In the manufacturing of machine tool castings, residual stresses arise during casting due to factors such as hindered shrinkage, and additional stresses are introduced during machining processes. These residual stresses, which persist within the casting, can relax and redistribute over time, often leading to deformation and compromising dimensional accuracy. With advancements in modern technology, there is an increasing demand for high dimensional stability in machine tool castings and precision instrument components. Therefore, research into reducing and stabilizing residual stresses to enhance the dimensional stability of castings has become a focal point globally. Years of production practice and scientific experiments have demonstrated that aging treatments are effective methods for reducing residual stresses and achieving stabilization. This article presents our findings from long-term research on the relationship between thermal aging processes, residual stress, and dimensional stability in machine tool bed castings, conducted in collaboration with partner units.
Rigidity describes a component’s resistance to deformation under external forces, influenced by material strength and the geometric shape and dimensions of the cross-section, the latter often referred to as structural rigidity. Poor structural rigidity is a significant factor in geometric accuracy changes in machine tool castings, such as beds. Due to inadequate structural rigidity, geometric accuracy can alter under load or even under self-weight. For instance, in a precision lead screw lathe bed, poor rigidity caused a sag of 23.5 micrometers under load, leading to machine tool rework. After improvement—adding a middle leg to a bed originally with only two end legs—the sag was reduced to 0.7 micrometers. Similarly, in a planer crossrail, before rigidity enhancement, sag was 20–25 micrometers when tool carriers moved to the middle, but after strengthening, it decreased to 7 micrometers. These examples highlight the substantial impact of structural rigidity on dimensional accuracy changes in certain machine tool castings. Moreover, poor structural rigidity can cause deformation during handling or storage under self-weight; thus, beam-type components like worktables should be supported or lifted appropriately.
Factors Influencing Deformation of Machine Tool Castings
As the foundation for machine tool installation, beds and other base components with precision fits (e.g., worktables, columns, saddles) directly affect machine tool accuracy through their deformation. Deformation during processing, transportation, assembly, and storage significantly impacts production, and deformation during use directly influences the accuracy of machined parts. Hence, this issue has garnered widespread attention. Our investigations into factors affecting casting deformation over the years indicate that the main factors include:
- Structural rigidity of the casting and load magnitude during use.
- Thermal deformation due to temperature variations. Changes in ambient air temperature or localized heating after machine startup are key factors causing deformation in machine tool castings. Thermal deformation is an elastic deformation resulting from temperature changes, an inherent physical phenomenon. Temperature variations affect straightness of machine tool guides due to: (a) Non-uniform cooling during casting leading to inhomogeneous microstructure, where different phases have different thermal expansion coefficients. For example:
- Pearlite: $\alpha = 10 \sim 11 \times 10^{-6} \, \text{mm/mm} \cdot \text{°C}$
- Ferrite: $\alpha = 12 \sim 12.5 \times 10^{-6} \, \text{mm/mm} \cdot \text{°C}$
- Cementite: $\alpha = 6 \sim 8.5 \times 10^{-6} \, \text{mm/mm} \cdot \text{°C}$
Non-uniform expansion occurs with temperature changes. (b) Temperature gradients in thick and thin sections due to different heat contents, causing uneven expansion and warping. Thermal deformation is predictable; for instance, in an M131W×710 universal cylindrical grinder, the bed guide convexity increases by 2–3 micrometers per °C temperature rise, reverting upon cooling.
- Foundation deformation. Since machine tools or bed castings are supported on foundations for accuracy measurement, creep in the foundation or underlying soil can affect geometric accuracy. Experiments show that foundations with different cement ratios, after curing for six months to 18 years, can cause installation level changes of 0.01–0.14 mm when people walk on them.
- Resistance of casting material to micro-plastic deformation at room temperature. Creep and dimensional instability under external forces arise from material inhomogeneity, elastic anisotropy of grains, varying grain orientations, and uneven load distribution, leading to intergranular stresses. In cast iron, the presence of graphite with strength around 2 kg/mm² facilitates micro-yield, especially at graphite tips due to stress concentration. Micro-yield involves dislocation processes in low-stress regions; enhancing resistance to dislocation improves resistance to micro-plastic deformation, achievable through heat treatment strengthening or stress relief.
- Magnitude and stability of residual stress in the casting. Residual stress (mainly Type I stress) in machine tool castings comprises:
- Casting stress: Formed during cooling into the elastoplastic range due to temperature differences and uneven contraction. Depends on material, molding工艺, pouring conditions, cooling uniformity, and shakeout temperature.
- Machining-induced stress: From metal removal disrupting the stress field and from localized plastic deformation during cutting, related to material properties and cutting parameters.
- Secondary stress from aging: Generated during cooling after aging due to temperature differences in the elastoplastic range, influenced by cooling rate, furnace temperature uniformity, and casting complexity.
These stresses accumulate and relax over time, affecting geometric stability.
Among these factors, structural design for adequate rigidity, constant-temperature conditions for precision machine tools, and proper foundation installation are essential for accuracy. However, resistance to micro-plastic deformation and residual stress reduction/stabilization are often overlooked, making them primary causes of machine tool deformation. Aging treatments, primarily thermal aging, are key to improving these aspects.
Thermal Aging Process for Machine Tool Castings
Thermal aging involves heating the casting to the elastoplastic temperature range, allowing residual stress relaxation and stabilization, followed by controlled cooling to avoid new stresses. Based on short-term (1.5-hour) creep tests on HT20-40 cast iron, we determined the elastoplastic temperature range and established a thermal aging specification suitable for small-to-medium precision machine tool castings.
The temperature-deformation curve for HT20-40 cast iron is shown in Figure 2 (not reproduced here, but described). It indicates that below 350°C, the material is essentially elastic; above 350°C, it enters the elastoplastic region, with intense deformation above 450°C. Aging above this temperature reduces residual stress significantly but is limited by hardness reduction. Since most residual stresses form in the elastoplastic range (350°C–450°C) due to uneven cooling, slow cooling is essential below 350°C to minimize secondary stresses.
The recommended thermal aging specification is as follows:
| Parameter | Value | Notes |
|---|---|---|
| Loading temperature | < 200°C | Avoid distortion from self-weight |
| Heating rate | ≤ 80°C/hour | For castings ≤2500 kg; adjust for complexity |
| Holding temperature | 530–550°C | To avoid hardness reduction |
| Holding time | 4–6 hours | For small-to-medium castings; ensures stress relaxation |
| Cooling rate above 350°C | Slow (e.g., ≤30°C/hour) | Prevent secondary stress |
| Cooling rate below 350°C | Can be faster | Based on elastoplastic curve |
Furnace temperature uniformity is critical; variations should be within ±25°C. Experiments on X62W bed castings showed that excessive cooling rates reduce stress relief effectiveness (Table 1).
| Cooling Condition | Residual Stress Relief (%) | Material |
|---|---|---|
| Slow cooling (per spec) | 54–87 | HT20-40 |
| Fast cooling | 14–65 | HT20-40 |
| Slow cooling | 55–86 | Phosphor-copper-titanium wear-resistant cast iron |
Furnace temperature differences greatly impact stress relief; Table 2 shows that large differences can even increase residual stress.
| Furnace Temperature Difference (°C) | Residual Stress Change (%) |
|---|---|
| 160–190 | 14–65 (relief) |
| 30–70 | Increase observed |
Proper arrangement of aging in the process sequence is vital. Measurements on T4240 coordinate boring machine beds and imitation C6225 lathe beds show stress variations across stages (Table 3).
| Stage | Stress Change Expression | Stress Change (%) for HT20-40 |
|---|---|---|
| As-cast (shakeout <200°C) | $\sigma_0$ (baseline) | 0 |
| As-cast (shakeout >200°C) | $\frac{\sigma_1 – \sigma_0}{\sigma_0} \times 100\%$ | +84 |
| After rough machining | $\frac{\sigma_2 – \sigma_1}{\sigma_1} \times 100\%$ | +250 (approx.) |
| After first aging | $\frac{\sigma_3 – \sigma_2}{\sigma_2} \times 100\%$ | -(54–87) |
| After second aging | $\frac{\sigma_4 – \sigma_3}{\sigma_3} \times 100\%$ | -(21–72) |
Where $\sigma_0, \sigma_1, \sigma_2, \sigma_3, \sigma_4$ represent residual stresses at different stages. This indicates that early shakeout increases stress, rough machining adds substantial stress, and aging after rough machining effectively reduces it. For precision machine tool castings, double thermal aging is recommended.
Relationship Between Thermal Aging, Residual Stress Elimination/Stabilization, and Dimensional Stability
Our experiments on T4240 coordinate boring machine beds and imitation C6225 lathe beds, using different aging methods (single thermal aging, double thermal aging, single thermal aging plus natural aging), measured residual stresses and periodically monitored guideway accuracy post-finish machining in constant temperature conditions. Results are summarized in Tables 4 and 5.
| Aging Method | Residual Stress (kg/mm²) | Annual Deformation (µm/year) | Notes |
|---|---|---|---|
| Single thermal aging | >1.99 | 3–4 | Over 15 months |
| Double thermal aging | 1.04 | <3 | Over 10–15 months |
| Combined aging (thermal + natural) | 2.0–1.7 | 3–4 | Over 15 months |
| Non-strict double aging* | ~1.5 | 6 | Over 8 months |
*Furnace温差 >50°C, lower holding temperature, uneven cooling.
| Material | Aging Method | Residual Stress (kg/mm²) | Annual Deformation (µm/year) |
|---|---|---|---|
| HT20-40 | Single thermal aging | 5.0–7.5 | 5.5–7 |
| HT20-40 | Double thermal aging | 1.8–3.0 | 2.4–3 |
| Phosphor-copper-titanium | Double thermal aging | 2.0–1.7 | <3 |
Double thermal aging on T4240 beds maintained geometric accuracy within 4 micrometers over 10–15 months, with residual stress below 2 kg/mm². For imitation C6225 beds, double aging improved stability by a factor of two compared to single aging. Thus, reducing and stabilizing residual stress through aging enhances dimensional stability, and double thermal aging is advisable for precision machine tool castings.
To assess the effect of aging on resistance to micro-plastic deformation, we tested imitation C6225 beds under load and temperature variations (Tables 6 and 7).
| Aging Method | Residual Stress (kg/mm²) | Deformation (µm) – Flat Guide | Deformation (µm) – V Guide |
|---|---|---|---|
| Single thermal aging | 1.59 + machining stress | Significant | Significant |
| Double thermal aging | 1.52 | Minimal | Minimal |
| Aging Method | Residual Stress (kg/mm²) | Deformation (µm) – Flat Guide | Deformation (µm) – V Guide |
|---|---|---|---|
| Single thermal aging | 1.59 + machining stress | Larger | Larger |
| Double thermal aging | 1.52 | Smaller | Smaller |
Double thermal aging reduces residual stress below 2 kg/mm², stabilizes it, and improves resistance to load and temperature-induced deformation, i.e., enhances relaxation resistance.
From experiments on dozens of bed castings for milling machines, grinders, coordinate boring machines, and lathes, we conclude:
- Shakeout temperature above 200°C increases residual stress by 84%; thus, early shakeout should be avoided in machine tool casting processes.
- Using the recommended thermal aging specification with small furnace温差, residual stress relief of 54–87% for HT20-40 and 55–86% for phosphor-copper-titanium wear-resistant cast iron is achieved.
- Machining, especially heavy rough machining, adds significant stress (about 2.5 times original stress). To eliminate this, aging should be scheduled after rough machining.
- Furnace temperature uniformity greatly affects aging quality; differences should be controlled within ±25°C.
- Double thermal aging on coordinate boring machine beds maintains accuracy within 4 micrometers over 10–15 months with residual stress below 2 kg/mm², and on imitation C6225 beds within 3 micrometers annually with stress below 1.2 kg/mm². Thus, double thermal aging is recommended for precision machine tool castings, as stress reduction and stabilization improve dimensional stability.
- Double thermal aging enhances static stability and resistance to load and thermal deformation by reducing and stabilizing residual stress.
Experimental Techniques Overview
Aging effectiveness is evaluated through residual stress measurement and dimensional stability observation. Residual stress measurement can be done on specimens like stress frames, open rings, or semi-rings, but direct measurement on workpieces is essential due to structural simplicity of specimens. Methods include X-ray, magnetic, ultrasonic, photoelastic, and mechanical electrical measurement techniques. For machine tool castings, mechanical electrical methods such as hole-drilling, ring-core, and layer-removal are common. These involve drilling or removing material to relax stress and measuring strain changes with high-sensitivity strain gauges. The original stress is calculated using elastic theory or experimental analysis. The hole-drilling method, for instance, measures strain before and after drilling a small hole. The stress components can be computed using formulas like:
$$
\sigma_x = -\frac{E}{2(1+\nu)} \left( \epsilon_a + \epsilon_b \right) + \frac{E}{2(1-\nu)} \sqrt{(\epsilon_a – \epsilon_b)^2 + (\epsilon_b – \epsilon_c)^2}
$$
where $\epsilon_a, \epsilon_b, \epsilon_c$ are strains at specific angles, $E$ is Young’s modulus, and $\nu$ is Poisson’s ratio. This method is accurate for low residual stresses (around 2 kg/mm²) but is destructive, time-consuming, and influenced by measurement location, microstructure homogeneity, and stress distribution. Non-destructive, economical, and rapid methods are under exploration.
Dimensional stability assessment is conducted in a constant temperature room at 20±1°C. Castings are acclimatized for over 12 hours before measurement. Supported on foundations, straightness of guides is measured periodically using instruments like autocollimators (minimum division 1 second or 0.001/200 mm) or coincidence levels (0.01/1000 mm). Deformation per month is compared to the initial post-scraping accuracy curve, with maximum deviation over the guide length considered. Since differences between aging treatments are small (few micrometers), measurement error must be less than deformation values. Error analysis for Zeiss coincidence levels and autocollimators shows total errors of 0.28–0.54 divisions and 0.2–0.36 divisions, respectively. Statistical analysis of 330 autocollimator and 410 level data points indicates standard errors of 0.3 and 0.36 divisions, with 87% and 80% of errors within 0.4 divisions. Thus, autocollimator accuracy is within ±0.5 µm, and level accuracy within 0.8 µm, ensuring reliable stability assessment for machine tool castings.
Other Aging Methods for Machine Tool Castings
Research on aging treatments, residual stress measurement, and dimensional stability is active globally. Aging aims to improve dimensional stability by stress relaxation and base strengthening. Methods can be categorized:
- Class I: Minimal stress change, primarily strengthening the matrix at graphite stress concentration points to improve relaxation rigidity. Includes natural aging, vibration aging, low-temperature annealing, hydro-electric effect, and shot peening.
- Class II: Significant stress change through stress relief with matrix strengthening. Includes static loading and thermal shock.
- Class III: Thermal aging, which substantially reduces stress.
In machine tool casting industries, thermal aging is predominant, but vibration aging shows promise. Vibration aging, using resonant frequencies for 10–60 minutes, claims better dimensional stability than natural or thermal aging, with advantages of shorter time, no furnace needed, and potential for automation. However, early attempts with high-frequency, long-duration vibration yielded poor stress relief and stability. Current trends favor natural frequency resonance for effective stress stabilization. As we continue research, critical吸收 of foreign technologies while adapting to domestic needs is essential for advancing machine tool casting precision in line with industrial development.
In summary, thermal aging plays a crucial role in managing residual stresses in machine tool castings. Through controlled heating and cooling, combined with proper process sequencing, significant stress reduction and stabilization can be achieved, directly enhancing dimensional stability. Our experiments confirm that double thermal aging, in particular, offers optimal results for precision applications. Future work should explore hybrid methods and non-destructive measurement techniques to further optimize the manufacturing of high-stability machine tool castings.