In the manufacturing of high-precision machine tools, cast iron parts serve as the foundational components for structural elements and key assemblies, including beds, tables, crossrails, columns, and saddles. These cast iron parts are predominantly made from gray iron, with common grades such as HT250 and HT300. Internationally, gray iron grades like HT300 and HT350 are also utilized, but for heavy-duty machine tools requiring enhanced accuracy, the demand for higher strength has led to the adoption of ductile iron grades like QT450, QT500, and QT600. The bed cast iron part alone constitutes approximately 70–80% of the total machine tool mass, ranging from several tons to tens of tons, and plays a critical role in ensuring machining performance, precision, and precision retention. Therefore, high-performance cast iron parts and their manufacturing technologies are pivotal determinants of machine tool functionality and quality.
During service, cast iron parts often face issues such as wear on working surfaces, distortion due to internal stresses, and cracking, which are primary causes of poor precision retention in machine tools. Hence, the quality of cast iron parts—encompassing structural design, mechanical properties, internal stresses, and dimensional accuracy—directly impacts the machine tool’s lifespan and reliability. My research focuses on optimizing casting processes, melting techniques, and aging treatments to achieve cast iron parts with high strength and stiffness while maintaining low initial stresses, thereby addressing challenges like deformation and cracking. The key technical aspects revolve around two critical indicators for machine tool stability: structural stiffness and material stiffness.
Structural stiffness refers to the design of the cast iron part based on operational conditions, considering factors like vibration resistance. Uniform wall thickness transitions, fillets, and reinforced ribs in specific areas are essential for structural integrity. A rational design is the foremost condition for machine tool stability. Material stiffness pertains to the physical properties of the cast iron part, including tensile strength, hardness, and elastic modulus. A high carbon equivalent forms the basis for low-stress cast iron materials, which, when combined with alloying elements such as Cu, Cr, Sn, and Sb, achieve high-strength, low-stress performance.
To illustrate common failure modes in cast iron parts, several case studies are examined. For instance, a large HT300 column exhibited cracking due to uneven wall thickness and lack of fillet transitions in thick sections, generating structural stresses. Another HT300 crossrail suffered from through-thickness cracks in rib plates because of inappropriate composition selection, where excessive pursuit of strength and hardness led to reduced carbon equivalent and high manufacturing stresses. A large HT300 ring-shaped cast iron part cracked due to insufficient holding time and temperature monitoring during cooling, creating thermal gradients. Additionally, an HT250 table developed multiple cracks in internal ribs because of rapid cooling during aging, resulting in uneven stress distribution. These cases highlight recurring issues in structural design, composition control, cooling practices, and aging processes—common challenges in foundry production.
The quality of large machine tool cast iron parts encompasses external appearance, internal integrity, and service performance. During structural design, considerations extend beyond geometric shape and dimensions based on operational conditions and material characteristics to include factors like shrinkage and internal stresses to minimize defects. For example, a lengthy bed cast iron part with dimensions of 16 m × 1.9 m, average wall thickness of 100 mm, and mass of 68 tons, made from HT300, was analyzed. Initial process reviews identified crack risks at junctions between rib plates and the main body. A semi-closed gating system was designed with pouring at both ends along the length to enhance filling speed. The melting process involved a charge mix of 10% pig iron, 45% scrap steel, and 45% returns, using high-temperature graphitizing carburizers. The composition was set with ω(C) at 3.05–3.10%, ω(Si) at 1.60–1.80%, ω(Mn) at 1.00–1.10%, and CE values of 3.60–3.70%, alloyed with ω(Cr) at 0.20–0.30%, accompanied by 0.1–0.2% instantaneous inoculation to boost strength.
Simulation software was employed to analyze filling and solidification. Results indicated high residual tensile stresses at riser roots, internal rib junctions, and side joints, with Mises coefficients exceeding 0.8, implying stress levels nearing the tensile strength and high crack probability. Structural optimization was then applied by adding transverse reinforcing ribs without increasing mass. Re-simulation showed Mises coefficients below 0.5, significantly reducing crack risk. This demonstrates the importance of simulation in refining cast iron part designs.
In melting process optimization, data from 2018 to 2020 revealed fluctuations in gray iron compositions: ω(C) ranged 2.9–3.2%, ω(Si) 1.4–2%, CE 3.4–3.7%, and Si/C ratio 0.4–0.6, leading to variable properties. Elastic modulus measurements for gray iron showed values from 93–164 GPa without alloying, and 117–142 GPa with 1–2 alloys, while ductile iron exhibited 138–190 GPa, indicating instability. Compared to international standards, domestic cast iron parts lag in carbon equivalent and elastic modulus. For instance, HT250 in industrial countries has CE of 3.95% versus 3.75% domestically, and elastic modulus requirements are higher. This gap underscores the need for improved melting practices.
The optimized melting process involves: (1) Synthetic cast iron technique: Using high scrap steel proportions (50–60%) with carburizers, minimizing pig iron to reduce coarse graphite inheritance. (2) High-temperature superheating: Holding molten iron at 1,500–1,550°C for 5–10 minutes to enhance purity. (3) Composition selection: Increasing CE for gray iron from 3.50–3.60% to 3.60–3.80%, and for ductile iron from 4.30–4.40% to 4.40–4.50% without graphite flotation, raising Si/C ratio, and adding Cu, Cr, Sn composites to promote pearlite and ensure strength. (4) Composition monitoring: Using spectrometers and thermal analysis for precise control. (5) Multi-stage inoculation: Combining base, stream, and pouring inoculation to enhance nucleation and silicon solid solution strengthening.
Experiments on HT300 material involved casting ϕ30 test bars with low-carbon high-silicon and composite alloying. Results showed that tensile strength and hardness improved with higher Si/C ratios within a range, as summarized in the table below. For example, when Si/C ratio increased from 0.61 to 0.72, tensile strength rose, while hardness remained relatively stable. This indicates that adjusting the Si/C ratio can optimize strength without compromising other properties.
| Si/C Ratio | Tensile Strength (MPa) | Hardness (HBW) | Graphite Type | Graphite Length Grade |
|---|---|---|---|---|
| 0.61 | ~370 | 186 | A | 5 |
| 0.63 | ~375 | 189 | A | 5 |
| 0.66 | ~380 | 188 | A | 4 |
| 0.72 | ~385 | 189 | A | 4 |
The relationship between tensile strength and Si/C ratio can be expressed empirically as: $$ \sigma_b = k \cdot \left(\frac{Si}{C}\right) + b $$ where $\sigma_b$ is tensile strength, $k$ is a constant, and $b$ is an intercept. Similarly, hardness shows a less pronounced correlation. For ductile iron QT600-3, experiments on molten iron pretreatment involved adding inoculated materials to the furnace before pouring. Results indicated that pretreatment increased graphite nodule count and reduced diameter, enhancing uniformity. Ultrasonic testing showed higher sound velocities in pretreated samples (5,671 mm/s vs. 5,454 mm/s), suggesting improved microstructure.
Alloying elements significantly impact the elastic modulus of ductile iron. For QT600-3, data shows that composite alloys like Cu+Sb or Cu+Sn stabilize elastic modulus above 160 GPa, as per GB/T 1348-2019. The table below summarizes effects of different alloys on elastic modulus distribution.
| Alloy Type | Elastic Modulus < 160 GPa (%) | Elastic Modulus ≥ 160 GPa (%) |
|---|---|---|
| None | 54.4 | 45.6 |
| +Cu | 22.8 | 77.2 |
| +Sb | 17.4 | 82.6 |
| +Cu+Sb | 0 | 100 |
| +Cu+Mo | 0 | 100 |
| +Cu+Sn | 0 | 100 |
Holding and aging processes are crucial for stress relief in cast iron parts. Measurements on beds of different masses showed cooling rates: below 3 tons, about 25°C/h; above 8 tons, about 13°C/h. Unloading temperatures below 300°C minimized deformation. Aging methods include natural aging, thermal aging, and vibration aging. Thermal aging is most effective, reducing stresses by 50–70%, but it consumes energy and may cause instability. Vibration aging reduces stresses by 30–50% with minimal energy use and good dimensional stability. Natural aging reduces stresses by less than 20% over months or years. Combining these methods can achieve stress reduction up to 78%.
Casting stresses in cast iron parts consist of thermal stress, phase transformation stress, and mechanical obstruction stress. Residual stresses are particularly dangerous as they continuously act on the cast iron part, potentially causing irreversible deformation. Initial measurements on HT300 beds showed as-cast stresses up to 175 MPa, unevenly distributed. After thermal aging, maximum stresses dropped to 79.7 MPa, but non-uniformity persisted. Through simulation and process adjustments, optimized cast iron parts were produced using high CE and composite low-alloy strengthening. Residual stress measurements on five gray iron cast iron parts (e.g., tables and beds) indicated predominantly compressive stresses with low tensile values, as shown in the table below.
| Cast Iron Part | State | Max. Principal Stress (MPa) | Min. Principal Stress (MPa) | Stress Type Dominance |
|---|---|---|---|---|
| Table 01# | As-cast | -105.9 | -74.1 | Compressive |
| Table 02# | Aged | -54.8 | -118.1 | Compressive |
| Bed 01# | As-cast | -152.7 | -234.4 | Compressive |
| Bed 02# | Aged | 112.5 | -227.3 | Mixed |
| Bed 03# | Aged | 25.7 | -82.8 | Compressive |
The elastic modulus of high-strength low-stress cast iron parts was evaluated. For QT600-3 parts like boxes, turntables, and slides, elastic modulus values were consistently ≥160 GPa, meeting standards. HT300 parts like columns and beds showed elastic modulus above 110 GPa. These results confirm that the optimized processes yield cast iron parts with stable mechanical properties and low stresses.
In summary, process simulation is key for optimizing cast iron part design and manufacturing. High carbon equivalent with low alloying ensures material stiffness, while proper aging treatments guarantee low stresses. The integration of these techniques produces cast iron parts suitable for high-precision machine tools, enhancing performance and longevity. Future work may focus on further refining alloy compositions and aging protocols to push the boundaries of cast iron part capabilities.