In the production of high-end machine tool castings, achieving superior material properties is paramount. These machine tool castings demand high strength, rigidity, excellent machinability, and low residual stress to ensure precision and durability in demanding applications. We have extensively utilized cupola furnaces for melting iron to produce various types of machine tool castings, such as those for grinders and machining centers. This article details our approach to producing high-quality iron melt for machine tool castings using cupola furnaces, focusing on key parameters, melting practices, and resulting material characteristics. The integration of advanced cupola technology enables the production of machine tool castings with high carbon equivalent (CE), enhanced mechanical properties, and optimal performance.
High-end machine tool castings typically employ gray iron grades like HT250, HT300, and HT350, which require specific microstructural features. The graphite morphology should be predominantly Type A, with a pearlitic matrix and minimal ferrite or phosphide eutectic. Additionally, hardness specifications vary based on casting size and application, as summarized in Table 1. For instance, castings with guideways longer than 2,500 mm or weighing over 3 tons require controlled hardness to prevent distortion and ensure stability. These machine tool castings must also exhibit high stiffness and low stress to maintain accuracy under operational loads.
| Casting Characteristic | Material Grade | Guideway Hardness (HBS) | Hardness Variation (HBS) |
|---|---|---|---|
| Guideway length ≤ 2,500 mm or weight ≤ 3 t | FC250 | 190–255 | 25 |
| Guideway length > 2,500 mm or weight > 3 t | FC300 | 180–241 | 35 |
| Weight 5–10 t | FC350 | 175–241 | – |
| Weight > 10 t | FC300 | 165–241 | – |
To meet these demands, the iron melt must have a high CE (above 3.8%), elevated tapping temperatures (over 1,500°C), and a significant scrap steel ratio in the charge. Proper inoculation and, for higher grades like HT350, low-alloying with elements such as copper and chromium are essential. The cupola furnace plays a critical role in achieving these parameters, as it allows for precise control over melting conditions and composition.
The cupola furnace used in our production features a dense-ribbed, water-cooled blast heating tank and large tuyere spacing, which enhances thermal efficiency and melt quality. Key structural parameters are outlined in Table 2. The furnace is equipped with a hot blast system to maintain consistent temperatures, and the design includes multiple tuyeres at different levels to optimize combustion and reduce oxidation. The effective height and melting zone diameter are critical for ensuring proper residence time and heat transfer, contributing to the production of high-quality iron for machine tool castings.
| Parameter | Value |
|---|---|
| Melting Zone Diameter (mm) | 1,100–1,150 |
| Wind Zone Diameter (mm) | 900 |
| Tuyere Row Distance (mm) | 800 |
| Lower Tuyere Diameter (mm) | 45 |
| Number of Lower Tuyeres | 8 |
| Upper Tuyere Diameter (mm) | 50 |
| Number of Upper Tuyeres | 8 |
| Melting Zone Tuyere Ratio (%) | 2.74–3.00 |
| Wind Zone Tuyere Ratio (%) | 4.47 |
| Effective Height (mm) | 7,400 |
| Number of Dense-Ribbed Hot Blast Tanks | 1 |
The melting process begins with careful selection of raw materials. We use high-quality foundry pig iron and carbon steel scrap, ensuring they are free from rust, impurities, and sand adhesion. The chemical composition of the pig iron is critical, as it influences the final properties of the machine tool castings. Table 3 provides the main chemical requirements for the pig iron, which includes controlled levels of silicon, manganese, phosphorus, and sulfur to minimize variability.
| Element | Composition Range (wt%) |
|---|---|
| C | >3.30 |
| Si | 1.25–3.60 (graded by category) |
| Mn | ≤0.50 |
| P | ≤0.060 |
| S | ≤0.03 |
For scrap steel, we specify carbon content between 0.2% and 0.6%, with low levels of silicon, manganese, phosphorus, and sulfur. The charge consists of 53% to 60% scrap steel, depending on the target iron grade, with the remainder being pig iron and returns. This high scrap ratio promotes a refined graphite structure and higher strength in the machine tool castings. Alloying elements like copper, chromium, and tin are added to enhance specific properties, such as hardness and wear resistance, particularly for grades like HT350. The charge makeup for different iron grades is detailed in Table 4.
| Material Grade Code | Scrap Steel (%) | Pig Iron + Returns (%) | Cu (wt%) | Cr (wt%) | Sn (wt%) |
|---|---|---|---|---|---|
| I (HT250) | 53 | 47 | – | – | – |
| M (HT300) | 60 | 40 | – | – | – |
| T (HT300 with Cu/Sn) | 60 | 40 | 0.5–0.6 | – | 0.02–0.03 |
| T1 (HT300 with Cu/Cr) | 60 | 40 | 0.5–0.6 | 0.15–0.25 | – |
| G (HT350 with Cu/Cr) | 60 | 40 | 0.5–0.6 | 0.25–0.35 | – |
Coke quality and charging are vital for maintaining stable melting conditions. We use high-quality foundry coke with low ash and sulfur content, as specified in Table 5. The base coke height is set at 1.8 m, with a layer coke mass of 100 kg per 1,000 kg of iron charge. Additional coke is added periodically to sustain the melting rate and temperature. The wind supply is controlled using a Roots-type blower, with an optimal blast intensity of 100 Nm³/(m²·min). As the furnace lining erodes during operation, the blast volume is adjusted from 78.5 m³/min initially to 138.9 m³/min to compensate for the increased hearth area. This ensures consistent combustion and heat distribution, crucial for producing uniform iron melt for machine tool castings.
| Parameter | Specification |
|---|---|
| Size (mm) | 60–120 for layer coke; 100–150 for base coke |
| Moisture Content (%) | ≤5 |
| Ash Content (%) | 8.01–10.00 |
| Volatile Matter (%) | ≤1.5 |
| Sulfur Content (%) | ≤0.8 |
| Crushing Strength (M40, %) | ≥81 |
| Abrasion Resistance (M10, %) | ≤6 |
During melting, we monitor key parameters such as tapping temperature, melting rate, and element recovery. The average tapping temperature exceeds 1,500°C, with variations throughout the campaign as shown in Table 6. Carbon increase typically ranges from 58% to 65%, while silicon and manganese losses are controlled between 17% and 24%. These factors contribute to a stable CE, which is critical for achieving the desired microstructure in machine tool castings. The CE is calculated using the formula:
$$ CE = \%C + \frac{1}{3} (\%Si + \%P) $$
where %C, %Si, and %P are the weight percentages of carbon, silicon, and phosphorus, respectively. For high-end machine tool castings, we aim for a CE above 3.8% to ensure good fluidity and reduced shrinkage, while maintaining high strength through proper inoculation and alloying.
| Sequence | Tapping Temperature (°C) |
|---|---|
| First Ladle | 1,463 |
| Second Ladle | 1,493 |
| Third Ladle | 1,510–1,520 |
| Second Last Ladle | 1,510 |
| Last Ladle | 1,510 |
Frontal control involves rapid thermal analysis to determine CE, carbon, and silicon content before inoculation. Based on this, inoculants such as ferrosilicon or silicon-barium are added to promote Type A graphite and reduce chilling. The white iron width in wedge tests is controlled between 3 mm and 5 mm, depending on casting section thickness and hardness requirements. For critical machine tool castings, we use silicon-barium inoculants for improved effectiveness. After inoculation, samples are poured for tensile testing, spectroscopic analysis, and hardness checks to verify quality.
The pouring system is designed based on large orifice flow theory to ensure smooth filling and minimize slag inclusion. For castings under 2 tons, the gating ratio (sprue:runner:ingate) is set at 1.2:1.4:1, while larger castings use a ratio of 2:1.5:1. For massive bed castings, a bottom shower gating system with a ratio of 2:1.5:1:1.5–2 is employed to reduce turbulence. A gate cup is used to prevent primary slag entry, and rapid pouring (within 30–90 seconds for castings up to 15 tons) is maintained to avoid gas defects. The pouring temperature is controlled between 1,380°C and 1,420°C to accommodate the thin-walled sections typical of machine tool castings.
Production results demonstrate the effectiveness of this approach. Tensile strength and hardness data for separately cast test bars are summarized in Table 7. The relationship between CE and tensile strength for φ30 mm test bars is illustrated in Figure 1, showing that higher scrap ratios and alloying improve strength at a given CE. For example, at a CE of 3.6–3.8%, the tensile strength increases by over 20 MPa when the scrap ratio is raised from 53% to 60%. Alloying with copper and chromium further enhances strength by approximately 40 MPa, while copper and tin additions provide a smaller gain. The quality coefficient (Q), defined as the ratio of tensile strength to hardness, exceeds 1.1 for all grades, indicating excellent machinability for machine tool castings.
| Grade Code | C (wt%) | Si (wt%) | Mn (wt%) | CE (wt%) | Tensile Strength (MPa) | Hardness (HBS) | Quality Coefficient (Q) |
|---|---|---|---|---|---|---|---|
| I | 3.09–3.36 | 1.67–1.88 | 0.83–0.91 | 3.66–4.02 | 289.6–322.9 | 202.5–213.7 | 1.10–1.14 |
| M | 3.02–3.36 | 1.63–1.71 | 0.85–0.89 | 3.58–3.94 | 306.8–363.3 | 209.4–221.3 | 1.14–1.16 |
| T | 2.98–3.33 | 1.66–2.12 | 0.88–0.91 | 3.55–4.04 | 292.5–385.0 | 213.0–231.3 | 1.14–1.18 |
| T1 | 3.08–3.28 | 1.55–1.79 | 0.88–0.93 | 3.61–3.89 | 334.2–407.5 | 223.3–232.5 | 1.16–1.26 |
The high CE values contribute to good castability and reduced stresses in machine tool castings. Elastic modulus measurements, as shown in Table 8, range from 135.6 GPa to 137.5 GPa for CE values of 3.8–3.85%, indicating high stiffness. Residual stress tests on a bed casting revealed a maximum stress of -159.5 MPa in the as-cast condition, which was reduced to 21.1 MPa after stress relief annealing, demonstrating the low-stress characteristics essential for precision machine tool castings.
| Sample ID | C (wt%) | Si (wt%) | CE (wt%) | Tensile Strength (MPa) | Elastic Modulus (GPa) |
|---|---|---|---|---|---|
| #1 | 3.51 | 1.68 | 4.08 | 250 | 114.2 |
| #10 | 3.11 | 2.02 | 3.80 | 355 | 137.5 |
| #14 | 3.20 | 1.84 | 3.83 | 340 | 135.6 |
Environmental considerations are integral to our cupola operations. The furnace is equipped with a water-cooled bag filter dust collection system, which reduces emissions to comply with industrial standards. Monitoring data show dust concentrations of 46.8 mg/m³ and SO₂ levels of 197 mg/m³, meeting Class II emission standards for industrial furnaces. This ensures sustainable production of machine tool castings without compromising environmental goals.
In conclusion, the use of cupola furnaces with optimized parameters and strict process controls enables the production of high-quality iron for machine tool castings. By selecting premium materials, maintaining precise melting conditions, and implementing effective pouring techniques, we achieve iron melts with high CE, strength, stiffness, and low stress. The resulting machine tool castings exhibit superior mechanical properties and machinability, meeting the rigorous demands of high-end applications. Continuous monitoring and environmental measures further enhance the sustainability and reliability of this approach for producing advanced machine tool castings.