In the production of high-end machine tool castings, achieving superior material properties is paramount. These castings form the backbone of precision machinery, demanding exceptional strength, stiffness, low residual stress, and excellent machinability. For decades, our foundry has relied on cupola melting to produce iron melts that meet these rigorous demands. Through meticulous control of melting parameters, charge materials, and operational practices, we consistently obtain high-quality iron suitable for advanced machine tool castings. This article details our comprehensive approach, from cupola design to final casting, emphasizing the critical role of each step in ensuring the performance of machine tool castings.
The requirements for high-end machine tool castings are stringent. Typically, grades such as HT250, HT300, and HT350 gray iron are employed, with specifications calling for Type A graphite morphology, a predominantly pearlitic matrix, and minimal ferrite or phosphide eutectic. Hardness values must be tightly controlled, as outlined in the following table, which summarizes typical hardness requirements for various machine tool casting configurations.
| Casting Characteristic | Material Grade | Guideway Hardness (HBS) | Hardness Variation (HBS) |
|---|---|---|---|
| Guide length ≤ 2500 mm or mass ≤ 3 t | FC250 | 190–255 | 25 |
| Guide length > 2500 mm or mass > 3 t | FC300 | 180–241 | 35 |
| Mass 5–10 t | FC300 | 175–241 | – |
| Mass > 10 t | FC300 | 165–241 | – |
| – | FC350 | 165–241 | – |
| – | FC300 | 180–241 | – |
| – | FC350 | 190–255 | – |
Additionally, machine tool castings must exhibit high stiffness, low residual stress, and good machinability, often necessitating a high carbon equivalent (CE), elevated melting temperatures, and precise alloying. To meet these needs, we employ a specifically designed hot-blast cupola equipped with a dense-ribbed blast heating tank and large tuyere spacing. The structural parameters of this cupola are critical and are summarized below.
| Parameter | Value |
|---|---|
| Melting Zone Diameter (mm) | 1100–1150 |
| Tuyere 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 |
| Tuyere Ratio in Melting Zone (%) | 2.74–3 |
| Tuyere Ratio in Tuyere Zone (%) | 4.47 |
| Effective Height (mm) | 7400 |
| Number of Dense-Ribbed Hot-Blast Tanks | 1 |
The cupola is integrated with a water-cooled bag filter system to meet environmental standards. The selection of charge materials is equally vital. We use high-quality foundry pig iron with strict chemical composition limits, as shown in the following table, ensuring minimal impurities and surface contamination.
| Pig Iron Grade | C (%) | Si (%) | Mn (%) | P (%) | S (%) |
|---|---|---|---|---|---|
| Z22 | >3.30 | >3.20–3.60 | ≤0.50 | ≤0.060 | ≤0.03 |
| Z18 | >3.30 | >2.80–3.20 | ≤0.50 | ≤0.060 | ≤0.03 |
| Z14 | >3.30 | >2.40–2.80 | ≤0.50 | ≤0.060 | ≤0.03 |
Steel scrap is selected from ordinary carbon steel with controlled composition: C 0.2%–0.6%, Si ≤0.6%, Mn 0.35%–0.9%, P ≤0.05%, S ≤0.05%. All charge materials must be free of rust, impurities, and adhered sand, with sizes below 300 mm and weights under 25 kg for pig iron and 10 kg for scrap. Returns are classified by iron grade and cleaned. Foundry coke from Zhenjiang is used, meeting stringent specifications: size 60–120 mm for layer coke and 100–150 mm for bed coke, moisture ≤5%, ash 8.01%–10.00%, volatiles ≤1.5%, sulfur ≤0.8%, crushing strength (M40) ≥81%, and abrasion resistance (M10) ≤6%.
The melting process begins with charging. The bed coke height is maintained at 1.8 m, corresponding to a mass of approximately 1000 kg. The layer charge consists of 1000 kg of metallic materials, with layer coke fixed at 100 kg. Every 8–10 charges, a supplementary coke addition of 3% of the iron charge weight is made to maintain bed coke height and temperature. Wind supply is controlled via a Roots blower with a maximum capacity of 157 m³/min at 29 kPa. The optimal blast intensity is calculated as 100 Nm³/(m²·min). Given the varying cupola diameter during melting, the blast volume is adjusted dynamically. Initially, with an average cross-sectional area of 0.785 m², the blast volume is set to 78.5 m³/min. As melting progresses and lining erosion increases the diameter to up to 1330 mm (area 1.389 m²), the blast volume is raised to 138.9 m³/min. In practice, accounting for pipeline leaks, the actual blast volume ranges from 100 m³/min at startup to 155 m³/min later in the campaign.
Chemical composition is tailored for each grade of machine tool castings. The internal designations and corresponding standard grades, along with their chemical control ranges, are provided below.
| Internal Code | Standard Grade | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Cu (%) | Cr (%) | Sn (%) |
|---|---|---|---|---|---|---|---|---|---|
| I | HT250 | 3.1–3.4 | 2.0–1.7 | 0.6–1.0 | <0.15 | <0.1 | – | – | – |
| M | HT300 | 3.0–3.3 | 1.9–1.6 | 0.8–1.1 | <0.15 | <0.1 | – | – | – |
| T | HT300 (Cu/Sn) | 3.0–3.3 | 1.9–1.6 | 0.8–1.1 | <0.15 | <0.1 | 0.5–0.6 | – | 0.02–0.03 |
| T1 | HT300 (Cu/Cr) | 3.0–3.3 | 1.9–1.6 | 0.8–1.1 | <0.15 | <0.1 | 0.5–0.6 | 0.15–0.25 | – |
| G | HT350 (Cu/Cr) | 2.9–3.2 | 1.8–1.6 | 1.0–1.2 | <0.15 | <0.1 | 0.5–0.6 | 0.25–0.35 | – |
The charge makeup for these grades emphasizes high steel scrap ratios to enhance strength and stiffness. Typical charge ratios are: 53% steel scrap for Grade I, and 60% for Grades M, T, T1, and G, with the balance being pig iron and returns. Alloying elements like Cu, Cr, and Sn are added as per the specifications. The carbon equivalent (CE) is a key parameter, calculated using the formula: $$CE = C + \frac{Si + P}{3}$$ For high-end machine tool castings, we aim for CE values above 3.8% to ensure good castability and reduced shrinkage tendencies, while maintaining high strength through proper inoculation and alloying.
Melting operations follow a strict sequence. After briefly blowing air to clear ash and prodding tuyeres to settle the bed coke, the remaining bed coke is added to achieve the 1.8 m height. Then, 60 kg of limestone (double the batch amount) is charged, followed by metallic materials in the order: steel scrap, pig iron, alloys, and returns. This sequence repeats until the charging door is reached. The cupola is then allowed to soak for 15–30 minutes before initiating the blast. During melting, tuyeres are kept clear and bright, charge level is maintained steadily, and any bridging is promptly addressed. Regular monitoring of iron temperature guides the addition of supplementary coke.
At the spout, rapid thermal analysis is used to measure the CE, C, and Si content of the base iron. Based on this, inoculant addition is determined. Typically, Fe-Si inoculant is used for general castings, while Si-Ba inoculant is employed for critical machine tool castings. Inoculated iron is poured into wedge test samples, spectroscopic specimens, and tensile test bars. The chill width on the wedge test is controlled according to casting section thickness and hardness requirements: 3–4 mm for Grade I and 4–5 mm for Grade M. Thicker sections or higher hardness guideways require larger chill widths. Adjustments, primarily via inoculation, are made if the iron does not meet specifications.
The gating system for machine tool castings is designed based on the large orifice outflow theory. For castings under 2 t, the gating ratio is: $$\Sigma F_{\text{sprue}} : \Sigma F_{\text{runner}} : \Sigma F_{\text{ingate}} = 1.2 : 1.4 : 1$$ For castings over 2 t, the ratio is: $$\Sigma F_{\text{sprue}} : \Sigma F_{\text{runner}} : \Sigma F_{\text{ingate}} = 2 : 1.5 : 1$$ Large bed castings employ a bottom rain gating system with a ratio: $$\Sigma F_{\text{sprue}} : \Sigma F_{\text{runner}} : \Sigma F_{\text{inner runner}} : \Sigma F_{\text{ingate}} = 2 : 1.5 : 1 : 1.5\text{–}2$$ Enlarging the ingate area reduces metal velocity, ensuring smooth filling and minimizing slag entrapment. A gate strainer cup is used to prevent primary slag entry. Pouring is rapid, especially at the start, to keep the pouring basin full. Pouring times for castings ranging from a few tons to 15 t are controlled between 30–90 s through system design. Pouring temperatures are maintained high, typically 1380–1420°C, to accommodate the thin-walled nature of many machine tool castings.
The production outcomes demonstrate the effectiveness of this approach. Key melting parameters from sample campaigns are shown below.
| Campaign Date | Layer Coke (t) | Bed Coke (t) | Supplementary Coke (kg) | Total Coke Rate (%) | Melting Zone Diameter (mm) | Tuyere Zone Diameter (mm) | Blast Volume (m³/min) | Blast Pressure (kPa) | Blast Intensity (m³/(min·m²)) |
|---|---|---|---|---|---|---|---|---|---|
| 20160826 | 10.5 | 1 (height 1.8 m) | 300 | 11.80 | Initial 1100, Final 1370 | Initial 900, Final 1290–1200 | 121.6 | 18–19 | 129.19 |
| 20160908 | 7.3 | 1 (height 1.8 m) | 0 | 11.36 | Initial 1100, Final 1300 | Initial 950, Final 1250 | 121.6 | 18–19 | 128.02 |
| Campaign Date | Average Tapping Temp (°C) | Iron Mass (t) | Melting Time (h) | Melting Rate (t/h) | Melting Intensity (t/(m²·h)) | FeO Content (%) | Carbon Increase Rate (%) | Si Burn-off Rate (%) | Mn Burn-off Rate (%) |
|---|---|---|---|---|---|---|---|---|---|
| 20160826 | 1515 | 100 | 10.03 | 9.97 | 8.33 | 5.5 | +58.3 | -18.7 | -23.8 |
| 20160908 | 1520 | 73 | 7.12 | 10.25 | 8.82 | 4.65 | +64.87 | -17 | -20.8 |
Tapping temperatures vary during the campaign: first tap ~1463°C, second tap ~1493°C, third tap 1510–1520°C, and remaining taps around 1510°C. Chemical composition fluctuations are minimal: C ±0.15%, Si ±0.1%, Mn ±0.05%. The tensile strength of separately cast test bars (φ30 mm) correlates with CE, as depicted in the data from numerous samples.
| Sample ID | Grade | C (%) | Si (%) | Mn (%) | P (%) | S (%) | Cu (%) | Cr (%) | Sn (%) | Tensile Strength (MPa) | Hardness (HBS) | CE (%) | Si/C | RG | RZ | RH | HG | RG/HG | Number of Samples |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| #1 | I | 3.09 | 1.67 | 0.83 | 0.04 | 0.09 | – | – | – | 322.9 | 213.7 | 3.66 | 0.54 | 0.96 | 1.25 | 0.88 | 0.87 | 1.10 | 15 |
| #2 | I | 3.18 | 1.67 | 0.88 | 0.04 | 0.08 | – | – | – | 314.5 | 213.4 | 3.75 | 0.52 | 0.99 | 1.22 | 0.90 | 0.90 | 1.10 | 71 |
| #3 | I | 3.25 | 1.72 | 0.88 | 0.04 | 0.08 | – | – | – | 315.1 | 211.0 | 3.84 | 0.53 | 1.05 | 1.25 | 0.92 | 0.92 | 1.14 | 77 |
| #4 | I | 3.35 | 1.74 | 0.91 | 0.04 | 0.07 | – | – | – | 289.6 | 209.0 | 3.94 | 0.52 | 1.04 | 1.18 | 0.95 | 0.95 | 1.10 | 25 |
| #5 | I | 3.36 | 1.88 | 0.91 | 0.12 | 0.07 | – | – | – | 291.3 | 202.5 | 4.02 | 0.56 | 1.12 | 1.25 | 0.95 | 0.95 | 1.17 | 5 |
| #6 | M | 3.02 | 1.63 | 0.85 | 0.04 | 0.08 | – | – | – | 363.3 | 218.7 | 3.58 | 0.54 | 1.03 | 1.35 | 0.84 | 0.87 | 1.18 | 4 |
| #7 | M | 3.09 | 1.65 | 0.87 | 0.04 | 0.08 | – | – | – | 348.2 | 221.3 | 3.66 | 0.54 | 1.03 | 1.26 | 0.88 | 0.90 | 1.14 | 26 |
| #8 | M | 3.17 | 1.68 | 0.91 | 0.04 | 0.08 | – | – | – | 332.6 | 216.4 | 3.75 | 0.53 | 1.05 | 1.26 | 0.88 | 0.91 | 1.15 | 71 |
| #9 | M | 3.26 | 1.69 | 0.89 | 0.05 | 0.08 | – | – | – | 321.9 | 214.0 | 3.84 | 0.52 | 1.08 | 1.25 | 0.89 | 0.94 | 1.15 | 77 |
| #10 | M | 3.36 | 1.71 | 0.89 | 0.04 | 0.08 | – | – | – | 306.8 | 209.4 | 3.94 | 0.51 | 1.11 | 1.24 | 0.89 | 0.96 | 1.16 | 25 |
| #11 | T1 | 3.08 | 1.55 | 0.90 | 0.04 | 0.08 | 0.57 | 0.23 | – | 407.5 | 232.5 | 3.61 | 0.51 | 1.17 | 1.36 | 0.83 | 0.93 | 1.26 | 2 |
| #12 | T1 | 3.19 | 1.65 | 0.88 | 0.04 | 0.08 | 0.55 | 0.20 | – | 370.0 | 228.9 | 3.76 | 0.52 | 1.17 | 1.27 | 0.87 | 0.97 | 1.21 | 11 |
| #13 | T1 | 3.28 | 1.79 | 0.93 | 0.04 | 0.08 | 0.54 | 0.20 | – | 334.2 | 223.3 | 3.89 | 0.55 | 1.16 | 1.20 | 0.90 | 1.00 | 1.16 | 6 |
| #14 | T | 2.98 | 1.66 | 0.90 | 0.04 | 0.08 | 0.53 | – | 0.025 | 385.0 | 231.3 | 3.55 | 0.56 | 1.07 | 1.29 | 0.86 | 0.91 | 1.18 | 3 |
| #15 | T | 3.09 | 1.69 | 0.88 | 0.04 | 0.08 | 0.50 | – | 0.025 | 362.2 | 231.2 | 3.67 | 0.55 | 1.08 | 1.22 | 0.89 | 0.95 | 1.14 | 18 |
| #16 | T | 3.15 | 1.77 | 0.91 | 0.04 | 0.08 | 0.52 | – | 0.025 | 353.6 | 227.1 | 3.75 | 0.56 | 1.11 | 1.23 | 0.89 | 0.96 | 1.16 | 47 |
| #17 | T | 3.24 | 1.78 | 0.91 | 0.04 | 0.08 | 0.54 | – | 0.025 | 336.6 | 223.6 | 3.84 | 0.55 | 1.13 | 1.20 | 0.90 | 0.98 | 1.15 | 67 |
| #18 | T | 3.32 | 1.80 | 0.91 | 0.04 | 0.11 | 0.55 | – | 0.025 | 322.2 | 221.9 | 3.94 | 0.54 | 1.15 | 1.17 | 0.92 | 1.01 | 1.14 | 16 |
| #19 | T | 3.33 | 2.12 | 0.91 | 0.04 | 0.07 | 0.51 | – | 0.025 | 292.5 | 213.0 | 4.04 | 0.64 | 1.14 | 1.14 | 0.94 | 1.01 | 1.13 | 4 |
In this table, RG represents the quality factor relating tensile strength to hardness, RZ is another quality index, and RH is a hardness factor. HG is a derived parameter. The quality factor RG/HG is critical, with values above 1.1 indicating excellent material quality for machine tool castings. The data reveals that at similar CE, higher steel scrap ratios (e.g., 60% in Grade M vs. 53% in Grade I) yield increased tensile strength—by over 20 MPa at CE 3.6%–3.8% and over 10 MPa at higher CE. Alloying with Cu and Cr (Grade T1) boosts strength by about 40 MPa at CE 3.6%–3.8%, while Cu and Sn addition (Grade T) provides a 15–20 MPa increase. The quality factor improves with higher scrap ratios and alloying: for CE 3.6%–4.0%, RG/HG ranges are: Grade I 1.10–1.14, Grade M 1.14–1.16, Grade T1 1.16–1.26, Grade T 1.14–1.18.
Elastic modulus and residual stress are also key for machine tool castings. Independent measurements on samples gave the following results:
| Sample No. | C (%) | Si (%) | Mn (%) | CE (%) | Tensile Strength (MPa) | Hardness (HBS) | Elastic Modulus (GPa) |
|---|---|---|---|---|---|---|---|
| #1 | 3.51 | 1.68 | 1.00 | 4.08 | 250 | 199 | 114.2 |
| #10 | 3.11 | 2.02 | 1.12 | 3.80 | 355 | 229 | 137.5 |
| #14 | 3.20 | 1.84 | 0.93 | 3.83 | 340 | 227 | 135.6 |
At CE 3.8%–3.85%, tensile strength reaches 340–355 MPa with elastic modulus of 135.6–137.5 GPa, indicating high stiffness essential for machine tool castings. Residual stress measurements on a large bed casting showed a maximum as-cast stress of -159.5 MPa, reduced to 21.1 MPa after stress relief annealing, ensuring dimensional stability.
Hardness tests on actual machine tool castings confirm uniformity. For instance, a 13,000 kg bed casting had guideway hardness values between 176–210 HBS, and a 2,570 kg bed casting showed values of 205–220 HBS on guideways and 205–219 HBS on thin-wall machined surfaces, all within specifications.
Environmental compliance is achieved with the water-cooled bag filter. Monitoring data showed dust emission at 46.8 mg/m³ and SO₂ at 197 mg/m³, meeting Class II standards of GB 9078-1996 for industrial furnace emissions.
In conclusion, the cupola melting process, when meticulously controlled, is highly effective for producing high-quality iron for machine tool castings. Key factors include: using a dense-ribbed hot-blast cupola with large tuyere spacing; selecting premium charge materials and coke; implementing precise blast control and charge composition; maintaining high melting and pouring temperatures; and employing proper inoculation and gating design. This approach yields iron with high CE, high strength, high stiffness, low stress, and excellent quality factors, meeting the demanding requirements of high-end machine tool castings. The process is also environmentally sustainable with appropriate filtration systems. Continued refinement in these areas ensures that cupola melting remains a viable and superior method for manufacturing critical components like machine tool castings.