Why Do Sheets Warp Again After Leveling? The Principles, Contributing Factors, And Countermeasures for Aging Deformation

Why Does Material Warp Again After Leveling? The Principles and Solutions for Aging-Induced Warping

After sheet metal is flattened on a leveling machine and passes flatness inspection, it may warp again after being left for two or three days. This situation is common in factories, and many people mistakenly believe it is due to improper adjustment of the leveling machine, but that is not the root cause. The fundamental reason for the sheet metal’s re-deformation after leveling is the redistribution of internal residual stresses—a phenomenon known as aging deformation.

The Principle of Aging Deformation

Residual stresses within sheet metal originate from various processes, including rolling, cutting, and transportation. The leveling machine induces plastic deformation in the sheet metal through repeated bending, thereby releasing a portion of the residual stresses and temporarily flattening it. However, the leveling process cannot completely eliminate all stresses; it merely reduces them to a temporary state of equilibrium.

A portion of “locked stress” still remains within the sheet; this stress was not fully released during the leveling process and behaves much like a compressed spring. Over time, these stresses slowly relax, causing changes in the stress distribution that disrupt the original equilibrium and result in the sheet deforming once again. This process is known as stress relaxation and is an inherent property of metallic materials.

The rate of stress relaxation is closely related to ambient temperature. At room temperature, stress relaxation in carbon steel is relatively slow, and it may take several days or even weeks to observe noticeable deformation. However, if ambient temperatures fluctuate significantly (such as due to diurnal temperature variations), thermal expansion and contraction accelerate the redistribution of stresses, making the deformation more pronounced. This is why sheet metal is more prone to springback after being flattened in winter than in summer.

Which materials are most prone to aging deformation?

High-strength steel: This is a “hotspot” for aging deformation. Due to its high yield strength, greater bending force is required during leveling, which leaves more residual stresses trapped within the material. After leveling, if left to stand for 48 hours, the change in flatness can reach 2–4 mm/m—enough to render precision parts unusable.

Stainless steel: This material is also prone to aging deformation. Its work-hardening properties cause the surface hardness to increase after leveling, resulting in a more complex distribution of internal stresses. After leveling, 304 stainless steel left to stand for one week may exhibit a flatness change of approximately 1–3 mm/m.

Carbon steel sheets: Relatively stable. For ordinary carbon steels such as Q235 and Q345, the flatness change 48 hours after leveling typically does not exceed 1 mm/m, which can generally be ignored under standard machining accuracy requirements. However, if the sheet has undergone flame cutting, the enormous residual stresses generated in the heat-affected zone can also lead to significant aging deformation.

Aluminum sheets: Ageing deformation is relatively minor. Aluminum has a low modulus of elasticity, allowing for more thorough release of residual stresses during flattening and resulting in fewer locked stresses. However, aluminum sheets are sensitive to temperature changes; in environments with large temperature differentials, thermal deformation may be more significant than the effects of stress relaxation.

Copper sheets: The behavior falls between that of carbon steel and aluminum sheets. The rate of stress relaxation is faster than in steel; the material typically reaches a stable state within 24 hours, after which deformation is minimal.

The Effect of Thickness on Aging Deformation

A counterintuitive phenomenon is that thin sheets are more prone to aging deformation than thick sheets.

Although thick plates have higher absolute residual stresses, they also possess greater rigidity. For the same change in residual stress, the bending moment generated in a thick plate is insufficient to cause deformation, whereas in a thin plate with lower rigidity, it can result in visible bending. For example, a 2-mm-thick steel plate will exhibit noticeable deformation with a stress change of 50 MPa, whereas a 20-mm-thick steel plate will show almost no visible effect from the same stress change.

Experience shows that age-hardening deformation is most pronounced in thin plates less than 3 mm thick, followed by medium-thickness plates between 6 and 12 mm, while thick plates 16 mm or thicker exhibit minimal deformation. Therefore, it is recommended to allow thin plates to rest for 24 hours after leveling before rechecking their flatness, whereas this is generally not necessary for thick plates.

Four Methods to Reduce Ageing Deformation

Allow the sheet to rest after leveling before further processing: Do not proceed to the next process immediately after leveling the sheet. Instead, lay it flat (avoid standing it upright or stacking it) for 24 to 48 hours to allow residual stresses to relax naturally. For high-strength steel and flame-cut sheets, it is recommended to extend this period to 72 hours.

Multiple Passes of Leveling with Interval Resting: Perform an initial leveling pass to release most of the stress, let the sheet rest for 24 hours to allow residual stress to relax, and then perform a second leveling pass to correct the deformation caused by stress relaxation. This method provides significantly better stability than a single leveling pass and can reduce aging deformation by more than 60 percent; it is particularly suitable for high-strength steel and thin sheets.

Control the ambient temperature: Store the flattened sheets in a constant-temperature environment, avoiding direct sunlight and drastic temperature fluctuations. For every 10°C change in temperature, the stress relaxation rate of the steel sheet changes by approximately 30%. For precision machining, it is recommended to maintain the workshop temperature at 20±5°C.

Stress-Relief Annealing: For sheets with particularly severe aging deformation, stress-relief annealing can be performed prior to leveling. Typically, holding the sheet at 550–650°C (with the holding time calculated based on sheet thickness—1–2 minutes per millimeter) can reduce residual stress by more than 70%, resulting in much greater stability during subsequent leveling. This method is relatively costly and is generally used for workpieces requiring high precision.

How to Determine If the Sheet Has Stabilized?

A simple method is to measure flatness every 12 hours after leveling, taking three consecutive measurements. If the variation among the three measurements does not exceed 0.5 mm/m, the sheet is considered essentially stable.

When measuring, place the sheet on a level platform and use a feeler gauge to check the gap between the sheet and the platform. Select at least five measurement points (one at the center and one at each corner), and use the maximum gap value as the flatness value.

In summary, the recurrence of deformation in sheets after leveling is not a problem with the leveling machine, but rather an inherent characteristic of the metal material. Only by understanding the principles of aging deformation and implementing proper placement, re-inspection, and process control can the sheets achieve true and lasting flatness.