In the manufacturing of high-precision machine tools, the quality of machine tool castings such as beds, columns, saddles, and tables is paramount. These components constitute over 80% of the machine’s weight and directly influence its stability, wear resistance, damping capacity, and rigidity. Among the materials used, gray iron is predominant for machine tool castings, while ductile iron is less common. For high-end applications, machine tool castings must exhibit high compressive and tensile strength, high elastic modulus (stiffness), low stress to prevent deformation, excellent wear resistance and vibration damping, and superior dimensional accuracy and surface finish. However, achieving high strength and stiffness while maintaining low stress is challenging, as high carbon equivalent (CE) promotes low stress but reduces strength, whereas low CE enhances strength but increases stress. This contradiction necessitates a balanced approach to obtain high strength and stiffness at elevated CE levels, which is a critical focus in production.
Initially, our production process for HT300 machine tool castings involved controlling CE between 3.60% and 3.70% to achieve high strength through a low CE approach. Low CE expands the austenite growth range, promotes austenite development, refines dendrites, and improves strength. The chemical composition was meticulously managed, as summarized in Table 1.
| CE | C | Si | Mn | P | S | Sb | Sn |
|---|---|---|---|---|---|---|---|
| 3.6-3.7 | 3.0-3.1 | 1.6-1.7 | 0.8-1.0 | <0.05 | 0.06-0.1 | 0.02-0.03 | 0.02-0.03 |
The melting process utilized a 2-ton medium-frequency induction furnace. To minimize the hereditary effects of pig iron on the casting material, we limited pig iron addition and adopted a synthetic cast iron approach by carburizing and siliconizing steel scrap. The charge composition consisted of 60-70% high-quality carbon steel scrap, 20-30% returns of the same material, and 10% Q10 pig iron. Silicon carbide (90% purity, 2-9 mm grain size) was added at 1.0-1.5% for silicon and carbon adjustment, while medium-temperature graphitizing carburizer (with S ≤ 0.25% and N ≤ 1000 ppm) was used at 1.5-2.0% for carbon enhancement. Silicon carbide and carburizer were added in batches with the charge. At 1450°C, the melt was sampled for carbon-silicon analysis and spectroscopic composition testing. After adjusting to the target composition, the temperature was raised to 1480-1500°C, followed by a 3-5 minute holding period before tapping for inoculation.
Inoculation was performed twice using silicon-calcium-barium长效孕育剂. The primary inoculation involved adding 0.4% of 3-8 mm granules during tapping via a funnel flow, while secondary inoculation during pouring used 0.05-0.1% of 0.2-0.7 mm granules. This two-step inoculation ensured sufficient graphite nucleation sites, minimized fading, and guaranteed the desired microstructure and mechanical properties. The barium in the inoculant enhanced graphite nucleation resistance to衰退, reducing chilling tendency and improving graphite morphology in gray iron for machine tool castings.
Performance evaluation of bed machine tool castings based on 100 data sets showed an average CE of 3.635%, carbon content of 3.077%, and silicon content of 1.652%. The control charts for these elements are represented below, indicating stable composition control. The tensile strength of separately cast test bars exceeded 300 MPa, averaging 364 MPa with a maximum of 400 MPa. Metallographic examination revealed over 90% Type A graphite, 98% pearlite, and graphite length of grade 3-4, demonstrating satisfactory initial quality for machine tool castings.
| Parameter | Average Value |
|---|---|
| CE (%) | 3.635 |
| C (%) | 3.077 |
| Si (%) | 1.652 |
| Tensile Strength (MPa) | 364 |
However, several defects emerged in the machine tool castings. First, bed castings exhibited a high linear shrinkage rate of 1.4%, leading to crack formation in internal ribs, particularly near guideways. These cracks, accounting for 10% of defects, were attributed to uneven solidification due to structural variations—thin ribs solidified rapidly, while thick guideways solidified slowly, generating high internal stresses. Second, table castings displayed shrinkage porosity in T-slots, as confirmed by SEM analysis showing defect areas with iron oxide and graphite, but no impurities or decarburization, indicating typical shrinkage in hot spots. Third, uneven hardness in bed castings caused edge chipping and drill bit breakage during machining, especially in edges and lugs, due to low CE-induced shrinkage tendency and stress concentration.
The relationship between CE and mechanical properties can be expressed using the carbon equivalent formula for gray iron: $$CE = \%C + \frac{1}{3}\%Si$$. Lower CE values generally increase tensile strength but also elevate shrinkage and stress. To address this, we optimized the process by increasing CE to 3.75-3.85% and incorporating copper and tin for microalloying. Copper reduces section sensitivity, improves graphite morphology, promotes pearlite formation, and refines pearlite, while tin enhances pearlite stability. The combined effect of Cu and Sn (with Cu + 10Sn < 0.8%) strengthens the matrix more effectively than single elements. The revised chemical composition is detailed in Table 3.
| CE | C | Si | Mn | P | S | Cu | Sn |
|---|---|---|---|---|---|---|---|
| 3.75-3.85 | 3.15-3.25 | 1.65-1.75 | 0.8-0.9 | <0.05 | 0.06-0.1 | 0.4-0.5 | 0.02-0.03 |
To further enhance melt purity, refine graphite and matrix structure, and mitigate charge heredity effects, the melting temperature was increased to 1500-1520°C with a 5-minute holding period before tapping. Other melting parameters remained unchanged. This adjustment improves fluidity and reduces inclusions, critical for high-quality machine tool castings.
Post-optimization, analysis of 100 data sets for bed machine tool castings showed an average CE of 3.768%, carbon content of 3.189%, and silicon content of 1.711%. The control charts for these elements demonstrate improved consistency. Tensile strength of test bars averaged 341 MPa, within the optimal 300-350 MPa range, minimizing residual stress. Elastic modulus measurements from 70 sets averaged 126.64 GPa, ranging from 110 to 150 GPa, indicating enhanced stiffness for machine tool castings. Metallography confirmed predominantly Type A graphite, over 99% pearlite, and graphite length of grade 4. Hardness tests on 20 consecutive bed castings showed an average of 188.85 HBW, within the required 180-200 HBW range, with improved uniformity. Linear shrinkage reduced to 1.2-1.3%, and no cracks were observed, confirming better machinability and structural integrity.
| Parameter | Average Value | Range |
|---|---|---|
| CE (%) | 3.768 | 3.7-3.8 |
| C (%) | 3.189 | 3.15-3.25 |
| Si (%) | 1.711 | 1.65-1.75 |
| Tensile Strength (MPa) | 341 | 300-380 |
| Elastic Modulus (GPa) | 126.64 | 110-150 |
| Hardness (HBW) | 188.85 | 180-200 |
The improvement in material properties can be modeled using the relationship between tensile strength (σ_t), CE, and alloy content. For gray iron, a simplified formula is: $$σ_t = k_1 – k_2 \cdot CE + k_3 \cdot (\%Cu + \%Sn)$$ where k1, k2, and k3 are constants derived from regression analysis. In our case, increasing CE while adding Cu and Sn maintained strength while reducing stress. Additionally, the hardness uniformity is influenced by the pearlite refinement equation: $$H = α \cdot P + β \cdot G$$ where H is hardness, P is pearlite content, G is graphite size, and α and β are coefficients. Microalloying refined both pearlite and graphite, leading to consistent hardness across machine tool castings.
In conclusion, the optimization of HT300 machine tool casting production involved elevating CE to 3.75-3.85% and incorporating Cu and Sn for microalloying. This approach improved the matrix structure, refined grains, and enhanced strength, hardness uniformity, and dimensional stability. The adjustments in melting temperature and inoculation further contributed to superior performance. Although challenges such as precise composition control and silicon-to-carbon ratio adjustment remain, the results confirm that high-quality machine tool castings can be achieved with reduced defects and improved machinability. Future work should focus on optimizing CE within the target range and measuring residual stresses to fully validate the process for machine tool castings.