Residual stress is inevitably generated during the cooling and solidification of machine tool gray iron castings such as machine beds, columns and worktables. Unrelieved residual stress will release slowly during long-term use, leading to permanent deformation and precision failure of machine tools. Residual stress can be eliminated or reduced through pre-casting structural optimization, post-casting aging heat treatment and auxiliary processing techniques.
1. Structural optimization during casting design (fundamental prevention)
This method reduces the generation of residual stress from the source before production.
Set smooth large fillets at wall thickness transitions, rib intersections and inner corners to avoid sharp right-angle stress concentration zones.
Design uniform wall thickness as much as possible; avoid excessive local thick bosses and isolated heavy hot spots that cause uneven cooling shrinkage.
Distribute reinforcing ribs symmetrically and evenly to balance shrinkage force during solidification and lower asymmetric tensile/compressive stress.
Add reasonable shrinkage relief grooves at large-size thick sections to buffer volume shrinkage and reduce locked internal stress.
2. Natural aging treatment (low-cost stress relief method)
After casting demolding and rough cleaning, place castings in open workshops for long-term natural placement.
Principle: Under alternating natural temperature changes, internal metal grains creep slowly to release partial residual stress.
Operation requirements: The aging cycle generally lasts 3–12 months according to casting size; large heavy machine tool castings need more than half a year. Turn the casting regularly to ensure uniform temperature exchange on all surfaces.
Advantages: Low cost, no heat treatment equipment required.
Disadvantages: Long cycle, low stress removal efficiency, only eliminates 30%–40% residual stress, unable to meet high-precision machine tool standards alone.
3. Artificial stress relief annealing (most mainstream and effective process)
It is the standard process for high-precision machine tool castings to eliminate most residual stress.
Heating stage: Heat the casting uniformly to 500–600 °C at a slow heating rate to avoid generating new thermal stress from rapid temperature rise.
Constant temperature holding stage: Keep the temperature stable for 4–10 hours depending on casting wall thickness and overall volume; long holding time enables full grain rearrangement and stress offset.
Slow cooling stage: Cool the furnace together with the casting at a controlled low speed to below 200 °C before air cooling. Fast cooling will form new secondary residual stress.
Effect: Can eliminate 70%–90% of casting residual stress, greatly improving long-term dimensional stability. All precision CNC machine tool castings must adopt this process.
4. Vibration aging treatment (energy-saving auxiliary stress elimination)
A green heat-treatment-free process widely matched with annealing for secondary stress relief.
Fix the casting on a vibration platform, install a vibration exciter at the main stress concentration position.
Adjust the excitation frequency to the natural resonance frequency of the casting and maintain continuous vibration for 30–120 minutes.
Alternate multi-point vibration to cover all rib intersections, thick wall areas and bosses.
Principle: Mechanical vibration applies alternating micro-strain inside the casting, activating metal creep to release locked residual stress.
Advantages: Low energy consumption, short cycle, no oxidation decarburization on casting surface; suitable for secondary stress relief after rough machining.
Application: Usually used after rough cutting to eliminate new processing residual stress generated during material removal.
5. Rough machining + secondary aging composite process
Cutting and rough milling remove excess allowance, which will produce new machining residual stress. The composite process thoroughly eliminates double stress of casting and machining:
Complete casting annealing aging first to remove primary casting residual stress.
Carry out rough machining to cut off large margins, release surface constraint stress.
Conduct secondary low-temperature annealing or vibration aging again to eliminate machining-induced residual stress before finish machining.
This process is mandatory for ultra-precision machine tool castings such as grinding machine beds and measuring machine bases.
6. Low-temperature tempering supplementary treatment
For small and medium-sized machine tool castings with low precision requirements, low-temperature holding at 200–300 °C for short-term tempering can relieve partial surface residual stress without changing casting metallographic structure, serving as an economical supplementary measure.
Key supporting notes
Even after full stress elimination treatment, improper storage and installation will reintroduce stress:
Store castings flat on multi-point uniform supports, avoid single-point bearing or long-term tilting placement.
After machine assembly, complete precision leveling regularly to prevent uneven foundation support from generating new bending stress.
References
GB/T 7714
Wang Q, Li C. Comprehensive residual stress elimination technology for heavy machine tool gray iron castings[J]. Foundry, 2021,70(11):1176-1181.
MLA
Wang, Qiang, and Chao Li. "Comprehensive Residual Stress Elimination Technology for Heavy Machine Tool Gray Iron Castings." Foundry, vol. 70, no. 11, 2021, pp. 1176-1181.
APA 7th
Wang, Q., & Li, C. (2021). Comprehensive residual stress elimination technology for heavy machine tool gray iron castings. Foundry, 70(11), 1176–1181.
