Grey cast iron has long been recognized as an excellent engineering material due to its superior wear resistance, damping capacity, machinability, and ease of casting formation, making it widely used in internal combustion engines and machine tool components. However, with technological advancements, traditional grey cast iron can no longer meet the demands of modern high-end machine tool equipment. Instead, grey iron with high strength, low stress, high rigidity, and high elastic modulus holds promising market prospects. In this article, I will discuss the production technology control for high-strength synthetic grey cast iron machine tool castings, focusing on key aspects such as molten iron composition, charge materials, melting processes, gas content control, and inoculation treatments. The goal is to achieve consistent production of high-quality machine tool castings that meet stringent performance requirements.
High-end machine tools must possess characteristics like high processing accuracy, efficiency, stability, reliability, and durability, which impose higher demands on the material properties of machine tool castings. The specific machine tool casting discussed here is a bed component for export to Japan, with overall dimensions of approximately 1400 mm × 850 mm × 350 mm and a weight of around 1000 kg. The microstructure and performance requirements include graphite in the form of randomly distributed Type A graphite with a graphite length of grade 3–6, a matrix with pearlite content ≥ 98%, tensile strength ≥ 300 MPa, bulk hardness between 190–210 HB, and Shore hardness between 32–38 HS. Particularly, the guideway sections require quenching treatment to ensure uniformity and consistency in microstructure. To achieve these specifications, control measures are implemented in several areas: selection of molten iron chemical composition, charge materials and melting process control, gas content management, and inoculation treatment selection.
The chemical composition of molten iron directly influences the microstructure and properties of the material. The carbon equivalent (CE) plays a crucial role in determining the structure and performance of cast iron. A high carbon equivalent increases graphite quantity, improves damping capacity, enhances fluidity, and reduces shrinkage tendency. However, excessive carbon equivalent can lower tensile strength and hardness. Therefore, low-alloy treatment is essential when operating with high carbon equivalent. For the machine tool casting requiring guideway quenching, copper (Cu) and tin (Sn) are selected as alloying elements (antimony, Sb, is prohibited). The total amount of alloying elements added during melting is strictly controlled according to the formula: $$\text{Cu} + 10 \times \text{Sn} \leq 0.8\%$$. Fluctuations in chemical composition significantly affect the uniformity and consistency of material properties, so it is necessary to tightly control composition ranges: ±0.05% for carbon (C), silicon (Si), and manganese (Mn). Based on product characteristics, usage requirements, and empirical experience in producing high-end machine tool castings, the composition ranges are selected as shown in Table 1.
| Element | Range (wt%) |
|---|---|
| C | 3.1–3.3 |
| Si | 1.6–1.9 |
| Mn | 0.8–1.0 |
| S | 0.06–0.1 |
| P | < 0.05 |
| Cu | 0.5–0.6 |
| Sn | 0.02–0.03 |
In the melting process, a 2-ton medium-frequency induction furnace is employed, utilizing a synthetic cast iron process with steel scrap carburizing and siliconizing. The raw materials and charge ratios are as follows: high-quality ordinary carbon steel plate scraps constitute 55–65% of the charge, while returns from the same material account for 25–30%. Semi-graphitized carburizer is added at 1.0–1.3%, and silicon carbide (SiC) is used as a silicon and carbon additive at 0.6–1.2%, with particle size controlled between 2–9 mm. This promotes the formation of numerous dispersed heterogeneous crystallization nuclei in the molten iron, reducing undercooling and encouraging the development of Type A graphite as the primary graphite structure. The limited use of pig iron minimizes harmful trace elements like titanium (Ti ≤ 0.05%) and lead (Pb ≤ 0.004%),削弱ing the genetic effects of pig iron and resulting in fine, uniform, and branched Type A graphite. Carburizing increases the number of graphite nuclei, improving graphite nucleation and growth conditions, and compensating for the reduction in graphite nuclei typical of electric furnace melting. The tapping temperature is controlled between 1500–1520°C, with a holding time of 3–5 minutes at high temperature in the furnace. The electromagnetic stirring action of the induction furnace helps remove gases and inclusions, enhancing metallurgical quality and fluidity of the molten iron.
Control of gas content, particularly nitrogen (N), is critical in grey cast iron production. Nitrogen acts as a micro-alloying element that refines graphite morphology and matrix structure by blunting graphite tips, increasing mechanical properties, refining eutectic cells, increasing pearlite content, and enhancing the microhardness of pearlite and ferrite. However, nitrogen levels exceeding 120 ppm can lead to nitrogen porosity. Nitrogen sources include steel scrap and carburizer, so carburizer selection is vital. In this process, metallurgical silicon carbide and medium-temperature graphitized carburizer are used, with nitrogen content controlled to ≤ 500 ppm in incoming materials, ensuring final product nitrogen levels between 50–100 ppm. Oxygen (O) content also influences graphite morphology, though research is limited. Based on practical experience with oxygen-nitrogen analyzer tracking, the optimal oxygen content in cast iron is maintained between 10–40 ppm. The relationship between gas content and properties can be expressed using empirical formulas such as: $$\text{Hardness} = k_1 \times \text{N} + k_2 \times \text{O} + C$$ where \(k_1\) and \(k_2\) are constants, and \(C\) is a baseline value. Table 2 summarizes the target gas content ranges for machine tool castings.
| Gas Element | Target Range (ppm) |
|---|---|
| Oxygen (O) | 10–40 |
| Nitrogen (N) | 50–100 |
Inoculation treatment is implemented using silicon-calcium-barium长效 inoculant during tapping at 0.4–0.6% addition, with particle size of 3–8 mm, followed by secondary stream inoculation with 75% ferrosilicon powder during pouring into the mold at 0.05–0.1% addition, particle size 0.2–0.7 mm. The silicon-calcium-barium inoculant leverages barium’s strong ability to promote graphite nucleation, increasing graphite nodule count in ductile iron and improving graphite morphology in grey iron, reducing chill tendency, and ensuring desired microstructure and mechanical properties. Secondary stream inoculation ensures sufficient graphite nucleation sites in high-purity molten iron, guaranteeing that material performance and structure meet product specifications. The effectiveness of inoculation can be modeled as: $$\text{Graphite Nucleation Density} = A \times \text{Inoculant Addition} + B$$ where \(A\) and \(B\) are parameters dependent on the inoculant type and process conditions.
For product trials, a resin sand hand-molding process is adopted. Melting involves controlling tapping temperature between 1500–1520°C, using a front carbon-silicon analyzer for preliminary control, and a spark emission spectrometer for final composition adjustment. After composition qualification, copper and tin are added to the ladle. Tapping includes inoculation with silicon-calcium-barium, pouring temperature is maintained at 1360–1390°C, slag is removed before pouring, and secondary stream inoculation with 75% ferrosilicon is applied during pouring. Pouring is conducted rapidly, steadily, and continuously, using a bottom gating + side inlet system to ensure material uniformity. Five heats of casting trials are performed, and the results are analyzed below.
Chemical composition and gas content analysis for the five heats are presented in Table 3. The results show that composition fluctuations are within the designed control ranges, such as ±0.05% for C, Si, and Mn, and Cu + 10 × Sn ≤ 0.9%. Oxygen and nitrogen contents average between 15–30 ppm and 50–70 ppm, respectively, indicating that gas contents are within the targeted ranges. All heats meet the technical control requirements for machine tool castings.
| Heat No. | C (wt%) | Si (wt%) | Mn (wt%) | P (wt%) | S (wt%) | Cu (wt%) | Sn (wt%) | O (ppm) | N (ppm) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 3.14 | 1.72 | 0.857 | 0.028 | 0.071 | 0.65 | 0.022 | 15 | 63 |
| 2 | 3.12 | 1.69 | 0.887 | 0.015 | 0.077 | 0.60 | 0.025 | 13 | 57 |
| 3 | 3.17 | 1.64 | 0.840 | 0.021 | 0.061 | 0.61 | 0.023 | 17 | 67 |
| 4 | 3.15 | 1.70 | 0.861 | 0.020 | 0.080 | 0.64 | 0.021 | 24 | 61 |
| 5 | 3.13 | 1.69 | 0.878 | 0.018 | 0.078 | 0.61 | 0.022 | 23 | 53 |
Mechanical properties and microstructure analysis of separately cast test bars and casting bodies are summarized in Table 4. The results indicate that all heats exhibit Type A graphite with non-directional uniform distribution, graphite size of grade 4, pearlite content ≥ 98%, tensile strength ≥ 360 MPa, test bar hardness ≥ 220 HB, and body hardness ≥ 34 HS. These meet the product technical requirements for machine tool castings. The microstructure analysis reveals consistent A-type graphite and high pearlite content, confirming the effectiveness of the production control measures. The relationship between tensile strength and composition can be approximated by: $$\sigma_b = \alpha \times \text{C} + \beta \times \text{Si} + \gamma \times \text{Mn} + \delta \times \text{Cu} + \epsilon \times \text{Sn} + \zeta$$ where \(\alpha, \beta, \gamma, \delta, \epsilon, \zeta\) are coefficients derived from regression analysis of production data.
| Heat No. | Graphite Morphology (Test Bar) | Graphite Size (Test Bar) | Matrix Structure (Test Bar, % Pearlite) | Tensile Strength (MPa, Test Bar) | Hardness (HB, Test Bar) | Graphite Morphology (Body) | Graphite Size (Body) | Matrix Structure (Body, % Pearlite) | Hardness (HS, Body) |
|---|---|---|---|---|---|---|---|---|---|
| 1 | A | 4 | 99.5 | 395 | 230 | A | 4 | 98 | 34 |
| 2 | A | 4 | 99.6 | 370 | 230 | A | 4 | 98 | 35 |
| 3 | A | 4 | 99.4 | 390 | 229 | A | 4 | 98 | 35 |
| 4 | A | 4 | 98.7 | 365 | 226 | A | 4 | 98 | 36 |
| 5 | A | 4 | 99.0 | 375 | 229 | A | 4 | 98 | 35 |
In conclusion, the appropriate addition of alloying elements such as copper and tin in synthetic cast iron microalloying effectively improves the matrix structure, refines grains, and enhances strength, hardness, and stability of machine tool castings. The control of gas content in molten iron is essential for high-end machine tool castings, with oxygen content ideally maintained between 10–40 ppm and nitrogen between 50–100 ppm. The production process described has enabled mass supply of machine tool bed castings with positive feedback from customers. It is important to note that actual production conditions may vary, so adjustments should be made based on specific circumstances. The consistent performance of these machine tool castings underscores the success of the controlled production approach, ensuring that high-strength synthetic grey iron meets the rigorous demands of modern machine tool applications. Further optimization could involve dynamic control models such as: $$\frac{d\text{Quality}}{dt} = f(\text{Composition}, \text{Temperature}, \text{Inoculation})$$ where quality metrics are continuously monitored and adjusted in real-time for even better consistency in machine tool castings production.