In the manufacturing industry, the demand for high-precision machine tools in sectors such as automotive, aerospace, rail, and defense has highlighted a critical dependency on imported equipment. One of the primary reasons for this reliance is the inadequate precision retention of domestic machine tool castings, particularly the bed components, which serve as foundational elements. The dimensional accuracy and long-term stability of these castings are heavily influenced by factors like stiffness—both material and structural—and residual stresses. Through extensive experimentation, we have investigated how carbon equivalent (CE), alloying techniques, and heat aging treatments can optimize these properties, ultimately improving the precision and durability of machine tool castings. This article delves into our methodologies, results, and conclusions, emphasizing the role of high carbon equivalent, composite alloying, and tailored heat treatments in achieving superior performance. By focusing on key aspects such as residual stress reduction and enhanced stiffness, we aim to provide a comprehensive framework for advancing the quality of machine tool castings, thereby boosting their competitiveness in global markets.
Our research centered on designing T-type bedway samples to represent typical machine tool castings, with variations in thickness to simulate different structural stiffness scenarios. We produced both gray cast iron and ductile iron specimens under controlled melting conditions, adjusting carbon equivalent levels and incorporating alloying elements like copper, tin, and chromium. The melting process involved using high-purity pig iron and pure iron in a medium-frequency induction furnace, with overheating temperatures maintained between 1510°C and 1520°C. Alloy additions were made during the melt, and compositions were verified using an ARL direct-reading spectrometer. For gray iron, inoculation was performed with high-calcium-barium inoculant, while ductile iron underwent spheroidization and inoculation treatments. After casting, the samples were allowed to cool naturally below 300°C before being machined and tested without shot blasting, ensuring that the results reflected the as-cast and heat-treated states accurately.
To assess the impact of structural stiffness, we employed SolidWorks SimulationXpress for linear static analysis on both thin and thick T-type bedway samples. By applying identical loads and constraints, we compared displacement patterns, which revealed that thicker designs exhibited significantly lower deformation, underscoring the importance of structural rigidity in machine tool castings. For material stiffness evaluation, we conducted tensile tests using an SHT4605 electro-hydraulic universal testing machine and measured elastic modulus with a RUSpec ultrasonic resonance spectrometer. Microstructural analysis was performed with a Leica-DMILM microscope, while residual stresses were quantified via the blind hole method using a ZDL-II drilling device and YC-III stress instrument. Precision retention was monitored over time by measuring straightness with a GAVA1000-B straightness measuring instrument on a grade 0 inspection platform, with readings taken monthly to track changes.
The chemical compositions and mechanical properties of the gray cast iron samples are summarized in Table 1. As observed, higher carbon equivalent levels, combined with composite alloying, resulted in maintained tensile strength and increased elastic modulus, which are crucial for the stiffness of machine tool castings. For instance, sample 2# with a CE of 3.66% showed a tensile strength of 346 MPa, while sample 4# with a CE of 3.88% achieved an elastic modulus of 129 GPa, indicating enhanced material rigidity. Similarly, ductile iron samples demonstrated even higher performance, with sample 6# reaching a tensile strength of 705 MPa and an elastic modulus of 176 GPa, highlighting the benefits of optimized compositions for machine tool castings.
| Sample | C (%) | Si (%) | CE (%) | Mn (%) | P (%) | S (%) | Cu (%) | Sn (%) | Cr (%) | Rm (MPa) | E (GPa) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 1# (Thin) | 2.758 | 1.777 | 3.36 | 1.174 | 0.033 | 0.019 | – | – | 0.323 | 365 | – |
| 2# (Thin) | 3.054 | 1.799 | 3.66 | 1.194 | 0.031 | 0.016 | 0.569 | 0.062 | 0.353 | 346 | – |
| 3# (Thick) | 2.695 | 1.504 | 3.21 | 1.188 | 0.034 | 0.016 | – | – | – | 357 | 114 |
| 4# (Thick) | 3.250 | 1.880 | 3.88 | 1.138 | 0.034 | 0.012 | 0.672 | 0.029 | 0.342 | 322 | 129 |
Residual stress measurements provided critical insights into the stability of machine tool castings. As-cast samples with lower carbon equivalent exhibited higher residual stresses, such as 56.5 MPa for sample 1# and 89.9 MPa for sample 3#. In contrast, those with higher CE and composite alloying showed reduced stresses, and after applying a “stepwise heating and cooling” heat aging process—illustrated in Figure 4—the stresses dropped below 20 MPa. This thermal treatment involved gradual temperature ramps and holds, effectively relieving internal stresses without compromising mechanical properties. The straightness data, calculated using the least squares method, further demonstrated the benefits of these approaches. The formulas for straightness evaluation are as follows:
First, the coefficients for the least squares midline are determined by:
$$ a = \frac{\sum Z_i \sum X_i^2 – \sum X_i \sum X_i Z_i}{(n+1) \sum X_i^2 – (\sum X_i)^2} $$
and
$$ q = \frac{(n+1) \sum X_i Z_i – \sum X_i \sum Z_i}{(n+1) \sum X_i^2 – (\sum X_i)^2} $$
where \( n \) is the number of segments, \( X_i \) represents the horizontal coordinates, and \( Z_i \) denotes the vertical coordinates of measured points. The straightness error \( f_{LS} \) is then given by:
$$ f_{LS} = d_{\text{max}} – d_{\text{min}} $$
with
$$ d_i = Z_i – a – q X_i $$
Over a three-month period, thin samples with lower CE and no heat aging (1#) showed a straightness increase of 31.5 μm, whereas those with higher CE and treatments (2#) increased by only 18.1 μm, indicating better precision retention for machine tool castings. Thick samples, benefiting from higher structural stiffness, exhibited straightness values nearly ten times lower than their thin counterparts, as shown in Table 2 for gray iron and Table 3 for ductile iron samples. This underscores how structural design optimizations can minimize deformation in machine tool castings.
| Sample | Measurement Month | Straightness (μm) | Change Over Period (μm) |
|---|---|---|---|
| 1# (Thin) | 1 | 324.3 | 31.5 |
| 2 | 344.6 | ||
| 3 | 338.8 | ||
| 4 | 355.8 | ||
| 2# (Thin) | 1 | 365.0 | 18.1 |
| 2 | 365.8 | ||
| 3 | 380.0 | ||
| 4 | 383.1 |
For ductile iron machine tool castings, the combination of high carbon equivalent, composite alloying, and heat aging yielded exceptional results. As detailed in Table 4, sample 6# achieved a tensile strength of 705 MPa and an elastic modulus of 176 GPa, with residual stresses reduced from 88.1 MPa in the as-cast state to 26.3 MPa after aging. The straightness measurements in Table 5 confirm that these samples had the lowest deformation among all tested, with values of 19.5 μm and 17.4 μm for samples 5# and 6#, respectively. This highlights the superior performance of ductile iron in applications requiring high precision and stability for machine tool castings.
| Sample | C (%) | Si (%) | CE (%) | Mn (%) | P (%) | S (%) | Cu (%) | Sn (%) | Rm (MPa) | A (%) | E (GPa) | HB | Residual Stress (MPa) – As-cast | Residual Stress (MPa) – Aged |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 5# (Thick) | 3.56 | 2.05 | 4.26 | 0.51 | 0.03 | 0.007 | – | – | 443 | 17.5 | 161 | 175 | 108.8 | – |
| 6# (Thick) | 3.77 | 1.99 | 4.44 | 0.53 | 0.03 | 0.009 | 0.496 | 0.042 | 705 | 3.0 | 176 | 234 | 88.1 | 26.3 |
| Sample | Straightness (μm) |
|---|---|
| 5# (Thick) | 19.5 |
| 6# (Thick) | 17.4 |
In summary, our findings demonstrate that increasing the carbon equivalent in machine tool castings, when combined with composite alloying and stepwise heat aging, effectively enhances both material stiffness and reduces residual stresses. This leads to improved precision retention, as evidenced by slower and smaller changes in straightness over time. Structural stiffness also plays a pivotal role; thicker designs with minimized wall thickness variations significantly reduce deformation. Ductile iron, in particular, offers a compelling advantage due to its high strength and modulus, making it ideal for high-performance machine tool castings. These strategies provide a solid foundation for advancing the quality and competitiveness of domestic machine tool castings, reducing reliance on imports and fostering innovation in precision manufacturing. Future work could explore additional alloying elements and real-world applications to further validate these approaches for machine tool castings.