensile <50 MPa, compressive <98 MPa). The optimized composition reduces shrinkage, minimizes section sensitivity, and enhances hardness uniformity, all critical for the precision and stability required in machine tool castings. While this approach shows promise, ongoing collaboration with manufacturers, experts, and research institutions is essential to further validate and refine the material for diverse applications in the machine tool industry. Future work may explore additional alloying elements or heat treatments to push the boundaries of performance for these foundational components.